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Article

Short-Term Effects of N Deposition on Soil Respiration in Pine and Oak Monocultures

by
Azam Nouraei
1,
Seyed Mohammad Hojjati
1,*,
Hamid Jalilvand
1,
Patrick Schleppi
2 and
Seyed Jalil Alavi
3
1
Department of Forest Sciences and Engineering, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari 4818168984, Iran
2
Swiss Federal Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland
3
Department of Forestry, Tarbiat Modares University, Tehran 1411713116, Iran
*
Author to whom correspondence should be addressed.
Forests 2025, 16(10), 1570; https://doi.org/10.3390/f16101570
Submission received: 8 September 2025 / Revised: 29 September 2025 / Accepted: 30 September 2025 / Published: 11 October 2025
(This article belongs to the Section Forest Soil)
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Abstract

Atmospheric nitrogen input has been a severe challenge worldwide. The influences of N deposition on carbon cycling, loss, and storage have been recognized as a critical issue. This study aimed to assess the immediate responses of soil respiration to different N deposition treatments in radiata pine (Pinus radiata D. Don) and chestnut-leaved oak (Quercus castaneifolia C. A. Mey) plantations within 12 months. N treatments were performed monthly at levels of 0, 50, 100, and 150 kg N ha−1 year−1 from October 2017 to September 2018. Litterfall was collected and analyzed seasonally for its mass and C content. Within the 0–10 cm depth of mineral soil in both plantations, parameters such as total nitrogen, pH, microbial biomass carbon (MBC), organic carbon (OC), and fine root biomass were measured seasonally. Soil respiration (Rs) was determined through monthly measurements of CO2 concentration in the field using a portable, closed chamber technique. The control plots exhibited the highest Rs during spring (2.96, 2.85 μmol CO2 m−2 s−1) and summer (2.92, 3.1 μmol CO2 m−2 s−1) seasons in oak and pine plantations, respectively. However, the introduction of nitrogen significantly diminished Rs in both plantations. Moreover, N treatments caused a notable reduction of soil MBC and fine root biomass. Soil microbial entropy and the C/N ratio were also significantly decreased by nitrogen treatments in both plantations, with the most prominent effects observed in summer. The observed decline in Rs in N-treated plots can be attributed to the decrease in MBC and fine root biomass, potentially with distinct contributions of these components in the pine and oak plantations. Our findings suggested that N-induced alteration in soil carbon dynamics was more pronounced in the oak plantation, which resulted in more SOC accumulation with increasing N inputs, while the pine plantation showed no significant changes in SOC.

1. Introduction

Atmospheric nitrogen (N) inputs have increased due to the widespread use of fuel combustion and the application of fertilizers, along with intensive animal husbandry [1]. Elevated N input has prominent effects on biogeochemical processes in forest soils. To our knowledge, soil respiration [2,3,4], soil chemistry [5,6,7], and soil microbial and enzyme activities [8] are important processes in the C cycle, which have been affected by N deposition.
The influences of N deposition on carbon cycling and storage have been recognized as a critical issue, leading to numerous experimental studies in recent decades aimed at evaluating its effects on C cycles in various ecosystems [9,10,11,12,13,14,15]. Given that the soil C pool substantially outweighs plant C storage in terrestrial ecosystems [16], its role in the global C cycle is pivotal. Storing soil C comprises the interactions of inputs via litterfall, throughfall, and losses through soil respiration [17], and leaching of dissolved organic C [18], and all these processes might be affected by the availability of nitrogen (for plants and soil microbes).
Since soil respiration (Rs) constitutes a major contributor to atmospheric carbon dioxide levels [19], even slight changes in Rs rates can lead to a significant variation in atmospheric CO2 concentration [16,20,21]. According to Fang et al. [22], in the past decades, the atmospheric CO2 concentration has increased by approximately 35% (from 285 to 400 ppm) and is predicted to reach 700–800 ppm at the end of this century [23]. With the rise in anthropogenic N deposition rates, there is an urgent need to understand the relationship between Rs and N inputs [24,25,26]. Notably, increased N deposition may decrease soil respiration in the naturally N-limited temperate forest ecosystems [3]. Documented studies revealed diverse effects of N additions on soil CO2 emission, including decline [27,28], rise [29], or no significant change [30,31,32]. Various factors can affect soil respiration rates in temperate forests, including moisture [33] and temperature [34], both affecting fine root biomass and activity as the source of autotrophic respiration [29,35], and soil microbial activity [36,37] as well. In previous studies, nitrogen addition experiments have demonstrated how variation in N availability affects soil carbon efflux and plant root growth [38]. Furthermore, it is illustrated that elevated N concentration may increase both the quantity and quality of litterfall [39,40,41]. Additionally, soil acidification resulting from N addition has the potential to alter microbial biomass and diversity [13,42,43] and influence soil enzyme activities [44,45,46]. As Yuan et al. [14] claimed, the combined effects of biological and microclimatic factors may control Rs under N deposition. Understanding these multifaceted impacts is crucial for unraveling the complexities of nitrogen-induced changes in soil respiration and its broader implications for carbon cycling in temperate forest ecosystems.
Hyrcanian forests are one of the last remnants of natural temperate deciduous forests in the world [47], which were recently inscribed on the UNESCO World Heritage List [48]. Despite this international recognition, the conversion of forest ecosystems to arable land, intensive livestock husbandry, and industrial pollution has led to excessive deposition of reactive N in this unique eco-region [49], potentially disrupting the carbon cycle. The present study was conducted to assess the short-term impact of experimental N additions on the soil CO2 efflux and its main drivers in pine and oak plantations. Since the provision of surplus available nitrogen via N deposition diminishes soil microbial activity and fine root biomass, we hypothesized that the higher levels of N input would result in lower rates of soil respiration. Specifically, the present work seeks to elucidate the short-term impact of experimental N addition on soil CO2 emission and to discern the predominant below-ground CO2 sources in these two distinct plantations.

2. Materials and Methods

This study was carried out in two 34-year-old neighboring monocultures of radiated pine (Pinus radiata D. Don) and chestnut-leaved oak (Quercus castaneifolia C. A. Mey), situated near Sari city, Mazandaran Province, north of Iran (latitude: 36°30′ N, longitude: 53°2′ E, and elevation 400 m). Both plantations were located in a flat area. The annual average precipitation and temperature are 950 mm and 17 °C, respectively. According to the USDA Soil Taxonomy, the soil type is classified as Alfisols. The average height and diameter of the oak trees were 20 m and 30 cm, and the pine trees were 35 m and 25 cm. The original natural forest was dominated by various native hardwood trees such as ironwood (Parrotia persica C. A. M.), chestnut-leaved oak (Q. castaneifolia C. A. M.), and hornbeam (Carpinus betulus L.), which had been degraded. In 1986, both stands were clear-cut and then reforested with pine and oak trees (plant spacing 3 m × 3 m). Herbaceous species dominated, including perforate woodruff (Asperula odorata L.), St. John’s wort (Hypericum perforatum L.), sweet and ann ala (Hedera pastuchovii Woronow), and butcher’s broom (Ruscus hyrcanus Woronow), which covered less than 10% of both plantations. Chestnut-leaved oak is a high-quality native hardwood tree in the Hyrcanian forests, but pine is an exotic, coniferous species that was introduced to the Hyrcanian region specifically for reforestation purposes [50].

2.1. Experimental Design

An N-addition experiment was set up according to the design explained by Mo et al. [51] and Tian et al. [52]. In October 2017, twelve 200-m2 (20 m × 10 m) experimental plots were selected as a randomized block design in each plantation. A buffer distance of 10 m between plots was implemented, as suggested by Mo et al. [51] and Ma et al. [53]. Salehi et al. [54] reported that Hyrcanian forests (altitudes < 300 m) receive about 56 kg N ha−1 year−1 through wet N deposition. Considering these values, four levels of ammonium nitrate (NH4NO3) were considered as control treatment (C, zero), low (LN, 50 kg N ha−1 year−1), medium (MN, 100 kg N ha−1 year−1), and high (HN, 150 kg N ha−1 year−1). Each treatment was applied in three replicates. To achieve the desired nitrogen application rates, an aqueous stock solution of NH4NO3 was prepared and mixed with 20 L of tap water. This solution was then sprayed monthly under the canopy for 12 months (from October 2017 to September 2018). Control plots received the equivalent of 20 L of tap water without NH4NO3 application (see Figure 1 for a visual representation of the experimental setup). Given the lack of reliable dry deposition data for the Hyrcanian region, we based our N application rates on reported wet deposition values, acknowledging that total atmospheric N input may be somewhat higher.

2.2. Soil Sampling

Within each plot, three samples of topsoil (0–10 cm) were collected seasonally using the coring method (10 cm in height; 8 cm inner diameter). The organic layer was removed from the sampling points before the samples were taken. The soil samples were stored at 4 °C for analysis using conventional methods. Soil pH was determined in a 1:2.5 soil-to-water suspension; the Walkley and Black technique was performed to estimate soil organic C (SOC) [55], and the combustion method was applied to measure litter carbon. Total N was determined using the semi-micro Kjeldahl technique by Olsen et al. [56]. The fumigation-extraction technique was employed to estimate soil microbial biomass carbon or MBC [57,58], and microbial entropy or ratio was calculated based on the MBC/SOC [59]. The soil properties prior to the application of nitrogen treatments are presented in Table 1.

2.3. Measuring Soil Respiration (Rs)

In September 2017, three PVC cylinders (15 cm inner diameter and 25 cm in height) were installed 10 cm into the soil (far enough from the edge of the plots). By using a portable, closed chamber technique, an infrared absorption sensor with a CO2PORT device (Messwert Company GmbH, Göttingen, Germany; a developed version of the infrared gas analyzer, Edinburgh Sensors-Gascard II), the concentration of emitted CO2 was measured as the soil respiration (Rs) monthly from October 2017 to September 2018 [4,60]. All Rs measurements were conducted from 10 am to 3 pm. Simultaneously, soil temperature was recorded at 10 cm depth with a temperature probe in each Rs measurement point.

2.4. Litterfall Collection

Three litter traps (0.5 m × 0.5 m) were installed 0.5 m above the ground in each of the 12 plots for monthly litterfall sampling [61]. The samples were transported to the laboratory and dried at 70 °C for 48 h. Monthly litterfall samples were pooled to create composite seasonal samples (three months), ground with a mill, and passed through a sieve of 0.5 mm. The combustion method was applied to measure the concentration of organic carbon (OC) of each litter sample.

2.5. Fine Root Biomass Measurement

To sample the fine roots, three soil cores (10 cm in length, 8 cm inner diameter) were taken from each plot seasonally (November 2017, February 2018, May 2018, and August 2018). Soil samples were immediately sealed in plastic bags. In the laboratory, fine roots (<2 mm) were removed from soil samples and cleaned with distilled water. The samples were dried to a constant mass and then weighed [62].

2.6. Statistical Analysis

The normality of the data was checked using the Shapiro–Wilk test, and Levene’s test was employed for testing the homogeneity of the variances. Repeated measures analysis of variance (ANOVA) was used to estimate the effects of sampling time (seasons) and N treatments on the soil characteristics (Rs, MBC, OC, litterfall mass, and C, fine root biomass). A significance level of p < 0.01 was applied throughout. Multiple comparisons were conducted using Bonferroni correction following repeated measures ANOVA. The effect of seasons on the soil characteristics (Rs, MBC, OC, litterfall mass, C, fine root biomass, C/N, and microbial entropy) was analyzed by repeated measurements with Bonferroni correction (pairwise comparisons) in each plantation. A one-way ANOVA followed by an LSD test was performed to compare changes under N treatments on the soil characteristics (Rs, MBC, OC, litterfall mass, and C, fine root biomass). Regression analyses were applied to assess the relationship between Rs and variables potentially influencing soil respiration. All statistical analyses were conducted in the SPSS 19.0 statistical software package.

3. Results

3.1. Microbial Biomass Carbon (MBC)

MBC exhibited a peak during the summer season and reached its minimum levels in both autumn and winter. N treatments significantly decreased the MBC in the soil, also in the order of the applied amounts (C > LN > MN > HN). The most substantial negative effect of the N treatments was observed in summer, which yielded a significant interaction between treatment and season. The pine plantation showed a significantly (p < 0.01) higher MBC than the oak plantation and, at the same time, a stronger decrease with the N treatments, making a significant interaction between N treatments and species (Figure 2; Table 2).

3.2. Soil Organic Carbon (SOC)

In the pine plantation, nitrogen additions did not have a statistically significant effect on soil organic carbon (SOC). However, significant differences were observed concerning seasons and the interaction of seasons/treatments. On the other hand, in the oak plantation, N treatments led to higher SOC levels during the summer season, resulting in a significant overall effect of the treatments. The impact of the treatments varied across seasons, with SOC levels being highest in summer, followed by spring, fall, and winter. The interaction between treatments and seasons was also found to be significant (Figure 3; Table 2). Overall, the amount of SOC in the pine plantation was significantly higher compared with the oak one.

3.3. Soil Microbial Entropy and C/N Ratio

Both soil microbial entropy and soil C/N ratio were significantly higher (p < 0.01) in the pine plantation compared to the oak plantation (Figure A1). After one year of N addition, soil microbial entropy decreased by 24% and 33% in MN and HN treatments in the pine plantation, respectively. Concurrently, medium and high N additions resulted in a decrease in soil C/N ratio by 20% and 26%, respectively, compared to the control. In the oak plantation, soil microbial entropy showed reductions of 23% and 29% in the MN and HN treatments, while the soil C/N ratio decreased by 14% in the HN treatment compared to the other treatments.

3.4. Soil Respiration (Rs)

The lowest Rs was observed in winter, while the highest one was recorded in summer. Both seasons and nitrogen (N) treatments, along with their interaction, exerted significant effects on soil CO2 emission. Nitrogen addition led to a decrease in soil respiration, particularly in the spring and summer, towards the end of the experiment. The Rs values in plots with different N levels followed the order of the amount of added nitrogen: C > LN > MN > HN, resulting in a reduction of up to 27% at high and medium N levels (Figure 4). There was also a significant difference (p < 0.01) in Rs between the two plantations and an interaction between treatment and species, as the pine plantation showed slightly higher respiration rates and a more pronounced decrease in Rs with increasing N inputs (Table 2).

3.5. Litterfall

The highest litterfall production and the associated C input to the soil were recorded during autumn and winter. The oak plantation exhibited a significantly higher mass and C flux via litterfall. N additions in both plantations showed no significant effect on litterfall production and the corresponding C input to the soil (Figure 5 and Figure 6; Table 2). These findings indicate that the litterfall dynamics and C input were primarily influenced by seasonal variations rather than short-term N treatments in the studied plantations.

3.6. Fine Root Biomass

In both plantations, higher fine root biomass was recorded during autumn (Figure 7; Table 2). Across all seasons and N treatments, the fine root biomass was significantly (p < 0.01) greater in the pine plantation compared to the oak one. N treatments had a significant (p < 0.01) effect on fine root biomass in both populations. The N addition reduced the fine root biomass, which was more pronounced in the summer (toward the end of the experiment). The impact of different N levels on the fine root biomass was in the order of C > LN > MN > HN (Figure 7).

3.7. Relationship Between Rs and Soil Properties Under N Treatments

Rs exhibited a positive correlation with soil temperature, elucidating 70% of the variations (R2) of soil respiration in control plots within the pine plantation. Corresponding R2 were 65%, 39%, and 29% in low, medium, and high N treatments in the pine plantation, respectively (Figure A2a). In the oak plantation, soil temperature explained 66%, 65%, 34%, and 25% of the variations (R2) in soil respiration in control, low, medium, and high N treatments, respectively (Figure A2b).
At the end of the N addition experiment, Rs was positively correlated with soil pH, microbial entropy, MBC, and fine root biomass under different N treatments in both plantations. Soil pH explained 47% and 26% of the variations in Rs under oak and pine plantations, respectively. The relationship between Rs and MBC was stronger (R2 = 44% and 58%) in both stands. Rs was positively correlated with fine root biomass in two plantations; fine root biomass explained 50% and 48% of the variations in Rs under pine and oak canopies, respectively. Rs was positively correlated with microbial entropy, but the correlation between Rs and microbial entropy was much weaker in the pine plantation compared with the oak one (Figure A3 and Figure A4). There was a significant linear relationship between fine root biomass and MBC in both plantations. Fine root biomass explained 46% and 40% of the variation in MBC in pine and oak plantations, respectively (Figure A5).

4. Discussion

Litterfall serves as a crucial pathway for the input of organic matter into the forest soils as the primary source of nutrients and energy [61]. The simulated short-term N depositions had no significant effect on litterfall production and its organic carbon content in our experimental plots. In line with Zhang et al. [28] and Peng et al. [3], short-term N additions did not affect litterfall amount and organic carbon input. Previous long-term studies showed higher N concentrations in above- and below-ground biomass in response to nitrogen addition [63,64]. However, litterfall decreased in response to the chronic or long-term extra nitrogen inputs in a temperate forest when the soil became N-saturated [65,66]. Thus, in different forest ecosystems, the quantity and quality of litterfall response following N input may vary according to the intensity and duration of N addition experiments [3]. Consistent with previous studies, an increase in forest soil OC content can occur owing to the increase in biomass production and litterfall by alleviating the N limitation [67] and/or decelerating litter decomposition [68]. However, in this study, litterfall was not affected by N deposition; thus, an increase in soil carbon content in the oak plantation might have been due to the reduction of the litter decomposition rate [3]. Although the pine plantation had a higher initial SOC content, the oak plantation demonstrated a significant increase in SOC under high N treatment, while the pine stand showed no significant change. This finding is explained by the N-induced suppression of microbial activity [37,69]. The added N led to soil acidification, which significantly reduced microbial biomass carbon (MBC) and microbial entropy via inhibiting organic matter decomposition rather than by increasing litter input, which was more pronounced in the oak plantation.
According to our findings, in both plantations, Rs showed a clear seasonal pattern in all treatments, with a peak in the summer (warm season) and the lowest rates recorded in winter. Similar results were obtained in many previous studies in coniferous [70,71] and temperate deciduous forests [30,72,73]. Our results imply that N treatments significantly reduced soil respiration. Similar findings (Rs reduction) were reported in a Larix mastersiana forest during the first year of the N additions experiment [74]. These results are similar to those found in several temperate forests [28,37,74,75]. Soil respiration originates from different processes (root respiration, faunal respiration, and microbial respiration) and is regulated by temperature, soil moisture, pH, and SOM amount and quality, the latter being influenced by vegetation type (land use) and disturbances [76,77]. In the present investigation, soil respiration was positively correlated with soil pH, MBC, microbial entropy, and fine roots in both reforested areas. These results are in accordance with those of other studies, such as Lee and Jose [78], Zhou et al. [75], and Zhang et al. [28].
The observed reduction in soil respiration after N additions may be attributed to changes in both autotrophic and heterotrophic components, constituting approximately 50% of the total CO2 emission (e.g., [29]). Firstly, autotrophic respiration (Ra) originating from roots and rhizosphere microorganisms may decrease after N additions [52]. In our study, fine root biomass declined with increasing N application in both plantations. Bowden et al. [30], Wang et al. [79], and Zhang et al. [28] also demonstrated that fine root biomass was significantly reduced in response to N treatments. In temperate forests, when N limitation is alleviated by N addition, trees tend to allocate less photosynthetic product into root systems [52,80]. Secondly, heterotrophic respiration arising from microbial communities may be decreased in N-treated plots. Our findings revealed that MBC significantly decreased in the N-treated plots, which was also claimed by Liu et al. [81] and Zhang et al. [28]. According to Janssens et al. [37], the reduction in MBC may be directly linked to the formation of stable compounds via the rapid reaction of NH4-NO3 with the low-molecular-weight organic matter, which led to a lower decomposition rate. In addition, Baath et al. [82] implied that there is a positive relationship between soil pH and microbial biomass production. Therefore, in line with the findings of Smolander et al. [83], the activity and abundance of the soil microbial community change due to variations in soil organic matter and soil pH. In our investigation, the reduced MBC is largely explained by the observed reduction in soil pH. Finally, N deposition can also lead to a decrease in the C/N ratio and the soil organic matter, making it less decomposable for microbes [84,85]. In our N treatment plots, soil N increased much more than C, indicating that the litter decomposition rate could be reduced due to the decrease in the C/N ratio of the substrate [7]. Nitrogen addition significantly lowered the C/N ratio compared with the control plots. The decline in the C/N ratio is attributed to the increase in nitrogen input during the nitrogen addition experiment. However, in the pine plantation, no significant effect of deposited N on the SOC was found, possibly because short-term N treatment was not sufficient to lead to a considerable increase in the SOC content [28] because needle leaves may be less immediately affected by N-induced acidification [83]. It’s worth noting that the SOC content was more variable in the pine plantation, making changes more challenging to detect.
In both plantations, soil respiration had high correlations with fine root biomass and MBC, which is consistent with the results of Zhou et al. [86] and Lee and Jose [78]. The MBC, SOC, and litter mass flux have been related to heterotrophic respiration [87,88]. Since N treatments clearly led to reductions in both fine root biomass and MBC, it can be inferred that these are the two dominating mechanisms explaining how N additions negatively impact soil respiration.
The weaker correlation between soil respiration (Rs) and microbial entropy in the pine plantation compared with the oak one (R2 = 0.199 vs. R2 = 0.566) likely reflects differences in belowground substrate quality and microbial community. Pine soils receive lignin-rich and recalcitrant litter, which reduces labile substrate availability, while oak soils receive more nutrient-rich, easily decomposable litter, fueling microbial activity and linking more strongly to Rs [89,90,91].
The positive correlation between Rs and soil temperature in our control and LN plots is in accord with the temperature sensitivity of soil respiration as a well-established principle of soil biogeochemistry [20,92]. However, the weakening of this relationship under medium and high N treatments suggests that extra N availability shifts microbial compositions and functions towards a slower decomposition process and consequently dampens the response of respiration to temperature [29,93]. This finding aligns with other studies [32,37,94] where N addition reduced the Q10 of soil respiration, likely by limiting labile carbon availability for microbes and shifting microbial community function.
In the HN plots of the oak plantation, soil organic C significantly increased, while N treatments did not impact soil carbon content in the pine plantation. The observed decline in soil MBC and the concomitant reduction in soil respiration are certainly the primary cause of SOC accumulation, as also claimed by Bowden et al. [30], Gundersen et al. [95], and He et al. [96]. Similar to our findings in the oak plantation, Nava et al. [97], Wei et al. [98], Ma et al. [53], and Chen et al. [99] also reported that increasing the N input led to an elevation in SOC.
An important contribution of our study is the contrasting responses observed between oak and pine plantations. Oak soils, receiving nutrient-rich and labile litter, accumulated more SOC under N enrichment due to reduced microbial decomposition, consistent with patterns reported in broadleaf forests [28,30,69]. In contrast, pine soils with recalcitrant litter showed no short-term SOC increase, reflecting slower decomposition and stronger long-term stabilization potential [3,70]. Microbial biomass declined more sharply in pine, suggesting higher sensitivity to N-induced acidification, while oak soils exhibited greater resilience [13,42,83]. Fine root biomass was higher in pine but decreased more strongly with N, indicating vulnerability in belowground carbon allocation [30,79]. Oak appears to favor short-term carbon sequestration, while pine supports longer-term stability, reinforcing the importance of species identity in managing C dynamics under elevated N deposition [10,86].
These contrasting mechanisms between a native broadleaf species (oak) and an exotic conifer (pine) emphasize the importance of species selection in forest management under the condition of elevated nitrogen deposition. In the Hyrcanian forests, where N inputs are projected to rise further due to both wet and dry deposition, understanding these species-specific responses is critical.

5. Conclusions

In conclusion, our study demonstrates that even short-term N additions can significantly affect soil pH, microbial biomass carbon, microbial entropy, fine root biomass, and soil respiration in pine and oak plantations. The reduction of soil respiration was driven by declines in both fine root biomass and microbial abundance, highlighting the role of belowground processes in mediating ecosystem C dynamics. We also found species-specific differences, with oak exhibiting higher SOC accumulation under N additions compared to pine, likely due to differences in litter quality and decomposition dynamics. These results have important implications for forest carbon cycling under continued N deposition, suggesting that native broadleaf species may contribute more labile carbon inputs while exotic conifers may promote long-term stabilization. Future long-term experiments are needed to verify these short-term responses and to better understand the cumulative effects of N deposition on forest soil C storage and ecosystem functioning.

Author Contributions

Conceptualization, S.M.H., H.J. and P.S.; methodology, S.M.H. and H.J.; formal analysis, H.J., S.J.A. and P.S.; measuring, sampling, lab and statistical analysis, and writing-original draft, A.N.; writing, review, and editing, S.M.H.; supervision, S.M.H. and H.J. All authors have read and agreed to the current version of the manuscript.

Funding

This study was financially supported by Sari Agricultural Sciences and Natural Resources University (NF-FD1396,15.08. 2017).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the Swiss Federal Institute for Forest, Snow, and Landscape Research for hosting the first author during a scientific stay.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Appendix A

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Figure A1. Mean (±ED) of microbial entropy (a) and C/N ratio (b) in different stands (lowercase letters) and N treatments (capital letters).
Figure A1. Mean (±ED) of microbial entropy (a) and C/N ratio (b) in different stands (lowercase letters) and N treatments (capital letters).
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Figure A2. Relationship between soil respiration rate (Rs) and soil temperature in N treatments in oak (a) and pine (b) plantations.
Figure A2. Relationship between soil respiration rate (Rs) and soil temperature in N treatments in oak (a) and pine (b) plantations.
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Figure A3. Relationship between respiration rate (Rs), soil pH (a), microbial entropy (b), MBC (c), and fine root biomass (d) in a pine plantation.
Figure A3. Relationship between respiration rate (Rs), soil pH (a), microbial entropy (b), MBC (c), and fine root biomass (d) in a pine plantation.
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Figure A4. Relationship between soil respiration rate (Rs), soil pH (a), microbial entropy (b), MBC (c), and fine root biomass (d) in an oak plantation.
Figure A4. Relationship between soil respiration rate (Rs), soil pH (a), microbial entropy (b), MBC (c), and fine root biomass (d) in an oak plantation.
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Figure A5. Relationship between fine root biomass with MBC in pine (a) and oak (b) plantations.
Figure A5. Relationship between fine root biomass with MBC in pine (a) and oak (b) plantations.
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References

  1. Galloway, J.N.; Townsend, A.R.; Erisman, J.W.; Bekunda, M.; Cai, Z.; Freney, J.R.; Martinelli, L.A.; Seitzinger, S.P.; Sutton, M.A. Transformation of the nitrogen cycle: Recent trends, questions, and potential solutions. Science 2008, 320, 889–892. [Google Scholar] [CrossRef]
  2. Wang, F.; Chen, F.; Wang, G.G.; Mao, R.; Fang, X.; Wang, H.; Bu, W. Effects of experimental nitrogen addition on nutrients and nonstructural carbohydrates of dominant understory plants in a Chinese fir plantation. Forests 2018, 10, 155. [Google Scholar] [CrossRef]
  3. Peng, Y.; Song, S.Y.; Li, Z.Y.; Li, S.; Chen, G.T.; Hu, H.L.; Xie, J.L.; Chen, G.; Xiao, Y.L.; Liu, L.; et al. Influences of nitrogen addition and aboveground litter-input manipulations on soil respiration and biochemical properties in a subtropical forest. Soil Biol. Biochem. 2020, 142, 107694. [Google Scholar] [CrossRef]
  4. Tafazoli, M.; Hojjati, S.M.; Jalilvand, H.; Lamersdorf, N.; Tafazoli, M. Effect of nitrogen addition on soil CO2 efflux and fine root biomass in maple monocultures of the Hyrcanian region. Ann. For. Sci. 2021, 78, 29. [Google Scholar] [CrossRef]
  5. Lu, X.; Mao, Q.; Gilliam, F.S.; Luo, Y.; Mo, J. Nitrogen deposition contributes to soil acidification in tropical ecosystems. Glob. Change Biol. 2014, 20, 3790–3801. [Google Scholar] [CrossRef]
  6. Tafazoli, M.; Hojjati, S.M.; Jalilvand, H.; Lamersdorf, N. Simulated Nitrogen Deposition Reduces the Concentration of Soil Base Cations in Acer velutinum Bioss. Plantation, North of Iran. J. Soil Sci. Plant Nutr. 2019, 19, 440–449. [Google Scholar] [CrossRef]
  7. Nouraei, A.; Jalilvand, H.; Hojjati, S.M.; Schleppi, P.; Alavi, S.J. Effects of artificial nitrogen deposition on the forest floor and soil chemistry under chestnut-leaved oak (Quercus castaneifolia C. A. Mey) plantation in the north of Iran. Can. J. For. Res. 2022, 52, 808–818. [Google Scholar] [CrossRef]
  8. Zhou, S.; Xiang, Y.; Tie, L.; Han, B.; Huang, C. Simulated nitrogen deposition significantly reduces soil respiration in an evergreen broadleaf forest in western China. PLoS ONE 2018, 13, e0204661. [Google Scholar] [CrossRef] [PubMed]
  9. Tu, L.H.; Hu, T.X.; Zhang, J.; Li, X.W.; Hu, H.L.; Liu, L.; Xiao, Y.L. Nitrogen addition stimulates different components of soil respiration in a subtropical bamboo ecosystem. Soil Biol. Biochem. 2013, 58, 255–264. [Google Scholar] [CrossRef]
  10. Hyvönen, R.; Ågren, G.I.; Linder, S.; Persson, T.; Cotrufo, M.F.; Ekblad, A.; Freeman, M.; Grelle, A.; Janssens, I.A.; Jarvis, P.G.; et al. The likely impact of elevated CO2, nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: A literature review. New Phytol. 2007, 173, 463–480. [Google Scholar] [CrossRef]
  11. Pregitzer, K.S.; Burton, A.J.; King, J.S.; Zak, D.R. Soil respiration, root biomass, and root turnover following long-term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3. New Phytol. 2008, 180, 153–161. [Google Scholar] [CrossRef]
  12. Zhong, Y.; Yan, W.; Shangguan, Z. Soil carbon and nitrogen fractions in the soil profile and their response to long-term nitrogen fertilization in a wheat field. Catena 2015, 135, 38–46. [Google Scholar] [CrossRef]
  13. Zhao, B.; Geng, Y.; Cao, J.; Yang, L.; Zhao, X. Contrasting Responses of Soil Respiration Components in Response to Five-Year Nitrogen Addition in a Pinus tabulaeformis Forest in Northern China. Forests 2018, 9, 544. [Google Scholar] [CrossRef]
  14. Yuan, Y.; Peng, M.; Tang, G.; Wang, Y. Effects of simulated nitrogen deposition on soil respiration: Evidence from a three-year field study of the Abies georgei (Orr) forest in the Jiaozi Snow Mountains National Nature Reserve, Southwest China. For. Ecol. Manag. 2023, 542, 121098. [Google Scholar] [CrossRef]
  15. Yang, X.; Ma, S.; Huang, E.; Zhang, D.; Chen, G.; Zhu, J.; Ji, C.; Zhu, B.; Liu, L.; Fang, J. Nitrogen addition promotes soil carbon accumulation globally. Sci. China Life Sci. 2025, 68, 284–293. [Google Scholar] [CrossRef] [PubMed]
  16. Schlesinger, W.H.; Andrews, J.A. Soil respiration and the global carbon cycle. Biogeochemistry 2000, 48, 7–20. [Google Scholar] [CrossRef]
  17. Creamer, C.A.; Filley, T.R.; Boutton, T.W.; Oleynik, S.; Kantola, I.B. Controls on soil carbon accumulation during woody plant encroachment: Evidence from physical fractionation, soil respiration, and δ13C of respired CO2. Soil Biol. Biochem. 2011, 43, 1678–1687. [Google Scholar] [CrossRef]
  18. Findlay, S.E.G. Increased carbon transport in the Hudson River: Unexpected consequence of nitrogen deposition. Front. Ecol. Environ. 2005, 3, 133–137. [Google Scholar] [CrossRef]
  19. Schimel, D.S. Terrestrial ecosystems and the carbon cycle. Glob. Change Biol. 1995, 1, 77–91. [Google Scholar] [CrossRef]
  20. Zhou, L.; Zhou, X.; Shao, J.; Nie, Y.; He, Y.; Jiang, L.; Wu, Z.; Bai, S.H. Interactive effects of global change factors on soil respiration and its components: A meta-analysis. Glob. Chang Biol. 2016, 22, 3157–3169. [Google Scholar] [CrossRef]
  21. Adachi, M.; Ito, A.; Yonemura, S.; Takeuchi, W. Estimation of global soil respiration by accounting for land-use changes derived from remote sensing data. J. Environ. Manag. 2017, 200, 97–104. [Google Scholar] [CrossRef]
  22. Fang, J.Y.; Zhu, J.L.; Wang, S.P.; Yue, C.; Shen, H.H. Global warming, human-induced carbon emissions, and their uncertainties. Sci. China Earth Sci. 2011, 54, 1458–1468. [Google Scholar] [CrossRef]
  23. IPCC. IPCC Climate Change 2021: The Physical Science Basis; Contribution of working group I to the sixth assessment report of the Intergovernmental Panel on Climate; IPCC: Geneva, Switzerland, 2021. [Google Scholar]
  24. Vitousek, P.M.; Aber, J.D.; Howarth, R.W.; Likens, G.E.; Matson, P.A.; Schindler, D.W.; Schlesinger, W.H.; Tilman, D.G. Human alteration of the global nitrogen cycle: Sources and consequences. Ecol. Appl. 1997, 7, 737–750. [Google Scholar] [CrossRef]
  25. Lu, X.; Vitousek, P.M.; Mao, Q.; Gilliam, F.S.; Luo, Y.; Turner, B.L.; Zhou, G.; Mo, J. Nitrogen deposition accelerates soil carbon sequestration in tropical forests. Proc. Natl. Acad. Sci. USA 2021, 118, e2024460118. [Google Scholar] [CrossRef]
  26. Liu, G.; Wang, Q.; Chen, J.; Yan, G.; Wang, H.; Xing, Y. Natural δ13C and δ15N abundance of plants and soils under long-term N addition in a temperate secondary forest. J. Soil Sci. Plant Nutr. 2024, 24, 3491–3503. [Google Scholar] [CrossRef]
  27. Maaroufi, N.I.; Nordin, A.; Hasselquist, N.J.; Bach, L.H.; Palmqvist, K.; Gundale, M.J. Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils. Glob. Change Biol. 2015, 21, 3169–3180. [Google Scholar] [CrossRef]
  28. Zhang, H.; Liu, Y.; Zhou, Z.; Zhang, Y. Inorganic Nitrogen Addition Affects Soil Respiration and Belowground Organic Carbon Fraction for a Pinus tabuliformis Forest. Forests 2019, 10, 369. [Google Scholar] [CrossRef]
  29. Li, Q.; Song, X.; Chang, S.X.; Peng, C.; Xiao, W.; Zhang, J.; Wang, W. Nitrogen depositions increase soil respiration and decrease temperature sensitivity in a Moso bamboo forest. Agri. For. Meteorol. 2019, 268, 48–54. [Google Scholar] [CrossRef]
  30. Bowden, R.D.; Davidson, E.; Savage, K.; Arabia, C.; Steudler, P. Chronic nitrogen additions reduce total soil respiration and microbial respiration in temperate forest soils at the Harvard Forest. For. Ecol. Manag. 2004, 196, 43–56. [Google Scholar] [CrossRef]
  31. Burton, A.J.; Pregitzer, K.S.; Reuss, R.W.; Hendrick, R.L.; Allen, M.F. Root respiration in North American forests: Effects of nitrogen concentration and temperature across biomes. Oecologia 2002, 131, 559–568. [Google Scholar] [CrossRef] [PubMed]
  32. Liu, X.; Yang, Z.; Lin, C.; Giardina, C.P.; Xiong, D.; Lin, W.; Chen, S.; Xu, C.; Chen, G.; Xie, J.; et al. Will nitrogen deposition mitigate warming-increased soil respiration in a young subtropical plantation? Agric. For. Meteorol. 2017, 246, 78–85. [Google Scholar] [CrossRef]
  33. Savage, K.E.; Davidson, E.A. Interannual variation of soil respiration in two New England forests. Glob. Biogeochem. Cycles 2001, 15, 337–350. [Google Scholar] [CrossRef]
  34. Makita, N.; Kosugi, Y.; Sakabe, A.; Kanazawa, A.; Ohkubo, S.; Tani, M. Seasonal and diurnal patterns of soil respiration in an evergreen coniferous forest: Evidence from six years of observation with automatic chambers. PLoS ONE 2018, 13, e0192622. [Google Scholar] [CrossRef] [PubMed]
  35. Dong, N.; Zhou, J.; Yan, G.; Liu, G.; Xing, Y.; Wang, Q. Effects of long-term nitrogen addition and precipitation reduction on the fine root dynamics and morphology in a temperate forest. Eur. J. For. Res. 2022, 141, 363–378. [Google Scholar] [CrossRef]
  36. Fisk, M.C.; Fahey, T.J. Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests. Biogeochemistry 2001, 53, 201–223. [Google Scholar] [CrossRef]
  37. Janssens, I.A.; Dieleman, W.; Luyssaert, S.; Subke, J.-A.; Reichstein, M.; Ceulemans, R.; Ciais, P.; Dolman, A.J.; Grace, J.; Matteucci, G.; et al. Reduction of forest soil respiration in response to nitrogen deposition. Nat. Geosci. 2010, 3, 315–322. [Google Scholar] [CrossRef]
  38. Zhang, C.; Niu, D.; Hall, S.J.; Wen, H.; Li, X.; Fu, H.; Wan, C.; Elser, J.J. Effects of simulated nitrogen deposition on soil respiration components and their temperature sensitivities in a semiarid grassland. Soil Biol. Biochem. 2014, 75, 113–123. [Google Scholar] [CrossRef]
  39. Elser, J.J.; Bracken, M.E.; Cleland, E.E.; Gruner, D.S.; Harpole, W.S.; Hillebrand, H.; Ngai, J.T.; Seabloom, E.W.; Shurin, J.B.; Smith, J.E. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecol. Lett. 2007, 10, 1135–1142. [Google Scholar] [CrossRef] [PubMed]
  40. Xia, J.; Wan, S. Global response patterns of terrestrial plant species to nitrogen addition. New Phytol. 2008, 179, 428–439. [Google Scholar] [CrossRef]
  41. Forsmark, B.; Nordin, A.; Maaroufi, N.I.; Lundmark, T.; Gundale, M.J. Low and High Nitrogen Deposition Rates in Northern Coniferous Forests Have Different Impacts on Aboveground Litter Production, Soil Respiration, and Soil Carbon Stocks. Ecosystems 2020, 23, 1423–1436. [Google Scholar] [CrossRef]
  42. Guo, J.H.; Liu, X.J.; Zhang, Y.; Shen, J.L.; Han, W.X.; Zhang, W.F.; Christie, P.; Goulding, K.W.T.; Vitousek, P.M.; Zhang, F.S. Significant acidification in major Chinese croplands. Science 2010, 327, 1008–1010. [Google Scholar] [CrossRef]
  43. Zhang, R.-T.; Liu, Y.-N.; Zhong, H.-X.; Chen, X.-W.; Xin, S. Effects of simulated nitrogen deposition on the soil microbial community diversity of a Deyeuxia angustifolia wetland in the Sanjiang Plain, Northeastern China. Ann. Microbiol. 2022, 72, 11. [Google Scholar] [CrossRef]
  44. Sinsabaugh, R.L.; Gallo, M.E.; Lauber, C.; Waldrop, M.P.; Zak, D.R. Extracellular enzyme activities and soil organic matter dynamics for northern hardwood forests receiving simulated nitrogen deposition. Biogeochemistry 2005, 75, 201–215. [Google Scholar] [CrossRef]
  45. Wang, C.-Y.; Lv, Y.-N.; Liu, X.-Y.; Wang, L. Ecological effects of atmospheric nitrogen deposition on soil enzyme activity. J. For. Res. 2013, 24, 109–114. [Google Scholar] [CrossRef]
  46. Li, Y.; Wang, C.; Gao, S.; Wang, P.; Qiu, J.; Shang, S. Impacts of simulated nitrogen deposition on soil enzyme activity in a northern temperate forest ecosystem depend on the form and level of added nitrogen. Eur. J. Soil Biol. 2021, 103, 103287. [Google Scholar] [CrossRef]
  47. Sagheb-Talebi, K.; Sajedi, T.; Pourhashemi, M. Forests of Iran, A Treasure from the Past, a Hope for the Future. Springer 2014, 1007, 978–994. [Google Scholar] [CrossRef]
  48. Hosseini, S.M. Outstanding universal values of Hyrcanian Forest, the newest Iranian property, inscribed in the UNESCO’s World Heritage List. Tourism Res. 2019, 10, 1–7. [Google Scholar]
  49. Asadian, M.; Hojjati, S.M.; Pourmajidian, M.R.; Fallah, A. Impact of land-use management on nitrogen transformation in a mountain forest ecosystem in the north of Iran. J. For. Res. 2013, 24, 115–119. [Google Scholar] [CrossRef]
  50. Marvi-Mohajer, M.R. Silviculture and Forest Tending; University of Tehran Press: Tehran, Iran, 2007. [Google Scholar]
  51. Mo, J.; Zhang, W.; Zhu, W.; Gundersen, P.; Fang, Y.; Li, D.; Wang, H. Nitrogen addition reduces soil respiration in a mature tropical forest in southern China. Glob. Change Biol. 2008, 14, 403–412. [Google Scholar] [CrossRef]
  52. Tian, J.; Dungait, J.A.J.; Lu, X.; Yang, Y.; Hartley, I.P.; Zhang, W.; Mo, J.; Yu, G.; Zhou, J.; Kuzyakov, Y. Long-term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil. Glob. Change Biol. 2019, 25, 3267–3281. [Google Scholar] [CrossRef]
  53. Ma, J.; Kang, F.; Cheng, X.; Han, H. Response of soil organic carbon and nitrogen to nitrogen deposition in a Larix principis-rupprechtii plantation. Sci. Rep. 2018, 8, 8638. [Google Scholar] [CrossRef]
  54. Salehi, A.; Gruber, F.; Geranfar, S.; Moniri, V.R. Investigation on nitrogen deposition in the greater Tehran metropolitan area and Caspian Sea lowland areas of Iran, up to altitude of 2200 meters. Int. J. Agric. Sci. 2014, 4, 426–431. [Google Scholar]
  55. Allison, L.E. Organic carbon. In Methods of Soil Analysis, Part 2; Black, C.A., Ed.; American Society of Agronomy: Madison, AL, USA, 1975; p. 1367. [Google Scholar]
  56. Olsen, S.R.; Cole, C.V.; Watanabe, F.S.; Dean, L.A. Estimation of Available Phosphorus in Soils by Extraction with Sodium Bicarbonate USDA Circular 030; U.S. Government Printing Office: Washington, DC, USA, 1954.
  57. Sparling, G.P.; Feltham, C.W.; Reynolds, J.; West, A.W.; Singleton, P. Estimation of soil microbial C by a fumigation-extraction method: Use on soils of high organic matter content, and a reassessment of the EC-factor. Soil Biol. Biochem. 1990, 22, 301–307. [Google Scholar] [CrossRef]
  58. Brookes, P.C.; Landman, A.; Pruden, G.; Jenkinson, D.S. Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in the soil. Soil Biol. Biochem. 1985, 17, 837–842. [Google Scholar] [CrossRef]
  59. Insam, H.; Domsch, K.H. Relationship between soil organic carbon and microbial biomass on chronosequences of reclamation sites. Microb. Ecol. 1988, 15, 177–188. [Google Scholar] [CrossRef]
  60. Hojjati, S.M.; Lamersdorf, N.P. Effect of canopy composition on soil CO2 emission in a mixed spruce beech forest at Solling, Central Germany. J. For. Res. 2010, 21, 461–464. [Google Scholar] [CrossRef]
  61. Hojjati, S.M.; Hagen-Thorn, A.; Lamersdorf, N.P. Canopy composition as a measure to identify patterns of nutrient input in a mixed European beech and Norway spruce forest in central Europe. Eur. J. For. Res. 2009, 128, 13–25. [Google Scholar] [CrossRef]
  62. Lima, T.T.S.; Miranda, I.S.; Vasconcelos, S.S. Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil. New Phytol. 2010, 187, 622–630. [Google Scholar] [CrossRef]
  63. Talhelm, A.F.; Pregitzer, K.S.; Burton, A.J. No evidence that chronic nitrogen additions increase photosynthesis in mature sugar maple forests. Ecol. Appl. 2011, 21, 2413–2424. [Google Scholar] [CrossRef]
  64. Wang, J.; Wang, G.; Fu, Y.; Chen, X.; Song, X. Short-term effects of nitrogen deposition on soil respiration components in two alpine coniferous forests, southeastern Tibetan plateau. J. For. Res. 2019, 30, 1029–1041. [Google Scholar] [CrossRef]
  65. Magill, A.H.; Aber, J.D.; Currie, W.S.; Nadelhoffer, K.J.; Martin, M.E.; McDowell, W.H.; Melillo, J.M.; Steudler, P. Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA. For. Ecol. Manag. 2004, 196, 7–28. [Google Scholar] [CrossRef]
  66. Peng, Y.; Chen, G.; Chen, G.; Li, S.; Peng, T.; Qiu, X.; Luo, J.; Yang, S.; Hu, T.; Hu, H.; et al. Soil biochemical responses to nitrogen addition in a secondary evergreen broad-leaved forest ecosystem. Sci. Rep. 2017, 7, 2783. [Google Scholar] [CrossRef] [PubMed]
  67. Field, C.D.; Evans, C.D.; Dise, N.B.; Hall, J.R.; Caporn, S.J. Long-term nitrogen deposition increases heathland carbon sequestration. Sci. Total Environ. 2017, 15, 426–435. [Google Scholar] [CrossRef]
  68. Zang, H.; Wang, J.; Kuzyakov, Y. N fertilization decreases soil organic matter decomposition in the rhizosphere. Appl. Soil Ecol. 2016, 108, 47–53. [Google Scholar] [CrossRef]
  69. Frey, S.D.; Ollinger, S.; Nadelhoffer, K.E.; Bowden, R.; Brzostek, E.; Burton, A.; Caldwell, B.A.; Crow, S.; Goodale, C.L.; Grandy, A.S.; et al. Chronic nitrogen additions suppress decomposition and sequester soil carbon in temperate forests. Biogeochemistry 2014, 121, 305–316. [Google Scholar] [CrossRef]
  70. Mattson, K.G. CO2 efflux from coniferous forest soils: Comparison of measurement methods and effects of added nitrogen. In Advances in Soil Science: Soils and Global Change; Lal, R., Kimnble, J., Levine, E., Stewart, B.A., Eds.; CRC Press: Boca Raton, FL, USA, 1995; pp. 329–342. [Google Scholar]
  71. Maier, C.A.; Kress, L.W. Soil CO2 evolution and root respiration in 11-year-old loblolly pine (Pinus taeda) plantations as affected by moisture and nutrient availability. Can. J. For. Res. 2000, 30, 347–359. [Google Scholar] [CrossRef]
  72. Zhang, X.-J.; Xu, H.; Chen, G.-X. Major factors controlling nitrous oxide emission and methane uptake from forest soil. J. For. Res. 2001, 12, 239–242. [Google Scholar] [CrossRef]
  73. Yan, G.; Xing, Y.; Wang, J.; Li, Z.; Wang, L.; Wang, Q.; Xu, L.; Zhang, Z.; Zhang, J.; Dong, X.; et al. Sequestration of atmospheric CO2 in boreal forest carbon pools in northeastern China: Effects of nitrogen deposition. Agric. For. Meteorol. 2019, 248, 70–81. [Google Scholar] [CrossRef]
  74. Liu, Y.; Chen, Q.; Wang, Z.; Zheng, H.; Chen, Y.; Chen, X.; Wang, L.; Li, H.; Zhang, J. Nitrogen addition alleviates microbial nitrogen limitations and promotes soil respiration in a subalpine coniferous forest. Forests 2019, 10, 1038. [Google Scholar] [CrossRef]
  75. Zhou, L.; Zhou, X.; Zhang, B.; Lu, M.; Luo, Y.; Liu, L.; Li, B. Different responses of soil respiration and its components to nitrogen addition among biomes: A meta-analysis. Glob. Change Biol. 2014, 20, 2332–2343. Available online: https://onlinelibrary.wiley.com/journal/13652486 (accessed on 29 September 2025). [CrossRef]
  76. Bunt, J.S.; Rovira, A.D. Oxygen uptake and carbon dioxide evolution of heat-sterilized soil. Nature 1954, 173, 1242. [Google Scholar] [CrossRef]
  77. Hojjati, S.M.; Tafazoli, M.; Asadian, M.; Baluee, A. Soil respiration and carbon stock responses to land use changes in the temperate forest of northern Iran. Env. Earth Sci. 2023, 82, 413. [Google Scholar] [CrossRef]
  78. Lee, K.H.; Jose, S. Soil respiration, fine root production, and microbial biomass in cottonwood and loblolly pine plantations along a nitrogen fertilization gradient. For. Ecol. Manag. 2003, 185, 263–273. [Google Scholar] [CrossRef]
  79. Wang, Q.; Zhang, W.; Sun, T.; Chen, L.; Pang, X.; Wang, Y.; Xiao, F. N and P fertilization reduced soil autotrophic and heterotrophic respiration in a young Cunninghamia lanceolata forest. Agri. For. Meteo. 2017, 232, 66–73. [Google Scholar] [CrossRef]
  80. Boxman, A.W.; Blanck, K.; Brandrud, T.E.; Emmett, B.A.; Gundersen, P.; Hogervorst, R.F.; Kjønaas, O.J.; Persson, H.; Timmermann, V. Vegetation and soil biota response to experimentally-changed nitrogen inputs in coniferous forest ecosystems of the NITREX project. Ecol. Manag. 1998, 101, 65–79. [Google Scholar] [CrossRef]
  81. Liu, L.; Greaver, T.L. A global perspective on belowground carbon dynamics under nitrogen enrichment. Ecol. Lett. 2010, 13, 819–828. [Google Scholar] [CrossRef]
  82. Baath, E.; Berg, B.; Lohm, U.; Lundgren, B.; Lundkvist, H.; Rosswall, T.; Soderstrom, B.; Wirén, A. Effects of experimental acidification and liming on soil organisms and decomposition in a Scots pine forest. Pedobiologia 1980, 20, 85–100. [Google Scholar]
  83. Smolander, A.; Kurka, A.; Kitunen, V. Microbial biomass C and N and respiratory activity in soil of repeatedly limed and N and P fertilized Norway spruce plantations. Soil Biol. Biochem. 1994, 26, 957–962. [Google Scholar] [CrossRef]
  84. Zhou, X.; Zhang, Y.; Downing, A. Non-linear response of microbial activity across a gradient of nitrogen addition to the soil from the Gurbantunggut Desert, northwestern China. Soil Biol. Biochem. 2012, 47, 67–77. [Google Scholar] [CrossRef]
  85. Chai, B.; Lv, J.; Li, Q.; Wu, J.; Song, X. Interaction of Biochar Amendment and Nitrogen Deposition on Soil Microbial Biomass Carbon and Enzyme Activity in a Torreya Grandis Orchard. Pol. J. Environ. Stud. 2019, 28, 3605–3614. [Google Scholar] [CrossRef]
  86. Zhou, X.; Talley, M.; Luo, Y. Biomass, litter, and soil respiration along a precipitation gradient in Southern Great Plains, USA. Ecosystems 2009, 12, 1369–1380. [Google Scholar] [CrossRef]
  87. Yoshitake, S.; Uchida, M.; Koizumi, H.; Nakatsubo, T. Carbon and nitrogen limitation of soil microbial respiration in a High Arctic successional glacier foreland near Ny-Alesund, Svalbard. Polar Res. 2007, 26, 22–30. [Google Scholar] [CrossRef]
  88. Iqbal, J.; Hu, R.; Feng, M.; Lin, S.; Malghani, S.; Ali, I.M. Microbial biomass, and dissolved organic carbon and nitrogen strongly affect soil respiration in different land uses: A case study at Three Gorges Reservoir Area, South China. Agric. Ecosyst. Environ. 2010, 137, 294–307. [Google Scholar] [CrossRef]
  89. Hojati, S.M.; Tafazoli, M.; Hashemi, S.A.; Tafazoli, M. The study of litter decomposition process of Caucasian oak and Red pine using litter bag technique. J. Plant Res. 2016, 28, 510–521. [Google Scholar]
  90. Sun, X.L.; Zhao, J.; You, Y.M. Soil microbial responses to forest floor litter manipulation and nitrogen addition in a mixed-wood forest of northern China. Sci. Rep. 2016, 6, 19536. [Google Scholar] [CrossRef] [PubMed]
  91. Park, H.; Park, Y.; Lee, S.; Woo, C.H. Decomposition of Pine and Oak litters as affected by their lignin and mineral concentrations. Korean J. Soil Sci. Fertil. 2018, 51, 377–387. [Google Scholar] [CrossRef]
  92. Lloyd, J.; Taylor, J.A. On the temperature dependence of soil respiration. Funct. Ecol. 1994, 8, 315–323. [Google Scholar] [CrossRef]
  93. Yan, T.; Song, H.; Wang, Z.; Teramoto, M.; Wang, J.; Liang, N.; Ma, C.; Sun, Z.; Xi, Y.; Li, L.; et al. Temperature sensitivity of respiration across multiple time scales in a temperate plantation forest. Sci. Total Environ. 2018, 688, 479–485. [Google Scholar] [CrossRef]
  94. Davidson, E.A.; Janssens, I.A.; Luo, Y. On the variability of respiration in terrestrial ecosystems: Moving beyond Q10. Glob. Change Biol. 2006, 12, 154–164. [Google Scholar] [CrossRef]
  95. Gundersen, P.; Christiansen, J.R.; Alberti, G.; Brüggemann, N.; Castaldi, S.; Gasche, R. The response of methane and nitrous oxide fluxes to forest change in Europe. Biogeosciences 2012, 9, 3999–4012. [Google Scholar] [CrossRef]
  96. He, C.; Ruan, Y.; Jia, Z. Effects of Nitrogen Addition on Soil Microbial Biomass: A Meta-Analysis. Agriculture 2024, 14, 1616. [Google Scholar] [CrossRef]
  97. Nave, L.E.; Vance, E.D.; Swanston, C.W.; Curtis, P.S. Impacts of elevated N inputs on north temperate forest soil C storage, C/N, and net N-mineralization. Geoderma 2009, 153, 231–240. [Google Scholar] [CrossRef]
  98. Wei, L.; Vosátka, M.; Cai, B.; Ding, J.; Lu, C.; Xu, J.; Yan, W.; Li, Y.; Liu, C. The role of arbuscular mycorrhizal fungi in nitrogen uptake by Plantago lanceolata in two soils with different levels of available nitrogen. Plant Soil 2012, 351, 325–336. [Google Scholar] [CrossRef]
  99. Chen, X.; Liu, J.; Deng, Q.; Yan, J.; Zhang, D. Effects of elevated CO2 and nitrogen addition on soil organic carbon fractions in a subtropical forest. Plant Soil 2012, 357, 25–34. [Google Scholar] [CrossRef]
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Figure 1. Location of study area in Mazandaran Province, north of Iran, and sampling method in oak and pine plantations in different N treatments.
Figure 1. Location of study area in Mazandaran Province, north of Iran, and sampling method in oak and pine plantations in different N treatments.
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Figure 2. MBC (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); Capital letters represent significant differences between season; Lowercase letters represent significant differences between N treatments (C: control; LN: low nitrogen; MN: medium nitrogen; HN: high nitrogen).
Figure 2. MBC (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); Capital letters represent significant differences between season; Lowercase letters represent significant differences between N treatments (C: control; LN: low nitrogen; MN: medium nitrogen; HN: high nitrogen).
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Figure 3. SOC (Mean ±SD) in different seasons and N treatments (mean value of four seasons) in oak (a,b) and pine plantations (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
Figure 3. SOC (Mean ±SD) in different seasons and N treatments (mean value of four seasons) in oak (a,b) and pine plantations (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
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Figure 4. Rs (Mean ± SD) in different seasons and N treatment (mean value of four seasons) in oak (a,b) and pine plantations (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
Figure 4. Rs (Mean ± SD) in different seasons and N treatment (mean value of four seasons) in oak (a,b) and pine plantations (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
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Figure 5. Litterfall production (Mean ± SD) seasonally and annually in different N treatments in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
Figure 5. Litterfall production (Mean ± SD) seasonally and annually in different N treatments in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
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Figure 6. Litterfall OC (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
Figure 6. Litterfall OC (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
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Figure 7. Fine root biomass (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
Figure 7. Fine root biomass (Mean ± SD) in different seasons and N treatment in oak (a,b) and pine stands (c,d); capital letters illustrate significant differences between seasons; lowercase letters illustrate significant differences between N treatments.
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Table 1. Physical and chemical properties of the soil before applying N treatments in pine and oak plantations.
Table 1. Physical and chemical properties of the soil before applying N treatments in pine and oak plantations.
PlantationSoil Properties
Pinus radiata D. DonMoisture (%)18.58
Bulk density (g cm−3)1.6
Lime (%)2.82
pH (1:2.5 H2O)6.6
EC (dS/m2)0.4
C (%)5.9
Quercus castanifoliaMoisture (%)29.5
Bulk density (g cm−3)1.35
Lime (%)2.85
pH (1:2.5 H2O)6.1
EC (dS/m2)0.49
C (%)4.8
Table 2. Results of repeated measures ANOVAs of the effects of different seasons and N treatments and their interactions in the soil Rs, MBC, soil OC, litter fall OC, litterfall production, and fine root biomass.
Table 2. Results of repeated measures ANOVAs of the effects of different seasons and N treatments and their interactions in the soil Rs, MBC, soil OC, litter fall OC, litterfall production, and fine root biomass.
Nutrient Concentrations in SoilSpeciesDifferent SeasonsNitrogen TreatmentsDifferent Seasons × Nitrogen Treatments
FpFpFp
Rs (mol CO2 s−1 m2)oak95.658<0.0150.537<0.015.336<0.01
pine121.007<0.0161.115<0.0111.359<0.01
MBC (mg kg−1)oak64.796<0.0123.536<0.015.340<0.01
pine38.509<0.0116.586<0.016.092<0.01
Soil OC (%)oak30.039<0.0140.529<0.017.754<0.01
pine92.706<0.014.987<0.013.680<0.01
Litterfall OC (%)oak262.287<0.010.4850.7140.560<0.01
pine25.048<0.010.4360.7280.1200.999
Litterfall mass (g m−2)oak2866.858<0.012.2080.1062.2530.025
pine2891.092<0.0110950.3651.5530.220
Fine root biomass (g m−2)oak87.670<0.0111.579<0.016.790<0.01
pine39.356<0.0117.920<0.016.450<0.01
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Nouraei, A.; Hojjati, S.M.; Jalilvand, H.; Schleppi, P.; Alavi, S.J. Short-Term Effects of N Deposition on Soil Respiration in Pine and Oak Monocultures. Forests 2025, 16, 1570. https://doi.org/10.3390/f16101570

AMA Style

Nouraei A, Hojjati SM, Jalilvand H, Schleppi P, Alavi SJ. Short-Term Effects of N Deposition on Soil Respiration in Pine and Oak Monocultures. Forests. 2025; 16(10):1570. https://doi.org/10.3390/f16101570

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Nouraei, Azam, Seyed Mohammad Hojjati, Hamid Jalilvand, Patrick Schleppi, and Seyed Jalil Alavi. 2025. "Short-Term Effects of N Deposition on Soil Respiration in Pine and Oak Monocultures" Forests 16, no. 10: 1570. https://doi.org/10.3390/f16101570

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Nouraei, A., Hojjati, S. M., Jalilvand, H., Schleppi, P., & Alavi, S. J. (2025). Short-Term Effects of N Deposition on Soil Respiration in Pine and Oak Monocultures. Forests, 16(10), 1570. https://doi.org/10.3390/f16101570

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