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Article

Aboveground and Belowground Input Effects on Soil Health in Urban Camphor Tree Forests

by
Xuejia Huang
1,2,
Yuanying Peng
3,
Wende Yan
1,4,5,
Tianyi Yan
1,6,
Xiaocui Liang
1,4,5,
Junjie Lei
1,
Xiaoyong Chen
7,* and
Yaqin Qi
8,*
1
College of Life Science and Technology, Central South University of Forestry and Technology, Changsha 410004, China
2
Art and Design College, Hunan City University, Yiyang 413000, China
3
College of Arts and Sciences, Lewis University, Romeoville, IL 60446, USA
4
National Engineering Laboratory for Applied Forest Ecological Technology in Southern China, Changsha 410004, China
5
Key Laboratory of Urban Forest Ecology of Hunan Province, Changsha 410004, China
6
College of Information Technology, Hunan Biological and Electromechanical Polytechnic, Changsha 410127, China
7
College of Arts and Sciences, Governors State University, University Park, IL 60484, USA
8
Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(14), 6358; https://doi.org/10.3390/su17146358
Submission received: 28 May 2025 / Revised: 7 July 2025 / Accepted: 9 July 2025 / Published: 11 July 2025
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Abstract

Urban forests provide essential ecosystem services, including improving soil health, sequestering carbon (C), and supporting biodiversity. However, the effects of anthropogenic litter and root management on soil biogeochemical processes in urban environments remain poorly understood. This study applied the Detritus Inputs and Removal Treatment (DIRT) framework to examine how aboveground and belowground organic inputs influence soil organic carbon (SOC), total nitrogen (TN), soil water content (SWC), and enzymatic activities in subtropical urban camphor tree forests in China. Six treatments were implemented: litter removal (LR), litter addition (LA), root exclusion (RE), combined litter and root removal (LR + RE), combined litter addition and root exclusion (LA + RE), and an undisturbed litter control (LC). The results showed that the LA treatment significantly enhanced SOC, TN, SWC, and key soil enzyme activities (protease, catalase, and urease) compared to the LC, highlighting the crucial role of litter in enhancing soil fertility and microbial functioning. These elevated enzyme activities suggest intensified microbial nutrient cycling and metabolic activity in response to organic matter inputs. In contrast, the combined LR + RE treatment reduced SOC and enzyme activities but unexpectedly increased TN, indicating disrupted nutrient cycling, possibly due to accelerated microbial nitrogen mineralization and decomposition of existing soil organic matter in the absence of fresh carbon inputs. The LA treatment also showed the highest carbon-to-nitrogen (C:N) ratio, reflecting a carbon-enriched environment that may favor long-term carbon stabilization. Additionally, SWC was most improved under the LA + RE treatment, suggesting its potential for enhancing soil moisture retention in urban settings. These findings underscore the complementary roles of litter and root inputs in maintaining soil health and biogeochemical balance in urban forests. The study provides insights into enzyme-mediated soil processes under varying organic input regimes and highlights the value of targeted organic matter management to enhance urban ecosystem services.

1. Introduction

Urban forests have become critical green infrastructures within cities, offering a wide range of ecosystem services that enhance environmental quality and human well-being. They help moderate urban microclimates, improve air quality, preserve biodiversity, and provide essential recreational spaces [1,2]. However, the ecological dynamics of urban forests differ markedly from those of natural forests, as they are continuously shaped by anthropogenic disturbances such as soil compaction, understory removal, forest fragmentation, and land development [3,4]. These disturbances not only reduce habitat connectivity but also disrupt key soil processes, including carbon (C) sequestration, nutrient cycling, and microbial activity, thereby increasing the risk of urban land degradation [5]. As such, understanding the ecological foundations of soil health in urban forests is crucial for developing sustainable urban land management strategies [6].
Organic matter inputs from plant litter and roots are among the primary drivers of soil structure, fertility, and microbial functioning in urban forests. Aboveground litterfall serves as a major source of organic C and nutrients, supporting essential processes such as humus formation [7], soil moisture retention [8], aggregate stabilization [9], and soil CO2 emission [10]. Moreover, the biochemical composition of litter, particularly its lignin and nitrogen (N) content, regulates microbial decomposition pathways. Litter rich in lignin is more resistant to microbial breakdown due to the complex structure of lignin polymers, leading to reduced activity of lignin-degrading enzymes such as lignin peroxidase and manganese peroxidase [11,12]. In contrast, nitrogen-rich litter can stimulate the production of N-transforming enzymes, such as urease and protease, thereby enhancing nutrient availability for plant uptake [13]. Thus, changes in litter quality not only influence microbial activity but also have broader implications for nutrient cycling in urban soils [14].
Simultaneously, belowground inputs, including root growth, exudation, and turnover, represent another crucial pathway that regulates soil biological activity and nutrient availability. Root exudates comprising sugars, amino acids, and organic acids serve as essential energy sources that stimulate microbial metabolism and enzyme production [15]. These exudates also influence microbial community composition, thereby affecting the rates and pathways of organic matter decomposition and nutrient mineralization [16,17]. Physical traits of roots, such as surface area and length, further govern nutrient acquisition and soil biochemical processes by modifying the rhizosphere environment [18]. Central to these interactions are soil enzymes, which catalyze key transformations of organic and inorganic matter, sustaining soil fertility and ecosystem resilience [19]. Enzymes such as hydrolases and oxidoreductases are sensitive indicators of soil health, responding rapidly to changes in organic inputs and microbial dynamics [20]. In urban forests, where soils are often subjected to significant anthropogenic pressures, the role of enzymes in maintaining nutrient cycling and organic matter decomposition becomes even more critical [1]. However, the contributions of litter and roots to enzyme activity and soil nutrient dynamics are often altered by human interventions like understory clearing or landscaping practices [3,21], which can have unintended consequences for soil functioning and the delivery of ecosystem services [4,14].
Camphor tree (Cinnamomum camphora (L.) J. Presl) forests play a vital role in maintaining ecological stability and enhancing biodiversity in subtropical regions. Renowned for their resilience to environmental stressors, these forests contribute to improved soil fertility and structure through leaf litter decomposition and root activity [22]. As a fast-growing evergreen species, camphor trees not only facilitate C sequestration but also deliver key ecosystem services such as water regulation, soil conservation, and habitat provision for diverse wildlife [23]. Their adaptability to a wide range of soil types and climatic conditions further underscores their importance in forest restoration projects, particularly in degraded or urbanized landscapes [24]. Previous studies have emphasized their capacity to promote nutrient cycling and support sustainable land management, especially in areas vulnerable to erosion or soil degradation [25]. Given these ecological benefits, camphor tree forests are integral to enhancing ecosystem resilience and supporting the sustained delivery of essential ecosystem services across subtropical and temperate regions.
Despite growing recognition of the importance of soil biological processes in urban forests, empirical studies examining how human-driven changes in litter and root inputs affect soil chemical properties and enzyme activities remain limited [26,27]. To address this gap, this study investigates the independent and combined effects of aboveground litter and belowground root inputs on soil quality in urban camphor tree forests. We hypothesized that litter addition (LA) would enhance soil organic carbon (SOC), N content, and enzyme activities due to increased resource availability. We further hypothesized that root exclusion would have a stronger impact on microbial processes and enzyme activities than litter removal, given the pivotal role of roots in sustaining microbial communities. Finally, we anticipated that temporal changes in microbial and enzymatic activities would reflect the dynamics of organic matter decomposition and nutrient release, influenced by the interaction of litter and root inputs. Specifically, the objectives of this study were: (1) to assess the effects of altered litter and root inputs on soil chemical properties, including SOC and N content, and (2) to evaluate changes in soil enzyme activities under different management treatments.

2. Materials and Methods

2.1. Study Site

This research was conducted in the Hunan Forest Botanical Garden, located in Changsha City, Hunan Province, China (28°10′ N, 113°00′ E) (Figure 1), which spans approximately 1200 hectares. The site features hilly topography and lies within a subtropical monsoon climate zone. Average annual temperatures range from 16 °C to 18 °C, with annual precipitation between 1300 mm and 1600 mm. The soils at the site are classified as typical red earth, derived from slate parent material, and categorized as Allitri-Udic Ferrosols according to the World Reference Base for Soil Resources (CRG-CST 2001). The soil texture ranges from clay loam to sandy loam, with a moderately acidic pH of between 5.0 and 6.0. To control for parent material effects and minimize background variability, all experimental plots were established within a single, uniform managed unit on similar slope positions. Soils across all plots shared the same parent material, texture class, and pH, thereby reducing confounding factors unrelated to litter or root treatments. Dominant canopy species include Cunninghamia lanceolata (Lamb. Hook.), Pinus massoniana Lamb., Chamaecyparis spp., C. camphora, and Liquidambar formosana Hance. The understory is floristically diverse, comprising species such as Sassafras tsumu Hemsl., Symplocos caudata Wall. ex A. DC., Clerodendrum cyrtophyllum Turcz, Nephrolepis auriculata Trimen, Lophatherum gracile Brengn., Miscanthus floridulus Warb., and Phytolacca acinosa Roxb. These characteristics contribute to a well-developed urban forest ecosystem where human activities and forest management practices exert significant influence on both biological and physical processes.

2.2. Experimental Design and Soil Sampling Collection

This study was conducted in camphor tree forests located in Changsha, Hunan Province, China. The forests were originally planted in 1992 at a spacing of 2 × 3 m and span approximately 12 hectares within the Hunan Forest Botanical Garden, representing a type of plantation forest. By October 2018, the trees had reached an average height of 12.6 m and an average diameter at breast height (DBH) of 15.1 cm, with a stand density of 1600 trees per hectare and a crown density of 0.9. Characterized by a typical subtropical monsoon climate, the site serves as a representative setting for urban forest research.
The experimental design followed the Detritus Inputs and Removal Treatment (DIRT) framework and included six distinct treatments: litter removal (LR), LA, root exclusion (RE), LA + RE, LR + RE, and litter control (LC) treatments. The litter control (LC) treatment maintained both aboveground litter and belowground root inputs (Figure 1). The DIRT experiment was designed to investigate the impact of input and exclusion of both aboveground and belowground plant-derived matter on the physical, chemical, and biological properties of forest soils [28]. To ensure representative sampling, three replicate plots (20 m × 20 m) were established within the selected forest stand, each sharing similar topography, vegetation, and soil conditions. Although the number of replicates may seem limited, this level of replication is consistent with similar field-based ecological studies [7,8], especially in long-term field manipulation experiments. Our experimental design prioritizes site homogeneity, controlled treatments, and within-block replication rather than spatially extensive sampling, in accordance with standard ecological field methodology [29]. Within each plot, six 3 m × 4 m subplots were randomly assigned to the various detritus (litter and root) manipulation treatments. Subplots were spaced at least 3 m apart to minimize treatment overlap and ensure that their effects were independent. For the RE treatment, a trench approximately 0.8 m deep was dug around each subplot and lined with a 1 mm-thick polyethylene barrier to block root intrusion, following standard DIRT protocol [30]. While complete exclusion of fine roots cannot be guaranteed, this method has been widely accepted and considered effective in limiting root intrusion in long-term forest soil studies [31,32].
In the DIRT experiment, the LR treatment involved clearing all litter from the forest floor at the beginning of the study. A 1-mm mesh collection device was installed 0.8 m above the ground to prevent new litter accumulation, with collected litter removed twice monthly. For the LA treatment, litter collected from an LR subplot was evenly redistributed over the LA subplots, with additions occurring twice monthly. The RE treatment was implemented by digging a trench approximately 0.8 m deep around each subplot, lining it with a 1 mm-thick polyethylene sheet to prevent root intrusion, and removing live plants to isolate the soil from root activity. The LA + RE treatment combined the approaches of both the LA and RE treatments, while the LR + RE treatment incorporated the methods used in the LR and RE treatments. The LC treatment served as the control, maintaining natural litterfall and root growth without intervention.
Litterfall production in the camphor tree forests varied seasonally, ranging from approximately 210 to 900 kg ha−1 year−1, providing a dynamic natural source of organic input. All treatments were applied biweekly from October 2018 to October 2019 to ensure consistent plot management throughout the experimental period. This regular manipulation enabled the study to capture both short-term and long-term effects of litter and root modifications on soil properties. Previous studies in subtropical monsoon climates have shown that leaf litter from broadleaf evergreen species such as Cinnamomum camphora typically decomposes within 12 to 18 months, depending on environmental conditions [33]. Therefore, the year-long duration of our experiment allowed sufficient time for the decomposition of added litter and the release of bioavailable compounds capable of influencing soil microbial and enzymatic processes.
Soil sampling was conducted four times throughout the year—in January, April, July, and October 2019. During the sampling event, soil was collected from the top 20 cm in each subplot using a 5 cm diameter soil auger. Samples were obtained from three randomly selected locations within each subplot, arranged diagonally, to ensure representativeness of each treatment. After collecting, the soil samples were passed through a 2-mm mesh to remove roots, stones, and other debris. To preserve sample integrity, they were transported to the laboratory in insulated coolers. In the laboratory, the samples were sieved again for uniformity and then divided into two portions: one was air-dried for physicochemical analysis, while the other was stored at 4 °C for enzyme activity analysis.

2.3. Determination of Soil Carbon and Nitrogen

Soil water content (SWC) was determined by drying fresh soil samples at 105 °C until a constant weight was achieved. The SOC content was measured using the K2Cr2O7-H2SO4 oxidation method, and total nitrogen (TN) was determined using the semi-micro Kjeldahl method [34]. The carbon-to-nitrogen (C:N) ratio was calculated by dividing the SOC content by the TN content, providing insights into the relative abundance of C and N in the soil. SWC was calculated using the following formula: SWC (%) = [W2 − W1)/W1] × 100, where W2 is the weight of the fresh soil sample and W1 is the weight of the oven-dried soil sample (dried at 105 °C until a constant weight was reached).

2.4. Determination of Soil Enzyme Activity

Soil enzyme activities, which play a vital role in soil nutrient cycling, were assessed using a combination of colorimetric and titrimetric methods. To minimize potential degradation of enzyme activity, fresh soil samples were stored at 4 °C immediately after collection and processed within seven days, without undergoing any freeze–thaw cycles. These handling procedures followed international guidelines for enzyme activity measurement in soils [35], which emphasize the importance of controlled storage conditions to ensure data accuracy. Soil peroxidase activity was quantified using the potassium permanganate titration method, with results expressed as milliliters of 0.1 N KMnO4 per gram of soil. Protease activity, involved in protein degradation, was measured using the indole-3-acetic acid colorimetric method and reported as milligrams of NH2-N per gram of soil following a 24-h incubation at 30 °C. Urease activity, essential for N cycling, was assessed using the indophenol blue colorimetric method, with results expressed as milligrams of NH3-N per gram of soil after a 24-h incubation at 37 °C. Acid phosphatase (ACP) activity, important for phosphorus cycling, was assessed using the p-nitrophenyl phosphate disodium method at a pH of 6.5. Lastly, dehydrogenase activity, an indicator of overall microbial metabolic activity, was determined by measuring hydrogen ions released using soil organic matter or glucose as a donor. Results were expressed as microliters of H+ per gram of soil after incubation at 30 °C for 24 h [36].

2.5. Statistical Analysis

The data collected from different C input treatments, including measurements of SOC, TN, C:N ratio, SWC, and enzyme activity, were analyzed using one-way analysis of variance (ANOVA). To meet the assumptions of normality and homoscedasticity, log transformations were applied to the raw data for SOC, TN, SWC, and enzyme activity where necessary. Post-hoc comparisons among treatment groups were performed using Tukey’s Honestly Significant Difference (HSD) test. In addition, pairwise t-tests were conducted to examine specific treatment differences. All statistical analyses were performed using the SAS v9.4 software package (SAS Institute Inc., Cary, NC, USA), with a significance level set at p < 0.05. The practical significance of observed differences was assessed by calculating effect sizes, and the results are presented as means ± standard error (SE).

3. Results

The analysis of SOC content revealed significant variation among the treatments compared to the LC treatment (p < 0.05) (Table 1). Specifically, SOC content was highest in the LA treatment (23.94 g kg−1), followed by the LR treatment (22.13 g kg−1) and the LR + RE treatment (19.09 g kg−1), while the RE treatment exhibited the lowest SOC content (13.85 g kg−1). The LC treatment had significantly lower SOC content (15.28 g kg−1) than all treatments except the RE treatment. No significant differences in SOC content were found among the LA, LR, and LR + RE treatments (p > 0.05).
For TN content, significant differences were also observed among the treatments. The highest TN levels were recorded in the LR + RE (2.10 g kg−1) and LR (2.03 g kg−1) treatments, followed by LA (1.92 g kg−1) (Table 1). These treatments showed significantly higher TN content than the LC treatment (1.49 g kg−1), while the RE treatment exhibited intermediate TN levels (1.57 g kg−1). However, no significant differences in TN content were detected among the LA, LR, and LR + RE treatments (p > 0.05).
Regarding the C:N ratio, the RE treatment exhibited the lowest ratio (8.64), indicating a higher N content relative to C. The highest C:N ratio was observed in the LA treatment (12.69), followed by LR (10.93) and LR + RE (9.52), while the LC treatment showed an intermediate ratio (10.2) (Table 1).
Soil water content (SWC) was significantly affected by the treatments, with the LA + RE treatment showing the highest SWC (23.16%) among all groups. The LC treatment had the lowest SWC (22.46%), while the LR (17.64%) and LR + RE (17.08%) treatments showed the lowest values among the litter manipulation treatments. The RE and LA treatments had intermediate SWC levels (20.34% and 20.63%, respectively), both significantly higher than those in the LR and LR + RE treatments.
Enzyme activity was significantly higher in the LA and LA + RE treatments compared to the LC treatment (p < 0.05), particularly during the growing season (April to October) (Figure 2). Protease activity (Figure 2a) showed significant variation across treatments and seasons. In January, the LA + RE treatment showed the highest protease activity (3.60 mg·g−1), significantly exceeding all other treatments. Dehydrogenase activity (Figure 2b) in the LA + RE treatment peaked at 3.80 mg·g−1 and remained relatively stable through July and October. In contrast, the LC treatment consistently showed the lowest dehydrogenase activity, reaching its minimum value (2.20 mg·g−1) in October. Acid phosphatase (ACP) activity (Figure 2c) displayed distinct seasonal and treatment-related variations. Across all treatments, ACP activity was generally highest in the early growing season (April) and declined progressively through July and October. The LR + RE treatment exhibited the highest ACP activity in April, reaching nearly 17 mg·g−1, significantly higher than in other treatments (p < 0.05). The LA + RE and RE treatments also maintained relatively high ACP activity throughout the study period. In contrast, the LC and LR treatments consistently displayed the lowest ACP activity, especially in the later months. Seasonal trends indicated a clear decline in enzyme activity from April to October across all treatments. Urease (URE) activity (Figure 2d) also varied significantly among treatments and sampling times. The LA treatment recorded the highest urease activity in April, peaking at approximately 13.5 mg·g−1, significantly higher than other treatments or time points (p < 0.05). The LA + RE and LR + RE treatments also exhibited elevated urease activity in April and July, followed by a decline in October. In contrast, the LC and RE treatments consistently showed the lowest urease activity throughout the study period, with the LC treatment reaching its lowest point in October. Overall, urease activity increased from January to July, then declined in October, indicating a strong seasonal response.
Soil enzyme activities exhibited significant variation across both treatments and time. Protease activity (Figure 2a) was significantly affected by treatment and time (p < 0.05). The LA and LR + RE treatments showed the highest protease activity, with a notable decline across all treatments over the 10-month period. Dehydrogenase activity (Figure 2b) also varied significantly among treatments (p < 0.05), with the LA and LR + RE treatments maintaining the highest DEH activity throughout the study. Temporal variation was significant, as DEH activity decreased from month 1 to month 10 across all treatments. The LR + RE treatment exhibited relatively higher DEH activity compared to others. Acid phosphatase activity (Figure 2c) was significantly influenced by both treatment and time (p < 0.05). The highest ACP activity was observed in the LA and LA + RE treatments, while the LC and LR treatments consistently exhibited lower ACP activity throughout the study. Urease activity (Figure 2d) showed significant treatment and temporal effects (p < 0.05). The LA + RE and LR + RE treatments demonstrated the highest urease activity, with a steady decline over time, from the highest levels at 1 month to the lowest at 10 months. Catalase activity (Figure 2e) remained relatively stable in the LA, LA + RE, and LR + RE treatments, showing minimal decline over time compared to the other treatments. Significant differences were observed between treatments (p < 0.05), with the LA + RE treatment maintaining the highest catalase activity throughout the study.
In addition to evaluating treatment effects on soil properties and microbial enzyme activities, correlation analysis and principal component analysis (PCA) were conducted to further explore the relationships among the measured variables. A significant positive correlation was observed between SOC and protease activity (r = 0.85, p < 0.01) (Table 2). In contrast, the carbon-to-nitrogen (C:N) ratio exhibited a significant negative correlation with protease activity (r = −0.70, p < 0.05). Additionally, SOC was positively correlated with TN (r = 0.78, p < 0.01), reinforcing the close relationship between soil organic matter and N content. Soil moisture content (SWC) also showed positive correlations with both SOC (r = 0.65, p < 0.05) and protease activity (r = 0.72, p < 0.01).
Principal component analysis (PCA) was applied to identify patterns within the dataset and reduce dimensionality. The first two principal components explained 75% of the total variance, with Principal Component 1 (PC1) primarily influenced by soil nutrient variables such as SOC and TN, and Principal Component 2 (PC2) mainly driven by enzyme activities, including protease and urease (Table 3). In contrast, the LC and RE treatments were separated along the PCA axes, indicating distinct effects on microbial activity and soil properties.

4. Discussion

The results of this study demonstrate that forest management practices, such as LA and RE, significantly influence soil quality in camphor tree forests, particularly in terms of SOC, TN, C:N ratio, and SWC. The LA treatment yielded the highest SOC content, underscoring the positive impact of organic matter inputs on SOC dynamics. Although the LR treatment showed slightly higher SOC than the LC control, this may reflect the residual effect of root turnover or baseline soil carbon levels rather than a treatment-induced gain [37,38,39].
In contrast, the RE treatment did not lead to a significant increase in SOC and showed a notably lower effect compared to the LA treatment. Root exclusion limits the input of organic matter from living roots, which can reduce microbial activity and alter nutrient cycling processes [40]. While root exclusion may not directly reduce existing soil C storage, the absence of root-derived organic inputs could disrupt the organic matter cycle and limit C sequestration potential. The RE treatment showed the lowest SOC content, reinforcing the important role of living roots in maintaining soil C. These findings support the hypothesis that both aboveground and belowground inputs must be considered when evaluating their combined impact on soil C sequestration [41].
Regarding TN, the LR + RE treatment exhibited significantly higher TN content, which could be attributed to reduced microbial decomposition, resulting in greater N retention in the soil. Root exclusion can also stimulate saprotrophic fungi and certain bacterial populations, potentially accelerate the decomposition of organic matter and the release of N [29,30]. These microbial shifts, driven by changes in organic matter availability, could facilitate N mineralization under certain conditions, although the overall impact on N retention depends on factors such as soil conditions and microbial community dynamics [42].
The highest C:N ratio was observed in the LA treatment, indicating a more carbon-rich soil environment, possibly due to slower decomposition of high-carbon organic inputs. A higher C:N ratio does not necessarily imply increased C content, as it could also reflect reduced N availability [43]. Conversely, the RE treatment exhibited the lowest C:N ratio, likely due to enhanced decomposition and N mineralization, making N more readily available for plant uptake and microbial processes [44]. The significant differences in the C:N ratio, particularly between LA and others, highlight the strong influence of LA on nutrient cycling, consistent with previous findings [45]. In terms of SWC, the LA + RE treatment showed the highest values, suggesting that this combined treatment improves soil moisture retention. This observation aligns with previous studies reporting that LA enhances soil structure, reduces evaporation, and promotes moisture retention [46]. Conversely, the LR + RE treatment displayed the lowest SWC, likely due to the absence of litter, which increases surface evaporation and limits the soil’s capacity to retain moisture. Although RE may reduce transpiration and contribute to moisture retention, the lack of litter in the LR + RE treatment appears to be the dominant factor driving reduced SWC [47]. The significant differences between the LR + RE and other treatments (e.g., RE and LC) further support previous studies suggesting that both LR and RE can significantly affect soil moisture dynamics [48].
The increased enzyme activities observed in LA, LA + RE, and LR + RE treatments underscore the critical role of organic matter inputs in supporting soil microbial processes. Specifically, the LA treatment resulted in significant increases in SOC and TN levels, which were associated with elevated enzyme activities involved in nutrient cycling. This finding is consistent with previous studies showing that organic inputs, such as litter, stimulate microbial enzyme activities by supplying essential nutrients and energy for microbial metabolism [49]. Notably, the enhanced protease (PRO) and urease (URE) activities in the LA treatment support earlier studies indicating that organic amendments promote N mineralization and improve soil fertility, thereby supporting plant growth [49].
In contrast, the LR + RE treatment exhibited a paradoxical pattern, higher TN content alongside reduced SOC. This may result from enhanced N mineralization under carbon-limited conditions [50]. The removal of litter and exclusion of roots has likely reduced external carbon inputs, compelling microbial communities to decompose existing soil organic matter as an alternative energy source [51]. This accelerated microbial degradation may have led to SOC loss, while the increased protease and urease activities promoted nitrogen mineralization and retention, thereby increasing TN. This decoupling of SOC and TN dynamics under detritus exclusion treatments highlights the complexity of microbial responses to altered resource availability [52].
Dehydrogenase (DEH) activity, a marker of microbial respiratory function, was highest in the LA and LR + RE treatments, reflecting increased microbial respiration and energy production driven by increased organic inputs. These findings are consistent with previous studies highlighting the positive relationship between organic matter additions and DEH activity, as DEH is closely linked to microbial biomass [53]. The relatively high DEH activity in the LR + RE treatments suggests that root exclusion, combined with organic inputs, may create favorable conditions for microbial respiration, potentially due to reduced competition for soil nutrients, allowing microbial communities to thrive [42].
The increased acid phosphatase (ACP) activity observed in LA and combined treatments (e.g., LA + RE and LR + RE) indicates enhanced phosphorus cycling, which is essential for plant growth. ACP facilitates the release of organically bound phosphorus, making it available for plant uptake. This increase in ACP activity, particularly in the LA treatments, aligns with findings by Schaap et al. [54], who reported that litter inputs enhance phosphorus cycling in forest soils by increasing the availability of organic substrates. These results suggest that combined LA and RE strategies can be particularly beneficial in phosphorus-limited forest ecosystems by improving phosphorus availability and cycling.
Catalase (CAT) activity, an indicator of oxidative stress and microbial health, remained relatively stable in the LA and RE treatments. This stability suggests that these treatments may help alleviate oxidative stress in the soil, likely due to increased availability of organic substrates and nutrients. Previous research has shown that organic matter inputs can enhance antioxidant enzyme activities, protecting microbes from environmental stressors and supporting microbial stability [55]. The higher CAT activity observed in the LA treatments suggests that enzymatic activity, as a reflection of microbial activity, can serve as a useful indicator of microbial response to organic amendments. These findings imply that such amendments may help mitigate oxidative stress and promote microbial community resilience over time.
Interestingly, all enzyme activities showed a significant decline over time, with the highest levels observed in the first month and the lowest at the tenth month after treatments. This pattern suggests an initial surge in microbial activity following the addition of fresh organic matter, followed by a gradual depletion of readily available nutrients and substrates, resulting in reduced enzyme activity. This temporal trend is consistent with findings from previous studies, which have reported an initial increase in soil enzyme activities after the application of organic amendments, followed by a decline as the organic matter decomposes and nutrient availability diminishes [56]. The observed time-dependent decrease highlights the transient nature of the benefits provided by organic inputs, indicating that periodic reapplication may be necessary to sustain soil health and microbial activity over the long term.
The positive impact of LA and RE treatments on soil enzyme activities suggests that these practices enhance microbial function, nutrient cycling, and overall soil health. These findings are in line with the work of Wan et al. [40] and Liu et al. [57], who demonstrated that litter manipulation and root management significantly influence soil biochemical properties. Together, these results underscore the essential role of organic matter in maintaining soil health and promoting nutrient availability, offering important implications for forest management practices aimed at improving ecosystem function and resilience. To improve soil health and microbial activity in urban environments, it is crucial to prioritize organic matter inputs, such as mulch or compost, and effectively manage moisture levels.
Furthermore, these findings highlight the importance of integrated management strategies in urban forestry that account for multiple environmental factors to preserve soil quality and microbial health [58,59]. Given the continued pressures of urbanization, improving soil structure and nutrient cycling in urban green spaces will be essential for sustaining ecosystem services and enhancing urban resilience [60]. While this study provides valuable insights into the short- to medium-term effects of litter and root inputs on urban soil processes, long-term impacts on carbon stabilization and microbial dynamics remain unclear. Future research should investigate a broader range of urban forest types and incorporate long-term monitoring to better understand the sustained effects of organic inputs across diverse ecological contexts.

5. Conclusions

This study highlights the critical role of C inputs, particularly through litter and root management, in shaping soil properties and enzyme activities within the subtropical camphor tree forests. The results show that LA treatment significantly increased SOC, achieving the highest levels among treatments. The LR + RE treatment resulted in the highest TN content, while the LA treatment also exhibited the highest C:N ratio, indicating a more carbon-enriched environment at this site. Soil water content (SWC) was most effectively increased in the LA + RE treatment, suggesting potential benefits for moisture retention in similar urban forest conditions. Enzyme activities, including protease, catalase, and urease, were notably elevated in the LA treatment during the growing season, reflecting enhanced microbial nutrient cycling and activity. These findings provide site-specific insights into how litter and root inputs can influence soil fertility and microbial processes in subtropical urban forests. However, we acknowledge that soil carbon dynamics and microbial activities are complex and influenced by multiple interacting factors that vary across ecosystems. Therefore, while our results are applicable within the context of this study, caution should be taken when generalizing to other forest types or environments without further research. Overall, this study contributes to a growing understanding of soil biogeochemical responses to organic matter management in urban forest ecosystems and may inform localized management practices aimed at maintaining soil health and supporting ecosystem functions.

Author Contributions

Conceptualization, X.H., W.Y., X.C. and Y.Q.; methodology, X.H., X.C. and Y.Q.; investigation, X.H. and T.Y.; validation, T.Y., X.L. and J.L.; formal analysis, Y.P., X.L. and Y.Q.; visualization, J.L.; resources, Y.P., Y.Q. and X.C.; writing—original draft preparation, X.H. and Y.P.; writing—review and editing, X.H., W.Y., Y.P., T.Y., X.L., J.L., X.C. and Y.Q.; supervision, W.Y., X.C. and Y.Q.; project administration, W.Y.; funding acquisition, W.Y., Y.Q. and X.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (32471842), the Creative Research Groups of the Provincial National Science Foundation of Hunan (2024JJ1016), and the ‘Overseas Talent Scholar’ grant from Central South University of Forestry and Technology (1997–1999).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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

Acknowledgments

We thank Xin Liu and Can Mao for their assistance in the fieldwork and lab analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the study site and the six treatments at the study site. LC—litter control; LA—litter addition; RE—root exclusion; LR—litter removal; LR + RE—litter removal combined with root exclusion; LA + RE—litter addition combined with root exclusion.
Figure 1. Location of the study site and the six treatments at the study site. LC—litter control; LA—litter addition; RE—root exclusion; LR—litter removal; LR + RE—litter removal combined with root exclusion; LA + RE—litter addition combined with root exclusion.
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Figure 2. Variation of soil enzyme activity under different treatments from the first month to the tenth month of the study period after treatments: (a) Protease (PRO), (b) Dehydrogenase activity (DEH), (c) Acid phosphatase activity (ACP), (d) Urease activity (URE), and (e) Catalase activity (CAT). Bars with different capital letters represent significant differences in soil enzyme activities among treatments (p < 0.05), while bars with different lowercase letters represent significant differences among sampling times (p < 0.05).
Figure 2. Variation of soil enzyme activity under different treatments from the first month to the tenth month of the study period after treatments: (a) Protease (PRO), (b) Dehydrogenase activity (DEH), (c) Acid phosphatase activity (ACP), (d) Urease activity (URE), and (e) Catalase activity (CAT). Bars with different capital letters represent significant differences in soil enzyme activities among treatments (p < 0.05), while bars with different lowercase letters represent significant differences among sampling times (p < 0.05).
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Table 1. Variation in soil physicochemical properties in camphor tree forests under different treatments at the study site.
Table 1. Variation in soil physicochemical properties in camphor tree forests under different treatments at the study site.
TreatmentSOC
(g kg−1)		TN
(g kg−1)		C:NSWC%
LC15.28 (1.51) c1.49 (0.03) b10.2 b22.46 (0.95) a
LR22.13 (2.78) a2.03 (0.12) a10.93 b17.64 (0.91) c
LA23.94 (2.11) a1.92 (0.13) a12.69 a20.63 (0.75) b
RE13.85 (2.82) c1.57 (0.13) b8.64 c20.34 (0.31) b
LR + RE19.09 (1.63) a2.1 (0.15) a9.52 b17.08 (1.23) c
LA + RE17.17 (1.44) b1.77 (0.01) b9.62 b23.16 (1.33) a
Note: The values are average during the study. The data in parentheses represent the standard error (SE). SOC—soil organic carbon; TN—total nitrogen; SWC—soil water content. Different lowercase letters indicate significant differences among different treatments at the same soil property (p < 0.05).
Table 2. Correlation analysis among soil properties under different treatments.
Table 2. Correlation analysis among soil properties under different treatments.
Factor 1Factor 2Correlation Coefficientp-Value
SOCProtease activity0.85<0.01
SOCC:N ratio−0.56<0.05
SOCTN0.78<0.01
C:N ratioProtease activity−0.70<0.05
SWCSOC0.65<0.05
SWCProtease activity0.72<0.01
Note: SOC—soil organic carbon; TN—total nitrogen; SWC—soil water content.
Table 3. Principal component analysis of soil properties under different treatments.
Table 3. Principal component analysis of soil properties under different treatments.
Principal ComponentVariance Explained (%)Associated Variables
PC145%SOC, TN, C:N ratio
PC230%Protease activity, Urease activity, SWC
PC325%Other enzyme activity
Note: SOC—soil organic carbon; TN—total nitrogen; SWC—soil water content.
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Huang, X.; Peng, Y.; Yan, W.; Yan, T.; Liang, X.; Lei, J.; Chen, X.; Qi, Y. Aboveground and Belowground Input Effects on Soil Health in Urban Camphor Tree Forests. Sustainability 2025, 17, 6358. https://doi.org/10.3390/su17146358

AMA Style

Huang X, Peng Y, Yan W, Yan T, Liang X, Lei J, Chen X, Qi Y. Aboveground and Belowground Input Effects on Soil Health in Urban Camphor Tree Forests. Sustainability. 2025; 17(14):6358. https://doi.org/10.3390/su17146358

Chicago/Turabian Style

Huang, Xuejia, Yuanying Peng, Wende Yan, Tianyi Yan, Xiaocui Liang, Junjie Lei, Xiaoyong Chen, and Yaqin Qi. 2025. "Aboveground and Belowground Input Effects on Soil Health in Urban Camphor Tree Forests" Sustainability 17, no. 14: 6358. https://doi.org/10.3390/su17146358

APA Style

Huang, X., Peng, Y., Yan, W., Yan, T., Liang, X., Lei, J., Chen, X., & Qi, Y. (2025). Aboveground and Belowground Input Effects on Soil Health in Urban Camphor Tree Forests. Sustainability, 17(14), 6358. https://doi.org/10.3390/su17146358

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