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Article

Larger Soil Water-Stable Aggregate May Exert a Negative Effect on Nutrient Availability: Results from Red Soil (Ultisol), in South China

by
Ming Feng
1,2,
Jian Xiang
1,
Xiaofang Ji
3 and
Jiang Jiang
1,*
1
Collaborative Innovation Center of Sustainable Forestry in Southern China of Jiangsu Province, Key Laboratory of Soil and Water Conservation and Ecological Restoration in Jiangsu Province, Nanjing Forestry University, Nanjing 210037, China
2
College of Ecology and Environment, Hainan University, Haikou 570228, China
3
Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Forests 2023, 14(5), 975; https://doi.org/10.3390/f14050975
Submission received: 12 March 2023 / Revised: 3 April 2023 / Accepted: 25 April 2023 / Published: 9 May 2023
(This article belongs to the Section Forest Soil)
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Abstract

:
Soil aggregates are the basic units of soil, which regulate soil carbon cycling and nutrient availability through the protective effect of soil aggregates on soil organic matter. It is still uncertain whether larger aggregates are more conducive to soil nutrient availability in red soil. This study explored the regulation of soil aggregates on soil nutrient availability by studying the distribution of soil aggregates, nutrient concentrations, nutrient availability and organo-mineral bonds in soil aggregates in a low-productivity Chinese fir forest, in south China. We sampled the 0–10 cm soil with nine repeated plots and analyzed the soil aggregate structure, total nutrients, available nutrients and organo-mineral bonds of soil aggregates. The results showed that the contribution of >2 mm soil aggregates to soil nutrients was highest, because the mass of >2 mm soil aggregates accounted for about 50% of the total mass of aggregates and was much higher than that of other aggregates. The availability (available nutrient/total nutrient) of nitrogen, phosphorus and potassium increased with decreases in soil aggregate size, indicating that soil aggregates with a larger particle size were more averse to nutrient availability. Strong organo-mineral bonds accounted for more than 80% of the total organo-mineral bonds in the soil aggregates of each size, and the proportion of weak organo-mineral bonds in the soil aggregate increased with decreases in the soil aggregate size. There was a significant negative correlation between the size of soil aggregates and the proportion of weak organo-mineral bonds in soil aggregates. The availability of carbon, nitrogen, phosphorus and potassium in soil aggregates was positively correlated with the proportion of weak organo-mineral bonds. These results suggest that Fe/Al oxides may play an important role in regulating nutrient availability, especially in red soil. A higher proportion of strong organo-mineral bonds in larger soil aggregates may exert a stronger negative effect on the accessibility of microorganisms to organic matter and result in a lower nutrient availability. In conclusion, this study shows that larger-sized soil macroaggregates may exert a negative effect on nutrient availability, owing to a higher proportion of strong binding bonds, which can better prevent microorganisms from mineralizing organic matter into effective nutrients in red soil.

1. Introduction

The Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), an evergreen conifer native to southern China and northern Vietnam, belongs to the Cupressaceae family. Because of its fast growth, desirable wood properties and high resistance to diseases, it has been widely cultivated in China for over 3000 years [1,2]. The Chinese fir is one of the most important timber plants in southern China [3]. The shortage of available nutrients is particularly acute and has become a major factor limiting the growth and productivity of Chinese firs [4,5].
The availability of soil nutrients plays an important regulatory role in plant productivity and growth [6,7,8]. Plant productivity will decline, because plants may adopt a resource conservation strategy by having slow growth in an environment with low nutrient availability [9]. Soil nutrient availability plays essential roles both in linking soil to plants and in regulating nutrient cycling in terrestrial ecosystems [10]. Nutrient availability in soils under natural conditions is controlled by several factors, such as microorganisms, enzyme activity, temperature, moisture, aggregates and other factors, which mainly affect nutrient availability by affecting the process of microbial mineralization [7,11,12,13,14]. The mineralization of organic matter is an important process for the conversion of organic matter into bioavailable nutrients. The protective effect of soil aggregates on organic matter is an important limiting factor in the mineralization process [15,16,17,18].
Studies suggest that aggregate-based approaches are a critical next step for developing a predictive understanding of how geochemical and community interactions govern microbial community structures and nutrient cycling in soil [19]. Microaggregates (<0.25 mm) can withstand strong mechanical and physicochemical stresses, allowing them to persist in soils for decades [20,21]. This means that, compared with larger size soil aggregates, microaggregates are less likely to break and release nutrients under external forces (or without forces), and the mineralization of organic matter into available nutrients is slower. Other research suggested that the protection of SOC from macroaggregates (0.25~2 mm) was important to the accumulation of SOC in forest soil by investigating the differences in δ13C between the light and heavy fraction in all aggregate size fractions [22]. Larger soil fractions (>53 μm), especially 250–53 μm aggregates, which contain more soil C and N, are associated with greater microbial biomass and higher fungi/bacteria ratios [23]. Scientists generally pay more attention to the influence of environmental factors or management factors on the nutrients in the aggregates, but it is unclear what the variation characteristics of nutrient availability in the aggregates are and what the drivers influencing the availability of nutrients in aggregates are.
Researchers generally believe that the presence of more macroaggregates will have a positive effect on soil nutrient cycling, given the SOC concentrations within aggregates were hypothesized to increase with increases in the size of aggregates according to the hierarchy theory [24,25]. Macroaggregates were considered to be the “preferential aggregate fractions” where most of the plant residue C was accumulated and can be used as an indicator of management effects [26,27,28]. In addition, the OC in microaggregate and silt-clay units was believed to be more stable and recalcitrant than that in macroaggregate and coarser fractions [29]. Therefore, plants have greater difficulty utilizing the organic matter in smaller-sized soil aggregates. However, Jha’s research found an interesting phenomenon that significantly higher soil carbon mineralization occurred in smaller-sized microaggregates (<0.25 mm) rather than larger-sized macroaggregates (>0.25 mm) [15], indicating that the organic matter in microaggregates may be more easily mineralized by microorganisms to provide available nutrients for plant growth. There exists uncertainty regarding whether larger-sized soil aggregates have a higher nutrient availability or produce more available nutrients.
Soil aggregates affect the degree of hierarchical inaccessibility of soil organic carbon (SOC) to decomposing organisms or catalytic enzymes [20,27,30], and the availability of SOC and nutrients [31]. A large number of studies have reported that larger-sized soil aggregates store abundant organic matter [25]. Therefore, larger-sized soil aggregates may have a higher nutrient availability, due to the greater accessibility of the substrate to decomposers in larger-sized soil aggregates. The mineral phases considered to be the most important as aggregate-forming materials are the clay minerals and Fe- and Al-(hydr)oxides bounding together primarily by physicochemical and chemical interactions [21,32]. Previous studies pointed out that soil aggregates consist largely of clay-polyvalent metal–organic matter complexes, which may be represented as [(C-P-OM)x]y, where C = clay mineral particle, P = polyvalent metal (e.g., Ca, Al, Fe) and OM = organo-metallic complex [33]. The nutrient availability of soil aggregates can be regulated by the binding bonds between the polyvalent metal and organic matters, due to the binding force generated by the interparticle bonds, which may hinder the mineralization, by reducing the accessibility of organic matter to microorganisms. Therefore, it is still uncertain whether larger-sized soil aggregates have a higher nutrient availability in red soil.
The aims of this study were as follows: (1) to explore the distribution characteristics of the nutrient availability in soil aggregates; (2) to explore the distribution characteristics of the concentration of the binding bonds between the polyvalent metal and organic matters in soil aggregates; (3) to illustrate the key factors affecting the availability of nutrients in soil aggregates.

2. Materials and Methods

2.1. Study Area

The study was conducted in the Guanshan Forest Farm of Yongfeng County (26°38′~27°32′ N, 115°17′~115°56′ E) in the central part of Jiangxi Province. The annual average temperature is 18 °C, the annual average rainfall is 1627.3 mm, and the frost-free period is 279 days. The soils in this area are the typical red soils that are colored by the high concentrations (>17%) of iron (hydro)oxides [34]. Yongfeng County is rich in forest resources, with a forest area of about 200,000 hectares. However, the problem of the low production efficiency of plantation forests is widespread. The average diameter at breast height of the 10-year-old Chinese fir forest in this study is only 9.4 cm, which is about 30% lower than that of Chinese firs of the same age in other regions [35].

2.2. Soil Sampling

The study area started forestry planting in 1963 and experienced two rotation plantations of Chinese firs. In 2007, the third-rotation Chinese fir plantation was established. This research selected nine sampling plots (20 m × 20 m) in the study area. The main characteristics of these sampling plots are shown in Table 1. Soil samples were collected in January 2018. After the removal of the litter, roots and small rocks in soil, ten undisturbed soil samples were collected in the 0–20 cm soil layer and mixed to form a bulk sample in each plot and stored in aluminum containers. The soil samples were air-dried and broken into aggregates along natural planes of weakness for aggregate measurement and aggregate nutrients.

2.3. Soil Aggregates and Measurements

The aggregates were fractionated using a wet sieving method [24]. For each sample, 50 g of air-dried bulk soil was fractionated through a series of sieves (2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.106 mm, 0.053 mm), resulting in the collection of six aggregate size fractions: >2 mm, 1 mm~2 mm, 0.5 mm~1 mm, 0.25 mm~0.5 mm, 0.106 mm~0.25 mm and <0.106 mm, using a Wet Sieving Apparatus (Eijkelkamp, The Netherlands). Each 50 g soil sample was presoaked in distilled water for 10 min before the nested sieves were moved up and down in the water column within a range of 3 cm (25 S min−1) for 5 min [36]. The weight of each aggregate fraction was determined after oven-drying at 60 °C to a constant moisture content. For every wet sieving operation, the total mass of the recovered aggregates is about 80% of the mass of the soil sample. Each wet sieving operation can obtain 6 size aggregates; we conducted 9 wet sieving operations and finally obtained 54 aggregate samples (6 sizes × 9 plots). There are 9 samples for each size aggregate.
The separated aggregates were used for various chemical analyses, including total carbon, total nitrogen, total phosphorus, total potassium, interparticle bonds and the availability of carbon, nitrogen, phosphorus and potassium in soil aggregates. We measured each chemical property 54 times (6 size × 9 plots), and finally we could obtain 9 replicated data of the same chemical property for each particle size. The following methods were used to analyze the soil samples: the soil organic carbon and total nitrogen contents were measured using an Elemental analyzer (Vario MACRO cube, Elementar, Langenselbold, Germany) [37]. Labile organic carbon (LOC) was determined using oxidation with 333 mmol·L−1 KMnO4 [38]. Briefly, dried soil aggregate containing 30 mg of C was added to a 50 mL centrifuge tube, shaken with 333 mmol·L−1 KMnO4 at 200 rpm for 1 h and centrifuged at 4000 rpm for 5 min. The supernatant was diluted with deionized water at 1:250, and the absorbance was measured at 565 nm spectrophotometrically [39]. The soil aggregates’ AN was determined by the alkaline hydrolysis diffusion method [40]. The soil aggregates’ TP was digested with sulfuric acid-perchloric acid (H2SO4–HClO4), and the AP of the aggregate was extracted with hydrochloric acid-ammonium fluoride (HCl-NH4F) [41] and then determined according to the molybdenum blue method at 700 nm. The soil aggregates’ TK was digested with hydrofluoric acid-perchloric acid (HF–HClO4) [42] and the soil aggregates’ AK was extracted with ammonium acetate (CH3COONH4) [43] and then quantified using flame photometry. In this study, the concentration of weakly organically bound Fe and organically bound Fe in soil aggregates is expressed as weak interparticle bonds and strong interparticle bonds in soil aggregates. The weakly organically bound Fe and organically bound Fe were extracted with 0.5 M of CuCl2 electrolyte [44] and a sodium pyrophosphate (Na4P2O7·10H2O) solution at a high pH (>9) [45], respectively, and then determined in the extract by atomic absorption spectrometer (AA-7000, SHIMDZU, Kyoto, Japan).

2.4. Statistical Analysis

Data preprocessing using Office 2016 and the R3.6.0. R package (agriculae) was used to analyze the differences (Tukey test) and correlation (Spearman) of soil total carbon, total nitrogen, total phosphorus, total potassium, labile organic carbon, available nitrogen, available phosphorus and available potassium.

3. Results

3.1. Distribution of Soil Aggregates and Nutrients

3.1.1. Distribution of Soil Aggregates

The data of this study showed that the proportion of the soil aggregate mass of particular sizes in total soil aggregate mass gradually decreases with decreases in soil aggregate size (Figure 1). The mass of >2 mm soil aggregate accounted for about 51% of the total soil aggregate mass, which is the largest proportion, and was about 4 times higher than that of other sizes. The fraction of <0.106 size soil aggregate is the smallest, which is about 6%.

3.1.2. Nutrient Concentration Distribution in Soil Aggregates

With the decrease in soil aggregates’ size, the concentration of total organic carbon, total nitrogen, total potassium and labile organic carbon in soil aggregates increased first and then decreased. The concentration of alkali-hydrolyzed nitrogen and available phosphorus decreased first and then increased, while the concentration of total phosphorus and available potassium did not show obvious regularity. The maximum concentrations of total carbon and total nitrogen were 13.37 g/kg and 1.31 g/kg at 0.25~0.5 mm size, and the minimum concentrations were 10.88 g/kg and 0.93 g/kg at <0.106 mm size, respectively. The concentration of labile organic carbon showed a maximum value of 11.4 g/kg at 0.5~1 mm size and a minimum value of 8.2 g/kg at <0.106 mm size. The concentrations of alkali-hydrolyzed nitrogen and available phosphorus showed lower values of 111.6 mg/kg and 5.6 mg/kg at 1~2 mm size, respectively (Figure 2).

3.1.3. Contribution of Soil Aggregates to Soil Nutrients

With the decrease in soil aggregates’ size, the contributions of various nutrients in soil aggregates showed a decreasing trend. The contributions of various nutrients in >2 mm soil aggregates to soil were the largest, which was significantly higher than that of other size soil aggregates, and more than 5 times higher than that of <0.106 mm size soil aggregates (Figure 3).

3.1.4. Nutrient Availability in Aggregates

The availability (available nutrient content/total nutrient content) of nitrogen, phosphorus and potassium in soil aggregates increased with decreases in soil aggregate size. The nitrogen, phosphorus and potassium availability showed maximum values of 26.7%, 13.4% and 2.6%, respectively. For <0.106 mm size soil aggregates, which were significantly higher than soil aggregates of other sizes. However, there was no clear trend in the availability of carbon with decreases in soil aggregate size. The carbon availability of soil aggregates with a size <0.106 mm is the highest, which is significantly higher than that of soil aggregates with sizes between 0.25 and 0.5 mm (Figure 4).

3.2. Organo-Mineral Bonds in Soil Aggregates

With decreases in soil aggregate size, the concentration of weak organo-mineral bonding carbon in the soil aggregates showed an increasing trend, and the concentration of weak organo-mineral bonding carbon was the highest in soil aggregates with the size <0.106 mm, 34.058 mg/kg. However, there were no significant differences in the content of strong organo-mineral bonding carbon among all sizes of soil aggregates. Larger soil aggregates show a smaller ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon (Figure 5). The ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon in the <0.106 mm size soil aggregates was the highest, reaching 0.238, which was about twice that of soil aggregates of other sizes.

3.3. Correlation of Soil Aggregates’ Nutrients and Size

Correlation analysis of various nutrient concentrations showed that there were significant correlations among the concentrations of total carbon, total nitrogen and organic matter in the aggregates but no correlation between total carbon and labile organic carbon, total nitrogen and alkali-hydrolyzed nitrogen, total phosphorus and available phosphorus, or potassium and available potassium. These results suggest that the mineralization of organic matters in the soil aggregates may be hindered by the soil aggregates (Figure 6).
The correlation analysis results of nutrient availability, soil aggregate size and the ratio of weak organo-mineral bonding carbon showed that the availability of nitrogen, phosphorus and potassium correlated with soil aggregate size, while the carbon nutrient availability was not correlated with soil aggregate size (Figure 6). In other words, the smaller the soil aggregates size, the higher the availability of nitrogen, phosphorus and potassium, and there were no significant relationships between the availability of carbon and that of other nutrients. The availability of nitrogen, phosphorus and potassium was closely correlated and showed similar trends in different soil aggregate sizes. The ratio of weak organo-mineral bonding carbon was correlated to the availability of carbon, nitrogen, phosphorus and potassium. Except for the availability of carbon, the availabilities of other nutrients were significantly correlated to the size of soil aggregates, and the r2 values were 0.72, 0.64 and 0.52, respectively (Figure 7). This result may be due to the fact that organo-mineral bonding carbon and organo-organic bonding carbon are important parts of soil aggregates [17] and some organic matter is mineralized as CO2 rather than labile organic carbon. The soil aggregate size was significantly correlated with the availability of nitrogen, phosphorus and potassium and the ratio of weak organo-mineral bonding carbon.

4. Discussion

4.1. Nutrient Availability in Soil Water-Stable Aggregates

Soil water-stable aggregates and their stability, which is regarded as one of the key influential factors, can affect soil water storage, soil carbon storage, soil porosity, interflow, nutrient cycle, microbial activity [46,47,48,49], soil erosion and soil quality [50,51]. The aggregate size distribution of soil is an important parameter to describe soil properties [52]. A previous study adopted numerous indices for evaluating soil aggregate stability, such as the structural stability (SI), fractal dimension (D), mean weight diameter (MWD) and geometric mean diameter (GMD) [53], which means that the higher the content of larger aggregates (2–0.25 mm), the better the soil structure, soil stability and erosion resistance. Our research results show that as the size of the soil water-stable aggregates decreases, the proportion of the dry mass of the soil water-stable aggregates in the total dry mass gradually decreases (Figure 1). In this study area, the water-stable large aggregates (>0.25 mm) accounted for about 90% of the soil aggregates, of which the size of >2 mm accounted for about 51%. In addition, similar proportions of large aggregates (>0.25 mm) in red soil were also reported by Liu [54]. The results of this study show that red soil has high soil aggregate stability. However, the study of soil aggregates in the northeast black soil area (Mollisol) found that macroaggregates (>0.25 mm) accounted for about 25% of the soil, and microaggregates (<0.25 mm) accounted for 83%~86% [55]. The Fe/Al oxides play an important role in the formation of soil aggregates [56]. Ultisol possessed high amount of variably charged soil oxides and Fe/Al oxides are primary agents of soil aggregates in Ultisol, which may be the main reason for the difference between the results of the study in Mollisol and red soil [57].
The nutrient status in soil water-stable aggregates directly affects the growth of plants. Moreover, the formation and distribution of soil aggregates can regulate the process of crops acquiring soil nutrients [58]. Previous research found that aggregate fractions indirectly affect nutrients by contributing to multifunctionality mainly by regulating the richness of total nematodes and trophic groups [58]. Soil aggregates indirectly mediate organic carbon, total nitrogen and microbial activity in a Karst ecosystem [59]. The hierarchical order of aggregates might lead to the differences in the distribution and availability of soil organic matter (SOM) [48]. Hence, investigating the distribution of soil water-stable aggregates and nutrient concentration in soil water-stable aggregates in Chinese fir forest is essential for thoroughly understanding the nutrient cycling process. This study showed that with decreases in the aggregate size, the concentration of total organic carbon, total nitrogen, total potassium and labile organic carbon all showed a trend of increasing first and then decreasing (Figure 2). This study is consistent with previous studies, which showed that small macroaggregates (0.25~2 mm) might have more active carbon dynamics [59]. However, the concentration of alkali-hydrolyzed nitrogen and available phosphorus showed a trend of first decreasing and then increasing (Figure 2).
The availability of nutrients can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients thus limiting the crop productivity [60,61]. Soil water-stable aggregate nutrient availability distribution characteristics directly regulate soil nutrient availability since soil aggregates are important carriers of nutrients. Studying the nutrient availability distribution characteristics of aggregates helps scientists to understand what role soil aggregates play in the formation of soil nutrient status. Previous studies have focused on the concentration of available nutrients in soil and the effects of other factors (such as fertilizer, irrigation, biochar, Rhizobacteria, etc.) on the concentration of available nutrients [7,62,63,64]. However, only a few studies have focused on available nutrient concentration in soil aggregates [65,66,67], and few scholars have studied the relationship between Chinese fir forest soil aggregates and nutrient availability. This study investigated this aspect and found that as the aggregate size decreased, the availability of nitrogen, phosphorus and potassium in the aggregates (available nutrient content/total nutrient content) showed an increasing trend (Figure 4). The availability of carbon shows no apparent regularity (in other words, carbon availability exhibited an insignificant rise, as the aggregate size decreased) (Figure 4). This result indicated that larger aggregates in the soil of Chinese fir forest in the red soil region are not conducive to the accumulation of available nutrients and may limit the plants’ obtainment of available nutrients. The mineralization of organic matter in the soil produces available nutrients that are more readily available to plants. However, the mineralization process of organic matter is hindered by the protective effect of aggregates on organic matter. Therefore, the protective effect of aggregates of different particle sizes on organic matter may regulate the distribution of nutrient availability in soil aggregates. Jha’s study found that with decreases in soil aggregate size, the mineralization rate of organic matter in the soil aggregates increased sequentially [15], indicating that larger soil aggregates have a stronger ability to hinder the mineralization of organic matter. This also supports our findings that larger aggregates hinder the mineralization of organic matter in soil aggregates and the release of available nutrients, which is not conducive to plant growth.

4.2. Organo-Mineral Bonds for Nutrient Availability

Associations between soil organic matter (SOM) and semicrystalline reactive iron (Fe) and aluminum (Al) mineral are recognized to contribute to long-term SOM persistence and accumulation across widely variable soil types [16,17,68,69]. Torn’s study found a positive relationship between non-crystalline minerals and organic carbon in soil, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial-scale cycling of mineral-stabilized carbon [69]. Previous studies have shown through experimental analysis that there are organo-organic and organo-mineral interfaces in soil [17]. Numerous studies have demonstrated the existence of organo-mineral bonds in soils that protect organic matters from microbial decomposition. Therefore, an increase in organo-mineral bonds may reduce soil nutrient availability, which is generated during the decomposition of SOM, by reducing microbial accessibility and the availability for decomposition [70,71]. Previous studies have focused on the protective effect of organo-mineral bonds on soil organic matter and the structure of organo-mineral bonds [72,73]; however, few studies have focused on the distribution characteristics of organo-mineral bonds in soil aggregates and the relationship between organo-mineral bonds and nutrient availability in soil aggregates.
Organo-mineral bonds may play an important role in aggregate formation. Previous studies have found that soil aggregates with larger particle sizes have higher concentrations of organic matter [27,28], and organic matter may be an important substance that promotes the formation of aggregates. Comprehensive previous studies suggest that SOM may promote the formation of larger soil aggregates from smaller particles in the form of organo-mineral bonds, while hindering the decomposition of SOM by microorganisms. The results of this study found that with decreases in particle size, the ratio of weak bonds to strong bonds increased, and there was a significant correlation between the soil aggregate size and the ratio of weak bonds to strong bonds (p < 0.001, R2 = −0.6, Figure 5, Figure 7). Our results indicate that soil aggregates of larger particle sizes produce a stronger binding capacity to bind minerals to organic matter, and this binding capacity may determine whether the soil aggregates can develop into larger-sized soil aggregates. In soil lacking SOM, the low concentration of SOM in soil aggregates of each particle size may be the main limiting factor for their development into larger soil aggregates, and the ability of the soil aggregates to combine minerals with SOM will determine whether the particles can be developed into soil aggregates with larger particle sizes. In conclusion, this study suggests that SOM may play an important role in soil aggregate formation in the form of organo-mineral bonds, especially in the nutrient-poor red soil regions. At the same time, environmental factors such as microorganisms, plant roots, soil moisture, enzymes, temperature and their interactions [74,75] will affect the formation process of soil aggregates by regulating the content of SOM and minerals through physical rupture and chemical reactions. In future studies, we will comprehensively analyze the influence of various environmental factors on soil aggregates’ formation.
Organo-mineral bonds may modulate the distribution characteristics of nutrient availability in soil aggregates of different particle sizes by controlling microbial accessibility to organic matter [70,71]. A study using cryo-electrons with electron energy loss spectroscopy showed that organo-mineral interfaces are enriched with more nitrogen and oxidized carbon and are more conducive to carbon sequestration [17]. Our results show that the soil aggregates with stronger organo-mineral bonds have a lower proportion of available nutrients (Figure 7), which is consistent with the assumption that organo-mineral bonds limit microbial access to the substrate. The results of this study showed that the nutrient availability in larger soil aggregates was lower, which may be caused by the fact that larger soil aggregates had a higher proportion of strong organo-mineral bonds leading to stronger obstacles to organic mineralization (Figure 5).

5. Conclusions

Our study suggested that larger soil aggregates contain a higher proportion of strong organo-mineral bonds and lower nutrient availability. This study suggested that the proportion of strong organo-mineral bonds may dominate the formation of soil aggregates of different sizes, protect soil organic matter from mineralization and lead to the lower nutrient availability of larger soil aggregates. Although there are shortcomings in this study and other factors are not considered, it is suggested that the status of organo-mineral bonds in soil may have a significant influence on the formation of soil aggregates and soil nutrient availability. A higher proportion of strong organo-mineral bonds in larger soil aggregates exerts a stronger negative effect on the accessibility of microorganisms to organic matter and results in lower nutrient availability. In conclusion, this study shows that larger-sized soil macroaggregates may exert a negative effect on nutrient availability in the red soil of Chinese fir forest. Additionally, our research linked the role of organo-mineral bonds in the formation of soil aggregates to the micrometer-scale theory of soil aggregates limiting microbial accessibility to organic matter and the nutrient availability of soil aggregates.

Author Contributions

Conceptualization, M.F., J.X. and J.J.; methodology, M.F. and J.X.; software, M.F. and X.J.; writing—original draft preparation, M.F.; writing—review and editing, M.F., J.X., X.J. and J.J.; visualization, M.F. and X.J.; supervision, J.J.; project administration, J.J.; funding acquisition, J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Science Foundation of China (32071612). We greatly appreciate the reviewers and handling editor for their perspective comments.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The percentages of aggregates of different sizes. Different lowercase letters indicate significant differences among aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
Figure 1. The percentages of aggregates of different sizes. Different lowercase letters indicate significant differences among aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
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Figure 2. The total carbon (TC) (a), total nitrogen (TN) (b), total phosphorus (TP) (c), total potassium (TK) (d), labile organic carbon (LOC) (e), available nitrogen (AN) (f), available phosphorus (AP) (g) and available potassium (AK) (h) concentrations of aggregates of different sizes. Different lowercase letters indicate significant differences in total nutrient concentration between aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
Figure 2. The total carbon (TC) (a), total nitrogen (TN) (b), total phosphorus (TP) (c), total potassium (TK) (d), labile organic carbon (LOC) (e), available nitrogen (AN) (f), available phosphorus (AP) (g) and available potassium (AK) (h) concentrations of aggregates of different sizes. Different lowercase letters indicate significant differences in total nutrient concentration between aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
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Figure 3. The contribution of total carbon (TC) (a), total nitrogen (TN) (b), total phosphorus (TP) (c), total potassium (TK) (d), labile organic carbon (LOC) (e), available nitrogen (AN) (f), available phosphorus (AP) (g) and available potassium (AK) (h) of different aggregates to soil. Different lowercase letters indicate significant difference among the contributions of nutrients of aggregates of different sizes to soil by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
Figure 3. The contribution of total carbon (TC) (a), total nitrogen (TN) (b), total phosphorus (TP) (c), total potassium (TK) (d), labile organic carbon (LOC) (e), available nitrogen (AN) (f), available phosphorus (AP) (g) and available potassium (AK) (h) of different aggregates to soil. Different lowercase letters indicate significant difference among the contributions of nutrients of aggregates of different sizes to soil by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
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Figure 4. The carbon (a), nitrogen (b), phosphorus (c) and potassium (d) availability (the ratio of available nutrients to total nutrients) soil aggregates of different sizes. Different lowercase letters indicate significant differences among the nutrient availability in aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
Figure 4. The carbon (a), nitrogen (b), phosphorus (c) and potassium (d) availability (the ratio of available nutrients to total nutrients) soil aggregates of different sizes. Different lowercase letters indicate significant differences among the nutrient availability in aggregates of different sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
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Figure 5. The ratio of weak bond to strong bond. Different lowercase letters indicate significant differences among weak–strong ratios in different aggregate sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
Figure 5. The ratio of weak bond to strong bond. Different lowercase letters indicate significant differences among weak–strong ratios in different aggregate sizes by Tukey test (p < 0.05). Error bars indicate standard deviation (n = 9).
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Figure 6. Correlation matrix of soil aggregates’ sizes and nutrient concentrations. Note: Pearson product-moment correlations on the mean (n = 9) are shown. The numerical matrix of the upper triangle represents the correlation coefficient. *, ** and *** indicate significant correlations at 0.05, 0.01 and 0.001 probability levels, respectively. N: total nitrogen, P: total phosphorus, K: total potassium, AN: available nitrogen, AK: available potassium, AP: available phosphorus, SOM: soil organic matter, LOC: labile organic carbon.
Figure 6. Correlation matrix of soil aggregates’ sizes and nutrient concentrations. Note: Pearson product-moment correlations on the mean (n = 9) are shown. The numerical matrix of the upper triangle represents the correlation coefficient. *, ** and *** indicate significant correlations at 0.05, 0.01 and 0.001 probability levels, respectively. N: total nitrogen, P: total phosphorus, K: total potassium, AN: available nitrogen, AK: available potassium, AP: available phosphorus, SOM: soil organic matter, LOC: labile organic carbon.
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Figure 7. Correlation matrix of soil aggregates’ size, the ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon and nutrient availability. Note: Pearson product-moment correlations on the mean (n = 9) are shown. The numerical matrix of the upper triangle represents the correlation coefficient. *** indicate significant correlations at 0.001 probability levels. C: carbon, N: total nitrogen, P: total phosphorus, K: total potassium, bond: ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon.
Figure 7. Correlation matrix of soil aggregates’ size, the ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon and nutrient availability. Note: Pearson product-moment correlations on the mean (n = 9) are shown. The numerical matrix of the upper triangle represents the correlation coefficient. *** indicate significant correlations at 0.001 probability levels. C: carbon, N: total nitrogen, P: total phosphorus, K: total potassium, bond: ratio of weak organo-mineral bonding carbon to strong organo-mineral bonding carbon.
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Table
Table 1. Description of the sampling plots.
Table 1. Description of the sampling plots.
NoCoordinateElevation/mMean Height/mMean DBH/cmDensity (*/ha)Basal Area (m2/ha)
127°14′20.81″ N, 115°34′23.68″ E1187.459.90317525.67
227°14′21.51″ N, 115°34′22.57″ E1166.539.27347524.56
327°14′21.94″ N, 115°34′21.94″ E1115.689.17257517.83
427°14′22.44″ N, 115°34′21.57″ E1117.109.29312522.03
527°14′22.98″ N, 115°34′20.95″ E1127.1610.43252522.61
627°14′23.50″ N, 115°34′20.53″ E1126.639.26302521.13
727°14′25.30″ N, 115°34′18.84″ E1135.678.77270017.30
827°14′25.57″ N, 115°34′18.24″ E1116.739.65250019.22
927°14′33.28″ N, 115°34′17.72″ E1136.089.71217516.84
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Feng, M.; Xiang, J.; Ji, X.; Jiang, J. Larger Soil Water-Stable Aggregate May Exert a Negative Effect on Nutrient Availability: Results from Red Soil (Ultisol), in South China. Forests 2023, 14, 975. https://doi.org/10.3390/f14050975

AMA Style

Feng M, Xiang J, Ji X, Jiang J. Larger Soil Water-Stable Aggregate May Exert a Negative Effect on Nutrient Availability: Results from Red Soil (Ultisol), in South China. Forests. 2023; 14(5):975. https://doi.org/10.3390/f14050975

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Feng, Ming, Jian Xiang, Xiaofang Ji, and Jiang Jiang. 2023. "Larger Soil Water-Stable Aggregate May Exert a Negative Effect on Nutrient Availability: Results from Red Soil (Ultisol), in South China" Forests 14, no. 5: 975. https://doi.org/10.3390/f14050975

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