Composition Of Soil Organic Carbon Research Articles

Introducing N2-fixing tree species into Eucalyptus plantations promotes soil organic carbon sequestration in aggregates by increasing microbial carbon use efficiency

The conversion of a monoculture plantation into a mixed forest can improve soil properties and increase soil organic carbon (SOC) sequestration. However, the impacts of introducing N2-fixing tree species into Eucalyptus plantations on SOC composition and transformation, as well as the associated changes in microbial properties remain poorly understood. Here, we examined the SOC fractions (LPI-C, LPII-C, and RP-C) in different aggregates (>2.0 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm) from 15-year-old plantations of pure Eucalyptus urophylla (PP) and mixed E. urophylla and Acacia mangium (an N2-fixing tree species) (MP). The results showed that the contents of SOC fractions and the proportion of recalcitrant C pool (fRP-C) in aggregates were significantly higher (P < 0.05) in MP than those in PP. Pearson correlation analysis indicated strong correlations among litter properties, soil properties, and microbial properties. N2-fixing species increased soil aggregate stability and N availability while altering litter quality and quantity; all of these factors strongly impacted microbial properties. Changes in nitrogen availability and microbial properties (e.g. carbon use efficiency) were the drivers of changes in SOC fractions. Our findings highlight the importance of introducing N2-fixing species in regulating soil microbial properties thereby increased SOC sequestration and stability in Eucalyptus plantations, which should be a relatively good option for future afforestation projects in the view of improving the carbon benefits of the ecosystem.

Changes in soil organic carbon and nutrient pools in aggregate‐sized fractions along a chronosequence of wolfberry (Lycium barbarum L.) plantations in arid areas of Northwest China

AbstractDifferent land use and management actions can affect soil aggregates (SAs) and nutrient stocks, which are crucial for sustainable agriculture. The impacts of various chrono‐sequences on the soil aggregate structure, soil organic carbon (SOC) and nutrients associated with aggregate fractions in wolfberry (Lycium barbarum L.) plantations are still not fully understood. This study examined the composition and stability of SAs, SOC, total nitrogen (TN), available phosphorus (AP) and exchangeable cations (K+, Na+, Ca2+ and Mg2+) in bulk soil and various aggregate‐size fractions from five wolfberry plantations with varying ages (1, 4, 6, 10 and 13 years) and a corn field (0 years) in the arid region of northwest Ningxia in China. The results indicated that silt–clay (&lt;53 μm) fractions were dominant in the soil, accounting for 51%–66%, under different plantation ages. The proportion of the macro‐aggregates (&gt;250 μm) increased significantly, by 40%–47%, over the 4 years of wolfberry plantation. Likewise, the soil aggregate stability was improved, and total exchangeable bases (TEB) along with numerous cations concentrations (K+, Na+, Ca2+ and Mg2+) in SAs were significantly reduced as the wolfberry plantings became older. Both concentrations of SOC and TN in the soil aggregates peaked in the 13th year. The silt–clay fractions stored a considerable amount of SOC and nutrients. However, short‐term (under 6 years) cultivation of wolfberry reduced the stocks of SOC, TN and AP in the soil, while long‐term (over 10 years) cultivation increased them, particularly in macro‐aggregates. These findings indicated that long‐term wolfberry farming had several advantages, such as enhancing soil structure, accumulating SOC and nutrients and ameliorating alkaline soils, especially after 10 years, in the arid northwest of China.

Duration of continuous cropping with straw return affects soil organic carbon

AbstractUnderstanding the impact of continuous cropping with straw return on soil organic matter functional groups is of great importance for maintaining chemical stabilization of SOC in arid regions. Infrared spectral characteristics of cotton field soil with different continuous cropping durations (0, 5, 10, 15 and 20 years) were determined with Fourier‐transform infrared spectroscopy for fields in Manasi River Basin, Xinjiang province, China. The effects of continuous cropping duration on the stability and chemical composition of soil organic carbon (SOC) were analysed. The results indicate an initial increase in the SOC, soil particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) content of soil under continuous cropping with straw return. However, as continuous cropping duration increased, the levels of SOC, POC, and MOC in soil began to decrease. The soil POC content and POC/MOC were highest after 5 years of straw return, and started to decrease as the continuous cropping duration increased. The SOC and MOC contents were highest after 10 years of continuous cropping, and were 3.30 and 1.84 times higher than the control, respectively. As continuous cropping duration increased, the relative peak intensities for polysaccharides and aromatic groups in soil organic matter decreased, while the relative peak intensities for aliphatic compounds and hydroxyketone increased. In conclusion, continuous cropping with straw return in cotton fields promotes SOC but only for about 10 years. An increase in straw return duration boosts the degree of esterification in soil organic matter. Moreover, enhanced protection by mineral binding was observed for soil organic matter, increasing organic matter stabilization. This study aimed to provide an empirical foundation for the management of the SOC pool and the establishment of rational straw return practices for cotton fields in arid areas.

Effects of tillage on soil organic carbon and crop yield under straw return

No-tillage with straw mulch is an effective way to achieve soil sustainability. However, the specific redistributions of soil organic carbon composition, stocks, and crop yield remain unclear. The hypothesis that no-tillage had a minor effect on soil organic carbon stocks in 0–50 cm but decreased crop yield compared to plow-tillage under straw return was tested in this study. Soils properties from two decades of rice-wheat rotation field experiment were related to the yield of rice and wheat depending on tillage: 1) plow-tillage without straw return, 2) no-tillage with straw mulch, and 3) plow-tillage with straw return. No-tillage with straw mulch had the highest mean weight diameter of soil aggregates, the contents of macro-aggregates (>0.25 mm) organic carbon, particulate organic carbon (>53 µm), and soil organic carbon across tillage practices in 0–5 cm. However, it decreased the contents of macro-aggregates (>0.25 mm) organic carbon in 5–15 cm, the particulate organic carbon in 5–50 cm, and soil organic carbon in 5–30 cm compared to plow tillage with straw return due to less carbon input from roots and straw. Organic carbon stocks in micro-aggregates (<0.25 mm) and mineral-associated organic matter (<53 µm) did not vary between tillage practices. Hence the soil organic carbon stocks of plow tillage without straw return (48 Mg ha−1) were similar to that of no-tillage with straw mulch (51 Mg ha−1) but lower than that of plow tillage with straw return (57 Mg ha−1) in 0–50 cm. The mean annual yields of wheat and rice under no-tillage with straw mulch (14 t ha−1) were lower than that of plow tillage without straw return (15 t ha−1) and plow tillage with straw return (16 t ha−1). No-tillage-induced high soil bulk densities limited the rice yields. The yield losses of no-tillage with straw mulch were higher in rice than in wheat. The rice yield losses of no-tillage with straw mulch compared to plow tillage with straw return increased with experiment duration due to decreased available nitrogen in 7–14 cm and phosphorus contents in 0–14 cm over time. Conclusively, plow-tillage is more efficient for carbon sequestration and yield increase than no-tillage under straw return in rice-wheat farming by increasing carbon input in deep soils, soil available nutrients, and decreasing soil compactness.

Moso bamboo expansion into a broadleaved forest alters the dominant soil organic carbon source

AbstractBoth microbes and plants contribute to soil organic carbon (SOC) formation and retention, but their roles in controlling SOC dynamics in forest soils under Moso bamboo (Phyllostachys edulis) expansion remain unclear. Here, amino sugars and lignin monomers were measured to represent microbial necromass and plant‐derived components, respectively. The observed decline in both amino sugars and lignin monomers during Moso bamboo expansion indicates a reduction in microbial necromass and recalcitrant plant contributions to SOC composition. This could be attributed to a limitation of microbial substrates and proliferation caused by the reduced litter inputs resulting from the expansion. The proportion of microbial necromass contributing to the SOC pool increased, but that of lignin monomers decreased, as SOC content decreased with Moso bamboo expansion. This suggests that the decrease of SOC during bamboo expansion was mainly due to the reduction of lignin, while the increased contribution of microbial‐derived carbon to SOC may serve to improve SOC stability. Our study sheds light on the altered SOC source inputs resulting from Moso bamboo expansion and emphasizes the need for sustainable forestry management practices that differentiate between microbial‐ and plant‐derived carbon pools.

Mapping soil organic carbon fractions for Australia, their stocks, and uncertainty

Abstract. Soil organic carbon (SOC) is the largest terrestrial carbon pool. SOC is composed of a continuous set of compounds with different chemical compositions, origins, and susceptibilities to decomposition that are commonly separated into pools characterised by different responses to anthropogenic and environmental disturbance. Here we map the contribution of three SOC fractions to the total SOC content of Australia's soils. The three SOC fractions, mineral-associated organic carbon (MAOC), particulate organic carbon (POC), and pyrogenic organic carbon (PyOC), represent SOC composition with distinct turnover rates, chemistry, and pathway formation. Data for MAOC, POC, and PyOC were obtained with near- and mid-infrared spectral models calibrated with measured SOC fractions. We transformed the data using an isometric-log-ratio (ilr) transformation to account for the closed compositional nature of SOC fractions. The resulting back-transformed ilr components were mapped across Australia. SOC fraction stocks for 0–30 cm were derived with maps of total organic carbon concentration, bulk density, coarse fragments, and soil thickness. Mapping was done by a quantile regression forest fitted with the ilr-transformed data and a large set of environmental variables as predictors. The resulting maps along with the quantified uncertainty show the unique spatial pattern of SOC fractions in Australia. MAOC dominated the total SOC with an average of 59 % ± 17 %, whereas 28 % ± 17 % was PyOC and 13 % ± 11 % was POC. The allocation of total organic carbon (TOC) to the MAOC fractions increased with depth. SOC vulnerability (i.e. POC/[MAOC+PyOC]) was greater in areas with Mediterranean and temperate climates. TOC and the distribution among fractions were the most influential variables in SOC fraction uncertainty. Further, the diversity of climatic and pedological conditions suggests that different mechanisms will control SOC stabilisation and dynamics across the continent, as shown by the model covariates' importance metric. We estimated the total SOC stocks (0–30 cm) to be 13 Pg MAOC, 2 Pg POC, and 5 Pg PyOC, which is consistent with previous estimates. The maps of SOC fractions and their stocks can be used for modelling SOC dynamics and forecasting changes in SOC stocks as a response to land use change, management, and climate change.

Response Characteristics of Soil Organic Carbon Pool and Its Chemical Composition During Secondary Forest Succession in the Loess Plateau

In order to explore the characteristics of the soil organic carbon(SOC)pool and its chemical composition during the succession of secondary forests in the Loess Plateau, samples of the primary stage (Populus davidiana forest), transition stage (Populus davidiana and Quercus wutaishansea mixed forest), and top stage (Quercus wutaishansea forest) of secondary forest succession in the Huanglong Mountain forest area of the Loess Plateau in Northern Shaanxi were selected as the research object. The variation characteristics of SOC content, storage, and its chemical composition at different soil depths (0-10, 10-20, 20-30, 30-50, and 50-100 cm) were analyzed. The results showed that:① the contents and storage of SOC increased significantly with the secondary forest succession process (P<0.05). The content of SOC decreased significantly with the increase in soil depth, and the storage of SOC increased from 64.8 Mg·hm-2 in the primary stage to 129.2 Mg·hm-2 in the top stage, with an increase of 99%. ② During the succession of secondary forests, in the surface (0-30 cm) soil organic carbon, the relative content of aliphatic carbon components that have a simple structure and can be decomposed more easily decreased, and the relative content of aromatic carbon components that have a complex structure and cannot be decomposed easily increased, indicating that the chemical composition of organic carbon stability of surface-layer soil increased significantly with the process of secondary forest succession. However, the stability of the chemical composition of SOC in the deep layer (30-100 cm) first increased and then decreased, that is, the transition stage>the top stage>the primary stage. ③In the process of secondary forest succession, the stability of SOC chemical composition in the primary stage and transition stage increased significantly with the increase in soil depth. The top stage tended to be stable, and the deep soil carbon stability decreased slightly. ④ Pearson correlation analysis showed that during the secondary forest succession process, SOC storage and chemical composition stability were significantly negatively correlated with soil total phosphorus content. In general, the content and storage of SOC in the 0-100 cm soil increased significantly during the secondary forest succession, playing the role of a "carbon sink." The stability of the chemical composition of SOC in the surface layer (0-30 cm) increased significantly, but in the deep layer (30-100 cm), it increased first and then decreased.

Necromass-derived soil organic carbon and its drivers at the global scale

Microbial residual carbon (MRC) plays an important role in maintaining soil carbon stability and is therefore a focus of soil organic carbon (SOC) estimation. However, the global pattern of MRC and its underlying determinants remains uncertain, mainly due to the lack of spatially explicit data on the ratio of gram-positive to gram-negative bacteria which are required to make the conversion from muramic acid to MRC. In the absence of such data, a constant value (65% gram-positive and 35% gram-negative) is commonly used, despite the fact that the ratio varies with environmental conditions. Here, we synthesized a comprehensive data set composed of field data on gram-positive and gram-negative bacteria from 487 sites across the globe, obtained from published literature. Our results show that the ratio is affected by a suite of environmental factors, such as soil moisture and pH. We developed a multiple linear regression model between the ratio and the influencing variables to recalculate the MRC based on amino sugar data from 654 sites. The global distribution pattern of the newly-obtained values of MRC and its driving factors were analyzed. We found striking differences in the estimation of the MRC contribution to SOC when using the newly generated coefficient compared with the currently widely used coefficient, notably in temperate grassland. MRC was greater in topsoil than in deep soil, while the converse was true for MRC/SOC. The MRC and the MRC to SOC ratio varied in different ecosystems, with the highest values of MRC and MRC/SOC occurring in temperate forest and tundra, and the lowest in desert and wetland. The key factors determining microbial residual C were microbial characteristics (microbial biomass C and N) and soil properties (SOC and total nitrogen). We provide a globally gridded map of MRC produced by the boosted regression-tree model, which further reveals that MRC in mid-high latitude areas is higher than in other regions of the northern hemisphere. These results emphasize that assessment of microbial necromass-derived SOC should consider the heterogeneity of the ratio of gram-positive to gram-negative bacteria, which would provide a huge boost to our depth of understanding of SOC composition and stability.

Preliminary investigation of soil organic carbon distribution and turnover patterns, and potential pollution sources in and around a typical coking plant in North China

The variation characteristics of soil organic carbon (SOC) in and around the coking plant area are still unclear. In this work, the concentration and stable carbon isotope composition of SOC in coke plant soils were investigated to preliminarily identify the sources of SOC in and around the plant area, and to characterize soil carbon turnover. Meanwhile, the carbon isotopic technique was used to initially identify the soil pollution processes and sources in and around the coking plant area. The results demonstrate that the SOC content (12.76 mg g−1) of the surface soil in the coking plant is about 6 times higher than that outside the coking plant (2.05 mg g−1), and the variation range of δ13C value of the surface soil in the plant (−24.63～-18.55‰) is larger than that of the soil outside the plant (−24.92～-20.22‰). The SOC concentration decreases gradually from the center of the plant outward with increasing distance, and the δ13C in the middle and north of the plant tends to be positive compared with the δ13C in the west and southeast of the plant. As the increase of soil depth, the SOC content and δ13C value in the plant increases. On the contrary, δ13C value and SOC content outside the plant decreases, with a minor variation. Based on the carbon isotope method, the SOC in and around the coking plant area is mainly from industrial activities (e.g., coal burning and coking), and partly from C3 plants. Notably, organic waste gases containing heavy hydrocarbons, light oils, and organic compounds accumulated in the northern and northeastern areas outside the plant due to south and southwest winds, which may pose an environmental health risk.

Microbial response to warming and cellulose addition in a maritime Antarctic soil

AbstractMaritime Antarctic King George Island (South Shetland Islands) has experienced rapid warming in recent decades, but the impacts on soil organic matter (SOM) decomposition remain ambiguous. Most vegetation cover is dominated by bryophytes (mosses), whereas a few vascular plants, such as Deschampsia antarctica and Colobanthus quitensis grow interspersed. Therefore, SOM is mainly enriched with carbohydrates and C‐alkyl, provided by mosses, which lack lignin as a precursor for aromatic compounds and humus formation. However, there is no clear answer to how substrate and temperature increase changes in Antarctic microbial respiration. We determined in what way SOM mineralization changes with temperature and substrate addition by characterizing the temperature sensitivity (Q10) of soil respiration in an open‐top chamber warming experiment. We hypothesized that: (a) cold‐tolerant microorganisms are well adapted to growing in maritime Antarctic soils (~ 0°C), so would not respond to low and moderate temperature increases because they undergo various metabolic mechanism adjustments until they experience increasing temperatures toward optimum growth (e.g., by enzyme production); and (b) cellulose, as a complex carbonaceous substrate of vegetated areas in Maritime Antarctic soils, activates microorganisms, increasing the Q10 of soil organic carbon (SOC) mineralization. Soils (5–10 cm) were sampled after four consecutive years of experimental warming for SOC composition, microbial community structure, and C mineralization at 4, 12, and 20°C with and without cellulose addition. Functional group chemoheterotrophs, represented mainly by Proteobacteria, decomposed more refractory SOC (aromatic compounds), as indicated by nuclear magnetic resonance (NMR) spectroscopy, in ambient plots than in warming plots where plants were growing. The C‐CO2 efflux from the incubation experiment remained stable below 12°C but sharply increased at 20°C. Q10 varied between 0.4 and 4 and was reduced at 20°C, whereas cellulose addition increased Q10. In conclusion, as confirmed during field studies in a climate scenario, cold‐tolerant microorganisms in maritime Antarctic soils were slightly affected by increasing temperature (e.g., 4–12°C), with reduced temperature sensitivity, as summarized in a conceptual model.

Structural changes in humic substances after long‐term fertilization of a calcareous soil with pig slurries

AbstractPig slurries are widely used on calcareous soils in European rainfed systems. Here we assess their impact on the amount of soil organic carbon (SOC) and on the composition of humic‐type substances (HTS). Seven doses of slurry (five from fattening pigs and two from sows) ranging from 1.0 to 4.8 Mg ha−1 yr−1 of organic matter were evaluated after a period of 12 years and compared with mineral fertilizer treatment. At the end of the last annual cropping season (September), SOC was quantified, and HTS were isolated by alkaline extraction followed by acid precipitation, and studied by visible spectroscopy (800–400 nm) and Fourier‐transformed infrared spectroscopy (4000–400 cm−1). Following the trend in the slurry organic matter applied rates, SOC increased from 9.5 g C kg−1 (mineral treatment) to 13.8 g C kg−1. This SOC increase was equivalent to c. 25.4% of the slurry organic carbon applied. The incorporation of aliphatic structures, mainly polyalkyl, from slurries into the HTS tends to modify the composition of the soil organic matter (SOM), which is reflected in a decrease in the intensity of FT‐IR peaks related to aromatic structures. Despite the trend of significant increase in SOC with fattening slurries, mainly from the organic matter rate of 1.6 Mg ha−1 yr−1 (c. 185 kg N ha−1), the composition of the HTS showed an important aliphatic enhancement. The FTIR results showed that using exclusively the relative intensities of specific peaks (alkyl, carboxyl, aromatic and amide groups) as variables for the discriminant analysis, it is possible to identify HA between different groups of soils treated with progressive levels of slurry. Although the new aliphatic components could be considered important to improve soil physical quality, after the incorporation of additional SOM, the spectroscopic characteristics of HTS in soils treated with slurries suggested a weak effect in long‐term C sequestration, as the newly incorporated OC forms are not qualitatively similar to the presumably stable native SOM. These potential changes in SOC and SOM composition at field level are constrained by the maximum allowed N rates from organic origin in some agricultural systems.

Habitats generated by the restoration of coal mining subsidence land differentially alter the content and composition of soil organic carbon.

The content and composition of soil organic carbon (SOC) can characterize soil carbon storage capacity, which varies significantly between habitats. Ecological restoration in coal mining subsidence land forms a variety of habitats, which are ideal to study the effects of habitats on SOC storage capacity. Based on the analysis of the content and composition of SOC in three habitats (farmland, wetland and lakeside grassland) generated by different restoration time of the farmland which was destroyed by coal mining subsidence, we found that farmland had the highest SOC storage capacity among the three habitats. Both dissolved organic carbon (DOC) and heavy fraction organic carbon (HFOC) exhibited higher concentrations in the farmland (20.29 mg/kg, 6.96 mg/g) than in the wetland (19.62 mg/kg, 2.47 mg/g) or lakeside grassland (5.68 mg/kg, 2.31 mg/g), and the concentrations increased significantly over time, owing to the higher content of nitrogen in the farmland. The wetland and lakeside grassland needed more time than the farmland to recover the SOC storage capacity. The findings illustrate that the SOC storage capacity of farmland destroyed by coal mining subsidence could be restored through ecological restoration and indicate that the recovery rate depends on the reconstructed habitat types, among which farmland shows great advantages mainly due to the nitrogen addition.

Altered Organic Matter Chemical Functional Groups and Bacterial Community Composition Promote Crop Yield under Integrated Soil–Crop Management System

Sustainable agricultural production is essential to ensure an adequate food supply, and optimal farm management is critical to improve soil quality and the sustainability of agroecosystems. Integrated soil–crop management based on crop models and nutrient management designs has proven useful in increasing yields. However, studies on its effects on the chemical composition of soil organic carbon (SOC) and microbial community composition, as well as their linkage with crop yield, are lacking. Here, we investigated the changes in SOC content, its chemical functional groups, and bacterial communities, as well as their association with crop yield under different farmland management based on four farmland management field trials over 12 years (i.e., FP: farmer practice; IP: improved farmer practice; HY: high-yield system; and ISSM: integrated soil–crop system management). The crop yield increased by 4.1–9.4% and SOC content increased by 15–87% in ISSM compared to other farmland management systems. The increased proportion of Methoxy C and O-alkyl C functional groups with a low ratio of Alkyl C/O-alkyl C, but high Aliphatic C/Aromatic C in ISSM hints toward slow SOC decomposition and high soil C quality. The relative abundances of r-strategists (e.g., Firmicutes, Myxobacteria, and Bacteroidetes) was highest under the ISSM. Co-occurrence network analysis revealed highly complex bacterial communities under ISSM, with greater positive links with labile SOC functional groups. The soil fertility index was the main factor fueling crop yields, as it increased with the relative abundance of r-strategists and SOC content. Our results indicated that crop yield advantages in ISSM were linked to the high C quality and shifts in bacterial composition toward r-strategists by mediating nutrient cycling and soil fertility, thereby contributing to sustainability in cropping systems.

Straw type and returning amount affects SOC fractions and Fe/Al oxides in a rice-wheat rotation system

Straw returning is a vital agronomic practice for soil organic carbon (SOC) sequestration and stabilization by changing the storage and fraction composition. Iron and aluminum (Fe/Al) oxides have a protective effect on SOC through adsorption and co-precipitation, which depends on the fraction composition of SOC itself, especially originated from externally added organic materials. However, the changes and interactions of SOC components and Fe/Al oxides in straw returning soil are not clearly understood. To evaluate the effect of straw returning on SOC composition and Fe/Al oxides, we have conducted an in situ field experiment for 10 yrs., which including five treatments: no rice and wheat straw returning (NRW, as control), total amount wheat straw returning (W), total amount rice straw returning (R), half amount rice and wheat straw returning (HRW), and total amount rice and wheat straw returning (TRW). The results showed that SOC and heavy fraction organic carbon (HFOC) increased under all five treatments, especially for R, HRW, and TRW treatments (P < 0.05). Compared with wheat straw returning, rice straw returning was more conducive to increasing the contents of SOC and its fractions, free Fe/Al oxides, complex aluminum oxides and the carbon utilization efficiency. Collectively, these properties were strongly affected by both the type and amount of straw returning. Soil organic carbon was significantly correlated with HFOC, water-soluble organic carbon, alkali-hydrolyzed nitrogen, available phosphorous, available potassium, and total nitrogen (P < 0.05). Of these, HFOC had the most significant effect on SOC and was vitally important for improving SOC sequestration and stability (P < 0.001). The increase of labile carbon fractions was more conducive to the absorption and utilization of nutrients by crops. Straw returning promoted the interaction between increasing organic matter and complex iron oxide (Fec). This served to strengthen the protective effect of SOC, which increased the content of Fec and stability of SOC, resulting in decreased soil pH. Taken together, these results highlight the importance of straw returning in promoting the storage and stability of SOC pool. Critically, rice straw returning can reduce soil pH and Fec content, therefore could be considered as an ideal model for a rice-wheat rotation system.

Fertilization makes strong associations between organic carbon composition and microbial properties in paddy soil

Fertilization changes the soil organic carbon (SOC) composition, affecting the carbon cycle of paddy soil. Understanding the mechanisms of physical fraction and chemical composition of SOC responding to fertilization can help regulate the nutrient release and carbon sequestration. However, it is unclear whether these changes in SOC composition to fertilization are consistent and how these are regulated by biotic and abiotic properties. Therefore, a positioning experiment in a rice field was conducted with a total of nine treatments. Chemical fertilizers (0, 337.5, and 675 kg ha−1; C0, C50, and C100, respectively) and fungal residue (0, 10,000, and 20,000 kg ha−1; F0, F50, and F100, respectively) were applied to evaluated (i) changes in the physical fraction and chemical composition of SOC, (ii) changes in soil properties, microbial biomass and community, and (iii) establish relationships among soil properties, microbial community, microbial biomass, and SOC composition. Our results showed that the application of fungal residue exhibited more significant effects on SOC physical fractions than those with the chemical fertilizers. Furthermore, the chemical composition of SOC was more respond to the application of chemical fertilizers than fungal residue. The partial least squares path model indicated that soil properties mainly affected the mineral-associated organic carbon (MAOC) by microbial biomass. In addition, bacterial diversity played an important role in improving the accumulation of MAOC. The SOC chemical composition was mediated by fungal community composition and bacterial diversity. In conclusion, fungal residue application affected SOC physical fraction by increasing soil properties, microbial biomass, and bacterial diversity. Chemical fertilizers application mainly mediated the chemical composition of SOC by altering fungal community composition and decreasing bacterial diversity.

The variation in organic carbon mineralization and its temperature sensitivity in soils after long-term planting of different tea varieties

Evaluating soil organic carbon (SOC) mineralization and its temperature sensitivity (Q10) is essential to understand SOC sequestration capacity. However, little is known about the changes in the SOC mineralization process in soils when planting different tea varieties. In the current study, we collected soil samples from a tea plantation planted with four tea varieties (Chuancha No. 3 (CC3), Chuanmu No. 217 (CM217), Chuannong Huangyazao (CN), and C. Sinensis ‘Fuding Dabaicha’ (FD)). SOC mineralization and Q10 were investigated through a 90-day laboratory incubation experiment. The results demonstrated that both SOC mineralization and Q10 were variable among the soils after planting different tea varieties. Overall, CM217 soil had the highest cumulative mineralization amount (Cm), mineralization rate of SOC (Cr), potentially mineralizable carbon (C0), and the proportion of C0 in SOC (C0/SOC) at each incubation temperature, and FD soil had the lowest C0 and C0/SOC. These results indicated that CM217 and FD soil had the lowest and highest SOC sequestration capacity, respectively. This difference was explained by varied SOC stability (SI), which was further dominated by the SOC composition and aggregate distribution. FD soil showed the highest aggregate stabilization index (mean weight diameter, MWD) in both soil layers, accumulated the greatest recalcitrant organic carbon (ROC) and further contributed the highest sequestration capacity. In contrast, the lowest SI of CM217 soil reflected a low SOC sequestration capacity, which would be detrimental to long-term SOC storage and ultimately explained the lowest SOC stock. In addition, the variations in Q10 among the soils planted with different tea varieties were explained by the different mechanisms in the topsoil and subsoil. The varied Q10 in topsoil and subsoil was predominantly explained by semi-labile organic carbon (LOCII) and ROC, respectively, and the contribution of aggregate protection to explain the varied Q10 in subsoil was greater than that in topsoil.

Long-term partial substitution of chemical nitrogen fertilizer with organic fertilizers increased SOC stability by mediating soil C mineralization and enzyme activities in a rubber plantation of Hainan Island, China

The partial substitution of chemical nitrogen (N) fertilizer with organic fertilizers is widely used to maintain soil fertility and enhance crop yield, although its effects on soil organic carbon (SOC) composition, mineralization and stability are not fully understood. For this study, a 12-year field experiment was conducted in a rubber plantation field of Hainan Island, China. Three fertilizer treatments included no fertilizer application as control (CK), chemical fertilizer application (NPK), and half of the chemical N fertilizer plus manure application equivalent to half of chemical N fertilizer (NPKM). We determined the soil physicochemical properties, four labile SOC compositions (microbial biomass carbon (MBC), light fraction organic carbon (LFOC), dissolved organic carbon, dissolved organic carbon extracted by hot water (HOC), particulate organic carbon (POC)), four C-associated soil enzymes (β-1, 4-glucosidase (BG), polyphenol oxidase (POX), catalase (CAT) and cellulase), and SOC mineralization (from a 46-day aerobic incubation experiment) in two soil layers (topsoil layer (0–10 cm) and subsoil layer (10–20 cm)). Compared with NPK, NPKM significantly increased soil pH, total nitrogen, total phosphorus, available phosphorus and potassium, NH4+-N, and NO3−-N contents, and SOC, LFOC, POC, HOC, and MBC concentrations. NPKM had higher cumulative CO2 production but lower SOC mineralization potential (Cp) than NPK, although the difference was not significant. In both soil layers, NPKM significantly increased POX activity, but decreased BG and CAT activities. Soil pH can regulate enzyme activities and synthesis in acidic soil in a tropical region with a significantly positive correlation with POX activity. Compared with NPK, the relatively lower Cp/SOC and higher carbon quality index (CQI) indicated higher SOC stability (Cp/SOC) in NPKM. The redundancy analysis results revealed that the dominant factors affecting SOC stability were NH4+-N, pH, POC and MBC in NPK, and available phosphorus, BG, and pH in NPKM. In the three fertilization treatments, the POX was the dominant factor affecting the CQI. Conclusively, half of the chemical N fertilizer plus half of the manure application could facilitate SOC stability by mediating soil C mineralization and enzyme activities in the rubber plantation of Hainan Island, China.

Aggregate-associated carbon compositions explain the variation of carbon sequestration in soils after long-term planting of different tea varieties

Strategies to increase carbon (C) sequestration in tea plantation soils are pertinent to mitigating global climate change, but little is known about the variation in C sequestration in soils planted with different tea varieties. In the current study, we collected 0–20 and 20–40 cm layer soil samples from a tea plantation planted with four tea varieties (Chuancha No.3 (CC3), Chuanmu No. 217 (CM217), Chuannong Huangyazao (CN), and C. sinensis ‘Fuding Dabaicha’ (FD)). Soil organic carbon (SOC) stock and composition in the bulk soil and aggregate fractions, as well as the SOC stability index (SI), were investigated. Both SOC stock and composition in the bulk soil or aggregate fractions were variable among the soils after planting different tea varieties. Overall, the highest SOC stock (0–40 cm) was observed in FD soil, followed by CN, CC3, and CM217 soil. This difference was dominated by the SOC stock associated with macroaggregates, and the highest macroaggregate-associated SOC stock was detected in FD soil in both soil layers. Moreover, FD soil showed the highest proportion of macroaggregates in both soil layers, accumulated the greatest recalcitrant organic carbon (ROC) and further contributed to the highest SI values of SOC associated with most aggregate fractions. In contrast, CN topsoil (0–20 cm) accumulated the greatest labile organic carbon (LOC) in most aggregate fractions, which had a positive correlation with the amount of C return by pruning litter. Ultimately, long-term planting of FD promoted macroaggregate formation, and ROC accumulation in aggregates greatly contributed to maintaining high C sequestration in the tea plantation soils and showed a high potential for future C budgets; in contrast, the tea plantation soil planted with CN could be a potential C source because of high C return.

Divergent carbon stabilization pathways in aggregates in Ultisols with and without organic amendments: Implications from 13C natural abundance and NMR analysis

Ultisols with high physio-chemical protection and high acidity prevent soil organic carbon (SOC) from microbial decomposition and accumulate plant-derived recalcitrant C by forming organo-metal complexes. Therefore, the C stabilization pathways in Ultisols may challenge the current common understanding, which is that the major pathway is the stabilization of microbial-derived C to minerals. In addition, organic amendments are expected to affect C stabilization pathways by promoting SOC decomposition and changing SOC composition. However, limited evidence has been found for Ultisols, and the underlying mechanism remains under discussion. C stabilization processes lead to 13C fractionation and changes in SOC composition among aggregate fractions. Therefore, 13C natural abundance and 13C nuclear magnetic resonance spectrometry approaches were applied to investigate 13C fractionation and SOC composition, especially the relative content of recalcitrant C, such as lignin and lipids, and the humification degree represented by the O-alkyl/alkyl ratio, between aggregate size classes. Five fertilization regimes from an 8-year field experiment were selected: no fertilization (CK), inorganic fertilizer (NPK), NPK + straw (NS), NPK + straw and manure (NSM), and NPK + biochar (NC). The bulk soil was separated into three aggregate fractions: >2000 μm, 250–2000 μm, and < 250 μm. In all treatments, 13C presented greater depletion in microaggregates (<250 μm) than macroaggregates (>2000 μm and 250–2000 μm). Regarding the changes in SOC composition, in non-organic-amended treatments (CK and NPK) and NC, recalcitrant C fractions, such as lignin and lipids, were more abundant in < 250 μm aggregates. With the fresh organic material (OM) application (NS and NSM), the lowest O-alkyl/alkyl ratio was found in the > 2000 μm aggregates, suggesting a high humification degree. Summarizing the δ13C and SOC composition results, in the non-organic-amended treatments and NC, the depletion of δ13C in microaggregates coincided with the enrichment of recalcitrant C, which suggests that the depletion of δ13C was primarily controlled by the preferential substrate utilization mechanism. Thus, labile C with heavier δ13C is preferentially decomposed while partially oxidized recalcitrant C with lighter δ13C was adsorbed to the mineral surface and protected in microaggregates. Because the δ13C value decreases with the aggregate size, C flow (modeled in a scheme) primarily occurred from macroaggregates to microaggregates. With fresh OM input (NS and NSM), the C stabilization pathway was primarily governed by the kinetic fractionation mechanism; that is, heavier δ13C remained after microbial utilization, which was reflected by the association of heavier δ13C and a lower O-alkyl/alkyl ratio. With macroaggregates exhibiting 13C enrichment, the C flow was primarily from microaggregates to macroaggregates. These results highlight the importance of the accumulation and protection of plant-derived recalcitrant C in microaggregates in the C stabilization pathway of Ultisols and emphasize that the C stabilizing mechanisms largely depend on organic amendments.

Fractionation of soil organic carbon in a calcareous soil after long-term tillage and straw residue management

No-tillage (NT) and straw return (S) collectively affect soil organic carbon (SOC). However, changes in the organic carbon pool have been under-investigated. Here, we assessed the quantity and quality of SOC after 11 years of tillage and straw return on the North China Plain. Concentrations of SOC and its labile fractions (particulate organic carbon (POC), potassium permanganate-oxidizable organic carbon (POXC), microbial biomass carbon (MBC), and dissolved organic carbon (DOC)), components of DOC by fluorescence spectroscopy combined with parallel factor analysis (PARAFAC), and the chemical composition of SOC by 13C NMR (nuclear magnetic resonance) spectroscopy were explored. Treatments comprised conventional tillage (CT) and NT under straw removal (S0), return of wheat straw only (S1), or return of both wheat straw and maize residue (S2). Straw return significantly increased the concentrations and stocks of SOC at 0–20 cm depth, but NT stratified them with enrichment at 0–10 cm and a decrease at 10–20 cm compared to CT, especially under S2. Labile C fractions showed similar patterns of variation to that of SOC, with POC and POXC more sensitive to straw return and the former more sensitive to tillage. Six fluorescence components of DOC were identified, mainly comprising humic-like substances with smaller amounts of fulvic acid-like substances and tryptophan. Straw return significantly decreased the fluorescence index (FI) and autochthonous index (BIX) and increased the humification index (HIX). No-tillage generally increased HIX in topsoil but decreased it and increased the FI and BIX below the topsoil. Relative abudance order of the chemical composition of SOC was: O-alkyl C>alkyl-C>aromatic-C>carbonyl-C. Overall, NT under S2 effectively increased SOC and its labile C forms and DOC humification in topsoil and microbially-derived DOC below the topsoil. Return of both wheat and maize straw was a decisive factor in promoting SOC in the plow layer. The stratification of SOC under NT may confer a long-term influence on carbon sequestration.