Land use change impacts on red slate soil aggregates and associated organic carbon in diverse soil layers in subtropical China

Responses of aggregate-associated carbon and their fractions to different positions in a karst valley of Southwest China

Changes in aggregate-associated carbon and their fractions are vital for soil organic carbon (SOC) sequestration. However, changes in SOC contents and their fractions in different soil aggregates under different landform positions in karst regions remain unclear. Soil samples were collected from the valley bottom (VAFL), dip slope (DIPS), and anti-dip slope (ANTD) in a karst valley of Southwest China. These soil samples were then divided into three aggregates including macroaggregate (> 0.25 mm), microaggregate (0.053-0.25 mm), and silt and clay (< 0.053 mm) using wet sieving methods. Contents of total SOC and their three oxidized carbon fractions (F1: easily oxidized carbon fraction, F2: oxidized carbon fraction, and F3: stable carbon fraction) were measured. Compared with ANTD, total SOC contents, microaggregate-associated carbon contents, and SOC contents in silt and clay fraction decreased by 37.90%, 38.41%, and 40.07%, respectively, under VAFL, and by 12.95%, 11.65%, and 15.60%, respectively, under DIPS. Contents of F1 and F2 fraction under VAFL were significantly lower than those under ANTD and DIPS in bulk soil, microaggregate, and silt and clay. The mean percentages of F2 in total SOC in bulk soil, macroaggregate, microaggregate, and silt and clay were 43.66%, 43.81%, 45.67%, and 41.70%, respectively, while the percentages of F1 were 25.08%, 25.04%, 25.68%, and 24.86%, respectively. Compared with DIPS, SOC stability under ANTD and VAFL increased by 6.59% and 8.44%, respectively. These findings emphasized the influence of landform positions on the carbon contents in different soil aggregates, and deepened the understanding of SOC accumulation mechanisms in Southwest China.

The conversion of tropical natural forests alters soil carbon fractions in aggregates and reduces aggregates stability

Tropical soils, which are characterized by high temperature and high humidity, play a significant role in the global carbon cycle. Soil aggregates are crucial for physically protecting soil organic carbon (SOC), and active organic carbon fractions exhibit a rapid response to changes in soil aggregates. Nevertheless, limited research has been conducted on the impacts of natural forest (NF) conversion on soil aggregates and aggregate-associated carbon fractions, particularly in tropical regions of China. In this study, we aimed to investigate the effects of converting NF to secondary forest (SF), feather pine (Podocarpus imbricatus) forest (FPF) and rubber tree (Hevea brasiliensis) forest (RF) in tropical regions on soil physicochemical properties, aggregate stability and aggregate-associated carbon fraction contents. The results indicated that the RF presented the lowest soil water content (SWC) and highest free iron oxide (Fed) concentration, as did aluminum oxide (Ald). Following the conversion of NF, the macroaggregate proportion exhibited a considerable decline across all soil depths, and there was an increase in the proportion of clay-sized aggregates, especially in FPF and RF. The mean weight diameter (MWD) and geometric mean diameter (GMD) decreased markedly after NF conversion. Additionally, the NF conversion caused a significant decrease in the aggregate-associated SOC and readily oxidizable organic carbon (ROC) contents at all aggregate sizes at all soil depths, especially in SF and RF. Pearson correlation and RDA revealed that Fed, Ald and SWC were the most important factors affecting aggregate stability and aggregate-associated carbon fractions, respectively, in macroaggregates, microaggregates and clay-sized aggregates. In summary, the conversion of NF not only reduced the stability of the soil aggregates, but also changed the state of the SOC and its fractions in the aggregates, resulting in the loss of soil aggregate-related carbon. These findings provide valuable references and recommendations following NF conversion in tropical regions. Therefore, proper policies must be implemented to standardize the utilization of NF resources and protect the soil carbon pool in tropical areas.

Long-term cultivation reduces soil carbon storage by altering microbial network complexity and metabolism activity in macroaggregates

Cultivation alters soil aggregation, microbial compositions and the potential for carbon sequestration in cropland soils. However, the specific effects of long-term cultivation and the underlying mechanisms on soil organic carbon (SOC) storage at different aggregate sizes remain poorly understood. We characterized the dynamics of SOC storage in macroaggregates (>0.25 mm) and microaggregates (<0.25 mm) across four paddy soils successively cultivated for 60, 100, 125, and 150 years. Microbial community compositions, network patterns, enzyme activities and carbon use efficiency (CUE) were examined to elucidate the underlying microbial pathways governing SOC storage. The results showed that prolonged cultivation led to an average reduction of 45 % in SOC storage, particularly in macroaggregates. Partial least squares path modeling revealed that shifts in microorganisms in macroaggregates explained almost 80 % of the variation in SOC storage. Specifically, variations in fungal composition and decreased complexity of microbial interaction networks were strongly correlated with SOC storage. Fungal community and microbial interactions also indirectly affected SOC storage by positively correlating with extracellular enzyme activity. Moreover, bacterial composition indirectly regulated SOC storage by positively correlating with carbon use efficiency. Our findings indicated that the macroaggregate-associated microbial interactions and the metabolism activities had significant implications for SOC sequestration in paddy fields. We suggest that implementation of management practices targeted at improvement of these microbial attributes could enhance agroecosystems sustainability.

Changes in soil aggregate stability and aggregate-associated carbon under different slope positions in a karst region of Southwest China

Soil aggregates are crucial for reducing soil erosion and enhancing soil organic carbon sequestration. However, knowledge regarding influences of different slope positions on compositions and carbon content for different soil aggregates is limited. Soil samples were collected from various slope positions including dip slope, anti-dip slope and valley depression in the Longtan karst valley of Southwest China. Contents of macroaggregate (> 0.25 mm), microaggregate (0.053-0.25 mm) and silt and clay fraction (< 0.053 mm), and aggregate-associated carbon contents under the three slope positions were measured. Compared to the anti-dip slope, the mean weight diameter under the dip slope and valley depression decreased by 28.48 % and 58.79 %, respectively, while the geometric mean diameter decreased by 39.01 % and 62.57 %, respectively. The mean carbon content in silt and clay fraction was 27.59 % and 21.00 % lower than the macroaggregate- and microaggregate-associated carbon content, respectively. Under the valley depression and dip slope, soil organic carbon contents in bulk soil (37.67 % and 10.36 %, respectively), microaggregate (37.56 % and 4.95 %), and silt and clay fraction (39.99 % and 12.84 %, respectively) were significantly lower than those under the anti-dip slope. However, the difference in macroaggregate-associated carbon content among the three slope positions was not significant. The silt and clay fraction was the major contributor to soil carbon pool in bulk soil in the study area because of its high content. Compared to the anti-dip slope, contribution of macroaggregates to soil carbon pool under the dip slope and valley depression decreased by 25.53 % and 47.95 %, respectively, whereas the contribution of silt and clay fraction increased by 22.68 % and 42.66 %, respectively. These results suggested that the anti-dip slope surpassed both the dip slope and valley depression in carbon sequestration and soil and water conservation in karst regions.

Twelve-year conversion of rice paddy to wetland does not alter SOC content but decreases C decomposition and N mineralization in Japan

Land-use change worldwide has been driven by anthropogenic activities, which profoundly regulates terrestrial C and N cycles. However, it remains unclear how the dynamics and decomposition of soil organic C (SOC) and N respond to long-term conversion of rice paddy to wetland. Here, soil samples from five soil depths (0-25 cm, 5 cm/depth) were collected from a continuous rice paddy and an adjacent wetland (a rice paddy abandoned for 12 years) on Shonai Plain in northeastern Japan. A four-week anaerobic incubation experiment was conducted to investigate soil C decomposition and N mineralization. Our results showed that SOC in the wetland and rice paddy decreased with soil depth, from 31.02 to 19.66 g kg-1 and from 30.26 to 18.86 g kg-1, respectively. There was no significant difference in SOC content between wetland and rice paddy at any depth. Soil total nitrogen (TN) content in the wetland (2.61-1.49 g kg-1) and rice paddy (2.91-1.78 g kg-1) showed decreasing trend with depth; TN was significantly greater in the rice paddy than in the wetland at all depths except 20-25 cm. Paddy soil had significantly lower C/N ratios but significantly larger decomposed C (Dec-C, CO2 and CH4 production) and mineralized N (Min-N, net NH4+-N production) than wetland soil across all depths. Moreover, the Dec-C/Min-N ratio was significantly larger in wetland than in rice paddy across all depths. Rice paddy had higher exponential correlation between Dec-C and SOC, Min-N and TN than wetland. Although SOC did not change, TN decreased by 14.1% after the land-use conversion. The Dec-C and Min-N were decreased by 32.7% and 42.2%, respectively, after the12-year abandonment of rice paddy. Conclusively, long-term conversion of rice paddy to wetland did not distinctly alter SOC content but increased C/N ratio, and decreased C decomposition and N mineralization in 0-25 cm soil depth.

Effects of the oversized microplastic pollution layer on soil aggregates and organic carbon at different soil depths

Soil aggregates (SAs) are the main site for soil organic carbon (SOC) fixation, and land plastic pollution is increasingly causing many soil problems. The effects of plastic on SAs and SOC seem to be significant, but there is still a lack of relevant research. This study investigated the effects of the "plastic contamination layer" (PCL) formed by the microplastic precursors (namely, oversized microplastics (OMPs)) on the content and properties of SAs of different particle sizes at different soil depths. The results showed that the PCL had an effect on SAs of different sizes at different depths: Compared with the control group, PCL mainly increased the content of SAs in 0-5 cm soil depth, about 28.08 mg macroaggregates, 13.79 mg microaggregates and 59.82 mg silt and clay aggregates per gram of soil. The presence of the PCL mainly down-regulates the organic carbon (OC) content in 0-5 cm macroaggregates, which is about 9.59 g/kg, the OC content in 10-20 cm microaggregates, which is about 16.41 g/kg, and the OC content in 0-5 cm silt and clay aggregates, which is about 4.16 g /kg, downregulated the expression of the key carbon metabolism genes (CMGs) coxL, and inhibited the contribution of the potential CMGs host bacteria Sphaerimonospora and Bacteroides to soil organic matter. This paper emphasizes that the presence of PCL reduced SOC sequestration.