Impacts of abandoned sloping farmland on soil aggregates and aggregate-associated organic carbon in karst rocky desertification areas.

Vegetation restoration after the abandonment of sloping farmland can effectively promote the sequestration of soil organic carbon (SOC), with soil aggregates playing a pivotal role. However, in abandoned farmlands in karst regions with varying degrees of rocky desertification, the relationship between soil aggregates, aggregate-associated organic carbon (AAOC), and total SOC content remains unclear. Taking abandoned sloping farmlands (5 years, 10 years, and 15 years) with different levels of rocky desertification (no rocky desertification, potential rocky desertification, slight rocky desertification, and moderate rocky desertification) in a typical karst area as research objects, this study investigated the dynamic characteristics of the particle size distribution of soil aggregates, total SOC, and AAOC. The results indicated that total SOC content in the 0-20 cm soil layer increased after abandonment in all levels of rocky desertification, peaking after 15 years. The abandoned sloping farmland with moderate desertification showed the best recovery effect. Post-abandonment vegetation restoration increased the content of 5-10 mm soil aggregates, but decreased those of 1-2 mm and < 0.25 mm aggregates. Particularly for 5-10 mm aggregates, the contribution of AAOC to total SOC significantly increased over time. Moreover, a strong correlation was observed between >1 mm aggregates and total SOC (p < 0.05). The increase in total SOC was primarily driven by the growth of AAOC in 5-10 mm aggregates. In general, vegetation restoration is an effective approach for enhancing total SOC content in abandoned sloping farmland with varying degrees of rocky desertification.

Characteristics and factors influencing soil organic carbon composition by vegetation type in spoil heaps.

Introduction: The variation of organic carbon content in spoil heaps is closely related to improving soil structure, maintaining soil fertility, and regulating soil carbon cycling balance. Analyzing the soil organic carbon content and related driving factors during the natural vegetation restoration process of spoil heaps is of great significance for promoting the accumulation of soil organic carbon in the spoil heaps. Methods: we selected spoil heaps with the same number of years of restoration to research the variations in soil organic carbon components under different vegetation types (grassland: GL, shrubland: SL, secondary forest: SF) and compared the results with those on bare land (BL). Results: Our results showed that vegetation type and soil depth significantly affect the content of soil organic carbon components. There was no difference in soil organic carbon components between SF and SL, but both were considerably superior to GL and BL (p<0.05), and the particulate organic carbon (POC) and light fraction organic carbon (LFOC) contents of SL were the highest. A significant positive linear correlation existed between SOC and active organic carbon components. Pearson's correlation and redundancy analysis showed that the available potassium (AK) and total nitrogen (TN) contents and gravel content (GC) in the BL soil significantly impacted soil organic carbon. When vegetation is present, TN, total phosphorus (TP), and Fine root biomass (FRB) significantly affect soil organic carbon. Structural equation modelling (SEM) shows that AK and soil moisture content (SMC) directly affect the organic carbon composition content of BL, When there is vegetation cover, fine root biomass (FRB) had the largest total effect in the SEM. Soil bulk density (BD) has a negative impact on soil organic carbon, especially in the presence of vegetation. Conclusion: These findings suggest that vegetation restoration can significantly increase soil organic carbon content, FRB, AK, and TN play important roles in enhancing soil organic carbon. Supplementation with nitrogen and potassium should be considered in the bare land stage, and shrubs nitrogen-fixing functions and well-developed roots are more beneficial for the accumulation of soil organic carbon.

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Soil heterogeneity is at a high level in the karst areas, which resulted from the complex habitat. On the one hand, plants have some adaptive strategies to such special habitats by forming certain morphological and physiological characteristics, which result in higher diversity of functional traits. One the other hand, plants improve the physical and chemical properties of soil through a series of life activities. The interactions between plants and soil drive ecosystem structure and function and its responses to global climate change. Here, we summarized the characteristics of soil hete-rogeneity in the karst areas, and reviewed the response of plant functional traits to soil and the feedback of plants to soil. It revealed the coupling mechanism between plants and soil in karst eco-system. We provided a future outlook, including future research contents and directions based on the current research status in this field, which aimed to provide theoretical reference for maintaining the structural and functional stability of fragile karst ecosystems.

Effects of rainfall intensity on runoff and nutrient loss of gently sloping farmland in a karst area of SW China.

Nutrient losses from sloping farmland in karst areas lead to the decline in land productivity and nonpoint source pollution. A specially tailored steel channel with an adjustable slope and underground hole fissures was used to simulate the microenvironment of the "dual structure" of the surface and underground of sloping farmland in a karst area. The artificial rainfall simulation method was used to explore the surface and underground runoff characteristics and nutrient losses from sloping farmland under different rainfall intensities. The effect of rainfall intensity on the nutrient loss of farmland on karst sloping land was clarified. The results showed that the surface was the main route of runoff and nutrient loss during the rainy season on sloping farmland in karst areas. The influence of rainfall intensity on the nutrients in surface runoff was more substantial than that on underground runoff nutrients. Nutrient loss was more likely to occur underground than on the surface. The losses of total nitrogen, total phosphorus, and total potassium in surface and underground runoff initially increased and then gradually stabilized with the extension of rainfall duration and increased with increasing rainfall intensity and the amount of nutrient runoff. The output of nutrients through surface runoff accounted for a high proportion of the total, and underground runoff was responsible for a low proportion. Although the amount of nutrients output by underground runoff was small, it could directly cause groundwater pollution. The research results provide a theoretical reference for controlling land source pollution from sloping farming in karst areas.