Soil aggregate stability and organic carbon contribution in Mountain Li along an elevational gradient

Background Soil aggregates can reflect the carrying capacity and carbon sequestration capacity of soil. Studying the elevation gradient pattern and influencing factors of aggregates stability and organic carbon contribution is of great significance for understanding the changes in soil structure and carbon sequestration potential in the vertical space of forest ecosystems at the local scale and their driving mechanisms.

Methods First, we adopted the five-point sampling method to sample the soil at 8 permanent forest sites (400 m²) along an elevational gradient ranging from 890 to 2200 m in Mountain Li, a warm-temperate forest region in Shanxi province. Then, we obtained three kinds of aggregates with different particle sizes by the dry sieving method and calculated their respective contents. Meanwhile, we determined the physical and chemical properties of the soil, such as pH, total nitrogen, organic carbon, etc. We also obtained soil microorganisms through high-throughput sequencing methods and calculated the diversity of plants and soil microorganisms. Finally, we used statistical software for data analysis and plotting.

Results 1) The composition of soil aggregates was mainly medium aggregates (0.25-2.00 mm), followed by large aggregates ( > 2.00 mm), and the least small aggregates (≤ 0.25 mm) at each elevation. 2) MWD and GMD showed a "U" -shaped variation trend at elevation (R² = 0.32, P < 0.001; R² = 0.40, P < 0.001). 3) The contribution of total organic carbon in aggregates was 0.03-0.08 g/g, and it showed a linear upward trend with the increase at elevation (R² = 0.48, P < 0.001). 4) MWD and GMD were significantly positively correlated with soil pH, total phosphorus, total nitrogen, soil water content, clay, as well as the content of large aggregates and the organic carbon contents in medium and small aggregates. It was significantly negatively correlated with silt, species richness, plant Simpson index, mean annual temperature and the content of small aggregates. In addition, GMD was also significantly negatively correlated with precipitation. 5) The partial least squares path model indicated that environmental factors can respectively explained 96.6% of the variation in soil aggregate stability with elevation and 87.7% of the variation in total organic carbon contribution with elevation. Soil physicochemical properties and plant diversity indirectly affected stability by influencing the content of large aggregates and the organic carbon contents in medium and small aggregates (P < 0.05, with a path coefficient of 0.92; P < 0.05, with a path coefficient of 0.17), which further affected contribution of total organic carbon (with a path coefficient of 0.06). In addition, the contribution of total organic carbon was not only directly affected by the organic carbon contents of medium and small aggregates (P < 0.05, with a path coefficient of 0.96), but also directly affected by the content of large aggregates (P < 0.05, with a path coefficient of −0.21).

Conclusions This study emphasizes the driving process of environmental factors on soil aggregate stability and organic carbon contribution and clarifies the direct and indirect effects of various environmental factors on aggregate stability and organic carbon contribution. The results may provide important basis for scientific forest management, enhancing soil carbon sequestration potential, effectively increasing forest productivity and soil and water conservation capacity, promoting ecosystem health and sustainable development in the region.