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

Abstract

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.