Effects of land abandonment on soil aggregate and organic carbon stability in cold waterlogged paddy fields

Soil structure, which is decided by the stability of soil aggregates to a great extent, significantly influences soil environment. Soil aggregates are secondary particles formed through the combination of particulate minerals with organic and inorganic substances. Soil organic carbon is critical for enhancing soil quality and sustainable production. Even a slight increase in soil organic carbon content could induce a substantial sequestration of excess atmospheric CO_2. Cold waterlogged paddy field is one form of low-productivity paddy fields in the central China. This type of paddy field is especially common in the middle and lower reaches of Yangtze River Basin. In recent years, large areas of cold waterlogged paddy fields have been abandoned due to high labor costs and low returns. Due to long-term floods, cold waterlogged paddy fields have very poor soil structures that are often described as mushy soils. Not only is the composition of aggregate proportions and organic carbon of marshy soils are vastly different from that of ordinary paddy fields, the aggregate sizes and organic carbon stability of marshy soils are unclear. It is also not clear how soil structure and organic carbon changed after cold waterlogged paddy field was abandoned. In this study, we investigated the effects of land abandonment on soil total organic carbon, soil aggregation fractions and organic carbon content in cold waterlogged paddy fields under continuous rice cultivation (CWC), abandoned for three years (CWA3) and abandoned for six years (CWA6). The results showed that the aggregate compositions were dominated by <53 μm particles, accounting for over 40% of the soil in cold waterlogged paddy fields. Then >250 μm particles accounted for over 35% and 53 250 μm particles accounted for less than 20% of the 0-25 cm soil layer in cold waterlogged paddy fields. Land abandonment significantly increased the <53 μm aggregate fraction and decreased the 53-250 μm aggregate fraction in the 0-25 cm soil layer. Also land abandonment improved labile organic carbon pool Ⅰ index (LIc-Ⅰ) of the <53 μm and >250 μm soil aggregates, decreased labile organic carbon pool Ⅱ index (LIc-Ⅱ) in the 53-250 μm and >250 μm soil aggregates, and reduced recalcitrant organic carbon index (RIc) in the <53 μm and 53-250 μm soil aggregates. Soil total organic carbon increased with increasing length of period of land abandonment.