Sensitivity of aggregate-associated soil organic carbon and total nitrogen to abandonment of paddy soil in subtropical China

Abandonment of paddy fields is a significant threat to soil organic carbon (SOC) stocks owing to the associated shift from anaerobic to aerobic conditions. However, the impact of this transition on the dynamics of soil total nitrogen (TN) and its relationship with SOC in bulk soil and soil aggregates remains unclear.

A long-term experiment was conducted to examine abandoned paddy fields with different fertilizer treatments over a 16-year period before abandonment, followed by an 8-year period after abandonment.

The abandonment of paddy fields led to a significant decrease in TN content by an average of 14.0%, resulting in a mean annual loss rate of 0.08 t N ha⁻¹. The loss of TN was as sensitive as that of SOC, and there was a positive correlation between SOC and TN in both bulk soil and soil aggregates. The loss of SOC and TN was mainly caused by reductions in the middle- and micro-aggregate-associated SOC and TN, which together explained approximately 87.3% of C loss and 81.3% of N loss. The weaker protective capacity of soil aggregates (> 53 μm) was evidenced by a significant decrease in aggregate-associated C (average of 8.7%) and N (average of 9.1%). Abandonment maintained stoichiometric stability, with bulk soil C:N ratios ranging from 9.4 to 9.6 following abandonment.

Paddy soil aggregate-associated SOC and TN were sensitive to loss owing to the weaker protective capacity of soil aggregates following the abandonment of paddy fields. The C:N ratios remained relatively consistent after abandonment.