Farmland abandonment and vegetation succession mediate soil properties but are determined by the duration of conversion

Farmland abandonment has continued to expand in China over recent decades due to the migration and aging of rural labour forces and the increasing industrial focus of the working population. Up till now, how farmland abandonment and vegetation secession operate on soil physical structure and chemical properties in the Three Gorges Reservoir Area featured by fragile hilly and mountainous landscapes remains incompletely understood. Accordingly, we used a space-for-time substitution approach to evaluate the changes of soil nutrients, aggregate stability, and aggregate-specific organic carbon for farmland abandonment with different durations of conversion. Composite soil samples were collected from present-day croplands (CL) and farmland abandoned for 3 years (AL3), 7 years (AL7), 15 years (AL15), and 20 years (AL20), respectively. A variety of soil metrics including soil organic carbon (SOC), total nitrogen (TN), alkaline hydrolyzed nitrogen (AN), total phosphorus (TP), available phosphorus (AP), water-stable aggregate percentage, mean weight diameter (MWD), geometric mean diameter (GMD) and aggregate-specific organic carbon were determined or calculated. It was found that the SOC, TN, and AN in topsoil (0-10 cm) of farmland abandoned for 20 years increased by 208 %, 187 %, and 128 %, respectively, compared with those of the present-day cropland, while TP and AP exhibited no significant differences. The percentage of macro-aggregates, MWD, GMD and aggregate-specific organic carbon in surface soils increased significantly with increasing durations of conversion. Specifically, these indicators for cropland abandoned for 20 years increased by 92 %, 87 %, 346 %, 161 %, and 275 %, respectively, compared with those of present-day cropland. Changes in these indicators were statistically insignificant in farmland abandoned for 0-7 years but were significant in farmland abandoned for 15-20 years. Soil nutrients, aggregate stability, and aggregatespecific organic carbon gradually decreased with increasing soil depths. It thus can be concluded that farmland abandonment is likely to be an effective natural restoration strategy to improve soil physical structure and recover soil nutrients and carbon storage in the Three Gorges Reservoir Area, but positive changes would take between one and two decades to manifest.