Temporal soil carbon and nitrogen accumulation after land use change from paddy rice to upland sugarcane cropping in Thailand

In this study it was hypothesized that accelerated soil organic carbon (SOC) mineralization caused by land use change (LUC) from lowland (paddy rice) to upland (sugarcane) cropping could be compensated for by increased inputs of new sugarcane biomass-derived C and N. We used the 13C natural abundance of SOC to follow dynamics of "old" rice-derived and "new" sugarcane-derived SOC after LUC. C and N dynamics in the top 100 cm soil layer as well as annual C and N inputs from plant biomass (residues, roots) and N input from chemical fertilizer were analysed using samples from farmers' fields: monocrop paddy rice cultivated for >40 years and sugarcane with 6, 13 and 17 years cropping after conversion from rice. LUC increased inputs of C and N by 91-141% and 39-48%, respectively, considering plant-derived biomass and applied chemical fertilizer. This resulted in temporal accumulation of total soil C and N stocks. Partitioning of soil C sources based on 13C measurements showed that the fraction of rice-derived C comprised 65% of the total that corresponded to a decrease of rice-derived C from 58.9 to 38.8 Mg C ha-1 after 17 years of LUC. Total content of sugarcane-derived C along the soil profile increased with time. Accumulation of new sugarcane-derived C in the soil was slower over time than decom-position of rice-derived soil C in upper and deeper soil layers. Our research revealed a fundamental under-standing of the transient C accumulation potential of agricultural soils after LUC from anaerobic to aerobic cropping.