Soil carbon sequestration efficiency under continuous paddy rice cultivation and excessive nitrogen fertilization in South China

Abstract

Long-term excessive applications of chemical fertilizers may result in adverse impacts on soil functions. This study was to evaluate soil organic carbon (SOC) sequestration efficiency under continuous paddy rice cultivation with an excessive nitrogen (N) fertilization over the period from1980 to 2017 in South China. The SOC and total nitrogen (TN) of total 108 soil samples from continuous paddy soils and new paddy soils collected in 2017 were accordingly compared with those of 54 samples from paddy soils and upland soils obtained in 1980. Results show a total SOC increase of 0.79 g kg−1 from the initial content of 12.82 g kg−1 over a 37-year period despite an increased input of about 550 kg N ha−1yr-1 from fertilizers and 2000 kg C kg−1 yr−1 from all straw incorporation after 1990. This small C sequestration rate (or 0.021 g C kg−1 yr−1) was also observed to couple with a significantly elevated C:N ratio and badly weakened SOC-N correlation as of 2017, which is almost impossible to be revealed by normal fertilization experiments. The SOC sequestration rate of 0.145 g C kg−1 yr−1 of the new paddy soils that were developed from uplands since 1980 implies a declining tendency of SOC sequestration efficiency with rice cultivation time, which could be mainly attributed to both low soil N content and microbial activity rather than to SOC saturation. This case reminds of a need for more dedicated plot studies coupled with field observations on farmers’ routine fertilization practices to elucidate why the low soil N content remains with an excessive N fertilizer input and how N interacts with SOC in the continuous paddy soils.