Effects of long-term chemical fertilization and organic amendments on dynamics of soil organic C and total N in paddy soil derived from barren land in subtropical China

For better understanding the development of infertile paddy soils in subtropical China, a long-term field experiment of paddy soil was developed from barren land in 1990. Experimental treatments including NPK, NPKRS (NPK and rice straw), NPK2RS (NPK and double amount of rice straw), NPKPM (NPK and pig manure), and NPKGM (NPK and green manure (Astragalus sinicus L.)) were employed with rice–rice (Oryza sativa L.) cropping system. Rice yields, soil organic C (SOC) and total N were analyzed. In all of the treatments, early rice yields increased along cultivation years steadily, however late and annual rice yield was fluctuant between different years. From 1991 to 2006, average annual yield ranged from 7795 to 8572 kg ha−1 among different fertilizer treatments. Organic amendments usually enhanced rice yields significantly except for the treatment with NPKRS. SOC and total N contents of surface soil increased linearly with cultivated years from 3.9–5.7 g kg−1 and 0.46–0.57 g kg−1 in initial stage to 7.1–9.2 g kg−1 and 0.87–0.95 g kg−1 in 2005 respectively. Quantity and quality of input organic matter affected soil C dynamics and N balance. SOC sequestration rates were well correlated to modified organic C input, while SOC mineralization rates were related with either organic C input or SOC contents. Annual soil N accumulation was 13–18% of total input. However the net lost N, which was calculated based on fertilizer inputs, crop outputs, and annual N accumulation, was 44–49% in our study. In general, after 17 years’ cultivation and fertilization management, rice yield reached a high level equivalent to the average yield of local high productivity paddy soils, whereas SOC and total N content were still less than half of those in high productivity paddy soils in this region.