Long-term fertilizer effects on organic carbon and total nitrogen and coupling relationships of C and N in paddy soils in subtropical China

Fertilizer application has the potential to promote the sequestration of carbon (C) and nitrogen (N) in agricultural soils and thus may mitigate the effects of atmospheric greenhouse gases. In this study, the effects of fertilizer practices [i.e., no fertilizer (CK), chemical fertilizer (NPK), and chemical fertilizer plus low or high rates of organic manure (LOM or HOM)] on soil organic carbon (SOC) and total nitrogen (TN) content in the plow layer (0–20 cm) of paddy soils were examined using the data from eight long-term field experimental sites (1986–2003) in Hunan Province, Southern China. The SOC and TN content with the treatments which included N fertilizer (NPK, LOM, and HOM) ranged from 16.2 to 38.6 g kg−1 and from 1.07 to 3.92 g kg−1, respectively. Compared with the CK treatment, the average SOC and TN content were 2.0 and 19.3%, 29.3 and 5.2%, and 19.5 and 27.1% larger, respectively, for NPK, LOM, and HOM. In addition, the average values for SOC with the four treatments (CK, NPK, LOM, and HOM) had increased by 13.1, 15.4, 35.0, and 46.3%, respectively, by 2003; for TN they had increased by 5.0, 10.5, 25.5, and 33.5%, respectively, above the values obtained in 1986. However, the increase in SOC and TN content varied substantially at the different experimental sites. Organic–chemical fertilization gradually increased SOC and TN content and then the values tended to be stable with the LOM and HOM treatments from 1986 to 2003; they also remained stable for the NPK and CK treatments. Soil TN contents were significantly correlated with SOC at each site (P < 0.001). Soil C/N ratios in 2003 were generally around 10 and usually ranged from 8.5 to 12.0. The soil C/N ratios among the four treatments were not significantly different at the eight sites in 2003. Nevertheless, the average C/N ratio at the eight sites was approximately 1.08 times higher than in 1986. The results indicate that there is a coupling relationship between SOC and TN in paddy soils in the subtropical region of China, a region that could provide C and N sinks under current fertilizer practices (i.e., combined chemical and organic fertilization) and cropping conditions. However, the results do suggest that the application of organic manure is the primary contributor to C and N sequestration.