Differential responses of crop yields and soil organic carbon stock to fertilization and rice straw incorporation in three cropping systems in the subtropics

Because the in situ incorporation of rice straw into paddy fields enhances CH4 emissions, the ex situ (or shifted) incorporation of rice straw into uplands may provide an alternative way of mitigating CH4 emissions and increasing crop productivity and soil organic carbon (SOC) accumulation. To evaluate the efficiency of this practice, three field trials were conducted in flooded paddies (FP), paddy-upland rotation (PU), and upland (UL) cropping systems in Taoyuan county, a subtropical region of China. All trials had three fertilization treatments: no fertilizer (Nil), chemical fertilizer only (NPK) and combined application of chemical fertilizer and rice straw (NPK + R in FP and NP + R in PU and UL). Results showed that the responses of crop yields to NPK in the UL trial (yields increased 2.4 to 4.1-folds relative to Nil) were greater than those of rice (increased 1.65 to 1.80-folds) in the FP and PU trial. Compared with NPK treatment, NPK + R constantly increased the grain yields of rice in the FP trial by 10% averagely, but not in PU trials. The effects of NP + R treatment on crop yields in the UL trial were significant (p < 0.05) during the first 5–6 years. NPK treatments increased the SOC accumulation at a rate of 0.48 Mg ha−1 yr−1 in the FP trial and 0.35 Mg ha−1 yr−1 in the UL trial, but not in the PU trial. NPK + R treatments resulted in SOC accumulation rates of 1.00, 0.68, and 0.24 t ha−1 yr−1, and 9.11%, 6.56%, and 6.45% of the total straw C input was converted to SOC in the FP, UL, and PU trials, respectively. The results suggested that the incorporation of rice straw was highly efficient on SOC accumulation and crop productivity in the uplands (as shown in the UL trial). We therefore recommend the ex situ incorporation of rice straw in the upland neighboring paddy fields as a way of utilizing excessive rice straw in the hilly area of subtropical China.