Soil Organic Carbon Pool under Diverse Chemical Fertilizer Management in Huang-Huai-Hai Plains, China

Soil analyses for 0–20 cm depth were conducted to assess changes in soil organic carbon (SOC) pool under long-term experiments conducted from 1979 to 2005 at eight sites in the Huang-Huai-Hai plains (HHH) of China. Different treatments were grouped into seven categories: (i) no chemical fertilizer as the control treatment (CK); (ii) chemical nitrogen (N); (iii) phosphorus (P); (iv) combined application of chemical fertilizer N and P (NP); (v) combined application of chemical fertilizer phosphorus (P) and potassium (K) (PK); (vi) Integrated chemical fertilizer N, P, and K management (NPK); and (vii) combined application of chemical fertilizer N and K (NK). The data indicated the following: (i) The magnitude of mean SOC pool value for N, P, NP, NK, PK, NPK, and CK was 20.4 ± 1.8, 20.1 ± 1.7, 20.3 ± 2.0, 20.3 ± 1.1, 19.5 ± 0.8, 21.2 ± 1.2, and 18.9 ± 1.8 Mg ha−1, respectively. (ii) Compared with CK, change in SOC pool was in the order of NPK > NP > N for the same application rate of N chemical fertilizer. (iii) In comparison with the baseline, the SOC pool increased with an increase in the rate of chemical fertilizer use. (iv) The average rate of change in SOC pool (kg ha−1 year−1) was −250.0 to 270, and the stable or steady rate of change was −253 to 142. (v) The threshold rate (kg ha−1) of chemical fertilizers for increasing the SOC pool in the HHH was 270 for N, 150 for P, and 150 for K.