Effects of Long-Term Fertilization on Soil Carbon and Nitrogen in Chinese Mollisols

Three field experiments were conducted to examine the influence of long-term (30-31 yr) effect of fertilizer applications on soil C and N in the mollisols of Northeast China. Each experiment represented a different latitude range: low latitude (LatL), middle latitude (LatM) and high latitude (LatH). Four types of fertilizer applications were considered: (i) CK: unfertilized (control); (ii) NPK: balanced application of inorganic fertilizer nitrogen, phosphorus, potassium; (iii) M: application of organic manure; and (iv) NPKM: fertilizer nitrogen, phosphorus, potassium, plus organic manure. Compared with CK treatments, applications of fertilizers increased soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), microbial biomass carbon (Cmic) and microbial biomass nitrogen (Nmic). On average compared with CK treatments, SOC in the NPK, M, and NPKM treatments increased by 2.74, 22.00, and 27.81%, respectively. Thus, applications of organic manure combined with inorganic fertilizers showed greater effects on the topsoil C and N content than inorganic fertilizers. Under the same fertilizer treatments, SOC, TN, Nmic, and Cmic contents were the lowest at LatM site. Available N content decreased for a latitude sequence of LatH > LatM > LatL for CK, M, and NPKM treatments and LatM > LatH > LatL for the NPK treatments. These results indicate that the magnitude of soil C and N among the three sites was not related to the latitude of the experiments.