Changes of soil microbial biomass carbon and nitrogen, and mineral nitrogen after a long-term different fertilization

A 17 year field experiment, located at Yangling, Shaanxi, was conducted to study effects of different fertilizer managements on changes of microbial biomass carbon and nitrogen (SMBC, SMBN) and mineral nitrogen in soils under the winter wheat and summer maize rotation system. There were 4 fertilizer managements, no fertilization (CK), mineral N, P, and K fertilizers (F), mineral N, P, and K fertilizers and manure (F+M), and mineral N, P, and K fertilizers and straw (F+S). The results show that contents of SMBC, SMBN and soil mineral nitrogen are 264.8-752.2, 37.5-114.8 and 3.8-38.5 mg/kg in the 0-10 cm soil layer, respectively. Contents of SMBC and SMBN in the F+M treatment are the highest among the different fertilizer treatments, and contents of SMBC and SMBN in the F+M treatment are 1.38-2.65 and 1.89-2.50 times of those in the no fertilizer treatment, respectively. Contents of soil mineral nitrogen of the F+S treatment are the highest among the different fertilizer treatments, and contents of SMBC, SMBN of the F+S treatment are higher than those of the F and CK treatments at all sampling times. Compared with the CK treatment, long-term application of mineral N, P, and K fertilizers increase SMBC and SMBN contents of0-10 cm soil layer at every sampling time. SMBN contents in each fertilizer treatment from elongation stage to grain filling stage of winter wheat are decreased; however, soil mineral nitrogen contents are very low during this period. Soil mineral nitrogen contents in the F, F+M, and F+S treatments from pre-tasselling to grain filling stages of summer maize are decreased sharply, while, SMBN contents are increased. It is concluded that if there is enough mineral nitrogen in soil, crop would absorb it firstly, while if mineral nitrogen is limited, SMBN would be decomposed for crop uptake.