Effects of excessive nitrogen fertilization on soil organic carbon and nitrogen and nitrogen supply capacity in dryland

【Objectives】 Excessive nitrogen (N) fertilization affects fractions and quantities of soil organic carbon (C) and nitrogen(N) and therefore influences soil nitrogen supply capacity. However, there were few studies on effects of long term excessive nitrogen fertilization on soil organic C and N in dryland of southern Loess Plateau. Therefore, a long-term experiment was carried out to determine changes of nitrate N, organic C and N, and microbial biomass C and N under different N treatments and to study effects of excessive N fertilization on soil carbon and nitrogen and soil nitrogen supply capacity. 【Methods】 A long-term experiment was established in fall of 2004 with five N levels, 0, 80, 160, 240 and 320 kg/ha. The plot size was 40 m2 and all treatments were arranged in a completely randomized block design with four replications, and winter wheat (Triticum aestivum L.) cultivar, Xiaoyan 22, was grown. The N levels of 0, 160 and 320 kg/ha were chosen as control (N0, with no N fertilization), normal N (N160) and excessive N (N320) treatments in this study. Soil samples were randomly collected from four sites at two depths (0-20 and 20-40 cm) using a 2.5-cm diameter auger in each plot on June 15, 2012 (at wheat harvest) and October 1 (at next winter wheat sowing) and then prepared for analysis. 【Results】 The excessive N fertilization treatment significantly increases soil nitrate N contents in 0-300 cm soil layers before the next winter wheat sown, i.e. from an average of 2.8 mg/kg (control) to 15.5 mg/kg which is higher than the normal N treatment by 8.1 mg/kg. Meanwhile, the nitrate N accumulation amounts in 0-60 cm and 0-300 cm soil layers are increased from 47.2 and 108.9 kg/ha of the control treatment to 76.5 and 727.7 kg/ha of the excessive N treatment, respectively, which are higher than the normal N treatment by 22% and 122%. During the summer fallow season, the excessive N fertilizer use also increases soil N mineralization, i.e. from 72.4 kg/ha of the control treatment to 130.7 kg/ha of the excessive N treatment. The excessive N treatment does not significantly increase soil organic C, but it does significantly increase organic N, and soil organic nitrogen C and N contents in the excessive N treatment are 9.24, 5.39 g/kg and 1.05, 0.71 g/kg (with increases by 52.2% and 54.3% than control, respectively) in 0-20 and 20-40 cm soil layers, respectively. In addition, the excessive N fertilization does not significantly affect the microbial biomass C in 0-20, 20-40 cm soil layer (with the average value of 253 and 205 mg/kg, respectively), and significantly increases the soil microbial biomass N, i.e. from 24.1 and 7.5 mg/kg in the control treatment to 43.6 and 16.1 mg/kg in the excessive N treatment. 【Conclusions】The excessive N fertilizer application significantly increases the nitrate N accumulation and mineralization in 0-300 cm soil profile, the quantities of soil N supply at the next winter wheat sowing and the soil microbial biomass N. However, the excessive N fertilization does not significantly affect soil total organic C and microbial biomass C, and increases the risk of nitrate N leaching in soil. Therefore, in order to reduce nitrate N leaching, input costs and the risk of environment pollution, high rate of N fertilizer is not suggested to apply to the dryland in southern Loess Plateau where the soil is low in organic matter concentration.