Effects of nitrogen application rates on grain yield, soil organic carbon and nitrogen under a rainfed cropping system in the loess tablelands of China

Nitrogen fertilization significantly influences soil organic carbon (SOC) and total soil N (TSN) in rainfed farming systems. Effects of N application rates on crop yield, SOC and TSN in a rainfed winter wheat (Triticum aestivum L.) cropping system without crop residues returned to soil were studied using a long term field experiment from 1984 to 2007 in a silt loam Heilu soil (Calcarid Regosols). There were five N application rates, 0, 45, 90, 135, and 180 kg/ ha N. The averaged grain yields of wheat are 1.2, 2.4, 2.9, 3.2, and 3.4 t /ha for the five treatments, respectively. For the treatment without N fertilization (N0), the grain yield gradually declines with years at a rate of 67 kg/ha (P 0.001). For the treatments with N fertilization, the grain yields are stable, and even improve with years. Fertilizer N use efficiencies are decreased from 40% to 28% for the N45 treatment and the N180 treatment. SOC contents in 0-20 cm depth are significantly influenced by the long-term N fertilization, and the SOC pools in 0-20 cm depth range from 16.9 t/ha for the N0 treatment to 19.1 t/ha for the N180 treatment under the 23 year N applications. Similarly, TSN contents and pools are also significantly affected by th N rates. The TSN pools for 0-20 cm depth range from 2.03 t/ha for the N0 treatment to 2.37 t/ha for the N180 treatment. There are significant correlation between crop yields and the N rates(R2＝0.993), and as well as the correlation between yields and SOC (R2=0.997). The grain yield increase rate is 29 kg/ha per 1 kg N fertilization, and the rates for SOC and TSN in 0-20 cm depth are 1.2 kg/ha and 0.13 kg/ha under crop residues removed from the ground, respectively. Root-derived C and N are the main sources of SOC and TSN under the rainfed wheat cropping system in the loess tablelands.