[Effects of nitrogen rates on soil respiration in winter wheat cropping system in semi-arid regions on Loess plateau].

To evaluate effects of nitrogen rates on soil respiration, we measured the diurnal and seasonal dynamic changes and the contents of the soil organic carbon (SOC), soil dissolved organic carbon (DOC), soil microbial biomass carbon (MBC), and total soil nitrogen (TSN) under different growing seasons of winter wheat under the continuous systems from March in 2008 to March in 2009 at Changwu ecological station of the Chinese Academy of Sciences. The experiment consists of five nitrogen rates: 0 (N0), 45 (N45), 90 (N90), 135 (N135), and 180 (N180) kg/hm2. Soil respiration rate significantly enhanced with increasing nitrogen rate ranging from 0 to 90 kg/hm2 and increased little from 90 to 135 kg/hm2, but decreased with increasing nitrogen rate from 135 to 180 kg/hm2. At the seasonal change scale, there were an active phase of soil respiration from March to October and a weak phase of soil respiration from November to February. At the annual change scale, the fluctuate range of the treatments N0, N45, N90, N135, and N180 were 0.27-2.01 micromol/(m2 x s), 0.36-2.26 micromol/(m2 x s) 0.58-2.56 micromol/(m2 x s), 0.65-2.94 micromol/(m2 x s), and 0.58-2.6 micromol/(m2 x s), respectively. Nitrogen fertilization significantly affected the dynamic changes of DOC and MBC. There were significant positive correlations between soil respiration rate and DOC and MBC, while significant negative correlations with the C/N ratio.