The Responses Of N Characteristics In The Soil-plant System To The N Addition In The Steppe On The Loess Plateau

To study the responses of N characteristics in the soil-plant system to the N addition in the steppe on the Loess Plateau, four level of nitrogen (N) treatments, NO (0 g/m2), N1 (4.6 g/m2), N2 (9.2 g/m2) and N3(13.8 g/m2), were conducted in a steppe in 2009 and 2010. Based on the field studies and laboratory experiment, the responses of a set of indices to the N addition were investigated. The indices included soil total nitrogen content (TN), ammonium nitrogen content (NH4+-N), nitrate nitrogen content (NO3--N), microbial biomass nitrogen (Micro-N), nitrogen mineralization, the biomass of plant, the plant nitrogen accumulation and the nitrogen utilizing efficiency. The relationships between the variation of inorganic nitrogen (IN) and the plant nitrogen accumulation, the N mineralization and the biomass and were studied. The main conclusions were as follows:(1) The responses of TN content to the N addition were consistent in two years study. In July and September,2009, no significant differences in TN content were found among different treatments and soil layers. The variations of TN content in 2010 showed the same pattern with that in 2009, except for that in 10-20 cm soil layer which increased significantly with the increase of N addition (the TN content for N3 was 11.7% higher than that for NO).(2) The responses of NH4+-N content to the N addition were not completely consistent in 2009 and 2010. In July 2009, NH4+-N content increased significantly with the increase of N addition, and NH4+-N content declined with the soil depth for each treatment except for N0; in September, there were no significant differences among the treatments and soil layers. In 2010, no significant differences among the treatments and soil layers were found except for N3, in which the soil NH4+-N in 0-10 cm was significantly higher than other treatments. In addition, NH4+-N significantly decreased from July to September in 2009, while it increased in 2010. For all treatments, NH4+-N contents in July,2009 were significantly higher than that observed on other occasions.(3) Soil NO3--N content increased significantly with the increase of N addition in July,2009 and July,2010, and it decreased with the soil depth. NO3--N content in September were significantly lower than that in July, while there were no significant differences among the treatments. In September,2010, NO3--N content increased significantly with the increase of N addition, and the values in 10-20 cm and 20-40 cm were higher than that in July. For the N2、N3 treatments, NO3--N contents along the soil profile were in the order:10-20 cm>0-10 cm>20-40 cm. N addition increased the possibility of the NO3--N leaching.(4) N mineralization significantly increased with the increase of N addition in 2009 and 2010, ranging between 0.087-0.234 mg/kg/d and 0.097-0.424 mg/kg/d in both two years. Nitrification was the main process of N mineralization in this area and N addition could promote this process. In addition, the results showed that the N addition could enhance the role of water on mineralization.(5) N addition significantly increased aboveground biomass, the values in the two year were 85.98 g/m2-154.96 g/m2 and 101.80 g/m2-177.98 g/m2, respectively. While content of underground biomass showed different change trends, with range of 161.34g/m2-236.63g/m2 and 198.83 g/m2-345.32 g/m2 in 2009 and 2100, respectively. After two years treatments N addition could decline the root shoot ratio.(6) N addition increased plant accumulation, the values of N accumulation in the above-ground biomass in the two years were 1.75-4.30 g/m2 and 1.33-3.32 g/m2, respectively, while that in the under-ground biomass were 1.16-3.00 g/m2 and 1.85-4.00 g/m2, respectively. The total N accumulation values were 3.37-7.71 g/m2 and 4.23-7.33 g/m2 in 2009 and 2010, respectively. The N utilizing efficiency declined with the increase of N addition.(7) The correlation between IN content and N accumulation in plant was different in the two years. The correlation was positive in 2009 (R2 values was 0.6687), while there was no significant correlation in 2010. Mineralized rate and the content of NO3--N had significhant positive correlation with aboveground biomass and totoal biomass (p<0.01), however, there was no significant correlation between NH4+-N content and plant biomass.