Response Of N₂O Emissions And Productivity Of Lucerne Stand To Nitrogen Addition

The Loess Plateau where has fragile ecosystems are sensitive to climate change. Lucerne（Medicago sativa L.） is an important legume forage to maintain the high production and efficiency of grassland-livestock industry. Optimum nitrogen fertilizer management is the fundamental of efficient production of lucerne stands, and unreasonable nitrogen additions induce lucerne stands produce more potent greenhouse gases-nitrous oxide（N₂O）. Therefore, the study conducted at 6 years aged lucerne stand at Lanzhou University qingyang loess plateau test station by setting four nitrogen addition treatments: N-0, N-50, N-100 and N-150（0, 50, 100 and 150 kg N ha-1）, LGR N₂O/CO gas analyser was used to observe N₂O emission fluxes and measured the effect of nitrogen addition to production performance of lucerne. In order to obtain optimum nitrogen addition amount that mitigating N₂O emissions and improving production performance of lucerne stands. The objects of the study were providing both the scientific basis of accurate assessment N₂O emission from lucerne stands in dryland and theoretical support coordinating production benefit and ecological benefit of lucerne stands. The main results were:1. Nitrogen addition induced N₂O emissions of 6 years aged lucerne stand, N₂O emission fluxes and emission variability increased with increase of nitrogen addition level. N₂O emission flux among the two years of N-0, N-50, N-100 and N-150 was 0.0255, 0.0410, 0.0730 and 0.0840 mg m-2 h-1, respectively. Effects of nitrogen addition on N₂O emission were significant in the first year, N₂O emission flux of N-50, N-100 and N-150 was 32%, 97%, and 158% higher than N-0（P<0.05）, respectively. Emission variability under nitrogen addition at the first year was higher than what at the second year. Seasonal dynamics of N₂O emission were described that N₂O emission fluxes were high at regreening stage and regeneration stage, more negative N₂O fluxes appeared at florescence and there was not a typical dynamic characteristics for diurnal N₂O emission. N₂O emissions were influenced mainly by soil moisture content and soil temperature. N₂O emission fluxes of N-0 and N-50 were positive with soil moisture but negative under N-100 and N-150.2. Nitrogen addition improved soil total nitrogen（TN） content and soil organic carbon（SOC） content, and the effects on TN were more significant than that on SOC. TN content at 0-10 cm under N-100 and N-150 was 15.63% and 18.75% higher than N-0 respectively（P<0.05）. The effects of nitrogen addition on SOC were significant in the second year. SOC at 0-10 cm under N-100 and N-150 was 5.29% and 8.47% higher than N-0 respectively. SOC in 0-10 cm soil layer under N-100 and N-150 in the second year was higher than that in the first year 4.74% and 6.22% respectively. Soil nitrate nitrogen content in 0-10 cm soil layer under N-50, N-100 and N-150 treatment in growing period was 140.55, 151.43 and 194.21% significantly higher than that under N-0（P<0.05）, respectively.3. Nitrogen addition promoted radiation use efficiency and aboveground biomass. Leaf area index under N-150 on early flowering stage in the first year was significantly higher than N-0, N-50 and N-100 22.43%, 17.61% and 14.53%（P<0.05）, respectively. Adding nitrogen reduced detergent fiber content, increased dry matter intake（DMI）, digestible dry matter（DDM） and RFV. Crude protein（CP） content also increased with increasing of nitrogen addition content.4. Compared with the control（N-0）, N₂O emission fluxes was not significant increasedunder N-50 treatment. Yield-scaled N₂O emission under N-100 and N-150 was significant higher than that under N-0. Net profit under N-150 was significant 5.28% higher than that under N-0（P<0.05）. Applied 50 kg N per ha was not significantly increased N₂O emission fluxes and carbon trade deficit, and providing the same production. N-100 and N-150 increased both yield and N₂O emissions.In conclusion, t the optimum nitrogen addition amount considering comprehensively both N₂O emissions and production benefits was 50 kg N ha.