Studies On Response Of Productivity And Nitrogen Allocation Of Artificial Grassland To Precipitation In Typical Steppe In Inner Mongolia

The typical grassland in Inner Mongolia is important production base and ecological barrier in northern China,which plays an important role in maintaining the ecosystem balance and stability.In the past half century,the grassland has been seriously degraded,and the quality of grassland has declined significantly,which restricts the sustainable development of animal husbandry.It is an important measure to maintain the sustainable development of animal husbandry that planting artificial grassland can alleviate the insufficient productivity of natural grassland and promote the natural restoration of degraded grassland.However,the precipitation in typical steppe of Inner Mongolia has the trend that decreasing and the interannual fluctuation increasing,which brings huge obstacles to the construction of artificial grassland.The productivity of artificial grassland under different cropping patterns has different responses to precipitation.In addition,precipitation has effect on the sources of available nitrogen by changing the soil nitrogen content in the artificial grassland and the nitrogen fixation capacity of legume herbage,then indirectly affecting the productivity of artificial grassland.Therefore,in order to clarify the different response mechanism of the artificial grassland productivity and nitrogen utilization to precipitation,in this paper,precipitation control experiments were carried out in single cropping Medicago sativa(Ms),single cropping Elymus nutans(En),intercropping Medicago sativa and Elymus nutans(In)in typical steppe of Inner Mongolia.The growth of herbage,the soil nitrogen distribution,and the source of nitrogen utilization under different water conditions were measured.The results showed that:(1)the height and biomass of Ms,En,and In had different response to precipitation condition.The height change rate of Ms of precipitation increasing was less than that when precipitation decreasing.In 2015,the height change rate of Ms was 9.6% and In-Ms was 15.5% when precipitation increasing,while Ms was 9.6% and In-Ms was 15.5% when precipitation decreased,indicating that the height of Ms was more sensitive to the soil moisture decrease.When the water decreased,the height change rate of En was significantly less than Ms,and the change rate of En was 0.7% and In-En was 4.4% when the water was reduced,indicated that En could be better adapted to the environment than Ms when the water condition was poor.The biomass change rate of En was the smallest when the water increased;the change rate of Ms was the largest when the water decreased.The change rate of In was between Ms and En when the water increased and decreased.The biomass of In did not show significant advantages to Ms,but it could be more stable than Ms under precipitation fluctuation.Leaf area and specific leaf area of En under different water conditions have significant difference.Intercropping and single cropping treatments had no significant effect on the leaf area and specific leaf area of the same herbage under the same water condition.The leaf area and the ratio of stem to leaf is more susceptible to soil carbon and nitrogen than the SLA.(2)The soil moisture increasing is beneficial to increase soil nitrogen content.In and Ms were beneficial to the accumulation of soil nitrate nitrogen.The soil total nitrogen content was the lowest in-50%,but there was no significant difference of soil total nitrogen content in Ms,En and In under the same water condition.The soil nitrate nitrogen content of Ms and In were significantly higher than that of En.(3)The higher the water content,the higher the nitrogen fixation rate of Ms and In-Ms.And the nitrogen fixation rate of In-Ms significantly higher than the nitrogen fixation rate of Ms.The proportion of nitrogen absorbed from soil nitrogen of In-Ms decreased with soil moisture increasing,which was 87.4%,85.1%,76.9%,and the proportion of nitrogen absorbed from soil of Ms was 91.9%,89.3%,82.3% in turn.Under the same water condition,In-Ms can get more nitrogen by biological nitrogen fixation compared with Ms.In intercropping artificial grassland,the transfer of nitrogen to In-En was limited by water,and the proportion of nitrogen absorbed from the soil of In-EN decreased with the precipitation increasing,which was 90.8%,84.4%,80.6% in turn,and the difference between different water treatments was significant.(4)Nitrogen content in root,stem and leaf had no significant difference under different water conditions.Nitrogen content in root,stem and leaf of Ms and In-Ms was significantly higher than that of En and In-En.And nitrogen content of root,stem and leaf of same forage had no significant difference under single cropping or intercropping.The nitrogen yield of stem and leaf increased significantly with the water increasing,but no significant difference was found under single cropping or intercropping.Precipitation and cropping model had no significant effect on the distribution of nitrogen in roots,stems and leaf.Precipitation only had significant effect on the nitrogen utilization efficiency(NUE)of In-En,but had no significant effect on NUE of Ms,En and In-Ms.Soil nitrogen content and leaf nitrogen yield had a significant negative effect on NUE of In-Ms.The nitrogen use efficiency of In-En was not only affected by the water content,but also negatively correlated with the leaf nitrogen content,and the soil nitrogen content could indirectly affect NUE of In-En.The leaf nitrogen content and leaf nitrogen yield had significant negative effects on NUE of Ms;and the leaf nitrogen content was negatively correlated with NUE of En.The water control experiment for three years of Ms,En and In artificial grassland showed that the precipitation increasing was beneficial to the productivity of artificial grassland,and the productivity of In artificial grassland was significantly higher than that En,and was more stable than Ms under the water wave condition.Intercropping can improve the nitrogen fixing ability of Ms,reduce the absorption and dependence of soil nitrogen,and could alleviate the limitation of nitrogen on the productivity of artificial grassland.