Response Of Biogenic Elements To Water And Densiy In Vegetated Legume Grassland In Water-Wind Erosion Crisscross Region

Water and plant density affected the formation of vegetation productivity as well as relevant ecological process in water-wind erosion crisscross region, they were important influence factors of vegetation construction in ecological environment vulnerable areas. In this paper, we studied the biogeochemical processes of biogenic elements in wind-water erosion crisscross region on the Loess Plateau under water and plant density treatments, analyzed the response of biogenic elements to water and plant density treatments, and investigated the relationships among biomass, soil respiration, plant carbon, nitrogen, phosphorus, potassium, soil water storage, and plant photosynthesis. The main conclusions were as follows:1. Water and plant density treatments enhanced aboveground biomass in alfalfa grassland and erect milkvetch grassland significantly. Water and plant density treatments had no obvious effects on underground biomasses in alfalfa grassland and erect milkvetch grassland, but the underground biomass in water treatments was less than it in no-water treatments. Water and plant density treatments changed aboveground biomass accumulation rate in the two grasslands significantly. Water treatments changed seasonal variation regularity of aboveground biomass accumulation rate in alfalfa grassland, but did not change that in erect milkvetch grassland. It indicated that influence of water treatments on aboveground biomass accumulation rate was related to vegetation type. Plant density treatments did not change aboveground biomass accumulation rate in alfalfa grassland and erect milkvetch grassland.2. The soil moisture content of alfalfa and erect milkvetch grassland varied significantly with different time, soil layers, water and density treatments. Water adding treatment changed soil water storage of artifical grassland, but the amplitude varied by different season and density.The response of soil nitrate nitrogen in 0-10cm layer for alfalfa grassland to water and density treatments was significant while for erect milkvetch grassland was insignificant. Water treatment had significant effect on soil mineral nitrogen content, ammonium and nitrate nitrogen content in each layer other than density treatment. Both water adding and density treatment had insignificant effect on organic carbon content of alfalfa and erect milkvetch, but the effect of water adding treatment on seasonal variation characteristics of organic carbon content for alfalfa and erect milkvetch related to vegetation type. Water and density had insignificant effect on total N content of erect milkvetch. Water had insignificant effect on total N content of alfalfa, while density was significantly effective. Water and density treatments had insignificant effect on total P and total K content of erect milkvetch and alfalfa. Total N and total K content had similar seasonal fluctuation for both erect milkvetch and alfalfa, while total P content increased with vegetation growing. Both water adding and higher density can increase the N, P, K uptake and accumulation of erect milkvetch and alfalfa significantly.3. Photosynthetic rate and soil respiration rate of vegetated grassland depended on the type of grassland. Effect of water treatment on photosynthetic rate of vegetated grassland also varied by vegetation type, water adding treatment increased the photosynthetic rate of erect milkvetch significantly but insignificant to photosynthetic rate of alfalfa. Density treatment had insignificant effect on photosynthetic rate of both erect milkvetch and alfalfa. Water adding increased soil respiration rate significantly but didn't change seasonal variation characteristics of vegetated grassland. Effect of density on soil respiration rate depended on the grassland type and doubled density increased the average soil respiration rate during erect milkvetch growing season, but had insignificant effect on soil respiration rate of alfalfa grassland.4. Biomass of alfalfa had high significant positive correlation with soil moisture and biomass of erect milkvetch had significant negative correlation with soil moisture, which related to higher water consumption of erect milkvetch; phosphorus was more effective on biomass of vegetated grassland than other elements; alfalfa grassland had better coupling relationship between water and nutrients than erect milkvetch. Soil respiration drivered by alfalfa photosynthesis was promoted in water adding treatment; water adding significant enhanced the soil respiration of erect milkvetch grassland, while it can not change the driver relationship between photosynthesis and soil respiration. Water is the limiting factor for soil respiration of both erect milkvetch and alfalfa. However, water adding reduced the dependence for soil respiration rate of alfalfa grassland on soil water and temperature, but increased reliance for soil respiration rate of erect milkvetch on soil water and temperature, which related to higher water consumption of erect milkvetch.