Effects Of Mowing And Nitrogen Addition On Soil Organic Nitrogen Fractions In Typical Steppe Of Inner Mongolia

Mowing is considered to be an important driver of changes in soil nutrient levels in the way of grass utilization.Nitrogen addition is a fairly common management method in grassland ecosystem.Appropriate intensity of N addition can effectively improve the effectiveness of nitrogen in the soil and has a positive effect on soil nutrient cycling.Soil nitrogen as an important limiting factor for plant growth and development as well as net primary productivity of grassland,exists mainly in the form of organic nitrogen,and its effectiveness is important for the function and stability of grassland ecosystems.At present,there are more studies on the effects of different environmental conditions or land use practices on the change pattern of soil organic nitrogen fractions and nitrogen cycle,but they mainly focus on agroecosystems.The process and mechanism of the effect of mowing and nitrogen addition on soil organic nitrogen fractions under different management practices in typical steppe are poorly studied.For this reason,in this study,the typical steppe in Inner Mongolia was used as the research object,and the split-plot design was adopted.Split-plot design was used with mowing treatments in the main plots and nitrogen addition treatments in the sub-plots.Four mowing gradients were set up in the experiment,namely,mowing once every two years M1,mowing for two years with a year off M2,mowing once a year M3 and mowing twice a year M4,with no mowing in the enclosure CK as the control.In each mowing and enclosed plot,three nitrogen addition gradients were set up in the experiment,namely,no nitrogen addition N0,medium nitrogen addition N5(add 5g N/m~2/yr)and high nitrogen addition N10(add 10g N/m~2/yr).This study analyzed the response of soil organic nitrogen fractions and soil environmental factors to different mowing systems and N addition gradients and their coupling relationships,explored the mechanisms of mowing and N addition on soil organic nitrogen fractions,and provided theoretical reference for the sustainable development model of typical steppe in Inner Mongolia.The main results are as follows:(1)High intensity mowing system significantly reduced soil water content and total carbon content.N addition significantly increased soil TN,ammonium-N and nitrate-N content,as well as soil protease and urease activities,and reduced soil pH,bulk density and C:N.(2)Mowing treatments had less effect on the content of all soil organic nitrogen fractions,but there was a significant main effect on the proportion of AN and AAN to TN,and the proportion of both to total nitrogen tended to increase and then decrease with the increase of mowing intensity,indicating that a suitable mowing intensity could increase the proportion of soil easily mineralized organic nitrogen.(3)Nitrogen addition has an important effect on the content and proportion of soil organic nitrogen fractions.N addition significantly increased the content and proportion of acid hydrolysable organic N,acid hydrolysable ammonium N and amino acid N,and decreased the content and proportion of Non hydrolysable N,N addition had a positive effect on increasing the content and proportion of soil easily mineralized organic nitrogen,and decreasing the content and proportion of soil difficult to mineralize organic nitrogen.(4)Under different mowing systems and N addition gradients,the main drivers of changes in organic nitrogen fractions content in typical steppe soil were total soil nitrogen content and soil pH,which explained 33.8%and 11.5%,respectively,with a combined contribution of 88.4%,while protease activity and nitrate nitrogen content also explained changes in organic nitrogen fractions content to a significant level of P<0.05.(5)Under different mowing systems and N addition gradients,the main driver of the change in the proportion of organic nitrogen fractions in typical steppe soil was soil pH,with a contribution of 58.0%.In addition,protease activity,total nitrogen and nitrate nitrogen contents explained the change in the proportion of organic nitrogen fractions to a significance level of P<0.05.