Effects Of Combined Application Of Nitrogen And Phosphorus On Soil Phosphorus Forms,Microorganisms And Phosphorus Use Efficiency In Alfalfa Field

Alfalfa（Medicago sativa L.）is an important legume quality forage in the development of animal husbandry,with the“grain to feed”proposed to further promote the development of alfalfa industry clustering.The application of nitrogen and phosphorus fertilizer is essential for the production of high quality alfalfa forage.However,excessive application of phosphorus fertilizer leads to excess phosphorus in the soil which is then adsorbed and fixed in the soil,resulting in low levels of available phosphorus and causing soil pollution and economic waste.How to determine the right ratio of nitrogen and phosphorus fertilization is particularly important in promoting soil phosphorus activation,increasing the available phosphorus content of the soil plant and improving alfalfa production performance.Objective:This study investigated the changes of inorganic and organic phosphorus components, rhizosphere soil microbial population and enzyme activity in drip irrigated alfalfa soils under different nitrogen and phosphorus combined application conditions,to clarify the changes of each phosphorus form in alfalfa soils under different application ratios,to elucidate the relationship between inorganic and organic phosphorus contents,rhizosphere soil microbial population and enzyme activity and alfalfa yield,and to determine the best nitrogen and phosphorus application ratio for alfalfa cultivation in Shihezi region.Methods:The experiment was conducted in a two-factor randomized group design with four fertilizer gradients for both nitrogen and phosphorus treatments:N:0（N₀）,60（N₁）,120（N₂）and 180 kg/hm² （N₃）,and P:0（P₀）,50（P₁）,100（P₂）and 150 kg/hm²（P₃）,with a total of 16 treatments of nitrogen and phosphorus.Each treatment was replicated three times,using urea（46%N）for nitrogen and monoammonium phosphate（52%P and 10%N）for phosphorus.Each fertilizer treatment was applied at the beginning of nutritional growth after re-greening and 3-5 days after mowing of each crop,using a drip irrigation pattern with water.Results:（1）Under the combined application of nitrogen and phosphorus,the proportion of inorganic phosphorus components in alfalfa soil was Ca₁₀-P>Ca₈-P>O-P>Al-P>Fe-P>Ca₂-P,and Ca₁₀-P accounted for about 42%of soil inorganic phosphorus components.Combined application of nitrogen and phosphorus promoted the activation of phosphorus in soil.The proportion of Ca₈-P,Al-P andCa₂-P that could be directly absorbed and utilized by plants in soil increased,and the proportion of Ca₁₀-P that was difficult to be absorbed and utilized by plants decreased. （2）Under the combined application of nitrogen and phosphorus,the content of organic phosphorus in alfalfa soil was MLOP>HROP>MROP>LOP,and the content of MLOP accounted for about 70%of the total organic phosphorus.The proportion of MLOP in alfalfa soils that is easily mineralized and then taken up by the crop increases with the application of nitrogen and phosphorus, and the proportion of MROP that is difficult to mineralize and then taken up and used by the plant decreases. （3）Under the combined application of nitrogen and phosphorus,the rhizosphere soil microbial number,rhizosphere soil enzyme activity and hay yield of alfalfa were increased,but excessive nitrogen and phosphorus fertilizer input led to the decrease of rhizosphere soil microbial number,enzyme activity and hay yield of alfalfa.The number of bacteria and actinomycetes,alkaline phosphatase,catalase activity and hay yield of alfalfa rhizosphere soil reached the maximum values under N₂P₂treatment,which were 8.17,6.49 lg cfu/g and 32253.89,955.17 U/g·d and 25.80 t/hm²,respectively.The number of fungi reached the maximum value under N₂P₁treatment,which was 6.29lg cfu/g.The urease activity reached the maximum value under N₃P₂treatment,which was 1831.01U/g·d.Conclusion:Combined application of nitrogen and phosphorus can promote the activation of phosphorus in alfalfa soil,increase the contents of Ca₂-P,Al-P,Ca₈-P and MLOP in soil inorganic phosphorus and organic phosphorus,and improve the phosphorus supply capacity of alfalfa soil.At the same time, combined application of nitrogen and phosphorus can improve the rhizosphere soil microbial quantity and enzyme activity,thereby promoting the growth of alfalfa.Comprehensive analysis showed that the optimal treatment was N₂P₂（N:120 kg/hm²,P:100 kg/hm²）.