Effects Of Nitrogen And Phosphorus Combined Application On Fine Root Turnover And Soil Organic Carbon Of Alfalfa

Objective:This experiment adopts the alfalfa as test object,explore common applied under the condition of nitrogen and phosphorus fine alfalfa root physiological and biochemical indicators in form and the characteristics of dynamic change in each soil layer analysis alfalfa fine roots between various indicators affect form,understand the alfalfa field component content and soil organic carbon in different dynamic change characteristics of the soil layer,clear the influence factors of alfalfa field soil respiration.Methods:A two-factor randomized block design was used to carry out the field experiment.Four phosphorus application levels[0（P₀）,50（P₁）,100（P₂）and 150 kg·hm^-2（P₃）]and two nitrogen application levels[0（N₀）and 120 kg·hm^-2（N₁）]were set up for a total of 8 treatments.The turnover characteristics of fine roots in 0-60 cm soil layer were monitored by micro-root canals.Results:1)Under the same nitrogen application condition,the root physiological indexes（total fine root stock,total surface area density,fine root growth and death amount）and root biochemical indexes（SS content,SP content,C,N,P stoichiometry characteristics of fine root and root neck）increased first and then decreased with the increase of Phosphorus application level.Alfalfa in 0～60 cm soil layer increased and then decreased,and alfalfa fine root in15～30 cm soil layer under different treatments.Under the same nitrogen application condition,the fine root turnover of alfalfa decreased first and then increased with the increase of phosphorus application.The fine root turnover rate was greatly affected by the dynamic changes of fine root stock and fine root death.Under N₀condition,P supplementation significantly decreased the C:N of fine roots under P₂treatment,but significantly increased the C:N of fine roots under P₁and P₃treatment（P<0.05）,and decreased the C:P of fine roots under P₂treatment,but significantly increased the N:P of fine roots under P₂treatment（P<0.05）,while P application had no effect on the C:N of root neck.However,the C:P and N:P of the root under P₁treatment were increased.In N₁condition,P application increased C:N and C:P in fine roots and root collar.2)Under the same nitrogen application condition,soil organic carbon and NCER increased firstly and then decreased with the increase of Phosphorus application rate,and reached the maximum value under P₂condition,and P（P₁,P₂,P₃）treatment was higher than that without P（P₀）treatment.In 0～60 cm soil layer,soil MBC content and effective rate increased firstly and then decreased;in 10～20 cm soil layer,all fertilization treatments reached the maximum value,and P₂treatment was significantly higher than other treatments（P<0.05）.In each fertilization treatment,the contents of soil DOC,EOC and POC gradually decreased with the increase of soil depth,and the soil MBC efficiency showed a trend of“increasing-decreasing-increasing-decreasing”with the increase of soil depth,while the soil DOC and EOC efficiency showed a trend of“decreasing-increasing-decreasing”with the increase of soil depth.Under the condition of N₀,POC efficiency decreased gradually with the increase of soil depth.There was significant difference in soil respiration rate before and after cutting in each crop（P<0.05）.NCER before cutting increased and then decreased in each crop,and reached the maximum in the third crop.The NCER varied from 1.4 to 6.5μmol·m^-2·s^-1and 2.9 to 6.5μmol·m^-2·s^-1,respectively.Conclusion:When the amount of phosphorus（P₂O₅）is 100 kg·hm^-2and nitrogen（N）is 120 kg·hm^-2,the existing stock and root surface area density of alfalfa fine roots can be significantly increased,and then the root turnover and growth of alfalfa can be promoted.At the same time,soil organic carbon accumulation was promoted and soil respiration rate was increased.