Study On Nitrogen And Phosphorus Utilization Characteristics Of Chinese Milk Vetch / Green Manure Rapeseed Mixed Cropping

The mixed green manure crops of Chinese milk vetch and Rapeseed is an important planting pattern to cultivate soil fertility and improve green manure production in the winter fallow area of southern China.In this study,we investigated the growth and nutrient uptake advantages of the mixed Chinese milk vetch and Rapeseed by comparing the differences among the treatments of monoculture,mixed crop,semi-isolated and fully isolated cultivation.We also investigated the biological nitrogen fixation and nitrogen transfer in the mixed crop of Chinese milk vetch and Rapeseed using the ¹⁵N natural abundance method,and explored the soil phosphorus nutrient status.The main results were as follows:（1）In the mixed cropping model of Chinese milk vetch/Rapeseed,the overall growth and development of the system are significantly promoted,and this model has significant advantages in mixed cropping.The mixed cropping mode significantly improved the overall biomass of the system,with an increase of 254.2%to 383.7%and 2.6%to 35.0%,respectively,compared to monoculture Chinese milk vetch and monoculture Rapeseed.Mixed cropping significantly improved the overall land equivalent ratio of the system,ranging from 2.13 to 2.94 throughout the entire growth period,all exceeding 1.Mixed cropping also significantly increased the overall accumulation of nitrogen,phosphorus,and potassium nutrients in the system.Compared to monoculture Chinese milk vetch and monoculture Rapeseed,nitrogen,phosphorus and potassium accumulation increasing by108.2%～133.9%and 10.8%～75.3%,153.9%～499.7%and 39.3%～124.0%,146.4%～179.2%and 3.8%～42.8%,respectively.（2）In the Chinese milk vetch/Rapeseed mixed cropping model,Chinese milk vetch is at a mixed cropping disadvantage position and Rapeseed is at a mixed cropping advantage position.Mixed cropping significantly reduced the plant height and potassium content of Chinese milk vetch,with a decrease of 21.9%～53.5%and 45.4%～69.2%respectively,compared to monoculture Chinese milk vetch.However,mixed cropping increased the nitrogen and phosphorus content of Chinese milk vetch,compared with monoculture Chinese milk vetch,increased by 23.4%～48.8%and 20.0%～75.0%,respectively.Mixed cropping significantly increased the plant height and phosphorus content of Rapeseed,with increases of 25.9%to 39.6%and 28.6%to 71.4%compared to monoculture Rapeseed,respectively.（3）The 15N natural abundance method clarifies that the nitrogen fixed by the Chinese milk vetch in the mixed cropping system can be transferred to Rapeseed.In the mixed cropping system of Chinese milk vetch/Rapeseed,the biological nitrogen fixation efficiency of Chinese milk vetch during the initial and full flowering stages was 42.9%and 42.6%,respectively.Compared to the biological nitrogen fixation rate of 25.9%and32.7%of monoculture Chinese milk vetch,it has a significant mixed cropping advantage.However,due to the inhibitory effect of mixed cropping mode on the growth of Chinese milk vetch,there was no significant difference in its biological nitrogen fixation compared to monoculture mode.In the mixed cropping mode,the nitrogen transfer rates from Chinese milk vetch to Rapeseed were 45.6%and 71.7%during the initial and full flowering stages of Chinese milk vetch,respectively.The nitrogen transfer amounts were269.56 mg/pot and 500.11 mg/pot,respectively.In the semi isolated root mode,the nitrogen transfer rates were 2.4%and 29.5%,and the transfer amounts were 0.72 mg/pot and 9.22 mg/pot,respectively.The mixed cropping system of Chinese milk vetch and Rapeseed significantly increased the content of soil organic matter,total nitrogen,and ammonium nitrogen.Compared to monoculture of Chinese milk vetch,the mixed cropping system of Chinese milk vetch increased soil organic matter,total nitrogen,and ammonium nitrogen by 119.0%～222.3%,122.2%～328.6%,and 50.6%～101.7%,respectively;Compared to monoculture Rapeseed,the soil organic matter and ammonium nitrogen increased by 31.7%～224.4%and 64.5%～97.2%,respectively.（4）The mixed cropping model of Chinese milk vetch and Rapeseed significantly increased the soil available phosphorus content.Compared to monoculture Chinese milk vetch,the available phosphorus content in the rhizosphere soil of mixed cropping Chinese milk vetch significantly increased by 111.9%to 157.4%.Although there was no significant difference in the available phosphorus content in the rhizosphere soil of Rapeseed,it could still maintain a high level under the conditions of plants taking away a large amount of phosphorus and no external phosphorus fertilizer application.Mixed cropping significantly improved the phosphorus nutrition status of the system,increased the available phosphorus components such as Ca₂-P,Ca₈-P,and Fe-P in the inorganic phosphorus components,and also increased the moderately active organic phosphorus component,increasing the potential source of soil available phosphorus.Among them,the content of Ca₂-P,Ca₈-P,Fe-P,and moderately active organic phosphorus in Chinese milk vetch increased by 149.3%to 173.8%,109.9%to 191.4%,110.5%to 216.2%,and 64.4%to 67.6%,respectively.In mixed cropping mode,soil p H decreased by 0.49 to 0.77 units compared to monoculture mode.At the same time,mixed cropping improved soil microbial activity,increasing soil MBN and MBP by 75.6%～81.5%and 118.8%～562.7%,respectively,compared to monoculture Chinese milk vetch,and 34.6%～479.7%,respectively,compared to monoculture Rapeseed.In summary,the mixed cropping model of Chinese milk vetch/Rapeseed can improve the nitrogen fixation rate of Chinese milk vetch,and some fixed nitrogen can be transferred to rapeseed.At the same time,it can better activate and utilize soil insoluble phosphorus,and the system has significant advantages in yield and nutrient absorption and accumulation.The mixed cropping system also increased the content of soil organic matter,ammonium nitrogen,total nitrogen,and available phosphorus,increased the effective component of soil phosphorus,and strengthened the soil’s phosphorus supply capacity.Therefore,the mixed cropping mode of Chinese milk vetch/green manure Rapeseed is a planting mode with complementary advantages and efficient resource utilization,which has broad application prospects in the rice fertilizer rotation mode.