Effects Of Reduced Nitrogen Application And Intercropping Soybean On The Productivity Of Sweet Maize Farmland System And AMF Community Diversity

Guangdong Province is the largest region for sweet maize production in China.However,continuous sole cropping and excessive chemical nitrogen fertilizer application can easily cause soil degradation and environmental pollution.In order to explore the effects of long-term nitrogen reduction on the productivity of sweet maize//soybean intercropping system and AMF community diversity.Here,a field experiment of 11 seasons was used to investigate the effects of four different cropping patterns[sole sweet maize（SS）,two rows sweet maize-three rows soybean（S2B3）intercropping,two rows sweet maize-four rows soybean（S2B4）intercropping,sole soybean（SB）]and three different nitrogen fertilizer application levels[0 kg N·hm-2（N0）,300 kg N·hm-2（N1）,360 kg N·hm-2（N2）]on crop yield,stability of crop yield,soybean nodulation and nitrogen fixation rate,soil fertility contribution rate and fertilizer contribution rate,greenhouse gas emissions,system nitrogen loss and sweet maize AMF（arbuscular mycorrhizal fungi）community diversity.The main results were as follows:During the experiment from autumn 2013 to autumn 2018（11 seasons）,the results showed that the total yield of sweet maize,soybean and system in each treatment showed obvious season dynamic changes.Different seasons had a significant effect on the total yield of sweet maize,soybean and intercropping system.The cropping pattern also had a significant effect on the total yield of sweet maize,soybean and intercropping system,but the nitrogen application of the experiment had no significant effects on the total yield of sweet maize,soybean and intercropping system.The experiment results in 11 seasons showed that there was no significant difference between W²（Wricke’s ecovalence）and SYI（production sustainability index）for sweet maize yields of different treatments,indicating no significant difference between sweet maize yield stability for different treatments.The W²value of soybean sole cropping pattern was significantly higher than that of intercropping pattern,indicating that the yield stability of sole soybean was lower than that of intercropping,but the SYI value showed no significant difference in soybean yield stability.The cropping pattern had a significant effect on the total yield stability of the system.W²of S2B3-N1,S2B3-N2 and S2B4-N1was lower than the SS of the corresponding nitrogen application level,indicating that the intercropping pattern increased the total yield stability of the system.The experiment results in 11 seasons showed that sweet maize was the dominant intercropping crop and soybean was the inferior intercropping crop.With the exception of some abnormal climatic years,the actual yield loss index of intercropping treatments were greater than zero,and the land equivalent ratio were greater than 1,indicating that sweet maize//soybean intercropping had obvious intercropping advantage and significantly improved land use efficiency.During the experiment from 2016 to 2018 year（6 seasons）,the contribution rate of soil capacity in the intercropping pattern was significantly higher than that in the monoculture sweet maize（except spring 2017）,indicating that intercropping soybean nitrogen fixation and straw mulching significantly increased soil capacity contribution rate.There was a significant negative correlation between the contribution rate of fertilizer and the contribution rate of soil capacity.There was no significant difference in the fertilizer contribution rate of sole sweet maize under the two nitrogen application levels and they were significantly higher than the two intercropping patterns.During the experiment from 2015 to 2018 year（8 seasons）,the annual season and nitrogen application level had extremely significant effects on the number of soybean root nodules,dry weight of root nodules,nitrogen fixation efficiency and biological nitrogen fixation amount.The planting pattern had extremely significant effects on the biological nitrogen fixation amount of soybean.Under the same croping mode,the nitrogen fixation efficiency and biological nitrogen fixation of soybeans without nitrogen treatment were significantly higher than those of nitrogen treatment,and there was no significant difference between the nitrogen application levels.During the experiment from 2015 to 2018 year（8 seasons）,the annual season and planting patterns had a very significant impact on the cumulative emission of soil CO₂,and the change characteristics of the cumulative emission of soil CO₂in the 8 seasons were consistent with the change range of the annual temperature,indicating that temperature was the main factor affecting the cumulative emission of soil CO₂;the annual season and nitrogen application levels had extremely significant impact on the cumulative emissions of the soil N₂O.The planting pattern had a significant impact on the cumulative emissions of soil N₂O,and the cumulative emissions of soil N₂O in the 8 seasons were positively correlated with the annual temperature and negatively correlated with the annual rainfall.The annual season,planting pattern and nitrogen application rate had a significant effect on the soil greenhouse gas emission intensity.During the experiment from 2015 to 2018 year（8 seasons）,the annual season,cropping pattern and nitrogen application levels had extremely significant effects on soil ammonia volatilization.The ammonia volatilization of intercropping treatment was significantly lower than that of sole treatment,and the amount of soil ammonia volatilization under the intercropping pattern was significantly lower than that of the conventional nitrogen application treatment.The soil ammonia volatilization was negatively correlated with the annual rainfall,and positively correlated with the annual temperature.The annual season and cropping pattern had extremely significant effects on soil nitrogen leaching.The soil nitrogen leaching amount of intercropping treatment was significantly lower than that of single cropping treatment,and soil nitrogen leaching amount was positively correlated with annual rainfall.The results of mid-autumn sample analysis in 2017 showed that under the same planting pattern,the AMF community richness index,diversity index,species composition and community richness of sweet maize root circumference soil,rhizosphere soil and root without nitrogen treatment were significantly higher than that of nitrogen treatment,indicating that nitrogen fertilizer was the main factor affecting the AMF community diversity of sweet maize.Environmental factors such as SOM,p H,AN,AP,NH₄⁺-N and NO₃^--N had significant effects AMF community structure of sweet maize rhizosphere soil and rhizosphere soil,and NO₃^--N and AP had significant effects AMF community structure of sweet maize root.The effects of cropping patterns on the diversity of AMF communities in sweet maize were not significantly reflected.In conclusion,reduced nitrogen application and sweet maize//soybean intercropping can maintain the intercropping advantage,improve the productivity and stability of farmland system,and reduce nitrogen application increaseed the species diversity of sweet maize AMF community.