Spatial Distribution Characteristics And Microbial Regulation Mechanism Of Anammox In Rice-crayfish Farming Model

The rice-crayfish co-cropping mode（RC）has the characteristics of"two harvests in one water,two harvests in one field,stable grain and increased efficiency",and has been widely promoted and applied in Hubei Province.However,in actual production,producers have gradually formed a production model of"high input,high output,high pollution,and low efficiency"in order to increase profits.Excessive nitrogen input will reduce the nitrogen utilization efficiency of crops,and will also harm the water quality of aquaculture.Anaerobic ammonium oxidation（anammox）is a key process of denitrification of soil nitrogen cycle,which can improve the water quality of crayfish by denitrifying the aquaculture ditch,but it may also accelerate the release of nitrogen in rice fields and reduce the nitrogen use efficiency of rice.At present,little is known about the role of anammox in the RC.Therefore,from 2019 to 2020,a study on the spatial distribution of anammox and its microbial regulation mechanism in the RC was carried out in Houhu Farm,Qianjiang City,Hubei Province.In this experiment,the rice field with the RC was used as the research object,and the control was the rice single-cropping model（RS）.From the perspective of soil anammox microbial community structure and abundance of functional genes hzs A and hzo,this study deeply analyzed the microbial regulation mechanism of the RC anammox.The main test results are as follows:（1）There was no significant difference in soil physicochemical properties between the RC and the RM.In the RC,the ammonium nitrogen（NH₄⁺-N）content in the culture ditch sediments increased by 1078.2%-2978.7%compared with the field surface soil,and the soil nitrate nitrogen content（NO₃^--N）content increased by 19.6%-148.6%,soluble organic carbon（DOC）content decreased by34.6%-79.4%,redox potential（Eh）decreased by 33.2%-2728.4%,dissolved oxygen content（DO）decreased by 40.2%-41.2%.（2）Compared with the RM,the total amount of phospholipid fatty acids（PLFAs）in the field surface of the RC increased by 55.1%-65.8%,the total amount of aerobic bacteria PLFAs increased by 28.7%-30.3%,and the total amount of anaerobic bacteria PLFAs increased by 27.5%-34.5%,and the ratio of the total amount of aerobic and anaerobic PLFAs decreased by 1.1%-3.1%.Within the RC,the total amount of PLFAs in the culture trench sediments decreased by 17.9%-29.9%,the total amount of aerobic bacteria PLFAs decreased by 27.1%-79.9%,and the total amount of anaerobic bacteria PLFAs increased by 39.1%-106.7%,the ratio of the total amount of aerobic and anaerobic PLFAs decreased by 47.5%-90.2%.（3）Anammox rate in the RC showed the spatial distribution characteristics of aquaculture furrows higher than that of paddy soil.The soil anammox rate and the contribution rate of anammox to nitrogen（N₂）production in the RC were not significantly different from those in the RM.Since the soil bulk density of the RC was significantly lower than that of the RM,the rate of N₂production due to anammox per unit area in the RC was lower than that in the RS.In the RC,the anammox rate of the culture ditch sediment was 18.05%-41.13%lower than that of the field surface soil,and the contribution rate of anammox to N₂production was increased by 24.52%-48.97%.The higher NH₄⁺-N content in the culture trench sediments inhibited the increase of the hzo abundance of the anammox functional gene,thereby reducing the anammox rate.The decrease in denitrification rate of the sediments in the breeding trench was larger than that of anammox,which resulted in the increase of the contribution rate of anammox to N₂production.（4）The total amount of ammonia（NH₃）volatilization in the RC was reduced by 16.7%-27.3%,the cumulative N₂production was reduced by 16.6%-17.6%,and the cumulative emission of nitrous oxide（N₂O）was reduced by 16.7%-27.3%,nitrogen output increased by 1.5%-1.7%,and nitrogen surplus increased by 32.7%-36.1%.Therefore,the RC increased the nitrogen uptake by rice by reducing the loss of gaseous nitrogen,thereby increasing the rice yield.To sum up,the anammox rate in the RC showed a spatial distribution characteristic that the breeding furrow was higher than the paddy soil.On the field surface,the anammox rate of the RC was not different from that of the RM.The RC increases rice yield by reducing the loss of gaseous nitrogen,including anammox,and increasing nitrogen uptake by rice.