Regulation Mechanisms Of Grasshoppers On Soil Nitrogen And Phosphorus Cycling And Coupling Relationship Under The Background Of Increased Precipitation In Songnen Grassland

In grassland ecosystems,nitrogen（N）and phosphorus（P）cycling are the two ones of most fundamental nutrient cycling processes,which carry out the transfer of N and P elements between biotic components,and biotic and abiotic components,and play important roles in the structure,processes,and functions of grassland ecosystems.Among them,the soil N and P cycling in grassland have been widely studied,including the cycling processes of N and P transfer,mineralization,and absorption.N and P are two key nutrient elements that often couple and play joint role in the cycling process.With a deeper understanding of soil N and P cycling in grasslands,the regulation of precipitation patterns on soil N and P cycling has widely studied.Most researches have focused on precipitation amount and frequency.Changes in precipitation patterns can significantly affect soil N and P cycling by altering vegetation and soil characteristics.Grasshoppers are important primary consumers that cause significant alteration in vegetation and soil characteristics through feeding,feces,and carcasses,thereby regulating soil N and P cycling.However,the understanding of the effects and mechanisms of grasshoppers on soil N and P cycling in grassland is limited.At the same time,grasshoppers are highly sensitive to changes in precipitation patterns,and changes in precipitation patterns may regulate the effects of grasshoppers on soil N and P cycling and their coupling relationship to varying degrees.However,the joint effects of precipitation pattern changes and grasshoppers on soil N and P cycling have not been considered in understanding soil N and P cycling and the coupling relationship between them.Therefore,we conducted a manipulated experiment to investigate the effects of grasshoppers on pools,mineralization of soil N and P,and functional genes involved in N cycling under increased precipitation conditions during 2019 to 2021 in Songnen grassland,analyzed the effects of increased precipitation and grasshoppers on grassland vegetation and soil characteristics,clarified the relationships between soil N and P pools,mineralization,and functional genes with vegetation and soil characteristics,and revealed the mechanism on regulate soil N and P cycling in grasslands.The main results of this study are as follows:（1）Grasshoppers have significant effects on both vegetation and soil characteristics.Increased rainfall amount regulated the effect of grasshoppers on soil characteristics,but had no significant effects on vegetation characteristics.Grasshoppers significantly reduced the height and aboveground biomass of Leymus chinensis through their feeding activities,subsequently affecting the nitrogen content of L.chinensis leaves,but not for plant community diversity.Grasshoppers had a positive effect on soil nutrients content and soil microbial biomass.Higher rainfall amount increased the height and aboveground biomass of L.chinensis,and not affected plant diversity;it was beneficial to the increase in soil water content,nutrient content,and microbial biomass.Additionally,the effect of increased rainfall amount on vegetation and soil characteristics varied with the year.Increased amount of rainfall regulated the effect of grasshoppers on soil characteristics.Under increased rainfall amount conditions,grasshoppers reduced soil water content,which was caused by the significant reduction in plant cover due to the enhanced feeding ability of grasshoppers under higher rainfall amount.The presence of grasshoppers in the context of increased rainfall amount increased soil nutrients due to the increased input of nutrient-rich feces from grasshoppers into the soil.（2）The grasshoppers significantly affected the soil inorganic nitrogen pool,nitrogen transformation rate,and abundance of functional genes on nitrogen cycling in grassland,and increased rainfall amount regulated the effects of grasshoppers.The presence of grasshoppers significantly increased soil NO₃^--N content,but had no significant effect on net nitrogen transformation rate,and its impact on functional gene abundance increased,decreased,or had no significant effect,depending on the type of functional gene.Increased rainfall amount increased soil inorganic nitrogen content and promoted the abundance of functional genes associated with nitrification（AOA amo A and AOB amo A）,and denitrification（nir K and nos Z）,but had no significant effects on net nitrogen transformation rate.In years with excessive rainfall（i.e.2020）,increased rainfall amount had a significant negative effect on net nitrogen nitrification,ammonification,and mineralization rates.Increased rainfall amount regulated the effects of grasshoppers on soil inorganic nitrogen pool,transformation rate,and functional gene abundance.Under increased rainfall amount conditions,grasshoppers had no significant effects on NO₃^--N content,because the positive effect of grasshoppers was far less than that of increased rainfall amount on soil microbial biomass carbon and total nitrogen,which affects NO₃^--N content.Increased rainfall amount enhanced the positive effect of grasshoppers on net nitrogen nitrification and mineralization rate,because interaction of increased rainfall amount and grasshoppers reduced the biomass of L.chinensis and promoted microbial biomass carbon and nitrogen,ultimately increasing net nitrogen nitrification and mineralization rate.In addition,the presence of grasshoppers under increased rainfall amount conditions reduced the abundance of functional genes associated with N₂ fixation（nif H）,nitrification（AOA amo A and AOB amo A）,and denitrification（nir K and nir S）.（3）The grasshoppers had no significant effects on soil available phosphorus content and organic phosphorus mineralization rate,which did not regulate by increased rainfall amount.The grasshoppers had no effect on soil available phosphorus content and organic phosphorus mineralization rate.Increased rainfall amount reduced soil available phosphorus content,but increased soil organic phosphorus mineralization rate.Under increased rainfall amount conditions,the increased microbial biomass nitrogen,soil moisture content,and activity of alkaline phosphatase were negatively correlated with soil available phosphorus content.The three factors cooperate to drive changes in soil available phosphorus.The positive relationship between activity of alkaline phosphatase and soil organic phosphorus mineralization rate indicated that activity of alkaline phosphatase is the main factor affecting soil organic phosphorus mineralization rate.（4）The grasshoppers significantly affected the coupling relationships of carbon,nitrogen,and phosphorus within L.chinensis,soil,and soil microorganisms,and between them.Increased amount of rainfall had inconsistent effects on the effects of grasshoppers on the coupling relationships.The grasshoppers reduced the soil C:N,but not for the C:N:P of L.chinensis and soil microorganisms.Increased rainfall amount decreased the C:N and C:P of soil microorganisms,but had no effect on the C:N:P of L.chinensis and soil.Both grasshoppers and increased amount of rainfall affected the coupling relationships of carbon,nitrogen,and phosphorus among microorganisms,soil,and L.chinensis.The presence of grasshoppers promoted the coupling of C and C:N between microorganisms and soil,while increased rainfall amount led to the coupling of C:N between microorganisms and soil.Increased rainfall amount regulated the negative effect of grasshoppers on the soil C:N,because increased rainfall amount alleviated the negative impact of grasshoppers on the soil carbon-nitrogen coupling relationship by enhancing soil carbon and nitrogen content.This indicates that increased amount of rainfall and grasshoppers could decouple the carbon,nitrogen,and phosphorus of microorganisms and soil,but promote the coupling of carbon,nitrogen,and phosphorus between microorganisms and soil.In summary,under the background of increased amount of rainfall,grasshoppers can significantly affect the nitrogen and phosphorus cycling and the coupling relationships between nitrogen and phosphorus elements by altering the characteristics of plants,soil,and microorganisms in grassland,but these effects varied across years.Therefore,the results from this study clarify possible mechanism on the effects of grasshoppers on the soil nitrogen and phosphorus cycle,and further reveal that increased precipitation can regulate the effects of grasshoppers on the soil nitrogen and phosphorus cycle in grassland.The findings not only provide experimental evidence for better understanding on the regulatory effects of precipitation patterns and herbivorous insects on the soil nitrogen and phosphorus cycle,but also contribute to better guide of the important role of herbivorous insects in nutrient cycling of grassland under changing precipitation patterns.