Effects Of Plant Diversity And Global Change Factors On Soil Multifunctionality In The Songnen Grassland And Its Mechanisms

A grassland ecosystem can support a wide range of ecological functions and services and is of great significance in maintaining the well-being of human society,however,it is also the type of ecosystem that is currently highly disturbed by human activities.Ecosystem multifunctionality is an essential ecological concept that describes the ability of an ecosystem to maintain multiple ecological functions or services simultaneously,such as productivity production,carbon storage,and nutrient cycling.Soil multifunctionality is a subset of ecosystem multifunctionality,which represents the ability of soil to provide multiple functions and services simultaneously.Among them,soil nitrogen(N)cycling is an important part of the biogeochemical cycle of terrestrial ecosystems.The excessive use of fossil fuels and N fertilizer has enormously increased atmospheric reactive N worldwide.Excessive N input can affect plant and soil microbial communities and ultimately impact ecosystem functions by altering N availability or soil acidification and toxicity.With global warming and changes in precipitation patterns,drought and extreme precipitation events occur frequently.Precipitation changes affect the functions of the semi-arid grassland ecosystem by changing soil water availability and microbial communities.At the same time,in the context of global change,biodiversity is being reduced at an unprecedented rate.Therefore,the impact of the reduction in biodiversity on ecosystem functions has become an ecological issue of great concern.In addition,soil microbial communities are an important driving factor of ecosystem functions.Global change and biodiversity loss can regulate the composition and diversity of soil microbial communities by changing nutrient availability and interactions.However,the interaction between plant diversity and atmospheric N deposition or between plant diversity and precipitation changes on grassland soil multifunctionality and its microbial regulation mechanisms are still unclear.In this study,field survey sampling and microcosm experiments of plant diversity and N addition interactions and plant diversity and precipitation change interactions were carried out in the Leymus chinensis meadow steppe of Songnen grassland.Realtime quantitative polymerase chain reaction technology,fluorescent enzyme labeling,weighing and drying methods and anaerobic culture methods were used for systematic analysis under the conditions of plant diversity loss and global change.The change trends of plant community characteristics and soil physical and chemical characteristics,soil microbial characteristics,soil N cycling,soil multifunctionality and ecosystem multifunctionality,and the regulatory mechanisms of soil microbial communities on ecosystem functions were discussed.The main results and conclusions of this study are as follows:(1)Field experiments were carried out in Songnen grassland,and six plots with different plant diversity but similar soil characteristics were selected to study the effects of plant diversity on soil multifunctionality and the regulatory mechanisms of soil microbial communities.The results suggested that microbial carbon limitation in low plant diversity was significantly higher than that in high plant diversity.The belowground biomass/aboveground biomass and fungal and bacterial richness in high plant diversity were significantly higher than those in low plant diversity.Fungal richness was negatively correlated with microbial carbon limitation.Using averaging,single threshold,and multiple threshold methods,it was found that soil multifunctionality was significantly positively correlated with soil fungal richness.Meanwhile,soil multifunctionality was significantly positively correlated with the relative abundance of saprophytic fungi.However,soil multifunctionality was significantly negatively correlated with the relative abundance of pathogens.In addition,the lowest threshold for the positive effects of fungal richness on soil multifunctionality was 16%,and the maximum effect was reached when the threshold was 36%;that is,the addition of 1 species of fungus increased the soil function by 0.016.Moreover,rare fungal taxa were the main predictors of soil multifunctionality.Finally,plant diversityinduced changes in plant biomass allocation patterns increase the ratio of plant belowground biomass to aboveground biomass,alleviate microbial carbon limitation,improve fungal richness and finally promote soil multifunctionality.These results suggest that soil fungi play a more important role than bacteria in regulating soil multifunctionality in field situ conditions.(2)Microcosm experiments for two years were carried out to explore the impact of the interaction of plant diversity and N addition on soil N cycling and soil multifunctionality and the regulatory mechanisms.The results showed that high diversity plant communities could mitigate the negative effects of high N addition on soil N cycling multifunctionality;under the condition of a high diversity plant community,high-level N addition activated various biological driving mechanisms,that is,increasing plant belowground biomass,soil total carbon content,AOB amo A gene abundance,and gaseous N loss potential,and ultimately promoted soil N cycling multifunctionality.The results showed that plant diversity and N addition had an interactive effect(P < 0.001)on soil multifunctionality.Both plant diversity(P = 0.012)and N addition(P = 0.041)significantly improved soil multifunctionality.Soil multifunctionality was positively correlated with bacterial abundance and diversity.Plant diversity and N addition indirectly increased the soil organic matter content,soil bacterial abundance,and soil respiration rate by increasing plant belowground biomass and ultimately enhanced soil multifunctionality in the semi-arid grassland.In addition,soil bacterial abundance and soil organic matter content were important predictors of soil multifunctionality.(3)Microcosm experiments for two years were carried out to explore the impact of the interaction of plant diversity and precipitation on soil N cycling and ecosystem multifunctionality and the regulatory mechanisms.This study found that plant diversity and precipitation interactively affected(P = 0.023)AOB amo A gene abundance,and increased precipitation reduced AOB amo A gene abundance and net N nitrification rate.Soil organic matter content and β-1,4-N-acetylglucosaminidase activities were the main driving factors affecting soil N cycling.Plant diversity increased ecosystem multifunctionality,but increased precipitation decreased ecosystem multifunctionality.Plant diversity increased above-ground biomass,α-1,4-glucosidase activities,and soil total organic carbon content.However,increased precipitation decreased plant community leaf N and P contents and β-D-cellobiohydrolase and β-1,4-Nacetylglucosaminidase activities.There was a significant positive correlation between ecosystem multifunctionality and soil p H,soil organic matter content,and bacterial abundance.The changes in soil bacterial abundance play a key role in regulating the impact of plant diversity and precipitation on ecosystem multifunctionality.Plant diversity increased bacterial abundance,which in turn increased ecosystem multifunctionality.In contrast,the increase in precipitation reduced bacterial abundance and indirectly inhibited ecosystem multifunctionality.In conclusion,this study was conducted under two different experimental conditions—a field in situ sampling and microcosm control experiment—to comprehensively analyze the impact mechanism of global changes on ecosystem functions from the predictive variables of plant diversity,the interaction between plant diversity and N deposition,and the interaction between plant diversity and precipitation to the response variables of soil N cycling,soil multifunctionality,and ecosystem multifunctionality.Plant diversity can mitigate the negative effects of N deposition and precipitation change on soil multifunctionality and is indirectly driven or regulated by changes in plant,soil and soil microorganisms.This study provides important insights into the maintenance of grassland ecosystem functions under global climate change and provides strong data support for the development of rational grassland adaptive management measures.