Effects Of Continuous Nitrogen And Phosphorus Additions On Soil Biological Communities And Their Functions In An Alpine Meadow On The Qinghai-Tibet Plateau

As one of the largest grassland ecosystems in the world,the Qinghai-Tibet Plateau（QTP）alpine grassland is also one of the main pastoral areas in China.The unique high altitude and cold environment of the QTP has created a fragile alpine grassland ecosystem,which is characterized by high altitude and low nutrition and is very sensitive to grassland management,regional climate change and human activities.In recent years,due to overgrazing and local rodent damage,the grassland on the QTP has been seriously degraded,reducing the local ecological diversity and function.Fertilization is one of the effective ways to restore grassland productivity and ecosystem function.At present,the mechanism of how fertilization affects soil functional microbial community and its driving mechanism is still unclear,which restricts the sustainable development of local agriculture and animal husbandry.This study took the alpine meadow soil ecosystem in the QTP as the research object,and set up different 9a fertilization measures manually,taking no fertilization（CK）as the control group,to study the response of carbon and nitrogen cycle-related functional genes,soil functional microbial community macrogenomes,and soil food web to the addition of nitrogen fertilizer（N）,phosphorus fertilizer（P）,and nitrogen and phosphorus combined fertilization（NP）in the soil ecosystem.The main results of this study are as follows:（1）Long-term addition of nitrogen fertilizer was beneficial to soil nitrogen cycle,but not conducive to soil carbon cycle and inhibited soil versatility.N addition reduced the abundance of genes related to soil carbon cycle,including gam involved in starch degradation,xyl A involved in hemicellulose degradation,cdh and exg involved in cellulose degradation,and naglu gene involved in chitin degradation.p H was significantly correlated with DC/4-HB and 3-HP cycles.N changed the soil p H,resulting in changes in CO₃^2-and HCO₃^-,which significantly promoted the DC/4-HB and 3-HP cycles.N addition can affect soil carbon cycle function genes by changing plant diversity and the composition and content of root exudates.However,N has no effect on carbon degradation.N significantly increased microbial diversity,but decreased the multifunction of soil.N significantly promoted soil nitrogen cycle by increasing soil microbial nitrification and denitrification.The abundance of bacteriophagous nematodes under N addition was significantly higher than that under other fertilization treatments,the abundance of phytophagous nematodes was significantly higher than that under CK,and the biomass carbon value of phytophagous nematodes was significantly higher than that under other fertilization treatments.N significantly reduced the biomass carbon content of the bacteriophagous nematode.This is related to the weakening of predation pressure in the soil food web caused by the addition of nitrogen fertilizer.The effect of phytophagous nematodes on grassland ecosystem productivity is greater than other factors.（2）Long-term addition of phosphorus fertilizer increased microbial diversity,but decreased soil function through changes in soil physicochemical properties and nematode mediated processes.P decreased the abundance of genes related to soil carbon cycle,and significantly decreased the abundance of methane genes.The addition of P significantly reduced the abundance of gam involved in starch degradation,xyl A involved in hemicellulose degradation,cdh and exg involved in cellulose degradation,and naglu gene involved in chitin degradation.P significantly promoted soil microbial carbon fixation,including r TCA,DC/4-HB and 3-HP cycles.P addition is an important factor affecting the multifunction of soil.Although P significantly improves the microbial diversity,it significantly reduced the multifunction of soil.The addition of P on the basis of N enhances the promotion of nitrogen cycle and has obvious synergistic effect.P significantly reduced the biomass carbon content of the soil bacteriophagous nematode,which was speculated to be related to phosphorus mineralization,and promoted the activity of phosphorus metabolism of related microorganisms.Fertilization was conducive to increasing the yield of grassland ecosystem by changing the activities of the mycophagous nematode.（3）Long-term NP is beneficial to increase aboveground biomass,but it will enhance the soil nitrification process and plant-mediated N emission,resulting in nitrogen loss,reduced the multifunction of soil,reduced the carbon degradation process,and improved the effectiveness of soil carbon.NP significantly changed the functional composition of microorganisms,significantly changed the functional composition of KEGG,and significantly reduced the multifunction of soil.NP addition significantly reduced the abundance of starch degradation genes amy A and apu,and there was no correlation between N and P for genes related to carbon cycle.NP significantly increased the relative abundance of nitrification and denitrification genes.NP increased the availability of nutrients,increased aboveground biomass,and was significantly related to carbon fixation pathway.At the same time,NP significantly reduced the carbon degradation process and reduced the carbon input from the aboveground part to the underground.Fertilization had no significant effect on the diversity index of soil nematodes,but the abundance of omnivorous nematodes under NP was significantly higher than that under CK,which significantly reduced the biomass carbon content of bacteriophagous nematodes.The biomass carbon value of omnivorous nematodes was higher than that under CK and P addition,and the increase of predation pressure reduced the abundance of phytophagous nematodes.Therefore,NP significantly reduced the nutrients entering the soil,thus reducing the loss of vegetation resources.At the same time,vegetation diversity is the main factor driving the soil multifunction.In general,N or P has a negative impact on forage yield.NP increased forage yield,but N,P,and NP both reduced the multifunction of the soil ecosystem.P and NP reduced the diversity of aboveground vegetation.It is recommended to select NP in future forage production activities.