| Many studies have focused on the stoichiometric characteristics of different components of grassland ecosystems,but little was known about the relationship between the stoichiometric characteristics of multiple different components(plants,litter,soil,microorganisms,extracellular enzymes)in grassland ecosystems.Nitrogen(N)and phosphorus(P)were important limiting nutrients in desert grassland.However,the response of plant-microorganism-soil to N and P addition in desert grassland was not clear.How did microorganisms and plants adapt to the changes of soil nutrient environment under N and P addition?Therefore,this study carried out a multi-year N and P nutrient addition control experiment in desert grassland(nutrient addition treatment began in 2018).The experimental treatments included control(CK),N addition(10 g·m2a-1,P addition(10 g·m-2a-1),and NP co-addition(10 g·m-2a-1 N+10 g·m-2a-1 P).The response of ecological stoichiometric characteristics of different components in desert grassland to various restrictive nutrient additions was studied,and the stoichiometric dynamics of plantmicroorganism-soil continuum and the synergy between components were elucidated.The results showed that:(1)N and P additions had significant effects on carbon(C),N and P nutrient contents and stoichiometric characteristics of plants and litter in desert grassland.NP co-addition significantly increased the N and P content of plant and litter,while significantly reduced plant C:N and C:P ratio as well as C:P and N:P ratio of litter.P addition significantly increased P content of plant and litter,while significantly reduced C:P and N:P ratio of plant and litter.The N content and N:P ratio of plant and litter were increased by N addition,but no significant change of plant N content was observed from N addition.(2)N and P nutrient addition significantly affected soil nutrient content and its ecological stoichiometric characteristics.In the fourth years of nutrient addition,soil total nitrogen(TN)content was significantly increased.Soil N:P ratio was significantly increased by N addition,soil organic carbon(SOC)content was significantly increased by P addition and NP co-addition.In the third and fourth years of nutrient addition,soil available N:P ratio was significantly increased by N addition;N addition and NP co-addition significantly increased the content of ammonium nitrogen(NH4+-N)and nitrate nitrogen(NO3--N),but significantly reduced soil available C:N ratio;P addition and NP co-addition significantly increased total phosphorus(TP),and soil available phosphorus(AP),while significantly reduced soil total and available C:P and N:P ratio.(3)The addition of N and P nutrients significantly affected soil microbial biomass content and its ecological stoichiometric characteristics.N addition significantly increased the content of soil microbial biomass C(MBC)and N(MBN),as well as decreased microbial C:N and C:P;P addition and NP co-addition significantly increased MBC,MBN and MBP,and significantly reduced soil microbial C:P and N:P.The response of soil extracellular enzyme activity and its ecological stoichiometric characteristics to N and P addition was different.N addition significantly increased the activity of N-acetylglucosaminidase(NAG)and reduced the ecological stoichiometry of extracellular enzymes.P addition significantly increased β-1,4-glucosidase(BG)activity and extracellular enzyme N:P,while decreased extracellular enzyme C:N and C:P;NP coaddition significantly increased extracellular enzyme activity and extracellular enzyme N:P.(4)N and P addition had significant effects on plant nutrient use efficiency,reabsorption efficiency and microbial carbon use efficiency(CUE)in desert grassland.N addition increased plant phosphorus reabsorption efficiency and reduced plant nitrogen utilization efficiency.P addition increased plant nitrogen reabsorption efficiency and reduced plant phosphorus utilization efficiency.The co-addition of N and P improved the plant nitrogen reabsorption efficiency and phosphorus reabsorption efficiency,and reduced plant nitrogen utilization efficiency and phosphorus utilization efficiency.N and P nutrient addition significantly reduced CUE,but had no significant effect on microbial nitrogen and phosphorus use efficiency(NUE,PUE).(5)The plant growth was limited by N in desert grassland.The N limitation of desert grassland was alleviated by N addition,while was aggravated by P addition and co-addition of N and P.Desert grassland plant could adapt to the change of soil N and P supply mainly by changing the nutrient utilization strategies and reabsorption efficiency.The growth of soil microorganisms was limited by C,P in desert grassland.C,P limitation of soil microorganisms in desert grassland were alleviated by N,P addition,while P limitation of soil microorganisms in desert grassland was aggravated by N addition.Soil microorganisms in desert grassland regulate their own C:N:P ecological stoichiometric balance to adapt to changes in soil N and P limitations;by improving the activity of extracellular enzymes related to carbon cycle and changing the utilization efficiency of carbon nutrients,we can adapt to the changes of C limitation in desert grassland.(6)There was a significant correlation between C,N,P content and stoichiometric ratio of plantmicroorganism-soil.The addition of N and P nutrients caused changes in soil nutrient content and ecological stoichiometric characteristics,while soil nutrient content and ecological stoichiometric characteristics could indirectly affect the ecological stoichiometric characteristics of extracellular enzymes by directly affecting the ecological stoichiometric characteristics of plants.However,different soil nutrient contents and ecological stoichiometry characteristics had different effects on microbial ecological stoichiometry.Soil available nutrients and stoichiometric characteristics directly affect microbial ecological stoichiometric characteristics,and soil total nutrients and stoichiometric characteristics indirectly affect microbial ecological stoichiometric characteristics by affecting plant ecological stoichiometric characteristics.In summary,N and P nutrient addition significantly changed the stoichiometric characteristics of plants,soil,microorganisms and extracellular enzymes,and changed the nutrient limitation of plants and microorganisms in desert grassland.Therefore,if the protection and restoration of desert grassland are promoted by N and P nutrient addition,the element balance and nutrient connection among components in desert grassland ecosystem should be considered to improve the efficiency of nutrient addition measures.At the same time,this study also enriches the theory of desert grassland ecosystem and provides a basis for the application of desert grassland ecosystem theory in chemometrics. |
