Response Of Microbial Functional Groups Involved In Soil N Cycle To N,P And NP Fertilization In Tibetan Alpine Meadows

Tibetan Plateau meadows are high altitude,low nutrient ecosystems sensitive to fertilization.Nitrogen（N）cycle is an important part of earth’s biogeochemical cycles and is strongly altered by human activities.But recently,human’s disturbances have contributed greatly to the severe degradation of alpine meadows,so fertilization was used as a way to the recorving of grassland.In the same time,the activity of soil N-cycle related microbial and nutrient cycling also were influenced by it.So understanding how fertilization manage influence soil microbial activity is so imporetant for us to conduct grassland managing and keep the ecology banlance of Tibetan Plateau meadows.Whereas the response to N-and more rarely P-fertilization of some microbial groups involved in soil N cycling has been studied,a comprehensive view of how the major microbial groups involved in soil N dynamics respond to combined N and P fertilization is lacking.Here we investigated the effects of different N,P and NP fertilizer levels（control without fertilization;3 N levels without P;3 P levels without N;and 3 P levels with constant N addition）on the abundances of 9 microbial groups involved in N dynamics.Real time PCR was used to target free N2 fixers,nitrifiers（bacterial and archaea ammonia oxidizers,AOB and AOA,respectively;and the nitrite oxidizers Nitrobacter and Nitrospira）,nitrate reducers,nirK-and nir S-nitrite reducers,and nitrous oxide reducers.The main results are as follows:1.The influence of three levels of N,P and NP fertilizer to the abundance of N-cycle microbial communitiesThe abundances of AOB and Nitrobacter increased exponentially from 10⁷ to 10⁹and linearly from 10⁵ to 10⁶,respectively,with increasing N level along the NP0 fertilization gradient.The abundances of AOA and Nitrobacter decreased and increased,respectively,with increasing P level along the N0P fertilization gradient.The abundance of AOB was highest for the fertilization of 2g-P m^-2 year^-1 along the same gradient.Along the N10P gradient,the abundance of AOA decreased linearly with increasing P level.The abundances of free N₂ fixers and nitrate reducers decreased,and the abundance of nitrous oxide reducers increased,with increasing P level along the N0P gradient.Along the N10P gradient,the abundance of the nirK-like nitrite reducers significantly decrease with increasing P level.2.Relationship between microbial abundances and soil environmental parametersWhen considering the overall data set across the 3 fertilization gradients,the abundances of AOA,free N₂ fixers,nitrate reducers and nirK-like nitrite reducers were all positively correlated to OC and TN and negatively correlated to available P.The abundances of AOB,Nitrobacter and nir S-like nitrite reducers were positively,although weakly,correlated to total N and OC.The abundance of nitrous oxide reducers was negatively correlated with p H.When focusing on the NP0 fertilization gradient,the abundances of AOB and Nitrobacter were negatively correlated to soil p H,while the abundances of AOA and free N₂ fixers were positively correlated to OC and TN.The changes in soil nitrate level were positively correlated to the changes in the abundances of both AOB and Nitrobacter.When focusing on the N0P fertilization gradient,AOB abundance was positively correlated to OC and TN,while the abundances of free N₂ fixers,nitrate reducers and nirK-like nitrite reducers were negatively related to available P or positively correlated to the AN/AP ratio.When focusing on the N10P fertilization gradient,Nitrobacter abundance was positively correlated to soil p H,while the abundances of AOA and nirK-like nitrite reducers were negatively related to available P and positively correlated to the AN/AP ratio.3.Level of coupling between microbial groupsStrong correlation between AOB and Nitrobacter abundances was observed for soils of the NP0 and N10P gradients,while the abundances of these two groups were not correlated to the abundances of other groups except nirK-nitrite reducers for the NP0 gradient.Strong correlations were nearly always observed between the abundances of AOA,N₂ fixers,nitrate reducers and nirK-like nitrite reducers.Lack of or weak correlation was observed between the abundances of different groups belonging to the same functional type,i.e.AOA and AOB（never correlated,each being correlated to different groups）;Nitrobacter and Nitrospira（correlated only along the N0P gradient）;and nir S-and nirK-like nitrite reducers（the abundance of nir S-nitrite reducers being often not or weakly correlated to the abundance of other groups,while the abundance of nirK-nitrite reducers was always strongly correlated with the abundances of AOA,N₂ fixers and nitrate reducers）.When considering all the soils from the 3 fertilization gradients,the abundances of AOB and Nitrobacter appeared strongly correlated,as were the abundances of AOA,N₂ fixers,nitrate reducers and nirK-like nitrite reducers.