The Effects Of Nitrogen And Phosphorus Additions On Soil Microbial Biomass In A Secondary Evergreen Broad-leaved Forest In Southwest China

Nutrient availability is a key regulator of soil microorganisms.Anthropogenic activities have dramatically accelerated the inputs of reactive nitrogen（N）,which may lead to phosphorus（P）limitation of soil microorganisms.However,many previous studies of soil microorganisms have focused more on the effects of N,and less on the effects of P and its interaction with N.Therefore,the objective here was to examine the roles of N availability,P availability and their interaction in determining soil microorganisms.In this study,we chose a secondary evergreen broad-leaved forest in Southwest China as the study object.Soil microbial biomass and soil chemical properties under nutrients addition were studied.Six nitrogen and phosphorus addition treatments were conducted,including control(CT,0kg N/P hm^-2·a^-1),low nitrogen treatment(LN,100 N hm^-2·a^-1)、high nitrogen treatment(HN,200 kg N hm^-2·a^-1),phosphorus treatment(P,50 kg P hm^-2·a^-1),low nitrogen plus phosphorus treatment(LNP,100 kg N hm^-2·a^-1+50 kg P hm^-2·a^-1)and high nitrogen plus phosphorus treatment(HNP,200 kg N hm^-2·a^-1+50 kg P hm^-2·a^-1).N was added as NH₄NO₃,and P was added as Na H₂PO₄.N and P supplies were applied monthly beginning in October2017.The soil microbial biomass was measured every three months from April 2018 to January 2020,and soil organic carbon,total nitrogen,total phosphorus,dissolved organic carbon,hydrolytic nitrogen,available phosphorus and p H value were measured in January2020.The results showed that:（1）LN treatment and HN treatment reduced soil microbial biomass carbon 11.7%（P>0.05）and 38.3%（P<0.05）,respectively.P treatment,LNP treatment and HNP treatment increased soil microbial biomass carbon 4.3%（P>0.05）,23.9%（P<0.05）and 1.5%（P>0.05）,respectively.LN treatment and HN treatment reduced soil microbial biomass nitrogen6.0%（P>0.05）and 20.6%（P<0.05）,respectively.P treatment,LNP treatment and HNP treatment increased soil microbial biomass nitrogen 5.9%（P>0.05）,35.3%（P<0.05）and6.1%（P>0.05）,respectively.LN treatment,HN treatment,P treatment,LNP treatment and HNP treatment reduced soil microbial biomass C/N 8.0%（P>0.05）、16.1%（P<0.05）、5.7%（P>0.05）、3.3%（P>0.05）and 5.8%（P>0.05）,respectively.The results suggested that soil microbial biomass carbon and nitrogen decreased gradually with an increase of N addition levels,unchanged following P addition,and enhanced following NP co-addition,in which LNP treatment had the most powerful positive effect on soil microbial biomass carbon and nitrogen.Under the same nitrogen addition level,additional application of the exogenous phosphorus significantly increased soil microbial biomass carbon and nitrogen.（2）There were significant interactions between N addition and P addition on soil microbial biomass carbon and nitrogen.The attributable proportion of interaction index suggested that approximately 25%of soil microbial biomass carbon and approximately 26%of soil microbial biomass nitrogen in the LNP plots originated from the interaction of LN addition and P addition and that approximately 31%of soil microbial biomass carbon and approximately 19%of soil microbial biomass nitrogen in the HNP plots originated from the interaction of LN addition and P addition.（3）Stepwise regression analysis suggested that 60.5%of total variance in soil microbial biomass carbon was explained by soil dissolved organic carbon independently and 68.7%by soil dissolved organic carbon and soil hydrolytic nitrogen,which indicated that soil microbial biomass carbon was controlled by soil dissolved organic carbon and soil hydrolytic nitrogen,and that 45.5%of total variance in soil microbial biomass nitrogen was explained by soil dissolved organic carbon independently and 66.3%by soil dissolved organic carbon and soil available phosphorus,which indicated that soil microbial biomass nitrogen was controlled by soil dissolved organic carbon and soil available phosphorus.（4）The direct path coefficients of soil dissolved organic carbon and hydrolytic nitrogen on soil microbial biomass carbon were 0.657 and-0.337,respectively,which suggested that the direct effect of soil dissolved organic carbon on soil microbial biomass carbon was positive and that the direct effect of soil hydrolytic nitrogen on soil microbial biomass carbon was negative.The direct path coefficients of soil dissolved organic carbon and available phosphorus on soil microbial biomass nitrogen were 0.591 and 0.476,respectively,which suggested that the direct effect of soil dissolved organic carbon and available phosphorus on soil microbial biomass nitrogen was positive and that the soil dissolved organic carbon had the most powerful positive effect on soil microbial biomass nitrogen.（5）Stepwise regression analysis suggested that soil C/N explained 30.8%of soil microbial biomass C/N variation,which indicated that nutrients addition changed soil microbial biomass C/N by regulating soil C/N.Pearson correlation analysis suggested that soil C/N was positively correlated with soil microbial biomass C/N.In conclusion,soil microbial biomass carbon and nitrogen decreased gradually with an increase of N addition levels,unchanged following P addition,and enhanced following NP co-addition,in which LNP treatment had the most powerful positive effect.Under the same nitrogen addition level,additional application of exogenous phosphorus significantly increased soil microbial biomass carbon and nitrogen.N and P additions changed soil microbial biomass by directly regulating soil available nitrogen and phosphorus and by indirectly regulating soil dissolved organic carbon.Given the dramatic increase in N disposition,we could apply the phosphate fertilizer in the secondary evergreen broad-leaved forest in southwest China,which may contribute to the increase in soil microbial biomass carbon and nitrogen.