Effects Of Nutrient Addition And Secondary Succession On Soil Microbial Community

The survival and growth strategies and community structures of soil microbes,the major decomposers in terrestrial ecosystems,influence decomposition of soil organic matter and carbon(C),nitrogen(N),and phosphorus(P)cycles and thus the functions of the ecosystems.Microbial ecology study progresses much slower than the counterparts of plant and community ecology,mainly due to the micro-scaled individual,large quantity,complex community structure of microbes.Therefore,scientists attempted to apply macro-ecological theories to explain the processes mediated by soil microorganisms.In this study,I used field experiment,laboratory incubation,and meta-analysis techniques to investigate the dynamics in soil microbes in a nutrient addition experiment and a secondary successional chronosequence,with the aims of applying the ecological stoichiometry and succession theories to microbial ecology.First of all,I conducted a nutrient addition experiment in a temperate deciduous forest and a Korean pine(Pnius Koraiensis Sieb.Et Zucc.)plantation in Maoershan area,Northeast China to investigate adaptive mechanisms of soil microbes in response to the stoichiometric imbalance(i.e.,the ratio of soil C:N:P to microbial C:N:P),and explore the possibility of applying the ecological stoichiometry theory to soil microbial study.Then I examined the temporal dynamics in soil microbial communities during forest secondary succession(i.e.,farmland-grass-shrub-forest)in the Maoershan area,and explore whether the ecological stoichiometry theory and/or the succession theory can predict the trends in soil microbial communities during the succession.To assess the generalities of the findings in the case studies.I conducted the global meta-analyses and examined effects of nutrient addition and secondary succession on soil microbes.The main results were as follows:Nitrogen addition significantly changed the microbial C:N:P stoichiometric ratio(reduced the C:N but increased the N:P)in the two temperate forests.indicating that microbial stoichiometry is not strictly homeostatic but matches the resource stoichiometry to some extent.Naddition inhibited the activity of the extracellular enzymes for N-acquisition,but increased the activity of the extracellular enzymes for C or P-acquisition;P addition inhibited the activity of the extracellular enzymes for P-acquisition,but increased the activity of the extracellular enzymes for C-or N-acquisition;these results support the theory of resource allocation that soil microlora maximizes their productivity by optimizing their allocation of resources to produce the extracellular enzymes for mining limiting elements and keeping elemental balances.N addition inhibited the microbial metabolic quotient(i.e.,microbial respiration per unit microbial biomass C),suggesting that N addition increases the microbial C use efficiency.Moreover,the meta-analysis of the global dataset that included 454 N addition experiments also showed that N addition reduced microbial biomass C:N and fungi to bacteria ratio.Further regression and principal component analyses suggested that the shifts in microbial communities induced by N addition mainly resulted from enhanced N availability(i.e.,the ecological stoichiometry approach)rather than soil acidification.Collectively,the findings suggest that soil microbial communities can adapt to the stoichiometric imbalance induced by N addition via adjusting the stoichiometric ratio of biomass,extracellular enzyme stoichiometric ratio,community compositions,and C use efficiency.The results from the secondary successional sequence in the Maoershan area showed that the microbial metabolic quotient increased as the succession proceeded.The microbial metabolic quotient was positively correlated with the soil C:P and N:P,but negatively correlated with the microbial C:N and C:P;in addition,the microbial metabolic quotient was significantly increased as the C:N:P stoichiometric imbalance increased.These results indicate that the ecological stoichiometry theory can predict the dynamics in soil microbes during the secondary succession in the Maoershan area.In order to assess the generality of the results above,I conducted a meta-analysis of 85 age.The results showed that the successional trends in soil microbes were coincident with the macro-ecological succession theory for plants and animals.Specifically,early successional stages tended to be dominated by r-strategists(bacteria)that had higher microbial metabolic quotient and lower microbial quotient(i.e.,microbial biomass per unit soil C),whereas late successional stages tended to be dominated by K-strategists(fungi)that behaved oppositely.The soil C:N increased with the successional stage,with a fast increasing soil C:N being accompanied by a fast increase of fungi:bacteria,a slow decrease of microbial metabolic quotient,and a slow increase of microbial quotient.These findings suggest that the stoichiometry theory may provide a feasible approach to explain the divergent successional trends in microbial communities.In conclusion,I used field experiment,laboratory incubation and meta-analysis techniques to explore applications of the ecological stoichiometry and succession theories to soil microbial ecology based on nutrient addition and secondary succession.The findings enhance the theoretical studies of microbial ecology,and open a window for applying traditional maero-ecological theories to understand the complex ecological process mediated by soil microbial communities.Future studies shoud focus on effects of changes in microbial community compositions,stoichiometry,elemental use efficiency under global change scenarios on soil ecological processes,and strengthen the understanding of ecological functions and services of terrestrial ecosystems provided by the soil microbial communities.