Response Of Soil Nitrogen Cycling Related Microbial Groups To Global Environmental Change And Fire Disturbance

Global environmental changes are main drivers of nutrient cycling in ecosystem.However,present studies mainly focus on evaluating the effects of single-and less often a few-global change factors on soil N cycling processes in grasslands.In addition,these studies have not recognized the importance of the response of grassland soil N cycling to co-occurring multiple global change factors and disturbance like fire.It remains unclear how N cycling response to fire could differ under different global change scenarios.This strongly restricts our ability to understand and predict global change effect on grasslands.In this work,two experiments were conducted:(i)a mesocosm experiment to assess the combined effects of increased N deposition and changes in both the precipitation amount and frequency on soil N cycling in a semi-arid Monsoon grassland;and(ii)an in situ experiment to assess the main and combined effects of elevated CO2,warming,increased precipitation,N deposition and fire on soil N cycling in a Mediterranean grassland.This allows studying the-possibly interactive-effects of several global change factors on the abundances of soil N-cycling microbial communities.The microbial groups studied were ammonia oxidizing bacteria(AOB)and archaea(AOA),nir Kand nir S-nitrite reducers,nos ZI-and nos ZII-N2 O reducers,plus Nitrobacter and Nitrospira for the Mediterranean grassland.The main results are as follows:1)The responses of different groups of soil(de)nitrifiers to global change scenarios differed strongly in both grasslands.In the Monsoon grassland,AOB abundance mostly responded to nitrogen,whereas AOA were more sensitive to soil water dynamics than nitrogen.The main effects of decreased precipitation amount and altered precipitation frequency differed between the four denitrifier groups studied.Nir K-and nir S-harboring nitrite reducers and nos ZI-harboring N2 O reducers were more sensitive to N deposition than nos ZII-harboring N2 O reducers,and nir K-and nir S-bacteria responded to reduced precipitation in an opposite direction.This highlights niche differentiation between these groups and indicates that the balance between them may be altered in the future.In addition,the study showed that N2 O emission was related to soil denitrification instead of nitrification in the Monsoon grassland.2)In the Mediterranean grassland,nitrogen deposition increased the abundance of AOB and to a lesser extent AOA and Nitrobacter(+12% to 182%),but not Nitrospira.Meanwhile,N deposition increased the abundance of nir K and nos ZI and to a lesser extent nir S(+ 10% to 46%),but not nos ZII.Instead,Precipitation had a negative main effect on Nitrobacter(up to-61%)and no significant effect on the three other nitrifier groups.Further,precipitation increased the abundance of nir S(+ 3% to 26%),whereas no significant effect was observed on the three other denitrifier groups.Further,burning had a negative main effect on AOB(-20% to 56%)and did not affect the abundances of soil denitrifiers immediately after fire,but it decreased the abundance of nir K(-6% to 37%)and nos ZI(-5% to 36%)almost three years after the initial disturbance.No main effect of elevated CO2 and heating was observed.Nitrobacter abundance was mostly affected by global change factors through their effects on AOB abundance,whereas Nitrospira abundance was more related to changes of AOA.The effects of multiple global changes and fire on soil(de)nitrifying microbial communities abundance were not additive and thus cannot be predicted by studies on single global change factor.These results demonstrate that for both grasslands studied,the effects of multiple global change factors and disturbances on soil N cycling could not be predicted simply by studying the effects of one or two factors.The observed interactive effects were explained by environmental variables like soil moisture,mineral N availability,p H and growth of plant belowground parts.This calls for more comprehensive studies in the global change biology domain.Modelling and evaluating the generality of these complex interaction effects is thus a high priority for research to predict the responses of soil N cycling processes to global change and feedbacks on climate in the future.