Opening the black box on soil warming: Microbial response to simultaneous warming and nitrogen fertilization

Background/Question/Methods

Soil warming studies are typically single factor experiments examining the effect of increased temperatures on soil carbon and nutrient fluxes.  Many of these studies do not quantify the microbial community that performs these fluxes.  In addition, most do not represent a scenario where increased temperatures take place with other disturbances such as nitrogen deposition.  The objective of this study was to examine the interactive effects of warming and nitrogen fertilization on the soil microbial community.   The research occurred at the Soil Warming ×Nitrogen Addition Experiment at the Harvard Forest in Petersham, MA.  The experiment included four treatments: control, heat, heat + N, and N only.   Since August 2006, heated plots have been warmed to 5°C above ambient using buried cables, and monthly additions of aqueous NH₄HO₃ have fertilized plots at a rate of 5 g N m^-2 y^-1.  The microbial response to warming and/or fertilization was assessed in soil and in decomposing red maple woody litter.  In soil, phospholipid fatty acid analysis was used to estimate microbial biomass and to obtain a community fingerprint.  In litter, fungal decomposers were identified by extracting, cloning, and sequencing fungal DNA, while litter decay was assessed as changes in mass, C, N, and lignin. 

Results/Conclusions

Initial results indicate that none of the experimental manipulations shifted soil microbial community composition.  The exception was at a single time point, in October 2008, when the N only plots showed a change in microbial fingerprint.  However, the heat, heat + N, and N only treatments all reduced soil fungal and bacterial biomass. The N only plots displayed the largest decrease in biomass, followed by heated + N, and then the heated only treatment.  These reductions could account for the long-term acclimation and/or decline of soil CO₂ respiration observed in many soil warming and N fertilization experiments.  In red maple litter, mass loss was suppressed in the N only treatment, while neither the heat nor the heat + N treatments affected litter decay.  Together, the lack of change in litter decay and the more moderate decrease in biomass in the heat + N plots as compared to the N only treatment suggests that increased temperatures alleviate the negative effect of N fertilization on the soil microbial community.