Climate warming and nitrogen (N) deposition occur on a global scale with unknown long-term consequences for microbial communities and the biogeochemical processes they mediate. The Soil Warming × Nitrogen Addition experiment at the Harvard Forest Long-Term Ecological Research (LTER) site (Petersham, MA, USA) was established in 2006 to examine these specific global pressures with four treatments: control, warming (+5°C above ambient), N addition (50 kg N ha-1 yr-1), and warming × N (+5°C and 50 kg N ha-1 yr-1). Multiple microbial and biogeochemical responses have been measured from organic horizon material and mineral soil over the past ten years with the objective to investigate the interactive effects of warming and simulated N deposition on microbial communities, their function, and soil C and N pools.
Annual estimates of field measured soil respiration show continuous enhancement of CO2 flux with warming (+18%) and warming x N (+21%). Enhanced respiration with N addition is +13%, but this response has acclimated in recent years. Fluctuations in microbial biomass and community composition occurred during the first 5 years, with warming increasing fungal biomass but N additions decreasing total microbial biomass. No treatment effects on microbial biomass or composition occurred in years 6-10. Instead, seasonal and interannual variation appears to have played a greater role in shaping microbial communities in this first decade of study. Microbial community function has changed, however, with C use efficiency declining when exposed to warming or warming x N (-46% and -26%, respectively). Concomitantly, fine root biomass has declined by 30% with warming treatments. Carbon stocks in mineral soil have also declined by 21% with warming, while N additions show an accumulation of soil C (+17%). Our results suggest warming is reducing soil C stocks by reductions in microbial efficiency and root growth. Alternatively, N additions mitigate soil C losses by suppressing microbial activity and biomass. We conclude that long-term, multi-factor experiments are necessary to understand the effects of global change on ecosystem-level C dynamics.