Ecosystem responses to long-term nitrogen additions in a temperate hardwood forest

Background/Question/Methods

Anthropogenic emissions of reactive nitrogen to the atmosphere (primarily due to fertilizer production, ammonia production for industrial purposes, and fossil fuel combustion) and the subsequent deposition to Earth’s surface have increased nearly 200% since the beginning of the industrial revolution and are projected to double yet again by 2050.  Nitrogen deposition rates over large regions of the world now exceed 10 kg N ha^-1 yr^-1, more than an order of magnitude higher than natural background levels.  This fertilization of historically N-limited ecosystems has the potential to saturate biotic demand for N thereby producing a cascade of negative impacts on ecosystem processes.  At the Harvard Forest LTER, we have, for the past two decades, been examining the effects of chronic N additions on above and belowground processes.  We have data on tree growth, foliar chemistry, litter fall, root dynamics, soil C and N pools and fluxes, and microbial biomass, community structure and function.  This talk will synthesize these results.

Results/Conclusions

Following two decades of research on the effects of long-term N additions on ecosystem processes in a temperate hardwood forest, we find that N additions have resulted in soil C accumulation of ~15-30 kg C / kg N added over 20 years.  We do not have strong evidence for a significant stimulation of aboveground productivity from N fertilization.  Tree biomass was a small sink for N deposition, with soils being the dominant sink for added N. Thus the effect of N addition appears to be greater for soils than for NPP.  Soil C accumulation is associated with reduced microbial biomass, enzyme activity, litter/wood decay, increased lignin concentrations, and a change in fungal community structure and function.