Soil Organic Matter Responses to Chronic Nitrogen Additions at the Harvard Forest
The Chronic Nitrogen Addition Experiment at the Harvard Forest was initiated in 1988 to better understand the process of forest N saturation due to anthropogenic N deposition. Presently, there is great interest in understanding and quantifying impacts of deliberate (forest fertilization) or inadvertent (atmospheric deposition) additions of nitrogen on forest growth as a means to enhance forest uptake of atmospheric C and subsequent storage within biomass and soil. For 20 years, plots within an approximately 100 year-old mixed hardwood forest have received N additions equivalent to 0 (Control), 50 (Low N), and 100 (High N) kg N ha-1 y-1. In 2008, as part of an extensive examination of long-term effects, we examined C dynamics at this site to better understand the response of these N additions on C stocks and processes. We have found that long-term N additions are increasing soil C stocks and altering important biological processes. Forest floor mass (Oe and Oa horizons) has increased from approximately 12500 g m-2 to over 20,000 g m-2. Mineral soil C, measured to 50 cm depth, has also increased. Total soil C including both forest floor and mineral soil has increased from less than 12500 g m-2 in the Control plot to over 20,000 g m-2 in the High N plot. Interestingly, litterfall mass is not consistently greater in the N addition plots than in the control, suggesting that alterations in decomposition are responsible for the additional stored C. In support of this, we find that microbial biomass in the forest floor of the High N plot is only approximately one-fourth of the mass found in the High N plot; the Low N plot falls in between. Relatedly, soil enzyme activity and soil respiration are dramatically lower in the N addition plots than it is in the Control plot. Our results suggest that in other mixed deciduous sites across the region, organic matter may accumulate in forest floor and mineral soil as atmospheric N deposition increases or where forest management involves N fertilization. However, the long-term trajectory of this accumulation, as well as long-term consequences on nutrient cycling and the quality of stored organic matter, remain unclear.