O horizon extracellular enzyme activity: Nitrogen limitation in decomposition

Modern agriculture and fossil fuel combustion contribute to the transfer of N from largely inert pools (atmospheric N2, fossil fuel reserves) to biologically reactive forms that can be transported downwind from agricultural or industrial areas to ecosystems that historically may have experienced low levels of N inputs. Understanding how increased N inputs alter the cycling of another biologically important element, C, has been impeded by uncertainties about N effects on the process of decomposition. To date, ecologists remain unable to predict when, where, and in what forms N addition stimulates rates of decomposition. For example, recent work showed that in eight low-N sites in Central Minnesota, litter N was positively correlated with decomposition, suggesting N limitation of decomposition, yet addition of inorganic N fertilizer increased decomposition in only two of eight sites. These paradoxical results call into question the assumption that the often-observed correlation between substrate N concentration and decomposition arises because N limits decomposition. Research is addressing three interrelated questions:* (1) Why do litter N and externally supplied N have contrasting effects on decomposition in low-N ecosystems? (2) Do different forms of N (organic vs. inorganic; substrate vs. externally supplied) affect the activity, function and composition of the decomposer community differently, and, if so, what are the consequences for decomposition? (3) What are temporal dynamics of the activity, function, and composition of the decomposer community and do these dynamics depend upon the amount and forms of N supplied to the decomposer community?* These questions will be addressed using a 4-y decomposition experiment manipulating the quantity and form of N available to decomposers via use of substrates ranging in N concentrations and of inorganic (ammonium nitrate) and organic (amino acids) N fertilizers. The response of microbial biomass, stoichiometry, efficiency