Effects of long-term nitrogen addition on microbial enzyme activity in eight forested and grassland sites: implications for litter and soil organic matter decomposition

Long-term nitrogen (N) addition experiments have found positive, negative, and neutral effects of added N on rates of decomposition. A leading explanation for this variation is differential effects of N on the activity of microbially produced extracellular enzymes involved in decomposition. Specifically, it is hypothesized that adding N to N-limited ecosystems increases activity of cellulose degrading enzymes and decreases that of lignin degrading enzymes, and that shifts in enzyme activity in response to added N explain the decomposition response to N fertilization. We measured litter and soil organic matter (SOM) decomposition and microbial enzyme activity in a long-term N fertilization experiment at eight forested and grassland sites in central Minnesota, USA, to determine (1) variation among sites in enzyme activity, (2) variation in the response of enzymes, litter decomposition, and soil respiration to added N, and (3) whether changes in enzyme activity in response to added N explained variability among sites in the effect of N on litter and SOM decomposition. Site differences in pH, moisture, soil carbon, and microbial biomass explained much of the among-site variation in enzyme activity. Added N generally stimulated activities of cellulose degrading and N- and phosphorus-acquiring enzymes in litter and soil, but had no effect on lignin degrading enzyme activity. In contrast, added N generally had negative or neutral effects on litter and SOM decomposition in the same sites, with no correspondence between effects of N on enzyme activity and decomposition across sites.