Wood Decomposition: Nitrogen and Phosphorus Dynamics in Relation to Extracellular Enzyme Activity

Because plant litter decomposition is directly mediated by extracellular enzymes (ectoenzymes), analyses of the dynamics of their activity may clarify the mechanisms that link decomposition rates to substrate quality and nutrient availability. We investigated this possibility by placing arrays of white birch sticks at eight upland, riparian, and lotic sites on a forested watershed in northern New York. For 3 yr, samples were analyzed for mass loss, protein, total Kjeldahl nitrogen (TKN), and total phosphorus (TP) accumulation, and the activity of 11 classes of extracellular enzymes involved in C, N, and P cycling. The relationship between lignocellulase activity and mass loss did not differ among sites. TKN and TP immobilization exhibited some spatial variation; rates of accumulation per 1% loss of initial mass, estimated from linear regressions, ranged from 2.2 to 4.4 mg/g OM for TP and from 43 to 139 mg/g OM for TKN, with maximum concentrations reached at °80% mass loss. The relationship between the activities of acid phosphatase (AcPase) and N—acetylglucosaminidase (NAGase), enzymes involved in the acquisition of P and N from organic sources, and mass loss displayed even greater variation among sites; the slopes of linear regressions relating mass loss and temporally integrated activity ranged from 0.019 to 0.135 activity—months per mass loss point and 0.107 to 0.775 activity—months per mass loss point, respectively, suggesting that edaphic rather than substrate quality factors were regulating activity. The extent of N limitation at each site was inferred by plotting TKN accumulation, defined as the slope of the linear regression TKN concentration vs. mass loss, in relation to NAGase activity accumulation, defined as the slope of the linear regression cumulative NAGase activity—months vs. mass loss. P limitation at each site was similarly assessed from an analogous plot of TP accumulation in relation to AcPase activity accumulation. Low N or P accumulation in conjunction with high acquisition activity was taken as an indication of nutrient limitation while the converse indicated surfeit. The diagrams suggested that decomposition at the upland hemlock and lotic sites, which displayed intermediate rates of OM loss (zero order k = 0.29 g/mo and 0.23 g/mo, respectively), was primarily N limited, while the riparian sites, which had the lowest rates of OM loss (k = 0.14 g/mo), appeared to be P limited. Relative to the others, OM loss at the upland deciduous sites (k = 0.38 g/mo) was not limited by either N or P. The concordance of field observations with predictions based on ectoenzyme regulation mechanisms suggest that enzyme activity assays in conjunction with nutrient concentration measurements may be a useful indicator of nutrient limitation. An economic model is proposed that directly links N and P availability to litter decomposition rates on the basis of microbial allocation of resources of extracellular enzyme production.