Abstract

Resolving the effects of nitrogen (N) on decomposition is ecologically critical for predicting the ecosystem consequences of increased anthropogenic N deposition. Although root litter is the dominant soil carbon (C) and nutrient input in many forest ecosystems, studies have rarely examined how the process of root decomposition is affected by N availability. In a field experiment, we studied the effects of N addition on fine root (<0.5 mm diameter) decomposition using five substrates ranging in initial gravimetric lignin concentrations (from 10.8% to 34.1%) over five years, and made a simultaneous characterization of effects of N on the enzymatic activity of the decomposer community in three temperate forests. Across substrates, asymptotic decomposition models best described the decomposition. The effects of N addition shifted over the course of fine root decomposition, regardless of initial lignin concentrations, with N speeding up the initial rate of decomposition, but ultimately resulting in a larger, slowly decomposing litter fraction (A). Such contrasting effects of N addition on initial and later stages of decomposition were closely linked to the dynamics of its extracellular enzyme activity. Our results emphasized the need for studies of N effects on litter decomposition that encompass the later stages of decomposition. This study suggested that atmospheric N addition may have contrasting effects on the dynamics of different carbon pools in forest soils, and such contrasting effects of N should be widely considered in biogeochemical models.

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