Effects of long-term nitrogen deposition on fine root decomposition and its extracellular enzyme activities in temperate forests

•Asymptotic decomposition models best described the decomposition.•N addition had contrasting effects on initial vs. later stages of decomposition.•Effects of N on decomposition were clearly linked to its microbial enzyme activities.•N addition may have contrasting effects on the dynamics of different soil carbon pools.

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.