Response of decomposing litter and its microbial community to multiple forms of nitrogen enrichment

Abstract

Despite the importance of litter decomposition for ecosystem fertility and carbon balance, key uncertainties remain about how this fundamental process is affected by nitrogen (N) availability. Resolving such uncertainties is critical for predicting the ecosystem consequences of increased anthropogenic N deposition. Toward that end, we decomposed green leaves and senesced litter of northern pin oak (Quercus ellipsoidalis) in three forested stands dominated by northern pin oak or white pine (Pinus strobus) to compare effects of substrate N (as it differed between leaves and litter) and externally supplied N (inorganic or organic forms) on decomposition and decomposer community structure and function over four years. Asymptotic decomposition models fit the data equally well as single exponential models and allowed us to compare effects of N on both the initial decomposition rate (ka) and the level of asymptotic mass remaining (A, proportion of mass remaining at which decomposition approaches zero, i.e., the fraction of slowly decomposing litter). In all sites, both substrate N and externally supplied N (regardless of form) accelerated the initial decomposition rate. Faster initial decomposition rates corresponded to higher activity of polysaccharide‐degrading enzymes associated with externally supplied N and greater relative abundances of Gram‐negative and Gram‐positive bacteria associated with green leaves and externally supplied organic N (assessed using phospholipid fatty acid analysis, PLFA). By contrast, later in decomposition, externally supplied N slowed decomposition, increasing the fraction of slowly decomposing litter (A) and reducing lignin‐degrading enzyme activity and relative abundances of Gram‐negative and Gram‐positive bacteria. Higher‐N green leaves, on the other hand, had lower levels of A (a smaller slow fraction) than lower‐N litter. Contrasting effects of substrate and externally supplied N during later stages of decomposition likely occurred because higher‐N leaves also had considerably lower lignin, causing them to decompose more quickly throughout decomposition. In conclusion, elevated atmospheric N deposition in forest ecosystems may have contrasting effects on the dynamics of different soil carbon pools, decreasing mean residence times of active fractions in fresh litter (which would be further reduced if deposition increased litter N concentrations), while increasing those of more slowly decomposing fractions, including more processed litter.

OMDY: a new model of organic matter decomposition based on biomolecular content as assessed by 13C-CPMAS-NMR

“…Some experimental data indicate that high initial decomposition rates can result in slower organic matter decay at the later stages (Hobbie et al 2012). Such observations are perfectly reproduced by our model implementation, in which high initial decomposition rates lead to a change in molecular composition of the decomposing matrix that translates into a larger value of the intermolecular protection parameter, which, in turn, alters the decay process.…”

Management practices amplify the effects of N deposition on leaf litter decomposition of the Moso bamboo forest

“…Lignin is a key factor in the regulation of litter decomposition, especially during the later decay stage (Austin and Ballaré 2010). Excessive N inputs have been demonstrated to suppress ligninolytic enzyme activity in the long term and correspondingly reduce litter decomposition (Fog 1988;Carreiro et al 2000;Thirukkumaran and Parkinson 2000;Hobbie et al 2012), although a few studies have found no evidence of negative effects (Hobbie et al 2006). In the present study, lignin degradation was faster under the N30 treatment and slower under the N90 treatment than the control and slowed after 18 months of decomposition under all four treatments (Fig.…”

“…The effects of N addition on the activity of the soil microbial decomposers varied with the decay stage (Hobbie et al 2012;Tu et al 2012). In the early stage, N addition may increase the decomposers' activity (Molina et al 1983;Madritch and Hunter 2003) and promote N release.…”

Lignin biochemistry and soil N determine crop residue decomposition and soil priming

“…The relative importance of residue chemistry, especially lignin, in the total amount and rate of residue decomposition is a function of N, which can both positively and negatively impact residue decomposition (Craine et al 2007;Hobbie et al 2012). These effects also differ depending on N source-residue N versus fertilizer N additions.…”