Despite recent decline, high nitrogen (N) inputs caused by anthropogenic activities (e.g. fossil fuel burning, fertilization) have been driving changes in ecosystems for decades, including soil acidification, loss or alteration of biodiversity, and reduction in microbial community activity. The effects of higher N deposition on forest soils and litter have been studied thoroughly, but we identified a research gap regarding the effect of higher N availability on coarse woody debris (CWD, d > 10 cm). Deadwood holds 8% of global carbon stocks, thus understanding the mechanisms of its decomposition is critical for modeling carbon cycling in forests. We hypothesized that chronically elevated N levels would result in higher mass loss from CWD due to increased enzymatic activity of wood-decomposing fungi, taking into account the much higher N limitation of the substrate compared to leaf litter.
We analyzed data from a nine year deadwood experiment in which half of all logs were exposed to artificially elevated N levels. The experiment comprised a total of 13 temperate tree species (nine angiosperms, four gymnosperms). Sampling included measurements of mass loss, respiration, lignocellulolytic enzyme activities and wood physicochemical parameters. We used non-parametric tests and GLMMs to test for and model treatment effects.
Results/Conclusions
The effects of enhanced nitrogen availability varied by tree species. Mass loss was significantly higher in gymnosperm logs (p ≤0.05, ~ 70%); similar applies to the respiration rate that was slightly higher in these logs as well (p ≤0.1, ~ 35%). Of the five enzymes measured, only laccase showed higher activity exclusively in gymnosperm logs (p ≤ 0.05). Responses to N addition were not consistent in angiosperm logs, but we observed higher percentages of carbon in treated logs (p ≤ 0.05).
Our results indicate that effects of increased N availability differ between logs of different phylogenetic backgrounds and their respective wood physicochemical properties such as C:N ratios. They illustrate that responses of CWD to higher N deposition cannot be extrapolated from litter-decomposition studies.
Given the strong responses in gymnosperm deadwood, we conclude that increased N deposition will have a lasting effect on CWD decomposition in forest ecosystems with a high proportion of conifer species.
