Reduced decomposition of coarse-woody debris under simulated chronic N deposition is associated with lower fungal biomass
Anthropogenic nitrogen deposition can affect forest ecosystem processes that may alter ecosystem carbon cycling and storage. Previous work has shown slower rates of wood density loss over time in coarse-woody debris (CWD) under simulated elevated N deposition, suggesting reduced fungal decomposition activity. This study focused on the effects of the N deposition treatment on fungal biomass within CWD in northern hardwood forests. The objective was to quantify impacts chronic N additions on CWD chitin content, an index of living and dead fungal biomass, to verify reductions in fungal decay as the cause of previous findings. Yearly tree growth and mortality have been recorded at four sugar-maple dominated forest sites in Michigan since 1987. Simulated elevated N deposition (3g N m-2 y-1) began in 1994. In 2011, all trees dying since 1994 were located, their decomposing wood was categorized into one of five visual decay classes, and samples from each were collected for density analysis. Archived samples from 2011 were analyzed for chitin content in 2014. Chitin, a constituent of fungal cell walls, was extracted chemically through acid hydrolysis from each sample and then measured colorimetrically to quantify fungal biomass present from each sample as D-(+)-Glucosamine hydrochloride (chitin equivalents).
Across all decay classes, chitin concentration of CWD from the N deposition treatment was lower than that for the control (P < 0.01). There were 32, 27, and 28% decreases in chitin concentration (P < 0.01) for the middle decay classes (classes 2-4 in a five-class system). These results are in agreement with treatment impacts on CWD mass loss over time. In the control treatment, a clear decrease in density of CWD occurs with time since tree death. This is in contrast to minimal changes in wood density over time for CWD from the N deposition treatment. Similar results exist for changes over time in CWD C:N ratios. The reduced fungal activity likely has contributed to a trend for increasing CWD biomass more for the N deposition treatment relative to the control (16% non-significant increase to date). Together these data suggest that future elevated atmospheric N deposition may have a suppressive effect on the wood-decaying fungal populations of natural forested systems, leading to reduced rates of wood decay. This could lead to increases in ecosystem C storage, in addition to those already documented due to increased woody biomass production and reduced decomposition of forest floor and surface soil organic matter.