Decomposition of trembling aspen leaf litter under long-term nitrogen and sulfur deposition: Effects of litter chemistry and forest floor microbial properties

Litter decomposition rates are affected by abiotic and biotic factors such as climate, soil physico-chemical properties, litter chemistry, nitrogen (N) availability, and activities of soil organisms. Elevated N and sulfur (S) deposition originated from oil sands mining and upgrading activities can change soil microbial properties, litter chemistry, and litter decomposition rates in the surrounding forest ecosystems in northern Alberta. We studied (1) the effect of long-term N and S deposition on litter chemistry and soil microbial properties, and (2) the effect of changed litter chemistry and soil microbial properties on litter decomposition (CO2 emission) in a 100-day laboratory incubation experiment using trembling aspen (Populus tremuloides) leaf litter and forest floor collected from a mixedwood boreal forest that has been subject to simulated N and S deposition for 10 years. Litter chemistry (lignin, total carbon (C) and N, and calcium (Ca), aluminum (Al), manganese (Mn), and magnesium (Mg) concentration) and forest floor microbial properties (microbial biomass C and N, and extracellular enzyme activities) were analyzed. Ten years of N and S addition increased N (P < .05 unless otherwise stated) and decreased lignin concentrations resulting in lower C/N and lignin/N ratios in the litter. In addition, N and S addition increased forest floor microbial biomass (P < .01) and enzyme activities. Cumulative CO2 emission (Ccum) from litter was greater from the N and/or S addition treatments than that from the control, probably due to decreased C/N and lignin/N ratios in litter from the N and S addition treatments; meanwhile, Ccum from litter was not affected by soil microbial activity. The results indicate that N and S deposition enhances decomposition of aspen leaf litter by decreasing C/N and lignin/N ratios, suggesting that long-term exposure to high levels of N and S deposition can significantly change C (and associated nutrients) cycling in forest ecosystems in the oil sands region.