Anthropogenic activities have increased the amount of nitrogen (N) entering terrestrial ecosystems, potentially altering important soil processes, such as the decomposition of leaf litter. Previous research indicates that increased N availability expedites the breakdown of labile litter constituents in early stages of litter decay and decreases lignin decay in later stages. Given the contrasting effects of increased N availability, it is unclear how N deposition will affect the trajectory of litter decomposition and ultimately the amount of carbon (C) sequestered.

To determine how increased N availability affects the decomposition of plant litter during different stages of litter decay, a laboratory incubation experiment was conducted with Acer saccharum litter and sandy, low C soil from the Oak Openings Region of Toledo, Ohio. Ammonium nitrate was added in the beginning of the experiment equivalent to the rate of 9 kg N/ha/yr (annual ambient deposition rate), including a soil only control, soil and litter control, and a soil, litter, and nitrogen treatment group. Replicates were harvested at several time points and analyzed for microbial respiration, biomass C and N, extracellular enzyme activity (ß-glucosidase, N-acetyl glucosaminidase, phenol oxidase, and peroxidase), and  extractable inorganic N.

Results/Conclusions

N addition increased ß-glucosidase and N-acetyl glucosaminidase activities in the +N treatment as compared to the soil and litter control. Phenol oxidase and peroxidase activity in the +N treatment varied with sporadic increases and decreases in enzyme activity. Microbial biomass C and N peaked early on in the experiment, and was lower in the nitrogen treatment as compared to the soil and litter control. Microbial respiration peaked by day three in the treatment and soil and litter control, with a higher peak in the nitrogen treatment. Thus, N addition had significant effects on microbial biomass, microbial respiration, and extracellular enzyme activities.