Short-term vs. long-term effects of warming and nitrogen addition on soil extracellular enzyme activity and litter decomposition in a grass-dominated system

Purpose In long-term global change experiments, while cumulative treatment effects on soil and plant responses can emerge over time, comparisons between short and long-term responses can potentially be confounded with interannual variability in the environment. We added new nitrogen addition and warming plots to a pre-existing nitrogen and warming field experiment in a grass-dominated field to compare the short-term (1–2 year; new plots) versus long-term (14–15 year; old plots) treatment effects on soil microbial activity and plant litter decomposition, while controlling for the effects of interannual environmental variability. Methods We assessed microbial activity by assaying the potential activities of five soil extracellular enzymes: three hydrolase enzymes (N-acetyl-glucosaminidase, phosphatase and β-glucosidase) and two oxidase enzymes (phenol oxidase and peroxidase). We measured mass loss from litter bags to assess the decomposition of grass and forb litter. Results Warming interacted with plot age, with increased hydrolase activity in spring in response to warming only occurring in the long-term plots; increases in peroxidase activity with nitrogen addition in spring occurred for all plots. By summer, there were no significant interactions between the treatments and plot age for enzyme activity. Decreased decomposition with warming, observed for forb litter, only occurred in the long-term plots, but increased decomposition with nitrogen addition, observed for grass litter, occurred in both the long-term and short-term plots. Conclusions Our results confirm an intensification of treatment effects on enzyme activity and litter decomposition over time for warming, but no interactions with plot age for nitrogen addition.