Grass litter responses to warming and N addition: temporal variation in the contributions of litter quality and environmental effects to decomposition

We explored how climate warming and increased atmospheric nitrogen (N) deposition may influence grass litter decomposition over time, how litter quality versus environmental effects contribute to these responses, and the importance of these responses over winter. We used litter bags to examine decomposition over 2 years in a warming and N addition field experiment, and examined the contributions of litter quality and environment to these responses by transferring litter reciprocally between the treatment plots and a common garden. Warming increased mass loss over the first year for Bromus inermis litter, which was consistent with the litter quality response, but by the second year there was no overall warming effect, and this change coincided with a negative environmental effect of warming. N addition increased mass loss and was more influential than warming in the early stages of Poa pratensis litter decomposition; the N effect appeared to be driven primarily by litter quality. Winter decomposition was not a substantial component of the treatment responses. Our results indicate that litter quality and environmental effects play different roles at different time scales in the decomposition responses of grass litter to warming and N addition, and these responses can be species specific.