Short- versus long-term effects of nitrogen addition and warming on aboveground and belowground nitrogen retention

Background and aims

Increased atmospheric nitrogen (N) deposition and climate warming can both exert strong, cumulative effects on N losses from ecosystems. However, given their potential interactions with interannual variability in environmental conditions or other factors, it is uncertain how effectively the comparison of short- vs. long-term results from N addition and warming field experiments can reveal these cumulative effects. Our aim was to compare the short-term versus long-term effects of N addition and warming on aboveground and belowground N retention while controlling for interannual variation in other factors.

Methods

We added ¹⁵N tracer to N addition and warming plots in a northern temperate grass-dominated old field in early spring, then assessed the amount of the added ¹⁵N recovered aboveground and belowground at the peak of the growing season. We controlled for interannual variation in background environmental conditions by comparing new N addition and warming plots (3 years old) to those of an existing field experiment (16 years old).

Results

Both N addition and warming significantly increased aboveground and belowground ¹⁵N tracer retention. However, there were no significant interactions between either of the treatments and plot age for ¹⁵N tracer retention, nor for plant production. The lack of interactions with plot age corresponded with the continued dominance of two non-native grass species in the plots.

Conclusion

Our results highlight the potentially important role of dominant species that resist replacement in buffering against the effects of global change on cumulative, long-term changes in ecosystem N retention.