Quantitative review of plant biomass and tissue nutrient concentration shows that response to nitrogen loading varies by wetland type

Background/Question/Methods

Wetlands are globally important as reservoirs of fixed carbon, sites of nutrient transformation, and habitat for a broad diversity of species.  Nitrogen (N) loads to North American wetlands have increased dramatically due to fossil fuel and fertilizer use, and considerable research effort has been directed over the last four decades toward understanding the extent and consequences of N addition.  We conducted a quantitative review of the literature on N addition in North American wetlands to assess the relationship between increasing N and changes in plant biomass and tissue nutrient content in inland and coastal ecosystems.  Of the 98 studies collected by Web of Science keyword and relational searches that quantify a biological response to nitrogen addition, 35 studies (n=347 total observations) contain information on plant biomass or tissue nutrient concentration.  We calculated weighted effect sizes (Hedge’s d) for all observations. 

Results/Conclusions

Initial analyses showed that in inland wetlands, the most significant effects (i.e. magnitude of change between control and N addition, p<0.001) and the largest increases in above-ground biomass (p=0.027), occurred in the middle range of N addition (total range: 4.6-480 kg N added hectare^-1 year^-1).   In contrast, plant tissue nutrient (primarily N and phosphorus, n=40) concentrations reached effect sizes indicative of nutrient levels lower than unfertilized controls at mid-range N loading (p=0.039) in inland wetlands.  In coastal wetlands, there was no significant relationship between N loads (total range: 112-4201 kg N hectare^-1 year^-1) and aboveground biomass, although there was a weak trend toward smaller increases in biomass (p=0.061) and smaller effect sizes (p=0.071) at mid-range N loads than at high or low N loads.  In coastal wetlands, plant tissue nutrient concentrations increased linearly with higher N loads (p=0.048), as did magnitude of effect size (p=0.032).  Our results indicate that inland versus coastal responses of wetland productivity and plant quality vary in their relationship to N load, with inland systems responding asymptotically and coastal systems responding linearly or not at all to increasing N.