Effects of wetland plant communities on denitrification rates: A meta-analysis

Background/Question/Methods

Natural communities are changing rapidly in composition and structure due to land-use alterations, climate change, sea-level rise, and species invasions. These changes are known to have profound impacts on ecosystem processes in wetlands. However, the magnitude and patterns of these impacts have thus far remained difficult to predict. In particular, changes to wetland plant communities may alter rates of denitrification, a process that mediates anthropogenic nitrogen loading by permanently removing nitrate to the atmosphere as inert dinitrogen gas. Developing a predictive understanding of denitrification is critical to the future management of aquatic ecosystems and for better understanding the long-term controls on coastal and global productivity. As a first step in exploring potential relationships between plant communities and denitrification rates, we conducted a systematic literature review and meta-analysis on all studies that have measured denitrification rates for clearly-defined wetland plant communities. The objective of this analysis was to determine whether vegetation has any general effect on denitrification and whether this effect varies among different plant communities. We also examined whether the type of wetland or the method used to measure denitrification influenced the magnitude of denitrification or the effect of vegetation detected among studies.

Results/Conclusions

The plant community, defined by the dominant species, explained a significant amount of the variation in denitrification rates, while methods used to measure denitrification and the types of wetland system had less influence. Because sediment characteristics may also affect denitrification, we controlled for such contextual factors in a second analysis, in which we analyzed the response of denitrification in vegetated relative to non-vegetated sediments. On average, denitrification was found to be 55% higher in vegetated sediments than in non-vegetated sediments. Variation in the effect of plants on denitrification was also best explained by plant community, with wetland type and method having no significant influence. Thus, developing an understanding of plant-mediated effects on denitrification should assist in predicting current and future patterns of nitrogen removal via denitrification. Because plants differ in traits that may influence denitrification, differences in community trait structure may help explain variation in denitrification rates. Few traits have been consistently reported for wetland plants. Still, we found significant effects of some plants traits on denitrification. A focus on key plant traits offers a general and, therefore, flexible way to link plant community composition and structure to important ecosystem process such as denitrification.