Field Experiment Reveals Unintended Consequences of Organic Matter Amendments in Restoring Wetlands

Abstract Wetlands are identified as key ecosystems in the UN Decade on Restoration initiative because of their water management and global warming benefits. Wetland restoration sometimes includes adding organic matter (+ OM) to accelerate hydric soil development and provide nutrients for new vegetation. We conducted a manipulative field experiment to identify the most effective type of + OM and loading rate for soil and vegetation development in a freshwater wetland restoration site that was a former agricultural field. The + OM included municipal waste biosolids, composted wood chips, and cow manure at various application rates. We found that + OM had little influence on soil bulk density or hydric soil indicators, and resulted in the growth of undesirable vegetation and increased greenhouse gas flux. For example, manure, which contains nitrogen (N) and phosphorus (P), increased aboveground biomass but favored non-native species that lowered plant diversity and floristic quality. All + OM increased methane (CH4), but the range varied greatly (1.04–7.3 times). Hydrology also had a strong influence on CH4 emissions, and areas that experienced wet-dry cycles had 1.7–3.0 times more estimated annual CH4 emissions that those that were constantly inundated. Organic soil amendments were not necessary to support wetland plant growth or develop hydric soils, and may unnecessarily exacerbate atmospheric warming and invasive species spread.