Consequences of organic matter amendments for methane emissions and soil and vegetation development in a restored wetland

Organic matter is sometimes added to soil in wetland mitigation projects, putatively to improve restoration outcomes. At a freshwater mitigation wetland, built in a former agricultural field to compensate for development-related wetland losses elsewhere, we conducted a manipulative field experiment using organic matter amendments to identify the effects different types and loading rates had on the development of soil (organic matter, bulk density, and hydric soil indicators), vegetation (root and shoot biomass, floristic quality), and methane (CH₄) emissions. The amendments included cow manure, composted wood chips, and hay at various loading rates, and municipal wastewater Class A biosolids. We found that there were trade-offs in desired restoration outcomes. Experimental loading rates of hay (226 m³ ha⁻¹) and manure (339 and 678 m³ ha⁻¹) produced more CH₄ (78–92 g m⁻² year⁻¹) than unamended plots (28 g m⁻² year⁻¹). These same amendments had little effect on hydric soil indicators (e.g., redox potential and reduced iron). Manure almost doubled vegetation biomass (937 g m⁻² versus 534 g m⁻²) compared to the unamended control, largely due to the growth of Typha sp. (cattail), an undesired plant at this site that resulted in lower floristic quality. Compared to unamended soils, only wood chips appeared to increase soil organic matter after one growing season. All amendments tended to reduce soil bulk density and penetration resistance, but these were not correlated with root growth. Unexpectedly, hydrology varied considerably due to patchy soil characteristics, despite little variation in elevation – this strongly influenced on our results. We qualitatively observed that constantly inundated plots had lower CH₄ emissions than areas with wet-dry cycles and that cattail proliferated mostly in wetter areas. Contrary to the prescription of organic matter amendments as a method for accelerating soil and vegetation development in wetland restoration projects, our findings demonstrate that amendments may not be necessary to support vegetation and hydric soil development and might unnecessarily exacerbate atmospheric warming and contribute to invasive species spread.