Reference: Yao, S. Q., Groffman, P. M., Alewell, C., & Ballantine, K. (2018). Soil amendments promote denitrification in restored wetlands. Restoration Ecology, 26(2), 294-302. https://doi.org/10.1111/rec.12573
The value of wetlands
Wetlands contribute many valuable ecosystem services across the world. For instance, wetlands serve as homes to many unique plants and animals. Wetlands also help prevent floods and improve water quality. Water quality improvements made by wetlands are especially important for aquatic systems, where nutrient pollution poses a severe threat to animal and plant life. In particular, excess levels of nitrate, a compound often found in pollutants, can cause eutrophication (a process in which excessive nutrients boost plant growth in aquatic systems, causing harmful algal blooms that can lead to fish/shellfish kills during decomposition). Wetland soils can remove more than half of the nitrate present in their soils through the process of denitrification, in which microorganisms convert nitrate into gaseous forms of nitrogen, which are then removed from the wetlands and released into the atmosphere.
Compensating for losses
Despite the valuable ecosystem services they provide, wetlands continue to face tremendous losses due to agricultural and urban development. Although efforts are being made to restore wetlands and recapture these ecosystem services, the complex physical, biological, and chemical properties of wetlands are very difficult to replicate. Researchers have found that restored wetland soils tend to accumulate organic matter very slowly. This slow accumulation can inhibit many ecosystem services provided by soil, such as denitrification, which relies on organic matter as a food/energy source for microorganisms driving this process.
In an attempt to compensate for insufficient organic matter, Si Qi Yao and several other researchers in New York have investigated whether adding organic matter amendments during wetland restoration projects could help promote denitrification in restored wetland soils. While they hypothesized that increases in organic matter would boost overall activity of the denitrifying community of microorganisms, they also decided to take a closer look at how different types of organic amendments would impact denitrification and denitrifier biomass. To do this, they utilized organic matter amendments that differed in their carbon and nitrogen contents. Although researchers did not expect the amended nor the unamended restored wetland soils to achieve denitrification rates comparable to natural wetland soils due to the relatively short length of their study (6 years), they hypothesized that organic amendments would at least result in some measure of boosted denitrification.
In this study, four New York wetlands of similar size, land use history, topography, and wetland classification were selected and restored in 2007. Researchers set up four treatment groups: control soils (soils with no amendments added), soils with straw amendments, soils with topsoil amendments, and soils with biochar amendments (charcoal that is produced from plant matter and stored in soils). Within each wetland, five replicate plots of each of the four treatment groups were established. Researchers also selected a neighboring natural wetland to be part of their study, so that they could compare denitrification in restored wetland soils versus natural wetland soils.
In 2013, soil samples were collected from all sites and analyzed for denitrification potential (used as a proxy for denitrification), carbon and nitrogen contents in microbial biomass, and soil properties known to be associated with denitrification, such as soil organic carbon content and pH (acidity). Measuring soil properties other than simply denitrification rates provided researchers with additional insight into the microbial processes taking place in the soil.
Researchers found that treatment type (amended vs. control) significantly influenced both denitrification and its products, as well as several soil properties. Specifically, soils with amendments had significantly higher denitrification potential than those without amendments (11 times higher in biochar plots, eight times higher in topsoil plots, and three times higher in straw plots). Soil amendments significantly altered the soil organic content, with soils amended with biochar having the highest organic content. While treatment type did not significantly influence microbial biomass carbon content, amendments did significantly increase biomass nitrogen content. Researchers found no significant relationship between treatment type and soil pH. Furthermore, researchers found that increases in soil organic content and microbial biomass nitrogen content resulted in increased denitrification potential. However, despite the positive impact that soil amendments had on wetland denitrification, denitrification potential in soils of the neighboring natural wetland was at least 50 times higher than that of the restored soils.
Results suggest that the use of organic soil amendments when restoring wetlands has the potential to boost denitrification activity in soils, thus enhancing restored wetland function. This is unsurprising, considering the important role that organic matter plays in the denitrification process. While differences in amendment carbon content and structure likely drove differences in their impact on denitrification potential, all amended soils reported overall higher denitrification potential than unamended control soils. Despite these positive gains, the significantly lower denitrification potential still reported by restored soils compared to natural soils highlights the importance of maintaining natural wetlands. While wetland restoration is a better solution than making no effort to compensate for wetland losses, it is important to point out that this option does not even come close to recapturing the full ecosystem function capability of natural wetlands. Future studies should expand the timeframe of studying restored wetlands, evaluate the economic impact of adding amendments to restored wetlands, and should also consider the implication that these organic amendments have on greenhouse gas emissions.