An evaluation of biochar as an effective soil amendment in coastal marsh restoration efforts

Brittany P. Wilburn

doi:10.17918/00001872

As climate change threatens to destroy the world's remaining coastal wetland habitat, restoration practitioners are exploring novel techniques and methods to improve current efforts to rebuild invaluable wetland habitat. Biochar may offer a sustainable method of not only enhancing wetland function but could also enhance long-term carbon sequestration in coastal restoration projects. Formed from anaerobic combustion (also known as pyrolysis) of organic material, biochar is a carbon-based soil amendment traditionally used in agricultural systems to improve soil fertility and vegetation biomass. However, coastal wetland ecosystems have significantly different characteristics, making it difficult to extrapolate and predict the effects of biochar incorporation. Additionally, wetland restoration is a costly process, in both time and money. Therefore, there is little room for error in incorporating new techniques and methodologies that may not be effective. This dissertation intends to elucidate the effects of biochar on various parameters related to wetland functions in the context of restoration. Through a greenhouse mesocosm study, Chapter 2 demonstrates that incorporation of softwood biochar (Pseudotsuga menziesii feedstock) or compost has no effect on salt marsh vegetation biomass; rather, it is sediment texture that has the greatest impact. Greenhouse gas emissions were found to be enhanced by the soil amendments over a single growing season, primarily by compost and not biochar, suggesting softwood biochar may have potential as a method of long-term carbon burial. I further found that while softwood biochar was ineffective in preventing acid sulfate production, the amendment did increase the total alkalinity of the sediments. Biochar amendments additionally impacted other sediment properties, such as salinity, eH, nutrients and, to some extent, pH. In Chapter 3, I determined that restoration sediments homogenized with hardwood biochar (Eucalyptus spp.) and aged for two years showed significantly lower denitrification potential than freshly amended restoration sediments and untreated restoration sediments. Two-year-aged amended restoration sediment denitrification rates were also statistically similar to natural marsh sediment rates. While sediment property changes were not found to be associated with differences in denitrification potential rates in amended sediments, I did find that microbial community make up was altered by amendments, suggesting that differences in nitrogen cycling were a result of microbial changes. Lastly, Chapter 4 describes the significant effects of depth, elevation, and temperature on the long-term stability of two hardwood biochars (Eucalyptus spp. and a mix of Quercus and Acer spp. feedstock) incubated in canvas bags in two coastal estuaries in Elkhorn Slough, CA and Tuckahoe Wildlife Management Area (WMA), NJ. While significant regional differences were not found, I did determine that biochar degradation was significantly affected by habitat elevation, with upland and low marsh elevations resulting in the highest degradation rates. Deeper depths of biochar incorporation resulted in significantly increased rates of biochar degradation compared to surficial incubations at both the CA and NJ wetland sites. Lastly, biochar degradation was significantly increased during the winter season at the NJ sites, likely as a result of physical fragmentation from free-thaw cycles. In this chapter, I further summarize suggested incorporation tactics for optimal sequestration capacity through long-term biochar stabilization. In Chapter 5, I provide an overview of the results of this dissertation, with potential future directions of this work.

An evaluation of biochar as an effective soil amendment in coastal marsh restoration efforts

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An evaluation of biochar as an effective soil amendment in coastal marsh restoration efforts

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