United States Department of Transportation
i
The Effectiveness of Organic Matter Amendments in Promoting Hydric Soil Conditions and Overall Health of Mitigation Wetlands – Phase II
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2021-05-01
[PDF-4.87 MB]
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Edition:SPR-B Final Report May 2019 - May 2021
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Abstract:Organic amendments did not promote hydric soil conditions in wetlands as originally hypothesized. Instead, soil saturation strongly determines if soils are hydric. Hydric soils can be tested using several different methods, and redox electrodes appeared to give more favorable test results than dipyridyl strips in unsaturated conditions. IRIS can also be used to test hydric soils and gave favorable results, but those results cannot be compared directly to the redox electrodes and dipyridyl. The research evaluated a broad range of wetland performance metrics, including soil bulk density, organic carbon storage, potential for methane generation, and plant growth and diversity. Overall, organic amendments often do very little. Sometimes there are benefits, like increased plant biomass, but the results can also be unfavorable, like loss of diversity and more invasive species. Often the perceived benefit (increased biomass) was of invasive species. Most organic matter amendments increase greenhouse gas (methane) production, but this can be mitigated by composting the material first and using a lower loading rate. Composted materials that have low pH value and are low in available nutrients (phosphorous and nitrogen) performed better than fresh, high nutrient options. Of the amendments used in the study, composted wood mulch and composted biosolids appeared the most favorable. The act of disturbing soil, in order to mix in amendments, also had negative side effects, so consideration should be given to placing amendments on the soil surface. The findings from the field and lab studies were generally consistent with a comprehensive literature review on the subject.
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