UC Merced

This dissertation investigates the impact of organic matter amendments (OMAs), with a focus on compost, on urban soil health and carbon storage. This research addresses key questions surrounding the impacts of OMAs not only in urban areas, but on soils generally, over month to decade timescales on soil biogeochemistry and management-relevant soil characteristics. Chapter two is a meta-analysis, focusing on how soil organic matter (SOM) and soil nutrients, such as nitrogen (N) and phosphorus (P), respond to different kinds of OMAs. We found that of the OMAs tracked – biochar, biosolids, and compost – biochar increased SOM% the most, by 6.5 percentage-points, while compost increased SOM% by 3.1. While biosolids saw the lowest SOM gains at 2.1, they had significantly greater gains in soil N and P. This chapter highlighted the urban OMA work done to this point, but stressed the importance of long-term and deep soil sampling, as most studies were 30cm or shallower and were monitored for less than three years, both of which informed our future work, particularly chapters three and four.Chapter three examines the short-term impacts of compost application on turfgrass soil biogeochemistry. A three-month greenhouse study was conducted to assess how recycled water and compost additions impact soil greenhouse gas emissions and plant growth. We found compost increased root biomass by over 50%, while recycled water increased aboveground biomass by up to 300%. These changes in biomass were partially driven by changes in available P and N, which significantly increased with compost and recycled water respectively.These results compliment chapter four, which delves into the multi-annual effects of compost application. We conducted a two-year field trial assessing soil greenhouse gas emissions and bulk soil properties over two annual compost applications. We found a no changes to soil C or N, potentially due to high decomposition rates of the applied compost, evident by an increase of 10% in cumulative CO2 emissions from our compost plots. Compost plots also saw reduced N2O emissions, important for whole system nutrient budgets. However, this study highlighted the need to monitor compost application on longer timescales, leading to our chapter four project.Chapter five dives into the long-term impacts of compost application in urban areas, specifically looking at changes in soil C to one meter and in different C fractions. We found bulk soil C to be increased by 50% with long-term compost additions, especially in the top 10cm and 75-100cm depth ranges. This increase was consistent in the mineral-associated fraction as well, which is thought to be slower to change and contain highly persistent soil C. In addition, the molecular makeup of this mineral fraction became more similar to compost, indicating the direct incorporation of compost C into even deep mineral C pools.In conclusion, we found significant gains in whole profile bulk soil C and soil C persistence with long-term compost application, and that short-term compost application can reduce N2O emissions while improving turfgrass growth.