Interaction between compost amendments, soil chemistry, and microbial activity in an urban agroecosystem

Gaston Small, Biology, University of Saint Thomas, Minnesota, St. Paul, MN, Paliza Shrestha, Ecology, Evolution, and Behavior, University of Minnesota, MN, Carolyn Zeiner, School of Engineering and Applied Sciences, Harvard University, Cambridge, MA and Griffin Swenson, University of St Thomas, Saint Paul, MN

Background/Question/Methods
Urban farms and gardens typically apply compost at high rates to meet crop nutrient requirements, presenting opportunities to recycle urban nutrients but also for nutrient loss through leachate. The rate of nutrient release from compost, driven by microbial mineralization, determines nutrient availability for crops and also nutrient loss rates. Understanding these controls for different compost types and application rates could be important for optimizing nutrient use efficiency in urban agriculture. We analyzed data from four years of a compost addition experiment using replicated garden plots, in which municipal compost or manure compost was added to meet anticipated crop nitrogen (N) demand (higher input) or phosphorus (P) demand (lower input). Additional treatments received synthetic fertilizer only, or no soil amendments. Characteristics of garden soil (organic matter, pH, Bray phosphorus), leachate (dissolved inorganic nitrogen and phosphorus), microbial activity (CO2 flux, teabag mass loss) and abundance, and crop yield were characterized over the course of the study.

Results/Conclusions
After four growing seasons, soil organic matter had increased from an initial value of 9% to 13% in both of the high-input compost treatments, while declining to 8% in the no-input control and synthetic fertilizer treatments. Plots receiving higher-input manure compost had soil CO2 fluxes 70% higher than other treatments early in the growing season. Teabag decomposition rates were 60% greater, and bacterial abundance was three-fold greater, in the higher-input manure compost treatment compared to other treatments. Crop yields were similarly high in all treatments receiving soil amendments, with lower values only observed in the no-input control treatment. Soil Bray P was highest in plots receiving high inputs of municipal compost, 63% higher than in the no-input control treatment. Phosphate concentration of leachate was highest in both of the higher-input compost treatments, more than two-fold greater than the no-input control treatment. Our results indicate that the more labile manure compost is rapidly mineralized and a higher fraction of nutrients are lost through leachate, compared to the more stable municipal compost, with slower mineralization rates and more nutrients retained in garden soil. Compost quality and application rate should be important considerations in recycling organics waste through urban agriculture.