COS 252-5 - Recycled organic fertilizers determine soil microbial community size and biochemical function in an urban agriculture garden

Abstract: Understanding soil microbial ecology is recognized as an important component of sustainable agriculture. While traditional mineral fertilizers can result in eutrophication and decreased soil microbial diversity, use of more recalcitrant (less bioavailable) organic fertilizers that require decomposition can stimulate biomineralization of nutrients by native microbial communities. Recycled organic material from urban waste streams is a promising source of organic fertilizer, yet the effects of these materials on microbial ecosystem function and crop yield remain poorly understood, particularly in urban settings. Here we quantified culturable microbial abundance, soil respiration, extracellular enzyme activity, and crop yield in an urban agriculture system in Minnesota, USA across two growing seasons. Garden plots received high or low application rates of two organic composts (manure and plant-based municipal) targeted to crop N and P demands, respectively. Controls included plots receiving inorganic fertilizer or no fertilizer. We found that differences in soil moisture across treatments was the primary driver of microbial community size and overall activity, with plots receiving a high application rate of manure exhibiting basal respiration rates up to 11-fold higher than the mean of all other treatments. We also show that the nutrient content (C, N, and P composition) of organic fertilizers dictated specific mining activity of key resources by microorganisms, and that plots receiving no compost were C limited. Finally, we found that microbial biochemical profiles were stronger predictors of crop yield than microbial community size, explaining 60% of the variability in the average yield of 4 crops. Our findings demonstrate that targeted application rates of organic composts, even those with nutrients in more recalcitrant forms, can promote natural microbial decomposition processes that maintain crop yield relative to traditional fertilizers, while simultaneously reducing detrimental environmental impacts and improving soil health.