Soil microbial activity profiles associated with organic compost fertilizers in an urban garden

Biological soil health is recognized as an important component of sustainable agriculture due to microbial biomineralization of nutrients. However, soil health can be difficult to assess consistently across urban agricultural systems due to diverse land use histories, soil heterogeneity, and lack of mechanistic links to agricultural management practices (e.g., recycled compost addition) and crop outcomes. In this study, we characterized soil microbial activity profiles in an urban agriculture system in Minnesota, USA, including microbial abundance, soil respiration, extracellular enzyme activity, and crop yield. Garden plots were fertilized with recycled organic compost (either manure or municipal) at high or low rates (ranging from 2.6 to 39 tons ha −1 ) targeted to crop N and P demands. Control plots received inorganic fertilizer or no fertilizer. We found that a high application rate of manure compost supported 6–10x higher basal respiration than municipal compost or inorganic fertilizer. Enzyme activity data demonstrated that soil microbial communities exhibited unique profiles of biochemical function that varied among fertilizers of different compositions. Microbial biochemical function predicted 50% of the variability in bell pepper ( Capsicum annuum ) yield, while soil microbial community size alone was a poor predictor of yield. Yield was highest in plots fertilized with municipal compost, outperforming inorganic fertilizer by threefold. High‐yield plots exhibited higher ratios of N to P enzyme activity compared to those with lower yield. Our findings demonstrate that “more is better” may not necessarily be true regarding soil microorganisms in biological soil health, and that measures of soil microbial biochemical function may be more important. High‐input manure compost supported higher soil microbial activity than municipal compost or inorganic fertilizer. Fertilizer composition supported unique soil microbial biochemical functions, even in communities of similar size. Pepper yield was highest in plots fertilized with municipal compost and lowest under inorganic or no fertilizer. Soil microbial enzyme activity was more important in predicting crop yield than soil microbial community size. Soil microbial activity predicted 50% of variability in crop yield; N:P enzyme activity ratio was best predictor.