Abstract
Urban agriculture soils are commonly amended with compost to improve fertility, yet the functional responses of these systems to biochar additions across application rates remain poorly understood. We conducted a field experiment at Three Sisters Gardens (West Sacramento, CA, USA) to evaluate how biochar applied at 0, 10, 20, and 30 t ha-1 influenced soil physical, chemical, and biological indicators in compost-amended plots under corn and squash production. Late-season soils were analyzed for various physical and biochemical properties. Results showed that biochar effects varied across application rates and functional domains. Intermediate application rates (10-20 t ha-1) improved aggregate stability and increased soil respiration, indicating moderate effects on soil physical structure and microbial activity. In contrast, the highest biochar rate (30 t ha-1) produced the strongest biological responses, including increases in microbial biomass, fungal:bacterial ratios, and extracellular enzyme activities involved in nitrogen and phosphorus cycling. Despite these biological responses, soil pH and cation exchange capacity did not differ significantly among treatments. Regression analyses indicated that nutrient availability was more strongly associated with microbial indicators than with bulk soil chemical properties, suggesting that biochar-associated changes in nutrient dynamics were consistent with microbial mediation of nutrient cycling. These patterns were broadly consistent across cropping systems despite crop-specific differences in nutrient pools and microbial biomass. Together, these results indicate that biochar application produced distinct functional responses along a dose gradient, with intermediate rates influencing soil physical properties and higher rates being associated with stronger microbial and biochemical responses related to nutrient cycling. Optimizing biochar application rates may therefore be important for enhancing soil biological activity and nutrient availability in compost-amended urban agricultural systems.