Biochar application rate regulates soil microbial processes and nutrient dynamics in compost-amended urban agricultural soils

Urban Farm DB summary

Urban agriculture soils are commonly amended with compost, yet how such systems respond functionally to biochar across application rates is poorly understood. In a field experiment at Three Sisters Gardens (West Sacramento, CA, USA), biochar was applied at 0, 10, 20 and 30 t/ha to compost-amended plots under corn and squash, and late-season soils were analysed for physical, chemical and biological indicators. Intermediate rates (10–20 t/ha) improved aggregate stability and increased soil respiration; the highest rate (30 t/ha) produced the strongest biological responses, including higher microbial biomass, fungal:bacterial ratios and extracellular enzyme activities in nitrogen and phosphorus cycling. Soil pH and cation exchange capacity did not differ significantly among treatments. Regression analyses indicated nutrient availability was more strongly associated with microbial indicators than with bulk soil chemistry, consistent with microbial mediation of biochar-associated nutrient dynamics, and the patterns held broadly across both cropping systems.