Irrigated urban agriculture (UA) supports the economy and health of urban inhabitants in low-income countries. This system is often characterized by high nutrient inputs and mostly utilizes wastewater for irrigation. Biochar has been proposed to increase crop yields and improve soil properties. In this study, we assessed the transient effect of rice husk biochar (20 t ha⁻¹) and/or fertilizer (NPK: 15-15-15) on microbial respiration, microbial biomass carbon and enzyme activities of irrigated (wastewater and tap water) soil from an UA field experiment in the Guinea savannah zones of Ghana. Our results showed an increase by up to 123% in soil organic carbon (SOC) after a year of biochar application, while hot water extractable carbon (HWEC) was increased by only 11 to 26% and microbial biomass carbon (MBC) by 34%. Basal respiration was significantly increased in mineral fertilized soil by up to 46% but decreased by 12–45% under wastewater irrigation. Overall, the metabolic quotient (qCO₂) indicated less stress for the microbial community and increased carbon use efficiency with biochar application and wastewater irrigation. Total enzymes activity was increased under wastewater irrigation and biochar treated soils exhibit a more diverse composition of C-cycling enzymes and a higher activity of aminopeptidases. Biochar and wastewater showed positive effects on biological soil properties and contributed to soil fertility. Our results suggest beneficial effects of biochar on non-biochar SOC stocks in the long term.

This work evaluated the agronomic and soil fertility effects of using municipal wastewater or anaerobically treated wastewater for irrigation and applying biochar to a soil from the Guinea savanna agroecological zone of Ghana. For this purpose, untreated municipal wastewater (WW), the effluent of an anaerobic wastewater filtration system (TWW), and clean water (CW) were used as irrigation water in a pot trial. Additionally, rice-husk biochar in the form of raw biochar (RB), water-washed biochar (WB), and biochar used as wastewater filter material (FB) were added to the soil, testing the influence on soil fertility and crop yield. Lettuce and carrot were selected for the pot study, grown on soil mixed with the biochar types at 20 t ha⁻¹ and irrigated with either WW, TWW, or CW. Our results indicated higher crop growth morphology and yields (up to 90% increase) by WW and TWW than CW. The average yield of carrot (34.1 g pot⁻¹) and lettuce (29.3 g pot⁻¹) with TWW irrigation were the highest, followed by 31.2 and 27 g pot⁻¹ with WW, then the lowest yields of 21.7 and 19.5 g pot⁻¹ of carrot and lettuce irrigated with CW respectively. Compared to WW, TWW was more beneficial to plant development, causing an up to 10% increase in crop yields. Soils with FB and WB produced similar agronomic effects and plant nutrient concentrations but were lower than pots amended with RB. Nevertheless, combining RB with TWW showed increasing effects on pH, CEC, and P availability in the highly weathered acidic soil. The results suggest a beneficial effect of biochar-filtered wastewater on soil fertility and crop growth, offering the potential to enhance resource use efficiency in irrigated urban agroecosystems.

Tropical acidic soils exhibit inherently low fertility and reduced microbial activity, driven by low pH and accelerated organic matter mineralization, phosphorus (P) fixation, and aluminum (Al³⁺) and iron (Fe³⁺) toxicity. These constraints limit agricultural productivity, necessitating sustainable and low-cost soil amendments essential for improving the soil fertility in such regions. This study investigated the effects of biochar, kitchen stove ash (KSA), and their combined application on the soil chemical properties, nutrient dynamics, and microbial functions in a tropical acidic soil. The treatment included the unamended control and two doses of 0.25% w/w (B₁₀) and 0.5% w/w (B₂₀) corncob biochar, 0.03% w/w kitchen stove ash (A_sh), and 0.027% w/w commercial-grade calcium carbonate (L_ime). Each biochar dose was added alone or in combination with either ash (A_sh + B₁₀ and A_sh + B₂₀) or calcium carbonate (L_ime + B₁₀ and L_ime + B₂₀). After eight weeks of laboratory incubation at 20 °C, the soil pH, N and P bioavailability, microbial biomass, and extracellular enzyme activities were measured. The combined application of 0.5% w/w biochar with 0.03% w/w KSA (A_sh + B₂₀) resulted in the most significant improvements in all of the examined soil fertility indicators than the individual amendments. Specifically, the soil pH was increased by 40% (+1.9 pH units) compared with the unamended control. Available phosphorus, mineral nitrogen, and total potassium were increased by 49%, 22%, and 36%, respectively, compared with the unamended control. Regarding the microbial parameters, the A_sh + B₂₀-treated soil showed the highest microbial respiration (+56%), microbial biomass (+45%), and extracellular C- and N-cycling enzyme activities compared with the unamended soil. The ash supplied minerals (P, K, and Mg) provided a more beneficial effect on the soil’s nutrient content and microbial functions than the calcium carbonate. The study demonstrated that underutilized kitchen ash may supplement biochar’s liming and nutrient supply potentials, even at a lower application rate, to improve the fertility of weathered acidic soil.