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.

Most crops grown in tropical savanna ecosystems have a 30%–60% yield gap, primarily due to poor soil fertility, low nutrient bioavailability, and limited effectiveness of conventional fertilization practices. This study used an integrated soil management approach involving Bradyrhizobium japonicum inoculant and rice husk biochar combined with compost or phosphorus fertilizer for soil fertility improvement, soybean ( Glycine max L.) growth, and grain production. In the Guinea savannah zones of Ghana, we conducted a two‐factorial pot experiment under greenhouse conditions. The treatments were as follows: (i) 1% w/w (20 t ha ⁻¹ ) rice husk biochar (B); (ii) 0.5% w/w (10 t ha ⁻¹ ) compost (CM); (iii) co‐application of 0.5% w/w (10 t ha ⁻¹ ) biochar with compost (B + CM); (iv) rock phosphate (B + RP); and (v) triple superphosphate (B + TSP) at 0.5% w/w (10 t ha ⁻¹ ), 0.003% w/w (0.06 t ha ⁻¹ ), and 0.003% w/w (0.06 t ha ⁻¹ ), respectively. Non‐inoculated and B. japonicum inoculated soybeans were grown for 91 days. Soybean yield indicators and soil chemical fertility were measured. A significant increase in pH, cation exchange capacity, and P availability was observed in combined biochar treatment with compost or P sources, resulting in increased grain production. Compared to the non‐inoculated plant, B. japonicum increased nodule numbers of plants grown in soils treated with B (+26), B + CM (+53), B + RP (+39), and B + TSP (+40). The combined biochar and compost amendment was superior, producing 93 nodules per plant compared with 51 in the control. Combined biochar and compost treatments most effectively minimized soil‐limiting factors for soybean nodulation and grain production in weathered tropical acidic soils. Core Ideas: Biochar and compost modified edaphic properties and increased chemical fertility of the weathered acidic soil. Biochar‐compost amendment improved Bradyrhizobium japonica ’s symbiosis with soybean roots and increased nodules. Combining biochar with either compost or P source increased soybean production in the acidic soil. Rock phosphate and triple superphosphate had similar effects on soybean yield in the biochar‐amended soil.

Previous studies have found that C turnover is bound to hotspots of microbial activity. The objective of this study was to analyze the effects of pure energy substrate (glucose), nutrient (mineral N or P) and combined substrate and nutrient (glucose + N, glucose + P, sterile DOC, artificial root exudate extract) additions to enzyme activity inside and outside hotspots as a proxy for microbial C turnover in a subsoil. By means of different substrate and nutrient additions, we tested how the limitations of our site were distributed on a small scale and depth-dependently to contribute to an increase in knowledge of subsoil mechanistics. The study site is a sandy Dystric Cambisol under an over 100-year-old beech forest stand in Lower Saxony, Germany. Forty-eight undisturbed soil samples from two depth increments (15–27 cm and 80–92 cm) of three profiles were sprayed homogeneously with easily available C, N and P sources to investigate the impacts of substrates and nutrients on three enzyme activities (acid phosphatase, β-glucosidase and N-acetylglucosaminidase) by using the soil zymography approach. Comparisons of upper and lower subsoils showed significantly fewer and smaller hotspots in the lower subsoil but with a high degree of spatial variation in comparison to the upper subsoil. Different patterns of enzyme distribution between upper and lower subsoil suggest microbial communities with a lower diversity are found in deeper soil regions of the site. Both substrate and nutrient additions stimulated enzyme activities significantly more outside the initial hotspots than within. Because of this, we conclude that microorganisms in the initial hotspots are less limited than in the surrounding bulk soil. Changes in enzyme activities owing to both substrate and nutrient addition were stronger in the lower subsoil than in the upper subsoil, showing differences in limitations and possible changes in microbial community structure with increasing depth. The results of our study emphasize the need to consider spatial factors in microbial turnover processes, especially in lower subsoil regions where stronger substrate and nutrient limitations occur.