Soil amended with biochar is considered a significant response to climate change, remediation of degraded soils, and agronomic improvements. An artisanal mobile pyrolysis kiln was developed for small-sized biomass inputs. Approximately 190 kg of biochar was produced in 21 carbonisation processes using acai residues (Euterpe oleracea Mart.) as raw material, as they are among the most abundant agro-industrial residues in the Amazon. It is a valuable and underutilised biomass resource, often inadequately discarded, causing environmental impact and health risks. The physicochemical and structural characteristics of four representative biochar samples from the pyrolysis processes were evaluated using different techniques. The produced biochar had an average pH of 8.8 and the ICP-OES results indicate that the most abundant elements were potassium (K) and phosphorus (P). Results of the elemental composition indicate that the produced biochar has a very stable carbon with an average H/C ratio of 0.23 and O/C ratio of 0.16, indicating that the pyrolysis performed was effective in transforming organic and volatile compounds into stable structures. Variations in nutrient contents call for soil application planning, as performed for other agricultural inputs. The developed mobile kiln can be adapted and favour the decentralisation of biochar production among small and medium-sized producers. Here, we show that even with variations in artisanal production, the biochar produced exhibits favourable characteristics for agronomic use and combating climate changes.

This study investigates the production and economic feasibility of biochar for smallholder and family farms in Central Amazonia, with potential implications for other tropical regions. The costs of construction of a prototype mobile kiln and biochar production were evaluated, using small-sized biomass from acai (Euterpe oleracea Mart.) agro-industrial residues as feedstock. The biochar produced was characterised in terms of its liming capacity (calcium carbonate equivalence, CaCO_3eq), nutrient content via organic fertilisation methods, and ash analysis by ICP-OES. Field trials with cowpea assessed economic outcomes, as well scenarios of fractional biochar application and cost comparison between biochar production in the prototype kiln and a traditional earth-brick kiln. The prototype kiln showed production costs of USD 0.87–2.06 kg⁻¹, whereas traditional kiln significantly reduced costs (USD 0.03–0.08 kg⁻¹). Biochar application alone increased cowpea revenue by 34%, while combining biochar and lime raised cowpea revenues by up to 84.6%. Owing to high input costs and the low value of the crop, the control treatment generated greater net revenue compared to treatments using lime alone. Moreover, biochar produced in traditional kilns provided a 94% increase in net revenue compared to liming. The estimated externalities indicated that carbon credits represented the most significant potential source of income (USD 2217 ha⁻¹). Finally, fractional biochar application in ten years can retain over 97% of soil carbon content, demonstrating potential for sustainable agriculture and carbon sequestration and a potential further motivation for farmers if integrated into carbon markets. Public policies and technological adaptations are essential for facilitating biochar adoption by small-scale tropical farmers.

Biochar is a multifunctional tool that enhances soil quality, with particularly positive effects on acidic soils with low nutrient content, common in tropical regions worldwide, such as in the Amazon region in Brazil. This study investigates the effects of açaí fruit waste biochar (Euterpe oleracea Mart.) amendment and phosphate fertilisation on the chemical characteristics of a Ferralsol and on the biological components of cowpea (Vigna unguiculata (L.) Walp). In a greenhouse setting, a randomised block design was employed, testing five doses of biochar (0, 7.5, 15, 30, and 60 t ha⁻¹) combined with four doses of phosphorus (P) (0, 40, 80, and 120 kg ha⁻¹), resulting in 20 treatments with three replicates and 60 experimental units. Cowpea responded to inorganic fertilisation, with lower doses of P limiting the biological components (height, leaves, leaf area, dry biomass, and dry root mass). Higher doses of biochar and P increased the soil’s available P content by up to 2.3 times, reflected in the P content of cowpea dry biomass. However, this increase in biochar and P levels led to a maximum increase of 7.7% in agronomic phosphorus efficiency (APE) in cowpea in the short term. The higher doses of biochar promoted increases in pH value, cation exchange capacity (CEC), and the contents of potassium (K), calcium (Ca), and total nitrogen (N). In contrast, a decrease in magnesium (Mg) and aluminium (Al) levels was observed, while the concentration of easily extractable glomalin (EE-GRSP) was not significantly affected during the evaluated period. We conclude that biochar altered the soil environment, promoting the increased solubility and availability of phosphorus.

Acidic tropical soils act as strong sinks for phosphorus (P) due to the high adsorption capacity of this nutrient by iron (Fe) and aluminium (Al) minerals. In this study, we investigated the effects of applying acai waste biochar (Euterpe oleracea Mart.) in combination with dolomitic lime on the P dynamics of a Ferralsol with cowpea (Vigna unguiculata (L.) Walp) test crop. Application of 12 t ha⁻¹ of biochar and 75% of the recommended lime rate increased soil pH by 1.77 units, reaching 6.77, and resulted in the lowest C:N ratio (18.99) at 0–5 cm depth. Inorganic P concentrations increased in Resin-Pi (2-fold), NaHCO₃-Pi (2-fold), NaOH-Pi (2.89-fold) and HCl-Pi (4-fold) fractions relative to the corresponding treatments without biochar, while NaHCO₃-Po decreased markedly, declining from 68% to 9% of the organic P fraction, NaOH-Po showed a less consistent response among treatments. In addition, P recovery in the Resin-Pi fraction increased, reaching 34.91% and 37.36% in the treatments with 12 t ha⁻¹ of biochar and both 75 and 100% liming, respectively. Combined use of alkaline biochar and lime is a practical strategy to raise pH and increase labile and moderately labile inorganic P, and improve P use efficiency in acid Ferralsols. These responses are consistent with a redistribution of P among the assessed fractions and with the absence of detectable short-term effects on arbuscular mycorrhizal fungi (AMF) colonisation and easily extractable glomalin-related soil protein (EE-GRSP) production.

Biochar derived from açaí (Euterpe oleracea Mart.) processing residues represents a sustainable strategy to improve fertility and mitigate acidity in highly weathered tropical soils. This study evaluated the effects of açaí biochar (0 and 12 Mg ha⁻¹), combined with dolomitic limestone (0, 75%, and 100% of the recommended rate), on chemical, biological, and agronomic attributes of a clayey Ferralsol cultivated with cowpea (Vigna unguiculata (L.) Walp) in the Amazon. A field experiment was conducted in a randomized block design with six treatments and four replicates. Soil samples were collected from the rhizosphere and from the 0–5, 5–10, and 10–20 cm layers to determine pH, exchangeable Al, pseudo-total concentrations of K, Ca, Mg, total carbon (TC), organic carbon (OC), microbial biomass carbon (MBC), β-glucosidase, and cellulase activity. Biochar increased soil pH (0–10 cm), reduced exchangeable Al, and increased pseudo-total K throughout the soil profile, whereas liming primarily increased Ca and Mg availability and contributed to acidity correction. A significant biochar × lime interaction was observed for exchangeable Al in surface layers, while Mg responses varied depending on depth and treatment combination. Biochar also enhanced cellulase activity, total carbon (TC), and microbial biomass carbon (MBC), while reducing β-glucosidase in surface layers, with no effect on organic carbon (OC) determined by the Walkley–Black method. Cowpea grain yield increased by 16% with biochar and showed additive response to lime, reaching 1460 kg ha⁻¹ under combined application, 13.6% higher than lime alone. These results indicate that açaí biochar acts as a complementary amendment for improving soil fertility, biological functioning, and crop performance in acidic tropical soils.