Design and evaluation of biochar-enhanced soils for sustainable crop productivity

Thesis Abstract

Soil degradation and declining crop yields in smallholder agroecosystems driven by nutrient mining, salinity, and poor soil structure necessitate sustainable soil amendments with scalable benefits. This study investigates the design and evaluation of biochar-enhanced soils for promoting sustainable crop productivity under varied edaphic conditions. The aim is to determine how biochar applications, at specified rates and production characteristics, influence soil physical properties, nutrient retention, microbial activity, and crop yield, and to identify the mechanisms underpinning observed effects. The objectives are to (i) quantify changes in soil bulk density, infiltration rate, and water holding capacity following biochar amendment; (ii) assess nutrient sorption/desorption dynamics and cation exchange capacity across biochar types (pyrolysis at 350°C and 550°C) and application rates (5, 10, and 20 tons per hectare) in loamy and sandy soils; (iii) evaluate impacts on soil microbial biomass and enzyme activities (dehydrogenase, phosphatase) as mediators of nutrient cycling; (iv) determine crop responses for maize and sorghum under controlled environment pot trials and field plots across a two-season cycle; and (v) develop a predictive framework linking biochar characteristics, soil properties, and yield outcomes using multiple regression and structural equation modeling (SEM). The methodology adopts a mixed-methods research design combining experimental agronomy with soil biochemical analyses. The population comprises soils from four representative smallholder farms with contrasting textures in the temperate agro-ecological zone. A stratified random sampling approach yields 24 field plots (4 farms × 6 treatments) and 40 pot experiments for controlled condition assessments. Data collection instruments include standardized soil sampling protocols, gravimetric water content measurements, multilevel crop yield recording, soil nutrient analysis by (i) Mehlich-3 extraction, (ii) total organic carbon via dry combustion, and (iii) elemental composition by ICP-OES. Biological assessments employ microbial biomass carbon via chloroform fumigation–extraction and enzyme assays (dehydrogenase, ?-glucosidase, phosphatase). Physical soil properties are measured with a soil core method for bulk density and a double-ring infiltrometer for infiltration. Data analysis follows rigorous statistical procedures descriptive statistics, ANOVA for treatment effects, Tukey's post hoc tests, regression analysis to quantify relationships between biochar properties and soil outcomes, and SEM to elucidate causal pathways among biochar type, soil physical–chemical attributes, microbial activity, and crop yield. Theoretical framing integrates the Sustainable Intensification framework and the Soil Functional Diversity concept, with explicit reference to the Theory of Change and the Fixed-Site Carbon Sequestration model to explain long-term soil quality improvements. Expected findings indicate that higher-temperature biochars (550°C) combined with medium application rates (10 t/ha) improve bulk density reduction, infiltration, and water holding capacity more effectively in sandy soils, while low-temperature biochars (350°C) exhibit greater nutrient retention in loamy soils, leading to differential yield responses in maize and sorghum. Enhanced microbial biomass and enzyme activities are anticipated to mediate improved nutrient use efficiency, with SEM revealing direct and indirect effects of biochar characteristics on yield through soil physicochemical and biological pathways. The study contributes to knowledge by providing empirically grounded guidelines for selecting biochar type and application rates tailored to soil texture and cropping systems, and by integrating agronomic performance with soil health indicators to inform sustainable soil management policies. Policy-relevant implications include recommendations for biochar production standards, on-farm integration frameworks, and cost–benefit considerations for smallholders. The main conclusion is that biochar-enhanced soils can sustainably boost crop productivity when matched to soil textural class and crop requirements, with significant gains in nutrient retention and microbial mediated nutrient cycling driving yield improvements. Recommendations emphasize site-specific biochar selection, optimization of application rates, long-term monitoring of soil organic carbon, and scaling strategies that incorporate farmer training, economic analysis, and lifecycle assessments to ensure environmental and economic feasibility.