Impact of agroforestry on soil carbon sequestration in smallholder farms

Thesis Abstract

Soil carbon sequestration potentials in smallholder agroforestry systems are investigated to address rising atmospheric CO2 and soil degradation risks faced by smallholder farmers in temperate and tropical regions, where conventional monocropping practices have limited soil carbon storage and resilience. The study addresses the gap in empirical evidence on how integrating trees with crops and livestock alters soil organic carbon (SOC) stocks, distribution across soil depths, and the stability of sequestered carbon under varying management intensities. The aim is to quantify and compare SOC stocks and sequestration rates between agroforestry and non-agroforestry smallholder systems, and to identify the mechanisms by which tree components influence carbon storage and soil quality. Specific objectives are (1) to quantify SOC concentration and stock in 0–20 cm, 20–40 cm, and 40–60 cm soil layers under traditional agroforestry, low-density and high-density systems, and adjacent conventional cropping controls; (2) to determine the influence of tree biomass, litterfall, root turnover, and soil covariates (moisture, pH, bulk density) on SOC dynamics using multivariate models; (3) to estimate annual SOC sequestration rates over a five-year period via the matched-control approach and equilibrium-based modeling; (4) to assess carbon stability using the particulate organic carbon (POC) and mineral-associated carbon fractions and their relationship with soil aggregate distribution; (5) to evaluate farmers’ perceptions, adoption drivers, and potential trade-offs through a mixed-methods lens. Methodologically, a quasi-experimental design with a longitudinal panel was employed in 120 representative farming households across three districts with established agroforestry practices and 60 matched controls. Stratified random sampling selected 40 farms for each agroforestry density tier (low, medium, high) and 60 conventional farms. Soil samples were collected annually for five years from all farms at depths of 0–20, 20–40, and 40–60 cm. SOC was measured via dry combustion using an elemental analyzer (LECO); fractionation into particulate organic carbon (POC) and mineral-associated carbon followed density fractionation and acidified density separation. Tree biomass carbon was estimated using allometric equations calibrated for dominant species, and litter input was quantified with litter traps. Complementary soil physical and chemical parameters (bulk density, pH, CEC, moisture, texture, total nitrogen, p-organic matter) were measured to control for confounders. Data collection instruments included structured household surveys, semi-structured interviews with a subset of 30 farmers, and field observation checklists. Validity and reliability were ensured through pilot testing, triangulation, and inter-laboratory cross-checks for carbon analyses. Data were analyzed using mixed-effects linear models to account for repeated measures, with fixed effects for agroforestry density, time, and interactions, and random effects for farm and district. Regression analyses identified the contribution of tree-related factors (biomass, litter input, root density) to SOC changes, while ANOVA tested differences among management systems. Structural equation modeling (SEM) examined pathways linking agroforestry practices to SOC via litter input, microclimate modification, and soil structure. The theoretical framework draws on the Soil Carbon Sequestration Theory and the Biodiversity-Ecosystem Functioning framework, with reference to the Carbon-Nitrogen Dynamic model to explain soil organic matter stabilization processes. Expected findings indicate higher SOC stocks and sequestration rates in agroforestry systems, particularly in the 0–20 cm and 20–40 cm layers, driven by increased litter inputs, root turnover, and improved soil structure in high-density canopies. POC and mineral-associated carbon fractions are anticipated to respond positively to elevated biomass input and stable aggregate formation, indicating enhanced carbon stability. The study is expected to reveal density-dependent effects, with diminishing returns beyond a threshold density due to potential competition for nutrients. The research will contribute to knowledge by providing robust, long-term empirical estimates of SOC gains attributable to agroforestry and by clarifying mechanisms of carbon stabilization in smallholder contexts. Policy and practice implications include evidence-based guidance on optimal tree-crop-livestock configurations to maximize soil carbon sequestration without compromising crop yields, together with farmer-centered recommendations for scaling agroforestry adoption. The study recommends integrating agroforestry into national land-use planning, monitoring soil carbon in smallholder programs, and aligning incentives with carbon credit frameworks. It also highlights areas for further research, such as long-term carbon persistence under climate variability and the socio-economic viability of agroforestry investments for smallholders.