Assessing Soil Health and Carbon Sequestration in Agro-Industrial Rubber Plantations, Sri Lanka

Thesis Abstract

Soil degradation and underutilization of carbon sequestration potential in rubber-dominated agro-industrial landscapes threaten long-term productivity and climate resilience in Sri Lanka, where extensive plantations experience nutrient depletion, reduced soil organic matter, and elevated greenhouse gas emissions from conventional management practices. This study aims to quantify soil health indicators and carbon sequestration dynamics within agro-industrial rubber plantations across varied agro-ecological zones in Sri Lanka, to identify drivers of soil quality change and to evaluate management practices that optimize soil carbon storage without compromising yield. The specific objectives are (1) to characterize baseline soil physical, chemical, and biological properties under different rubber management regimes (tapping intensity, fertilization, cover cropping, and residue management); (2) to quantify total and fractionated soil organic carbon (SOC) stocks and soil inorganic carbon along depth profiles (0–30 cm, 30–60 cm, 60–100 cm); (3) to evaluate the impact of management practices on soil biological activity, including microbial biomass carbon and enzyme activities (arylsulfatase, dehydrogenase); (4) to model relationships between soil health indicators and rubber yield, latex quality, and related ecosystem services; and (5) to provide evidence-based recommendations for improving soil health and increasing carbon sequestration in the sector. A mixed-methods design is employed, integrating quantitative soil analysis and qualitative stakeholder insights. The population encompasses rubber plantations in three districts representing wet, intermediate, and dry zones. A stratified random sample of 120 plots is selected, with 40 plots per district, comparing conventional management (n=60) versus enhanced soil-conserving practices (n=60). Data collection instruments include soil sampling for laboratory analyses, in-field soil moisture and bulk density measurements, multi-parameter soil health kits, and structured interviews with estate managers and agronomists. Laboratory analyses adopt standardized methods soil texture by hydrometer, pH and electrical conductivity by glass electrode, available phosphorus by Olsen extraction, exchangeable cations by ammonium acetate, SOC by dry combustion using a CNS analyzer, and inorganic carbon via acid neutralization. SOC is partitioned into particulate organic carbon and mineral-associated carbon using density fractionation. Microbial biomass carbon is determined by substrate-induced respiration, and enzyme activities are quantified colorimetrically. Gas flux measurements for CO2 and N2O are performed with closed-chamber methods at quarterly intervals to estimate soil-atmosphere carbon exchange. For data analysis, descriptive statistics summarize baseline properties; analysis of variance (ANOVA) tests effects of management regime and district; multivariate multiple regression links soil health indicators to yield and latex metrics; generalized linear models assess relationships between SOC stocks and depth, and factor analysis identifies patterns among soil properties. Structural equation modeling (SEM) evaluates causal pathways among management practices, soil health, carbon sequestration, and productivity. The thematic analysis of interview transcripts identifies perceived barriers and enablers of adopting soil-improving practices. The theoretical framing draws on the Soil Health Concept and the Ecosystem Services framework, complemented by the Theory of Planned Behavior to interpret adoption decisions. Expected findings indicate higher SOC stocks and enhanced microbial activity in plots implementing cover crops, return of crop residues, reduced tillage, and optimized fertilization, with SOC increases most pronounced in the 0–30 cm layer but contributing to deeper carbon accrual under specific practices. Enhanced soil structure and higher enzyme activities are anticipated to correlate with improved yield stability and latex quality, alongside reduced nitrous oxide emissions in conservative management systems. The study contributes to knowledge by delivering context-specific estimates of SOC sequestration potentials in Sri Lankan rubber plantations, linking soil health indicators to productivity, and quantifying the effects of practical management interventions on carbon dynamics. It provides a validated decision-support framework for policymakers and plantation managers to design soil-health–centric interventions aligned with climate mitigation targets and production objectives. Conclusions emphasize the necessity of integrating cover cropping, residue retention, and reduced-tillage strategies with precise nutrient management to maximize soil health benefits and carbon storage without compromising rubber output. Policy and practice recommendations include scaling up soil-health audits across estates, incentivizing adoption of residue-retention and cover-cropping programs, and incorporating SOC monitoring into certification schemes. Further research is recommended to explore long-term SOC trajectories under landscape-level interventions and to evaluate economic viability through life-cycle assessment.