Impact of conservation tillage on soil organic carbon in temperate agroecosystems.

Thesis Abstract

Soil organic carbon (SOC) stocks in temperate agroecosystems are increasingly influenced by tillage practices that disrupt soil structure and carbon dynamics, with conservation tillage proposed as a mitigation strategy for SOC decline under intensive cropping. The study addresses the gap between widespread adoption of reduced-till practices and robust, field-based evidence of their long-term effects on SOC across soil profiles and seasonal dynamics, particularly under varying residue management and crop rotations. The aim is to quantify the impact of conservation tillage on SOC sequestration, evaluate associated changes in soil physical properties, and identify drivers at the field scale that modulate SOC responses. Specific objectives are (i) to compare surface and subsoil SOC stocks (0–30 cm, 30–60 cm) under conventional tillage (CT) and conservation tillage (CTv) across three temperate agroecosystem sites over a five-year period; (ii) to assess soil organic carbon fractions (total SOC, particulate organic carbon, mineral-associated organic carbon) and related microbial biomass using CHLOS-1 and microbial PLFA analysis; (iii) to evaluate changes in soil aggregation, bulk density, and moisture retention as mediators of SOC dynamics; (iv) to model SOC changes using a mixed-effects regression framework incorporating litter input, crop rotation, and climatic variables; and (v) to test the applicability of the 'Primed SOC' concept and the theory of tillage-induced carbon redistribution in temperate soils. A longitudinal, multi-site experimental design will be employed, comprising three temperate agroecosystem sites with comparable climatic conditions but distinct soil types (loam, silt loam, and silty clay loam). At each site, paired plots will compare conventional tillage (CT) with conservation tillage (no-till or reduced-till) under standardized residue retention and three-year rotating crops (winter wheat, maize, and oilseed rape). The population includes mineral soil profiles and associated microbial communities. A total of 60 plots (10 per treatment per site) will be instrumented with soil sampling cores and in-situ sensors. Soil samples will be collected annually from 0–10 cm, 10–20 cm, 20–30 cm, and 30–60 cm depths. Analytical techniques will include dry combustion for total SOC (elemental analyzer), density fractionation to quantify particulate and mineral-associated SOC, and PLFA/Microbial Biomass methods to characterize microbial community structure. SOC stock changes will be calculated using equivalent soil mass adjustments. Residue-derived carbon inputs will be quantified via 13C natural abundance where feasible. Soil physical properties will be assessed through aggregate stability tests, wet sieving, bulk density measurements, and in-situ soil moisture sensors. Climatic data (precipitation, temperature) will be recorded from site-equipped weather stations. Data analysis will use linear mixed-effects models to compare SOC stocks and fractions between tillage treatments, accounting for site, depth, Year, and crop, with random effects for plot and block. Regression analyses will identify the relative contributions of residue retention, microbial biomass, and soil structure to SOC sequestration. Structural equation modeling will examine causal pathways among tillage, soil aggregation, microbial dynamics, and SOC outcomes. Sensitivity analyses will test the robustness of SOC stock estimates to depth and soil type. Key expected findings include higher SOC stocks and fractions under conservation tillage relative to conventional tillage at 0–30 cm and significant, albeit site-dependent, increases down to 60 cm. It is anticipated that improved aggregate stability and higher microbial biomass under conservation tillage will correlate with increased mineral-associated SOC, while particulate SOC may show faster turnover under CT. The study is expected to reveal differential SOC responses driven by soil texture and crop rotation, with the greatest sequestration potential in loam soils with dense residue inputs. Theoretical contributions will advance the Primed SOC framework and refine the understanding of tillage-driven carbon redistribution in temperate soils, integrating microbial and physical soil processes into SOC models. The study will contribute to knowledge by providing robust, field-based evidence on SOC responses to conservation tillage across soil depths, informing land management policies and carbon budgeting in temperate agroecosystems. Practical implications include guidance on residue management and rotation planning to maximize SOC sequestration while maintaining yields. Recommendations will address adoption strategies for farmers, including site-specific tillage advice, monitoring protocols for SOC and soil health indicators, and integration of SOC targets into agronomic decision-making under changing climate conditions.