Impact of organic amendments on soil carbon dynamics in temperate agroecosystems

Thesis Abstract

Soil carbon dynamics in temperate agroecosystems are influenced by organic amendments, yet quantifying their temporal contributions to soil organic carbon pools and stabilization pathways remains a key challenge for sustainable management under variable climate and soil types. This study aims to quantify how different organic amendment strategies affect soil carbon stocks, mineralization rates, and stabilization mechanisms across representative temperate soils, and to identify the conditions under which amendments most effectively enhance soil carbon sequestration and fertility. The specific objectives are (1) to compare the effects of farmyard manure, compost, and biochar amendments on total soil organic carbon (SOC), light fraction carbon, and mineral-associated carbon over a five-year field experiment; (2) to assess priming effects and CO2 efflux dynamics using isotopic tracing (13C) to distinguish amendment-derived carbon from native SOC; (3) to evaluate changes in soil microbial biomass, community structure, and enzyme activities driving carbon cycling; (4) to model SOC dynamics under varying climate and soil texture scenarios using the CENTURY/RothC coupled framework and relate model outputs to observed data; and (5) to develop guidelines for amendment selection and application regimes to maximize carbon sequestration without compromising crop yields. The study adopts a factorial field experiment in a temperate agroecosystem with controlled plots (n = 4 replicates per treatment) across three soil types (sandy loam, loam, silty clay) at two sites, with a randomized complete block design. Amendments include poultry manure, mature compost, and pine biochar at agronomically relevant rates, alongside a lose-no-amendment control. Data collection will span five cropping cycles and involve periodic soil sampling (0–20 cm, 20–40 cm), SOC fractionation, and total carbon analyses using dry combustion (Elemental Analyzer, LECO). Isotopic 13C tracers will quantify amendment-derived carbon incorporation into SOC pools. Microbial analyses will include substrate-induced respiration, phospholipid fatty acid profiling, and quantitative PCR targeting functional genes involved in carbon degradation (mcoA, ligninolytic genes). Enzymatic activities (?-glucosidase, cellulase, peroxidase) will be measured to link microbial processes with carbon mineralization. Gas flux measurements (CO2 efflux) will be conducted monthly using static chamber methods, with airborne CO2 isotopic signatures aiding source partitioning. Statistical analysis will employ ANOVA to test treatment effects, repeated-measures MANOVA for temporal trends, and structural equation modeling to elucidate causal pathways among amendments, microbial dynamics, and SOC changes. Regression analyses will quantify relationships between amendment rate, microbial indicators, and SOC pools. Theoretical grounding will draw on the Priming Effect theory and the Stabilization framework, complemented by the microbial efficiency–multiplier concept to interpret SOC stabilization pathways; these will be integrated within a process-based carbon model. Anticipated findings include (i) differential enhancement of SOC pools by compost and biochar relative to manure, with biochar showing greater stabilization in clayey soils; (ii) measurable priming effects that attenuate over time but are mitigated in biochar-amended plots; (iii) shifts in microbial community structure favoring oligotrophic communities under biochar, and higher enzyme activities under compost and manure treatments; (iv) improved model fit when amendment-specific parameters are included, enabling scenario analysis under climate variability. The study expected to contribute new empirical evidence on the long-term carbon sequestration potential of diverse organic amendments in temperate systems, refine process-based modeling of SOC dynamics with amendments, and inform land management policies for climate-smart agriculture. The practical implications include actionable recommendations for amendment selection, timing, and dosages that optimize SOC gains while sustaining crop productivity. In conclusion, the research will provide robust, site-responsive guidelines for leveraging organic amendments to enhance soil carbon sequestration, improve soil health, and support resilient agroecosystems under temperate climatic conditions. Recommendations for future work include longer-term monitoring beyond five years, exploration of interactions with cover crops, and expansion to additional soil textures to generalize findings.