Relations between soil organic carbon, soil structure and physical processes in an agricultural topsoil : The role of soil mineral constituents

Sammanfattning: A better understanding of the interactions between soil organic carbon (SOC) and mineral constituents (e.g. clay and reactive oxide phases) and their consequences for soil structure and physical processes is important for assessing the potential for, and benefits of, carbon sequestration in arable soils. This thesis investigated the factors determining topsoil SOC content at the field scale for an arable field with large var-iations in soil properties. Relationships between SOC, soil pore size distributions, macropore network characteristics, water flow and solute transport were also exam-ined using intact soil samples from the field.The spatial variation in SOC content at the Bjertorp field was mainly explained by the oxalate-extractable aluminum (Alox) content followed by carbon input from crops that was estimated from crop yield. In contrast, clay and oxalate-extractable iron (Feox) seemed not to play a major role in SOC stabilization/accumulation, pos-sibly due to the occurrence of stagnant water in soils with larger clay contents. It was concluded that reactive Al phases may be important for physico-chemical stabiliza-tion of SOC for arable topsoils in humid continental climates. Multiple linear regression analysis revealed that an increase of SOC was associ-ated with relatively large increases of porosities in the 0.2–5 µm and 480–720 µm diameter classes, which can contribute to enhancing both water supply to crops and water flow rates. The degree of preferential solute transport under steady state near-saturated conditions was reduced with larger volumes of small macropores (240–480 µm diameter) and mesopores (30–100 µm diameter), whereas it was not corre-lated with measures of macropore connectivity. The statistical analysis indicated that SOC had only limited effects on the degree of preferential transport, being overshad-owed by the large variation in clay content across the field.

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