The effect of freeze-thaw cycles on soil respiration and nitrogen dynamics as in-fluenced by soil structure, aggregate size and water content
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The role of soil structure and aggregation in influencing soil respiration and N dynamics in soils undergoing freeze-thaw cycles, as influenced by water content, is poorly known. Surface soil with a texture of sandy loam was collected from an agriculture field in Atlantic Canada. Two studies were undertaken. The first study determined the influence of soil structure (whole / crushed soil) and water content (35%, 60%, and 85% water-filled pore space) on soil respiration and nitrogen dynamics during freezing and thawing of the soil. The second study determined the role of different aggregate size fractions (0~0.25, 0.25~4, and 4~8 mm) on the freeze-thaw effect. The research found microbial metabolism is more limited by environmental conditions than by the substrate availability. Crushing altered structural characteristics and caused changes in substrate solubilization and / or microbial utilization of substrates during freezing and thawing. Furthermore, freezing and thawing did not influence the denitrification of the whole soil, but enhanced denitrification in soils where aggregates were crushed. At 60% WFPS, the interaction among aggregate crushing and freeze-thaw increased denitrification. Also, the impact of freeze-thaw was greater on soil respiration and N mineralization in medium size aggregates (0.25~4mm). Freezing and thawing improved de-nitrification in aggregates on all three size fractions. This project provided new information on the effects of freeze-thaw on soil carbon and nitrogen dynamics as influenced by soil structure and water content. This information will be critical in assessing the impact of climate change in soil carbon and nitrogen dynamics in temperate regions.