Regulation of carbon mineralization rates by soil structure and water in an agricultural field and a prairie-like soil

Soil organic carbon (SOC) mineralization is influenced by soil structure such as pore size distribution and aggregation, both of which result in heterogeneity in the distribution of soil water and microbial activity. This study investigated the effects of soil structure and its interaction with soil water content on C mineralization. Dry aggregate samples (0-2, 2-4, 4-8, 8-16 mm) obtained from agricultural field (AGRIC) and an adjacent less-disturbed area maintained under prairie vegetation (PRAIRIE) were subjected to a short-term incubation where soil water was maintained at four gravimetric water contents ranging from 5% to 50%. More water was needed to maximize C mineralization in larger sized aggregates supporting the notion that biological activity is located at the surface of aggregates or within pores located adjacent to their surface. Mean C mineralization rate was 54% greater from the PRAIRIE than the AGRIC soils, which contained 66.1 and 24.9 mg SOC g^{- 1} soil, respectively. However, mean specific C mineralization rates (mg C-CO₂ / mg-SOC ) were 45% lower from the PRAIRIE than from the AGRIC treatment, suggesting that physical protection of SOC was greater in that soil. The greater volume of macropores (> 300 μm) in the PRAIRIE aggregates may have contributed to its accumulation of humified SOC by limiting microbial usage of C in air filled pores. The volume of water holding pores (< 30 μm), which was lower in the aggregates from the PRAIRE than AGRIC treatment, was saturated in the PRAIRE aggregates at the wettest treatment. As a result, localized anaerobism restricted C mineralization in the PRAIRE but not in the AGRIC aggregates where water holding pores were not yet saturated. Differences in size distribution of pores in aggregates collected from the two soils considered affected the physical availability of substrates and optimum soil water conditions for biological activity. By considering macropores as regions of where C decay is restricted and by assessing the status of pores < 30 μm (water held at - 10 kPa), we can better understand spatial constraints on C mineralization.