Particulate Organic Carbon Content and Potential Mineralization as Affected by Tillage and Texture

Conservation of soil organic carbon (SOC) is important for improving soil quality. Increasing the pool of particulate organic carbon (POC), thought to have a “slow” or medium turnover rate, with conservation tillage may be a critical step in improving the quality of agricultural soils. We determined the standing stock and potential mineralization rate of POC (defined as material >53 µm diam.) at depths of 0 to 50, 50 to 125, and 125 to 200 mm in four Boralfs (loam, silt loam, clay loam, and clay) under conventional shallow tillage (CT) and 4 to 16 yr of zero tillage (ZT) in northern Alberta and British Columbia. Standing stock of POC was consistently different between tillage regimes only at a depth of 0 to 50 mm, averaging 0.63 kg m −2 under CT and 0.76 kg m −2 under ZT. However, the ratio of specific POC mineralization to specific whole‐SOC mineralization averaged 23% greater under ZT than under CT, suggesting that POC was of higher quality (i.e., more mineralizable) under ZT relative to other pools of SOC. With increasing clay content of the original soil, specific mineralization rate of POC increased after clay was removed by dispersion. This result suggests that clay may play an important role in sequestering POC by protecting its decomposition. Particulate organic C content was a more sensitive indicator of tillage‐induced changes in SOC than the total amount of SOC. Zero tillage in this cold semiarid climate may increase both the active and slow pools of SOC within several years.