Soil organic matter pools with conventional and zero tillage in a cold, semiarid climate

Conservation of soil organic matter (SOM), especially in high-SOM surface soils of the Canadian Prairies, is essential for maintaining soil quality and regulating soil CO, emissions. We determined the depth distribution (0-50, 50-125, and 125-200 mm) of soil organic C (SOC), soil microbial biomass C (SMBC), basal soil respiration (BSR), and net N mineralization in four Boralfs subjected to shallow conventional tillage (CT) and zero tillage (ZT). Tillage regimes were initiated 4, 4, 7, and 16 years prior to these measurements on a limed clay loam, unlimed clay loam, loam, and silt loam, respectively, located in northern Alberta and British Columbia. Soils under ZT were an average of 4% denser and 24% wetter than under CT. SOC content to a depth of 200 mm was 6.5 kg m(-2) under CT and 11% greater under ZT in the limed clay loam, but not different between tillage regimes in the unlimed clay loam (7.1 kg m(-2)), silt loam (5.1 kg m(-2)), and loam (4.3 kg m(-2)). BSR averaged 4.0 and 3.1 g CO2-C per square meter per day under CT and ZT, respectively. The portions of SOC and SMBC as BSR under ZT were lower than under CT. However, net N mineralization averaged 3.4 and 4.1 g m(-2) per 24 days under CT and ZT, respectively. The cold, semiarid climate of the Canadian Prairies appears to have hindered any acceleration of decomposition of SOM and crop residues owing to soil disturbance with CT. Therefore, SOC and active soil C pools were not enriched by minimized soil disturbance, unlike results obtained from more temperate and humid climates.