Soil Organic Matter Pools during Early Adoption of Conservation Tillage in Northwestern Canada

Abstract

Changes in soil organic matter (SOM) pools during adoption of reduced (RT) or zero tillage (ZT) can influence soil physical properties, nutrient cycling, and CO 2 flux between soil and atmosphere. We determined soil organic C (SOC), soil microbial biomass C (SMBC), basal soil respiration (BSR), and mineralizable N to a depth of 200 mm at the end of 3, 5, and 6 yr after implementation of tillage management on a Falher clay (fine, montmorillonitic, frigid Typic Natriboralf) near Rycroft, Alberta, in a canola ( Brassica campestris L.)‐wheat ( Triticum Aestivum L.)‐barley ( Hordeum vulgare L.)‐fallow cropping system. At the end of 6 yr, SOC was not different among tillage regimes and averaged 8.6 kg m −2 . At the end of 3 and 5 yr, SMBC was not significantly different among tillage regimes, but at the end of 6 yr SMBC was 7% greater in RT and 9% greater in ZT than in conventional tillage (CT). Basal soil respiration and mineralizable N at the end of 6 yr were not different among tillage regimes following barley and averaged 2.7 g CO 2 ‐C m −2 d −1 and 5.0 g inorganic N m −2 24 d −1 , respectively. However, BSR following fallow was 2.2, 2.5, and 2.6 g CO 2 ‐C m −2 d −1 in CT, RT, and ZT, respectively. Mineralizable N following fallow was 5.8 g inorganic N m −2 (24 d) −1 in RT and ZT and 7.3 g inorganic N m −2 (24 d) −1 in CT. At 0 to 50 mm, there was no significant increase in SOC at the end of 6 yr, a 17 to 36% increase in SMBC, and a 12 to 69% increase in BSR with ZT compared with CT, depending on rotation phase. Relatively small changes in SOM pools with adoption of conservation tillage may be attributable to the large amount of SOM initially present and the cold, semiarid climate that limits SOM turnover.