Tillage and Crop Rotation Impact on Soil Microbial Activity.

Soil microbial biomass and their activity are sensitive indicators of C sequestration. A randomized block design in 2 (tillage system) x 3 (crop rotation) factorial arrangement was laid-out to evaluate the impact (1990 to 2007)of tillage and crop rotation on soil microbial activity. Tillage treatments included conventional (CT) vs. and no-till (NT) were factored into continuous corn (CC), corn-soybean (CS), and corn–soybean-wheat-cowpeas (CSW) rotation. Composite soil samples were collected at 0-7.5, 7.5-15, 15-22.5 and 22.5-30 cm, 2-mm sieved, and analyzed for total microbial biomass (C_mic), basal (BR) and specific respiration rates (qCO₂), potentially mineralizable C (C_min), total C (C_org), pH, electrical conductivity, and bulk density (rb). Tillage significantly impacted C_mic, BR and qCO2 with an interaction from soil depth and crop rotation. NT had more than 56% C_mic, 9% Cmic:Corg, and 29% CMin compared to CT. More C (29%) mineralized under CT compared to NT. Likewise, C_mic, Cmic:Corg, BR, and qCO2 were significantly influenced by crop rotation. The CSW had more than 25% C_mic and 3% BR associated with lower qCO2 compared with CC and CS. The C_mic and BR rates have shown similar or a slightly different pattern when expressed on a mass per unit area basis at different depths or within soil profile calculated by taking the differences of rb into account. Overall, NT with CSW had greater microbial activity over CT or CC and CS.