Impacts of a Perennial Pasture on Organic Matter and Nitrogen in a Rainfed Crop Rotation.

Soil degradation and erosion take place during years with annual crops, especially if soil is subjected to tillage. If a perennial pasture follows, soil organic matter and soil structure may recover again. In Uruguay, before the advent of no-till farming, mixed crop rotations were widely adopted (i.e. 3 years of cash crops followed by 3 years of a perennial pasture), with average erosion rates that were considered tolerable. Long-term, well managed no-till systems also allow for successfully controlling soil erosion and improving soil quality. Therefore, would a perennial pasture still provide improvements in soil organic matter when soil is under no-till?
A long-term experiment was started in 1996 on a silty clay Vertisol in southwestern Uruguay to study the effects of tillage and crop rotation on total and labile soil organic matter. Treatments are no-till vs. reduced tillage and mixed vs. continuous cropping rotations. The design is a 2x2 factorial, with three staggered replications. The basic sequence has 5 crops and lasts 3 years: wheat – oats and maize – barley and sunflower. In the continuous cropping rotation the same sequence is repeated indefinitely, while in the mixed rotation a 3-yr perennial pasture (composed of white clover, lotus, and fescue) follows the basic crop sequence. Data from the last 10 years were analyzed as a factorial design with repeated measures and with year as a random effect. No-till had significantly higher soil organic matter than reduced tillage in the 0-7.5cm soil. However, under continuous cropping, the increase due to no-till was of lower magnitude than in the mixed rotation. Potentially mineralizable nitrogen had a 5–fold increase that coincided with the pasture. Residual effects of the pasture were of higher magnitude under no-till than with reduced tillage.