Responses of soil organic carbon, aggregate stability, carbon and nitrogen fractions to 15 and 24 years of no-till diversified crop rotations

Diversification within a cropping system together with no-till (NT) soil management can help to improve soil organic carbon (SOC). The present study was conducted to assess the impacts of crop diversity through crop rotations on SOC and other selected soil properties. The long-term experimental sites were located in Beresford and Brookings, South Dakota, USA. The Beresford site was initiated in 1991 (24 years) on Egan soil series (fine-silty, mixed, superactive, mesic Udic Haplustolls), whereas, the Brookings site was established in 2000 (14 years) on a Barnes clay loam soil (fine-loamy, mixed, superactive, frigid Calcic Hapludolls) under a randomised complete block design with four replications. Treatments at both sites consisted of a 2-year (corn (Zea mays L.)–soybean (Glycine max L.)), and a 4-year (corn–soybean–winter wheat (Triticum aestivum L.)–oat (Avena sativa L.)) rotation, all managed under NT soil management. Soil samples were collected in the fall of 2015 after crop harvest under the corn phase. Data showed that 4-year rotation increased SOC stock (8.3% in Brookings and 22% in Beresford) compared with that under 2-year rotation (not always significant) in the soil profile 0–60 cm. Soil particulate organic matter and organic matter were always higher under 4-year rotation than under 2-year rotation at 0–5 and 5–15 cm depths at both sites. Surface soil aggregate stability was improved in both locations under 4-year rotation (12% in Brookings, 4% in Beresford). Additionally, at 0–5 cm depth, the 4-year rotation increased light fractions of carbon (18% in Brookings, and 32% in Beresford) compared with 2-year. Results from this study showed that the use of diverse crop rotations (4-year) for longer (>24 years) duration enhanced SOC, carbon and nitrogen fractions, and soil aggregation compared with those under corn–soybean (2-year) rotation.