Soil carbon and nitrogen changes as influenced by tillage and cropping systems in some Iowa soils

Soil organic C (SOC) and total N (TN) contents play a crucial role in sustaining agricultural production systems. Short-term ( 10-year) management effects on SOC and TN dynamics are often complex and variable. Three experiments were conducted to evaluate short-term tillage and cropping system effects on SOC and TN within the 0–30 cm soil depth across Iowa. The first experiment with no-tillage and chisel plowing treatments was established in 1994 on Clarion-Nicollet-Webster (CNW), GalvaPrimghar-Sac (GPS), Kenyon-Floyd-Clyde (KFC), Marshall (M), and Otley-Mahaska-Taintor (OMT) soil associations under a corn (Zea mays L.)–soybean (Glycine max (L.) Merr.) rotation. The second experiment with no-tillage, strip-tillage, chisel plowing, deep ripping, and moldboard plowing treatments was initiated in 1998 on the CNW soil association in a corn–soybean rotation. The third experiment consisting of smooth bromegrass (Bromus inermis Leyss.), switchgrass (Panicum virgatum L.) and corn–soybean–alfalfa (Medicago sativa L.) treatments was established in 1991 on Monona-Ida-Hamburg (MIH) soil association under no-tillage management. Short-term tillage effects on SOC and TN occurred primarily at the 0–15 cm soil depth. Tillage effects did not vary significantly with soil association. No-tillage resulted in greater SOC and TN contents than chisel plowing at the end of 7 years of tillage practices averaged over the CNW, GPS, KFC, M, and OMT soil associations. The increase in SOC and TN with no-tillage was not related to SOC and TN stratification in the soil profile or annual C and N inputs from crop residue, but most likely due to decreased mineralization rate of soil organic matter. However, tillage effects on SOC and TN were negligible at the end of only 3 years of tillage practices on the CNW soil association. Smooth bromegrass and switchgrass systems resulted in greater SOC and TN contents at both 0–15 cm and 15–30 cm soil depths than a corn–soybean– alfalfa rotation after 10 years of management on the MIH soil association. Smooth bromegrass and switchgrass systems increased SOC by 2.3 and 1.2 Mg ha 1 yr 1 at the 0–15 cm soil depth, respectively. We conclude from these short-term experiments that reducing tillage intensity and increasing crop diversity to include perennial grasses could be effective in improving C and N sequestration in Midwest soils. # 2004 Elsevier B.V. All rights reserved.