Cropping System and Nitrogen Effects on Mollisol Organic Carbon

Time, fertilizer, tillage, and cropping systems may alter soil organic carbon (SOC) levels. Our objective was to determine the effect of long‐term cropping systems and fertility treatments on SOC. Five rotations and two N fertility levels at three Iowa sites (Kanawha, Nashua, and Sutherland) maintained for 12 to 36 yr were evaluated. A 75‐yr continuous corn ( Zea mays L.) site (Ames) with a 40‐yr N‐P‐K rate study also was evaluated. Soils were Typic and Aquic Hapludolls and Typic Haplaquolls. Four‐year rotations consisting of corn, oat ( Avena sativa L.), and meadow (alfalfa [ Medicago sativa L.], or alfalfa and red clover [ Trifolium pratense L.]) had the highest SOC (Kanawha, 32.1 g/kg; Nashua, 21.9 g/kg; Sutherland, 27.9 g/kg). Corn silage treatments (Nashua, ≤ 18.9 g/kg; Sutherland, ≤23.2 g/kg) and no‐fertilizer treatments (Kanawha, 25.3 g/kg; Nashua, ≤20.9 g/kg; Sutherland, ≤23.5 g/kg) had the lowest SOC. A corn‐oat‐meadow‐meadow rotation maintained initial SOC (27.9 g/kg) after 34 yr at Sutherland. Continuous corn resulted in loss of 30% of SOC during 35 yr of manure and lime treatments. SOC increased 22% when N‐P‐K treatments were imposed. Fertilizer N, initial SOC levels, and previous management affected current SOC levels. Residue additions were linearly related to SOC (Ames, r 2 = 0.40; Nashua, r 2 = 0.82; Sutherland, r 2 = 0.89). All systems had 22 to 49% less SOC than adjacent fence rows. Changing cropping systems to those that conserve SOC could sequester as much as 30% of C released since cropping began, thereby increasing SOC.