Crop Rotations and Nitrogen Fertilization to Manage Soil Organic Carbon Dynamics

Sustainability is influenced in many production systems by the variation of soil organic C (SOC) content and dynamics, and crop rotations. We hypothesized that arable layer SOC under conventional tillage can be managed through the amount of residue C (RC) returned to the soil as affected by tillage and fertilization. Soil organic C dynamics of a complex of Typic Argiudoll and Petrocalcic Paleudoll soils under conventional tillage between 1984 and 1995 at Balcarce, Argentina was studied for 16 crop sequences. Crops included were spring wheat ( Triticum aestivum L.), soybean [ Glycine max (L.) Merr.], sunflower ( Helianthus annuus L.), and corn ( Zea mays L.). Eleven years of conventional tillage decreased SOC 4.1 to 8.8 g kg −1 without supplemental N and 2.8 to 7.2 g kg −1 when N fertilizer was applied. Soil organic C loss increased when soybean (1.2 Mg RC ha −1 yr −1 ) was present in the sequence and decreased when corn (3.0 Mg RC ha −1 yr −1 ) was present. The amount of RC returned by the sequences correlated with SOC in 1995( r = 0.84 , P < 0.01 )and with SOC at equilibrium( r = 0.76 , P < 0.01 ), but the sequences with two summer crops (soybean, sunflower, or corn) every 3 yr showed lower SOC in 1995 (28.9–33.8 g kg −1 ) and at equilibrium (24.0–34.4 g kg −1 ) than sequences with none or one summer crop (29.7–35.0 g kg −1 either in 1995 or at equilibrium) for the same range of RC (1.4–2.6 Mg RC ha −1 yr −1 ). The difference between sequences in the relationship between RC and SOC were attributed to tillage timing. Under conventional tillage, arable layer SOC can be managed through the selection of the crops in the rotation and N fertilization, but the timing and intensity of tillage have to be taken into account.