Continuous corn with moldboard tillage: Residue and fertility effects on soil carbon

Greenhouse gas emissions from soil depend on land use, cropping systems, and tillage methods. The impact of 30 years of continuous corn (Zea mays L.) with moldboard plow tillage was evaluated from four treatments and a control: silage removal versus grain removal, each with low [83 kg N ha?1 (74 lb N ac?1)] and high [166 kg N ha?1 (148 lb N ac?1)] fertility, and no added fertilizer with grain removal. Soil organic carbon (SOC) changes over a 30-yr period were measured, as well as tillage-induced CO2 loss immediately after moldboard plowing, in the spring of 1996. The 24 h cumulative tillage-induced CO2 loss was not significantly different among treatments (excluding the control). Total C, total N, and C:N ratio in the soil remained virtually unchanged after 30 yr in fertilized treatments. All four treatments produced the same SOC content [21.9 g kg?1 (2.2%)] in the 0 - 20 cm (0 - 8 in) depth. The cumulative total input of 241 Mg ha?1 (107 t ac?1) of aboveground stover from the high fertility grain treatment, compared to none from the high fertility silage treatment, yielded no differences in SOC. Fertilizer N rates of 83 and 166 kg ha?1 (74 and 148 lb ac?1) produced no difference in SOC or associated C:N ratios. Moldboard plow tillage caused rapid soil degassing that masked fertilizer and stover removal and/or return effects on SOC. This uncontrolled SOC decline agreed with other studies in this region, indicating that the soils were sources of CO2 regardless of other agronomic practices, as long as moldboard plow tillage was used.