Tillage, Nitrogen, and Cropping System Effects on Soil Carbon Sequestration

Soil C sequestration can improve soil quality and reduce agriculture's contribution to CO 2 emissions. The long‐term (12 yr) effects of tillage system and N fertilization on crop residue production and soil organic C (SOC) sequestration in two dryland cropping systems in North Dakota on a loam soil were evaluated. An annual cropping (AC) rotation spring wheat (SW) ( Triticum aestivum L.)–winter wheat (WW)–sunflower (SF) ( Helianthus annuus L.) and a spring wheat‐fallow (SW‐F) rotation were studied. Tillage systems included conventional‐till (CT), minimum‐till (MT), and no‐till (NT). Nitrogen rates were 34, 67, and 101 kg N ha −1 for the AC system and 0, 22, and 45 kg N ha −1 for the SW‐F system. Total crop residue returned to the soil was greater with AC than with SW‐F. As tillage intensity decreased, SOC sequestration increased (NT > MT > CT) in the AC system but not in the SW‐F system. Fertilizer N increased crop residue quantity returned to the soil, but generally did not increase SOC sequestration in either cropping system. Soil bulk density decreased with increasing tillage intensity in both systems. The results suggest that continued use of a crop‐fallow farming system, even with NT, may result in loss of SOC. With NT, an estimated 233 kg C ha −1 was sequestered each year in AC system, compared with 25 kg C ha −1 with MT and a loss of 141 kg C ha −1 with CT. Conversion from crop‐fallow to more intensive cropping systems utilizing NT will be needed to have a positive impact on reducing CO 2 loss from croplands in the northern Great Plains.