Long‐Term Cropping System Effects on Carbon Sequestration in Eastern Oregon

Soil organic carbon (SOC) has beneficial effects on soil quality and productivity. Cropping systems that maintain and/or improve levels of SOC may lead to sustainable crop production. This study evaluated the effects of long‐term cropping systems on C sequestration. Soil samples were taken at 0‐ to 10‐, 10‐ to 20‐, 20‐ to 30‐, and 30‐ to 40‐cm soil depth profiles from grass pasture (GP), conventional tillage (CT) winter wheat ( Triticum aestivum L.)–fallow (CTWF), and fertilized and unfertilized plots of continuous winter wheat (WW), spring wheat (SW), and spring barley ( Hordeum vulgare L.) (SB) monocultures under CT and no‐till (NT). The samples were analyzed for soil organic matter (SOM) and SOC was derived. Ages of experiments ranged from 6 to 73 yr. Compared to 1931 SOC levels (initial year), CTWF reduced SOC by 9 to 12 Mg ha −1 in the 0‐ to 30‐cm zone. Grass pasture increased SOC by 6 Mg ha −1 in the 0‐ to 10‐cm zone but decreased SOC by 3 Mg ha −1 in the 20‐ to 30‐cm zone. Continuous CT monocultures depleted SOC in the top 0‐ to 10‐cm zone and the bottom 20‐ to 40‐cm zone but maintained SOC levels close to 1931 SOC levels in the 10‐ to 20‐cm layer. Continuous NT monocultures accumulated more SOC in the 0‐ to 10‐cm zone than in deeper zones. Total SOC (0‐ to 40‐cm zone) was highest under GP and continuous cropping and lowest under CTWF. Fertilizer increased total SOC only under CTWW and CTSB by 13 and 7 Mg ha −1 in 13 yr, respectively. Practicing NT for only 6 yr had started to reverse the effect of 73 yr of CTWF. Compared to CTWF, NTWW and NTSW sequestered C at rates of 2.6 and 1.7 Mg ha −1 yr −1 , respectively, in the 0‐ to 40‐cm zone. This study showed that the potential to sequester C can be enhanced by increasing cropping frequency and eliminating tillage.