Winter Wheat-Summer Fallow Cropping Systems Deplete Soil Organic Carbon.

Soil organic matter (SOM) is essential for sustaining crop productivity. SOM consists of carbon (C) based compounds that improve soil structure, nutrient storage and water holding capacity. In agricultural lands, cropping systems that maintain or increase SOM sequester C, thereby reducing emission and buildup of carbon dioxide (CO2) in the atmosphere. Tillage, crop rotations, and cropping intensity influence the rate at which C is added to or removed from soil. Cropping systems that result in C gain can lead to sustained soil and crop productivity. SOC (58% of SOM) has been monitored at 10-year intervals in the Pendleton, OR and Moro, OR long-term experiments (LTEs) since their inception in 1931 and 2004, respectively. The most recent sampling was conducted in 2010. At Pendleton, with mean annual precipitation of 400 mm, SOC in the 0-60 cm depth profile continued to decline in winter wheat-summer fallow (WW-SF) based cropping systems except where 22.4 Mg ha^-1 of steer manure was added biennially or under annual cropping. SOC from a nearby grassland plot was significantly higher than all cultivated treatments. At, Moro, with a mean annual precipitation of 280 mm, SOC from the grassland plot was significantly different from SOC in WW-SF but not from SOC in no-till annual cropping systems in the 0-60 cm soil depth profile. Differences in SOC from grassland and from cultivated plots were more pronounced in Pendleton, a higher precipitation zone, than at Moro, a dryer site. In conclusion, tillage and crop intensity influences SOC accretion; WW-SF system depletes SOC; annual cropping maintains SOC even under conventional tillage; no-till or direct seeding has the potential to increase SOC; increasing cropping frequency and reducing or eliminating tillage increases the potential for SOC accretion and for sustaining soil productivity.