Use of crop residues for biofuel production raises concerns on how removal will impact soil organic carbon (SOC). Information on the effects on SOC is limited and requires long-term experimentation. Fortunately, Pendleton long-term experiments (LTEs), dating to the 1930s, provide some answers. This study compared crop residue inputs and SOC balance in conventional tillage (CT) winter wheat ( Triticum aestivum L.)-summer fallow (WW-SF) systems with annual rotation of WW and spring pea ( Pisum sativum L.). The WW-SF consisted of crop residue (CR-LTE) (0-90 N ha -1 yr -1, 11.2 Mg ha -1 yr -1 of steer ( Bos taurus) manure and 1.1 Mg ha -1 yr -1) of pea vines additions, residue burning, and tillage fertility (TF-LTE) (tillage-plow, disc, sweep, and N (0-180 kg ha -1)). Winter wheat-pea (WP-LTE) rotation treatments included maxi-till (MT-disc/chisel), fall plow (FP), spring plow (SP), and no-till (NT). Soils were sampled (0-60-cm depth) at 10-yr intervals, and grain yield and residue data collected every year. In WW-SF systems, SOC was maintained only by manure addition and depleted at a rate of 0.22 to 0.42 Mg ha -1 yr -1 in other treatments. In WP-LTE, MT, FP, SP, and NT treatments increased SOC at the rate of 0.10, 0.11, 0.02, and 0.89 Mg ha -1 yr -1, respectively. Minimum straw biomass to maintain soil organic carbon (MSB) in the CR-LTE, TF-LTE, and WP-LTE was 7.8, 5.8, and 5.2 Mg ha -1 yr -1, respectively. Winter wheat-SF straw production was lower than MSB, therefore residue removal exacerbated SOC decline. Harvesting straw residues under NT continuous cropping systems is possible when MSB and conservation requirements are exceeded.
