Soil Organic Carbon Dynamics in the Pendleton Long‐Term Experiments: Implications for Biofuel Production in Pacific Northwest

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−1of 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.