Long-Term Tillage Effects on Soil Organic Carbon, Nitrogen, and Grain Yields of Winter Wheat - Spring Pea Rotation in the Pacific Northwest.

Questions pertaining to soil quality changes [soil pH, bulk density, organic carbon (SOC), total nitrogen (TN)] and grain yield responses to tillage management practices still exist. Data from a winter wheat (Triticum aestivum L.) - spring pea (Pisum sativum L.) rotation experiment (WP-LTE) in a Walla Walla silt loam was used to evaluate long-term (1963-2010) effect of four tillage systems [(i) fall moldboard plow after wheat and pea (FP), (ii) spring moldboard plow after wheat and fall moldboard plow after pea (SP), (iii) fall disking after wheat and fall chisel plow after pea (DT/CT), and (iv) no-till after wheat and fall sweep after pea (NT)] on pH, bulk density, SOC, TN, and grain yields. In addition, the soil parameters from the WP-LTE were also compared to an undisturbed grass pasture (GP). Intensive cultivation has significantly declined soil pH, SOC, and TN within 60 cm soil profile relative to GP. Soil bulk density did not differ among the managements at any soil depth. Within the cultivated system (WP-LTE), soil pH at surface 0-10 cm depth were significantly lower under DT/CT (5.14) and NT (5.14) than under FP (5.35) and SP (5.34). Also, DT/CT increased SOC at surface (0-10 cm) soils than FP by 17%. Both, SP and DT/CT stored greater SOC in 0-60 cm profile than FP by 8.0 and 6.0%, respectively. The NT maintained SOC levels similar to disturbance tillage systems at all soil layers. Tillage had no influence on TN at any soil layers, except when whole 0-60 cm soil profile was considered had the DT/CT higher total N than the FP by 7.5%. Over 15 years (1995-2010), the FP, SP, DT/CT, and NT treatments increased SOC by 2.6, 9.5, 12.1, and 8.6%, and increased TN by 0.3, 3.9, 8.3, and 8.8 %, respectively, in 0-60 cm soil profile. Mean grain yields (2005-2010) of wheat were 4.25, 4.20, 3.95, and 3.64 Mg/ha in the order of FP = SP > DT/CT > NT, respectively. Mean pea yields (2005-2010) under SP, DT/CT, and NT were not different, and were higher on average by 0.34 Mg/ha than under FP (1.82 Mg/ha). In a winter wheat - spring pea rotation under dryland agroecosystem, conservation tillage systems have potential to accumulate greater SOC and total N, and increase pea yields than traditional FP; however, wheat yields may be limited probably due to heavy weed (downy brome) infestation without moldboard plowing.