Soil chemical properties and wheat yields under different tillage and nitrogen rates in eastern Oregon
Tillage and nitrogen (N) application influence soil‐chemical properties and crop productivity. In a long‐term (76‐yr) wheat (Triticum aestivum L.)–fallow system in northeast Oregon, we assessed the effects of tillage (subsurface‐sweep, sweep; offset‐disk, disk; and moldboard plow, plow) and N application rates (0, 90, and 180 kg N ha−1) on soil organic matter (SOM), soil‐chemical properties in 0‐to‐30‐cm soil profile, and wheat yields. Sweep and disk exhibited more pronounced vertical gradient in soil chemical properties within the 0‐to‐30‐cm soil profile than plow due to shallow soil mixing. Across N rates, at 0‐to‐10‐cm depth, sweep and disk had lower pH (0.60), calcium (Ca; 0.32 g kg−1), magnesium (Mg; 83 mg kg−1), and estimated cation exchange capacity (CECe; 1.8 cmol kg−1), but higher SOM (9 g kg−1) and aluminum (Al; 26 mg kg−1) than plow. Below 10 cm, plow had lower pH (0.67–0.83), Ca (0.29–0.31 g kg−1), and CECe (1.7–1.8 cmol kg−1), but higher Al (13–16 mg kg−1) than disk and/or sweep. Across tillage treatments and depths, pH, Ca, Mg, and CECe generally decreased, whereas phosphorus, potassium, and Al increased with increasing N rates. Wheat yields increased by 14% from N application but did not differ between 90 and 180 kg N ha−1. Sweep and disk produced 9% less wheat yield than plow. Long‐term N application and grain removal of basic cations resulted in surface soil acidification under sweep and disk, which if not remedied, can decrease wheat productivity in this region.