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.