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Soil acidifi cation has become a major environmental challenge for crop production in the inland Pacifi c Northwest (iPNW). We evaluated the eff ects of tillage and N-fertilizer management on soil pH, soil organic carbon (SOC), soil N, and crop yields from 1995 through 2010 in an ongoing long-term experiment in eastern Oregon. Tillage systems included moldboard plow (MP), disk plow (DP), and subsurface sweep (SW) and N-fertilizer rates were 0, 45, 90, 135, 180 kg N ha -1 crop -1 in a dryland winter wheat (Triticum aestivum L.)-summer fallow (WW-SF) system. Soil pH, SOC, and N were monitored in 1995 and 2010, and crop yields were monitored every other year. Soil pH was lower in the higher N rate treatments. Long-term N fertilizer application increased soil acidity in 0-to 10-cm depth by 0.3, 0.2, and 0.3 units in MP, DP, and SW, respectively, for every 1000 kg N applied through ammonical N fertilizers. Soil pH was higher in DP than MP in 10-to 20-and 20-to 30-cm depth profi les. Th e SOC and N concentrations in the top 30-cm depth were lower in 2010 than in 1995 across all treatments. Wheat yield increased signifi cantly with increase in N rates from 0 to 90 kg N ha -1 crop -1 . Th ere were no further yield increases above 90 kg N ha -1 crop -1 . Soil acidifi cation, SOC and nutrient dynamics should be carefully monitored in cropping systems using ammonical N fertilizers, particularly under high rate of N application and reduced tillage.

Biochar can improve soil health and crop productivity. We studied the response of soil properties and wheat growth to four rates of wood biochar (0, 11.2, 22.4, and 44.8 Mg ha−1) and two fertilizer rates [no fertilizer and fertilizer (90 kg N ha−1, 45 kg P ha−1, and 20 kg S ha−1)]. Biochar application increased soil organic matter (SOM), soil pH, phosphorus (P), potassium (K), sulfur (S) contents, and the shoot and root biomass of wheat. However, these responses were observed at biochar rates below 22.4 Mg ha−1, particularly in treatments without fertilizer. In fertilizer-applied treatments, soil nitrate levels decreased with an increase in biochar rates, mainly due to better crop growth and high nitrate uptake. However, without N addition, the high C:N ratio (500:1) possibly increased nutrient tie-up, reduced plant biomass, and SOM buildup at the highest biochar rate. Based on these results, we recommend biochar rates of about 22.4 Mg ha−1 and below for Walla Walla silt loams.

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