Combined nitrate and phosphorus application promotes rhizosphere alkalization and nitrogen uptake by wheat but not canola in acid subsoils

Purpose: Excess anion uptake in the form of nitrate has been shown to reduce soil acidification. The question is to what extent the deep placement of calcium nitrate can increase this root-induced alkalization in reducing subsurface soil acidity. Methods: Wheat and canola were grown for 35 days in reconstructed soil columns comprising of topsoil (pHCaCl2 5.4) in 0–10 cm and subsurface soil (pHCaCl2 4.8) in 10–50 cm. Two forms of 15N-enriched fertilizers (urea versus calcium nitrate at 237 mg N per column) with or without P fertilizer (NaH2PO4 at 99 mg P per column) were placed at 0–10, 10–20, or 20–30 cm depth. Root proliferation, rhizosphere pH, and shoot 15N recovery were quantified. Results: Uptake of Ca(NO3)2 increased pH up to 0.5 and 0.2 units in the rhizosphere and bulk soil, respectively, in all treated layers compared to those with urea. The combined application of nitrate and P fertilizer facilitated plant nitrate uptake and hence rhizosphere alkalization. Significant increases up to 10% and 22% in shoot and root biomass, respectively, were observed in the combination of nitrate and P treatments compared with the combined urea and P treatments. The nitrate treatment significantly increased the 15N recovery of fertilizer in both wheat and canola up to 20% compared with the urea treatment. Conclusions: The increased nitrate uptake reduced subsurface acidity. The combination of nitrate and P treatment can facilitate alkalinity movement downward, hence ameliorating subsurface soil acidity.