Gross Nitrogen Transformations, 15N assimilation and Recovery at the Tree Scale.

Advanced fertilization systems adopted by almond [Prunus dulcis (Mill.) D.A. Webb] growers in California deliver nitrogen (N) in solution with irrigation water to meet peak tree demand. In summer 2010, we applied this technique using ¹⁵NH₄¹⁴NO₃ and ¹⁴NH₄¹⁵NO₃ (10% a.e.) to quantify gross N transformation rates and to trace ¹⁵N assimilation by microbes and fine roots over two days after fertilization (DAF) and into the almond crop for three successive years. We deployed a transect of closed chambers to spatially constrain ¹⁵N₂O flux and sampled soil to 50 cm at 10 cm intervals to extrapolate measurements to the tree scale. At 1 DAF, gross nitrification (171 g N tree^‑1) exceeded dissimilatory nitrate reduction to ammonium (DNRA; 16.2 g N tree^-1) while gross mineralization (15.5 g N tree^-1) was lower and NH₄⁺ consumption (116 g N tree^-1) and NO₃^- consumption (244 g N tree^-1) were greater than at 2 DAF.  At 2 DAF, both DNRA (24.0 g N tree^-1) and gross mineralization (37.1 g N tree^-1) increased while gross nitrification (67.9 g N tree^-1), NH₄⁺ (70.5 g N tree^-1) and NO₃^- (101 g N tree^-1) consumption decreased compared to 1 DAF.  These results support the notion that fertilization stimulates oxidation and consumption of N within 1 DAF and that this system shifts progressively toward greater soil N supply from mineralization and soil N retention by DNRA within 48 hours.  At 1 DAF, microbes assimilated more N than at 2 DAF and greater N than tree roots.  Despite greater competition for N, peak ¹⁵N₂O flux was observed at 1 DAF and was substantially greater from ¹⁵NH₄¹⁴NO₃ compared to ¹⁴NH₄¹⁵NO₃.  We conclude that effective N management may be attainable from increased utilization of native soil N, increased retention of soil N, and inhibiting nitrification that leads to N losses from agricultural ecosystems.