Print PageThe effective management of nitrogen (N) fertilizers is one of the most urgent issues confronting the agricultural community. On the arid lands in California, almond growers use advanced fertilization practices including micro-irrigation systems that deliver N in solution with water targeted to meet peak tree demand. In summer 2010, we examined gross N transformation rates and N assimilation by microbes and tree roots over two days after fertilization (DAF) using a combined isotope dilution and tracer approach. We deployed a transect of closed chambers to spatially constrain 15N2O flux during the experimental period and sampled soil to 50 cm at 10 cm intervals to extrapolate measurements to the tree scale.
Results/Conclusions
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 up to an order of magnitude greater 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 inhinbiting nitrification that leads to N losses from agricultural ecosystems.
phone 202-833-8773