NITROGEN DYNAMICS UNDER GREENHOUSE CONDITIONS AS INFLUENCED BY METHODOF ALFALFA TERMINATION - 1 - VOLATILE N LOSSES

Use of herbicide to terminate alfalfa (Medicago sativa L.), while redu cing soil erosion, could conceivably enhance volatile N losses from al falfa residues. A controlled environment study was conducted to determ ine the effect of termination method on the quantity of NH3 volatilize d from residues of alfalfa. A factorial combination of two termination methods (chemical, mechanical) and two methods of residue placement ( incorporated, surface applied) was applied to alfalfa. Treatments were incubated in a flow-through chamber in the greenhouse for 95 d during which NH3 and CO2 evolution was measured periodically. After 95 d, 8% of surface applied legume N was lost as NH3 in tillage treatments com pared to 12% in herbicide treatments. These losses amounted to 3% and 4% of total legume N in tillage and herbicide treatments, respectively . Incorporation of alfalfa residue essentially eliminated volatile N l osses. Termination method had comparatively less effect on NH3 evoluti on than residue placement although herbicide application tended to inc rease the proportion of alfalfa N lost via volatilization. Residue inc orporation increased cumulative CO2 evolution regardless of terminatio n method. Results of this study suggest that termination methods in wh ich alfalfa residue is retained on the soil surface may result in sign ificant volatile N losses which could diminish the fertilizer value de rived from alfalfa residues. However, these N losses do not necessaril y preclude the adoption of herbicide termination of alfalfa given the potential reductions in soil erosion and moisture loss which may be as sociated with this management practice.