Nitrogen dynamics under greenhouse conditions as influenced by method of alfalfa termination. 1. Volatile N losses

Abstract

Use of herbicide to terminate alfalfa (Medicago sativa L.), while reducing soil erosion, could conceivably enhance volatile N losses from alfalfa residues. A controlled environment study was conducted to determine the effect of termination method on the quantity of NH3 volatilized 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 compared 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 losses. Termination method had comparatively less effect on NH3 evolution than residue placement although herbicide application tended to increase the proportion of alfalfa N lost via volatilization. Residue incorporation increased cumulative CO2 evolution regardless of termination method. Results of this study suggest that termination methods in which alfalfa residue is retained on the soil surface may result in significant volatile N losses which could diminish the fertilizer value derived from alfalfa residues. However, these N losses do not necessarily preclude the adoption of herbicide termination of alfalfa given the potential reductions in soil erosion and moisture loss which may be associated with this management practice. Key words: Volatile N losses, NH3 evolution, termination method, alfalfa, herbicide, tillage, greenhouse