Volatile loss of nitrogen during decomposition of legume green manure

Significant amounts of volatile ammonia (NH3) may be lost from agricultural ecosystems. While NH3 volatilization from fertilizers has been well-documented, corresponding losses from crop residues, particularly legume green manures, have not been adequately quantified. Ammonia losses from decomposing lentil (Lens culinaris Medik.) green manure were measured under controlled conditions by applying residue to soil inside sealed chambers, establishing air flow and periodically measuring accumulated NH3 loss using acid traps. Three consecutive experiments were conducted to determine the effect of residue placement, air flow rate and green manure composition, respectively. The first experiment, using a relatively slow flow rate (0.07 chamber displacements min(-1)), demonstrated significant volatilization of NH3 (5% of applied N after 56 days) from green manure placed on or suspended above the soil. Incorporating the green manure into soil almost eliminated NH3 losses. Drying and rewetting the residues after the initial 28 days had only a small stimulatory effect on subsequent volatile losses. A second experiment indicated that maximum volatilization could be achieved at air flow rates of 0.3 chamber displacements min(-1) or higher. A third experiment, using an optimum flow rate (0.5 displacements min(-1)), demonstrated significantly higher volatile N losses from field-grown lentil material (14% over 14 days) than from hydroponically cultured lentil material (8% over 14 days). This difference was attributed, in part, to higher soluble N concentrations in the former residue. Ammonia volatilization consistently demonstrated similar temporal patterns: a rapid initial flush, apparently from the ammonification of labile N, followed by an indefinite period of slow volatilization, probably from the mineralization of more recalcitrant N fractions. The volatile loss of labile N from decomposing green manure may appreciably diminish its fertility benefit and represent an important contribution to atmospheric N concentrations.