Gaseous nitrogen losses from plants

The identification, measurement and understanding of the different loss pathways of N from the plant-soil system are prerequisites in efforts to increase the efficiency of N added to the system through biological and industrial fixation. Of the many pathways of loss, those from plants have in the main been ignored, or were thought to be extremely low (Allison 1955, 1966). Recently, however, Wetselaar and Farquhar (1980) have reviewed data that suggest that net losses from above-ground parts of plants can be substantial, up to 75 kg N/ha in 10 weeks (corresponding to 18 nmol (m2 leaf surface)−1 sec−1 from a crop with a leaf area index of 5), from a wide variety of species, in many geographical areas and under a variety of environmental conditions. They further pointed out that such losses are highest for plants with high N contents and take place mainly between anthesis and maturity. During this physiological period proteins break down in senescing leaves, liberating NH3 (Thimann 1980). Gaseous losses of NH3 are therefore possible, and have indeed been found. Gaseous losses of other reduced forms and of oxidized forms have also been found. In addition, losses as N2 have been postulated. Stutte and coworkers (Stutte and Silva 1981, Stutte and Weiland 1978, Stutte et al. 1979, Silva and Stutte 1979a, b, 1981a, b, Weiland and Stutte 1978a, b, 1979a, b, 1980) have reported gaseous fluxes from leaves that would account for the above losses. However, Wetselaar and Farquhar (1980) have suggested that the magnitude may have been overestimated.