Nitrogen losses from soils caused by Rhizobium-dependent denitrification

Abstract

Biological nitrogen fixation by the Rhizobium-legume symbiosis is of considerable agronomic importance, and most research on rhizobia is aimed at a better understanding of symbiotic nitrogen fixation (e.g. Hardy er nl.. 1979; Hardy and Silver. 1977: Hardv and Gibson. 1977). However the d I significance of early observations on denitrification by rhizobia (Rajagopalan, 1938; Ishizawa, 1939; Wilson, 1947) appears to have been overlooked; it has recently been shown to be widespread among Rhizobium species (Zablotowicz et al., 1978; Daniel ef al., 1980a. 1982) and to occur in soils under laboratory conditions (Daniel el al., 1980b). We report here field experiments showing the loss of fixed nitrogen from soils by denitrification carried out by rhizobia at rates comparable to those at which rhizobia fix nitrogen. This occurs in aerated soils, unlike previously described soil denitrification systems which are considered to be anaerobic (Knowles, 1982). Rhizobia are widespread in agricultural ecosystems (Parker et al., 1977) and an understanding of the conditions under which they denitrify may convey substantial benefits.