NITROGEN-FIXATION IN A WHITE CLOVER-GRASS PASTURE IRRIGATED WITH SALINE GROUNDWATER

Nitrogen (N2) fixation in an irrigated white clover-grass sward was es timated using the N-15 isotope dilution technique following the additi on of (KNO3)-N-15 at 0.5 g N m-2 and 80 atom % N-15 in a field study d uring the 1990-91 season. Two water salinity treatments (channel water ; EC(w) = 0.07 and groundwater; 2.4 dS m-1) and four irrigation freque ncies were included in a factorial design with four replicates. The ch annel water treatments were irrigated when pan evaporation minus rainf all equalled 50 mm, whereas the groundwater treatments were irrigated at deficits of 40, 50, 65 or 80 mm. Cumulative dry matter of the clove r was significantly less in treatments irrigated with saline groundwat er compared to channel water at day 164, and soil salinities (EC(e)) i ncreased on average from 2.3 to 5.07 dS m-1. In contrast, salinity of the irrigation water had no effect on the cumulative yield of grass. C umulative dry matter of the grass and clover were not affected by grou ndwater irrigation frequency. Total N accumulation by the grass did no t differ significantly between treatments. However, total N accumulati on in white clover was significantly less (P < 0.05) in all treatments irrigated with groundwater compared to channel water. Neither the N c oncentrations of the grass nor the clover differed significantly betwe en the salinity treatments. Salinity and irrigation frequency had no e ffect on the proportion of clover N (P(atm)) derived from N2 fixation. The values of P(atm) were high throughout, and increased progressivel y from 0.78 at day 39 to 0.91 at day 164 (P < 0.01). However, the yiel d of fixed N was lower in clover when watered with groundwater compare d to channel water (P < 0.01). Thus low to moderate soil salinity did not affect the symbiotic dependence of clover, but the yield of biolog ically-fixed N was depressed through a reduction in the dry matter yie ld of the legume. The efficiency of the dairy industry in the Murray-G oulburn Irrigation region is dependent on the white clover component o f the pasture supplying adequate nitrogen (N) for the intensive grazin g system (Cockroft and Martin 1981; Mundy 1987). In conjunction with w ater, the flux of biologically-fixed N, into the farming system determ ines the yield and productivity. Greater amounts of N are essential if the higher yield potential afforded by irrigation is to be realised. Limited data are available on the inputs of atmospheric N2 fixed by wh ite clover grown in mixed pastures of the region (Mundy 1987). A shall ow saline watertable has developed throughout the irrigation region of northern Victoria, with commensurate rises in soil salinity (Mehanni and Bleasdale 1983; Noble et al. 1987). The re-use of the moderately s aline groundwater for irrigation is one salinity management strategy u nder consideration. However, this option is only practicable if produc tivity losses are minimal. Studies have been undertaken to evaluate th e effect of irrigation with saline water on the growth of pasture (Meh anni and Repsys 1980, 1986) and lucerne (Noble et al. 1987), and recen tly Mehanni and West (1992) reported positive pasture yield response t o N application when irrigated with saline water. However, no measurem ents have been made of the effect of irrigation with saline water on N 2 fixation. Consequently, the impact of changes in the irrigation wate r quality and soil salinity on N2 fixation must be determined. The obj ective of this work was to determine the effect of irrigation with sal ine groundwater on the proportion of N derived from the atmosphere (P( atm)) in white clover, estimated by the N-15 isotope dilution techniqu e using grass as the non-fixing reference plant.