NATURAL ABUNDANCE OF N-15 IN BARLEY AS INFLUENCED BY PRIOR CROPPING OR FALLOW, NITROGEN-FERTILIZER AND TILLAGE

The N-15 abundance of nitrogen was measured in barley grown with 0, 50 and 100 kg/ha of applied nitrogen after pretreatments of either fallow or grain sorghum, where sorghum stubble was either incorporated, removed or retained on the soil surface (zero-till). Barley N-15 abundance was assumed to reflect that of assimilated soil mineral nitrogen. Fallowing soil prior to establishment of barley accumulated 226 kg NO3-N/ha to a depth of 120 cm and resulted in an 88% increase in N-15 enrichment of nitrogen in barley compared with that grown after sorghum. N-15 enrichment was assumed to be mostly the result of isotope fractionation between N-14 and N-15 during denitrification of the large excess of NO3-N present prior to and during the experiment. Nitrogen fertilizer additions caused N-15 depletion of nitrogen in barley. However, where fertilizer additions resulted in excess availability of NO3-N, subsequent denitrification and N-15 enrichment of this NO3-N partially counterbalanced the N-15 depleting effect of fertilizer additions. Where soil NO3-N levels were low (< 25 kg NO3-N/ha) following sorghum there were no difference in N-15 abundance of nitrogen in barley between tillage treatments. With additions of nitrogen fertilizer and the availability of excess NO3-N for denitrification, differences between tillage treatments occurred with some being significant. The ranking of stubble management treatments in terms of their effect on N-15 enrichment of nitrogen in barley was incorporated > removed > surface retained.