Natural abundance of ¹⁵N in barley as influenced by prior cropping or fallow, nitrogen fertilizer and tillage

Abstract

The 15N 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 15N 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 15N enrichment of nitrogen in barley compared with that grown after sorghum. 15N enrichment was assumed to be mostly the result of isotope fractionation between 14N and 15N during denitrification of the large excess of NO3-N present prior to and during the experiment. Nitrogen fertilizer additions caused 15N depletion of nitrogen in barley. However, where fertilizer additions resulted in excess availability of NO3-N, subsequent denitrification and 15N enrichment of this NO3-N partially counterbalanced the 15N depleting effect of fertilizer additions. Where soil NO3-N levels were low (<25 kg NO3-N/ha) following sorghum there were no differences in 15N 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 15N enrichment of nitrogen in barley was incorporated > removed > surface retained.