Use of residual fertiliser 15N in soil for isotope dilution estimates of N2 fixation by grain legumes

Estimates of the proportion of nitrogen (N) derived from the atmosphere (pNatm) by chickpea and lentil in the alternate phase of a cereal-legume 2-year rotation, for each of 3 seasons (1993, 1994, and 1995) in northern Syria, were obtained from isotope dilution methodology using residual fertiliser 15N in the soil (IDres). The 15N had been immobilised, during the year antecedent to the legume, from 15N-enriched fertiliser which had been applied at sowing to wheat in the cereal phase of the rotation at 30 kg N/ha. For lentil in 1994, and for chickpea in 1993 and 1994, the IDres estimates of pNatm were compared with those obtained by using the classical 15N isotope dilution method (ID) where 15N-enriched fertiliser (either 30 or 10 kg N/ha) was added at sowing to both the legume and non-fixing reference crops. Estimates of pNatm for 1994 from the 2 methodological approaches were significantly (P < 0 ·01) different for lentil, with ID resulting in a higher estimate than IDres (0·92 v. 0·85). For chickpea in the same season (1994) the IDres estimate was significantly higher than the ID estimate (0· 88 v. 0·78) at 30 kgN/ha because the N fertiliser inhibited biological N fixation (BNF). However, using a lower fertiliser rate (10 kg N/ha) for ID the estimate of pNatm obtained for chickpea in 1994 was 0·91, which was slightly higher than the IDres estimate. Proportional reliance on BNF was estimated to be greater in spring than at harvest for both lentil and chickpea. The estimates of p Natm obtained at harvest were greatest (>0·82) for both crops in 1994 and less, but similar, for both crops (0·64-0·79) in the other 2 seasons (1993 and 1995). Although substantial amounts of residual fertiliser N were present in the soil, only a small proportion of the original fertiliser N added (<5%) was utilised by plant uptake plus any losses in the residual year, indicative of a slow remineralisation rate for the immobilised labelled N. Nevertheless, the crops in the residual year were suciently enriched to allow for estimation of pNatm. The 15N abundance, at harvest, of wheat shoots from the 15N IDres method was similar to that of the soil nitrate and ammonium pools, suggesting that plant N uptake through the season had been from an N pool of reasonably constant enrichment. This was in contrast to wheat receiving 15N-labelled fertiliser at sowing, where the shoots at harvest had a higher 15N abundance than the plant-available N pool, indicating a declining15N enrichment of plant-available N in the soil through the season. Furthermore, variability in the 15N abundance of plant-available N with soil depth was also demonstrated to be greater where the 15N IDres method was used, for ammonium N at least. These differences in 15N enrichment patterns of the plant-available N pool for the 2 methods resulted in significantly different estimates for pNatm of lentil in 1994 but for all the other comparisons there were no major differences between estimates obtained using either ID or IDres.