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

Abstract

Estimates of the proportion of nitrogen (N) derived from the atmosphere (pN_atm) 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 (ID_res). 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 ID_res estimates of pN_atm 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 pN_atm for 1994 from the 2 methodological approaches were significantly (P < 0 ·01) different for lentil, with ID resulting in a higher estimate than ID_res (0·92 v. 0·85). For chickpea in the same season (1994) the ID_res 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 pN_atm obtained for chickpea in 1994 was 0·91, which was slightly higher than the ID_res estimate. Proportional reliance on BNF was estimated to be greater in spring than at harvest for both lentil and chickpea. The estimates of p N_atm 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 pN_atm. The ¹⁵N abundance, at harvest, of wheat shoots from the ¹⁵N ID_res 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 ID_res 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 pN_atm of lentil in 1994 but for all the other comparisons there were no major differences between estimates obtained using either ID or ID_res.