Use of residual fertiliser 15N in soil for isotope dilution estimates of N2 fixation by grain legumes
Authors: Pilbeam C. J. , Harris H. C. , Swift R. S. , McNeill A. M. 
Published: August 2004

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.