Am. Mcneill et al., USE OF IN-SITU N-15-LABELING TO ESTIMATE THE TOTAL BELOW-GROUND NITROGEN OF PASTURE LEGUMES IN INTACT SOIL-PLANT SYSTEMS, Australian Journal of Agricultural Research, 48(3), 1997, pp. 295-304
A leaf-feeding technique for in situ N-15-labelling of intact soil-pas
ture plant systems was assessed, using subterranean clover (Trifolium
subterraneum L.) and serradella (Ornithopus compressus L.) grown under
glasshouse conditions. Total recoveries of fed N-15 were 87-100% foll
owing leaf-feeding of plants at flowering but were lower (74-84%) foll
owing the feed at the vegetative stage. Below-ground recovery of fed N
-15 ranged from 7 to 26%, with serradella partitioning a greater propo
rtion of labelled N below ground than subterranean clover. Additionall
y, plants of both species fed at the vegetative stage accumulated a gr
eater proportion of the N-15 label below ground than did those fed at
flowering. Dry sampling procedures, which utilised freeze-drying, enab
led fractionation of the below-ground portion of the system into 'clea
n' nodulated macro-roots with no adhering soil, residual uncleaned roo
t, rhizosphere, and bulk soil. Calculated specific enrichment for the
'clean' roots at different depths demonstrated a relatively uniform di
stribution of N-15 label in the subterranean clover roots: whereas the
presence of large indeterminate nodules in the crown region of serrad
ella roots contributed to apparent uneven distribution of label. Appro
ximately half of the N in the residual fraction of both species consis
ted of labelled material, postulated to be mostly fine root. Additiona
lly, 5-20% of the rhizosphere N and 0.5-3% of the N in Sulk soil was l
egume root-derived, with some N-15 detected in the extractable total s
oluble N and microbial N pools. Rhizodeposition of N represented appro
ximately 10% of total plant N and 17-24% of total below-ground N for s
ubterranean clover, whereas values for serradella were 20 and 34-37%,
respectively. Estimated total below-ground N of subterranean clover re
ached a maximum value of 177 mg N/plant at 98 days after sowing, which
corresponded with a peak shoot N of 243 mg N. Maximum below-ground N
for serradella attained 196 mg N/plant 84 days after sowing with a cor
responding shoot biomass of 225 mg N. There was a decline in the total
below-ground N of serradella at maturity. Overall, recovered clean ro
ot N represented 30-62% of estimated total below-ground N; so it was c
oncluded that standard root recovery procedures might be likely to und
erestimate severely the total below-ground N accretion and N turnover
by legumes. The implications of these results for field estimation of
total legume N yield, biological N fixation, and the N benefit from le
gumes in rotations are discussed.