Abstract
Subterranean clover (Trifolium subterraneum L.) was grown in separate associations with annual ryegrass (Lolium rigidum Gaud.) and with phalaris (Phalaris aguatica L.) in reconstituted soil profiles (0-400 mm depth), and N2 fixation was measured by 15N isotope dilution and 15N natural abundance methods. In all experiments, two values were determined, viz. P, the percentage of clover nitrogen (N) iixed from atmospheric N2, and R, the ratio of N assimilated from added 15N-labelled N to N assimilated from indigenous soil N. Estimates of P that were negative were obtained for the clover/phalaris association, using 15N isotope dilution, during the first 16 days after 15N addition. This was due to the R value being higher for clover than for phalaris, which in turn was due to differences in their temporal pattern of growth interacting with a declining ISN enrichment of the plant-available soil N. The R values for clover and ryegrass, when grown together, were similar throughout the experiment, and estimates of P by 15N isotope dilution and natural 15N abundance were similar. The activity of plant roots in different soil layers was examined by injecting a solution of 15N-labelled nitrate at 50, 150 and 300 mm depths. Uptake of 15N was similar at all depths for phalaris and clover, whereas ryegrass assimilated a greater amount at 150 and 300 mrn. However, since the amounts of roots and indigenous plant-available soil N were small in the 100-400 mm zone relative to the 0-100 mm zone, the greater activity of ryegrass roots at depth had no significant effect on the estimates of P. In both plant associations, the estimates of P, by both of the 15N methods, increased with time, and were higher in the clover/ryegrass association than with clover/phalaris. Since this was associated with lower levels of inorganic soil N in the clover/ryegrass association, it must be recognized that the reference plant can induce real changes in P by influencing the soil N status, in the association, as well as causing erroneous estimates of P by the 15N isotope technique.
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Claims 9
Claim · model-written summary
Isotope dilution gave negative clover fixation estimates with phalaris in the first 16 days.
- Taxon
- Subterranean clover
Trifolium subterraneumt0 - Outcome
- Estimates of Pe0
- Value
- negativee0
- Comparator
- phalarise0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Estimates of P that were negative were obtained for the clover/phalaris association, using 15N isotope dilution, during the first 16 days after 15N addition.”
t0“Subterranean clover”
Claim · model-written summary
Clover R values exceeded phalaris R values.
TrifoliumOrganisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the R value being higher for clover than for phalaris”
Claim · model-written summary
Clover and ryegrass R values were similar throughout the experiment.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The R values for clover and ryegrass, when grown together, were similar throughout the experiment”
Claim · model-written summary
The two 15N methods gave similar fixation estimates for clover with ryegrass.
- Taxon
- Subterranean cloverTrifolium subterraneumt0
- Outcome
- estimates of Pe0
- Direction
- similare0
- Comparator
- natural 15N abundancee0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“estimates of P by 15N isotope dilution and natural 15N abundance were similar.”
e1“clover and ryegrass, when grown together”
t0“Subterranean clover”
Claim · model-written summary
Phalaris and clover took up similar 15N amounts at all tested depths.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Uptake of 15N was similar at all depths for phalaris and clover”
Claim · model-written summary
Ryegrass assimilated more 15N at 150 and 300 mm depths.
LoliumOrganisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ryegrass assimilated a greater amount at 150 and 300 mrn.”
Claim · model-written summary
Greater deep ryegrass root activity had no significant effect on clover fixation estimates.
- Taxon
- Subterranean cloverTrifolium subterraneumt0
- Outcome
- estimates of Pe0
- Direction
- no significant effecte0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the greater activity of ryegrass roots at depth had no significant effect on the estimates of P.”
t0“Subterranean clover”
Claim · model-written summary
Fixation estimates increased with time in both clover associations by both methods.
Verbatim
e0“In both plant associations, the estimates of P, by both of the 15N methods, increased with time”
Claim · model-written summary
Clover fixation estimates were higher with ryegrass than phalaris.
- Taxon
- Subterranean cloverTrifolium subterraneumt0
- Outcome
- higher in the clover/ryegrass associatione0
- Direction
- highere0
- Comparator
- clover/phalarise0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“were higher in the clover/ryegrass association than with clover/phalaris”
t0“Subterranean clover”