Nitrogen fixation by subterranean clover (Trifolium subterraneum L.) growing in pure culture and in mixtures with varying densities of lucerne (Medicago sativa L.) or phalaris (Phalaris aquatica L.)
Authors: Dear B. S. , Peoples M. B. , Cocks P. S. , Swan A. D. , Smith A. B. 
Published: September 2002

The proportions of biologically fixed (Pfix) plant
nitrogen (N) and the total amounts of N2 fixed by
subterranean clover (Trifolium subterraneum L.) growing
in pure culture and in mixtures with different densities (5, 10, 20, or
40plants/m2) of newly sown phalaris
(Phalaris aquatica L.) or lucerne
(Medicago sativa L.) were followed over 3 years in a
field study using the 15N natural abundance technique.
The amount of fixed N in subterranean clover was linearly related to shoot
biomass. Over the 3-year period, subterranean clover fixed 23–34 kg
N/t shoot biomass compared with 17–29 kg N/t shoot biomass in
lucerne. Based on above-ground biomass, pure subterranean clover fixed 314 kg
N/ha over the 3 years compared with 420–510 kg N/ha by
lucerne–clover mixtures and 143–177 kg N/ha by
phalaris–clover mixtures. The superior N2 fixation
by the lucerneŒsubterranean clover mixtures was due to the N fixed by
the lucerne and the presence of a higher subterranean clover biomass relative
to that occurring in the adjacent phalaris plots.
In the first year, 92% of subterranean clover shoot N was derived from
fixation compared with only 59% of lucerne. The reliance of clover upon
fixed N2 remained high (73–95%) throughout
the 3 years in all swards, except in pure subterranean clover and lucerne in
August 1996 (56 and 64%, respectively). Subterranean clover usually
fixed a higher proportion of its N when grown in mixtures with phalaris than
with lucerne. The calculated Pfix values for lucerne
(47–61% in 1995 and 39–52% in 1996) were
consistently lower than in subterranean clover and tended to increase with
lucerne density. Although lucerne derived a lower proportion of its N from
fixation than subterranean clover, its tissue N concentration was consistently
higher, indicating it was effective at scavenging soil mineral N.
It was concluded that including lucerne in wheat-belt pastures will increase
inputs of fixed N. Although lucerne decreased subterranean clover biomass, it
maintained or raised Pfix values compared with pure
subterranean clover swards. The presence of phalaris maintained a high
dependence on N2 fixation by subterranean clover, but
overall these swards fixed less N due to the lower clover herbage yields.
Perennial and annual legumes appear compatible if sown in a mix and can
contribute more N2 to the system than where the annual
is sown alone or with a perennial grass. These findings suggest that increases
in the amount of N2 fixed can be achieved through
different legume combinations without interfering greatly with the N fixation
process. Different combinations may also result in more efficient use of fixed
N2 through reduced leaching. Further work looking at
combinations of annuals possibly with different maturity times, different
annual and perennial legume combinations, and pure combinations of perennial
(e.g. lucerne) could be investigated with the aim of maximising
N2 fixation and use. Grazing management to encourage
clover production in mixtures with phalaris will be necessary before the
potential of subterranean clover to contribute fixed N2
in these swards is fully realised.