Sf. Ledgard et al., NITROGEN-FIXATION BY 9 WHITE CLOVER CULTIVARS IN GRAZED PASTURE, AS AFFECTED BY NITROGEN-FERTILIZATION, Plant and soil, 178(2), 1996, pp. 193-203
Forage production and N-2 fixation were determined for nine cultivars
of white clover (Trifolium repens L.) grown with perennial ryegrass (L
olium perenne L.) and receiving nitrogen (N) fertilizer at either 0 or
390 kg N ha(-1) yr(-1). The site was grazed by sheep at 3 or 6 (in wi
nter and summer) week intervals and N fertilizer was applied at 30 kg
N ha(-1) after each grazing. Annual white clover production showed a 2
-fold variation between cultivars and was positively correlated with t
otal pasture production in the 0 N treatment. Nitrogen fertilizer appl
ication increased average total pasture dry matter (DM) production fro
m 12830 to 16010 kg ha(-1) yr(-1), but decreased average white clover
production from 3600 to 2970 kg DM ha(-1) yr(-1). Fertilizer N applica
tion decreased annual N-2 fixation from 111 to 47 kg N ha(-1) (mean fo
r all cultivars, using N-15 dilution), with the decline occurring pred
ominantly in spring and summer. The decrease in N-2 fixation in spring
from 47 to 18 kg N ha(-1) was due largely to a decrease in clover DM
production. In contrast, N application decreased average N-2 fixation
in summer from 36 to 14 kg N ha(-1) due mainly to a large decrease in
the proportion of clover N derived from atmospheric N-2 (from 49 to 24
%), with clover DM production falling by only 10%. Clover cultivars sh
owed a variation in annual N-2 fixation of about 3-fold under both N r
egimes. During winter and spring, the amount of N fixed by the differe
nt cultivars was determined predominantly by their DM production in bo
th N regimes. In contrast, during summer and autumn there was a marked
variation between cultivars in tolerance of N-2 fixation to increased
soil inorganic N due to N fertilizer application. This was evident fr
om a significant cultivar x N interaction for the proportion (P-N) of
clover N derived from N-2 fixation. During summer/autumn, P-N for Kopu
was similar in the 0 and 390 N treatments, whereas P-N declined by up
to two-thirds for the other cultivars. Consequently, in summer/autumn
the amount of N fixed by Kopu decreased by only 20% (from 65 to 52 kg
N ha(-1)) due to N application whereas it decreased by 40-80% (to 15-
34 kg N ha(-1)) for the other cultivars. Thus, N-2 fixation during win
ter/spring was highest for the most productive large-leaved cultivars
(Kopu, Aran and Pitau) either in the absence or presence of added N. I
n contrast, in summer/autumn the cultivars differed in tolerance to ad
ded N, and N-2 fixation in the 390 N treatment was higher for the more
tolerant cultivar Kopu than for the other cultivars.