, Volume 178, Issue 2, pp 193-203

Nitrogen fixation by nine white clover cultivars in grazed pasture, as affected by nitrogen fertilization

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Abstract

Forage production and N2 fixation were determined for nine cultivars of white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium 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 winter 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 total pasture production in the 0 N treatment. Nitrogen fertilizer application increased average total pasture dry matter (DM) production from 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 application decreased annual N2 fixation from 111 to 47 kg N ha-1 (mean for all cultivars, using 15N dilution), with the decline occurring predominantly in spring and summer. The decrease in N2 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 N2 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 N2 (from 49 to 24%), with clover DM production falling by only 10%.

Clover cultivars showed a variation in annual N2 fixation of about 3-fold under both N regimes. During winter and spring, the amount of N fixed by the different cultivars was determined predominantly by their DM production in both N regimes. In contrast, during summer and autumn there was a marked variation between cultivars in tolerance of N2 fixation to increased soil inorganic N due to N fertilizer application. This was evident from a significant cultivar × N interaction for the proportion (PN) of clover N derived from N2 fixation. During summer/autumn, PN for Kopu was similar in the 0 and 390 N treatments, whereas PN 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, N2 fixation during winter/spring was highest for the most productive large-leaved cultivars (Kopu, Aran and Pitau) either in the absence or presence of added N. In contrast, in summer/autumn the cultivars differed in tolerance to added N, and N2 fixation in the 390 N treatment was higher for the more tolerant cultivar Kopu than for the other cultivars.