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

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 ⊸ 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