Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha⁻¹ and received urea at 0, 200 or 400 kg N ha⁻¹ yr⁻¹ in 8-10 split applications. A fourth farmlet received 400 kg N ha⁻¹ yr⁻¹ and had 4.4 cows ha⁻¹. There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha⁻¹ yr⁻¹. Despite this variation, total pasture production in the 0 N treatment remained at 75-85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years. Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (PN; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43-48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha⁻¹ yr⁻¹ to 39-53 kg N ha⁻¹ yr⁻¹. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons. In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease PN from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.