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

Abstract

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.