UPTAKE AND LEACHING OF NITROGEN FROM ARTIFICIAL URINE APPLIED TO GRASSLAND ON DIFFERENT DATES DURING THE GROWING-SEASON

Artificial urine, equivalent to 30 g N m-2, was applied to replicated plots in a perennial (Lolium perenne L.) sward, each plot receiving a single application on one of six dates between July and November 1990. Recoveries of urine-N in herbage up to the end of the growing season in November decreased linearly for consecutive application dates, ranging from 40% of the urine-N applied in July to a negligible proportion of the final application. In contrast, contents of urine-derived N remaining in the soil (to 1-m depth) in November increased from 3% of the N applied in July to 66% for the final application. Almost all of this was present as nitrate + nitrite-N. Only soils that had received urine in September or later contained significantly greater quantities of mineral-N than the control plots. The mineral-N content of soils collected the following April indicated that most of this urine-derived N had been lost from the soil over the winter. Estimates of the quantities of N leached ranged from 0.7 g N m-2 from untreated plots to 18.6 g N m-2 from plots treated with urine in November. Although grass yields and N uptakes in March and April provided evidence of a residual effect from the previous year's urine applications, contents of mineral-N and of 'potentially mineralisable N' in urine-treated soils in April were not significantly different from those in untreated soils.