The high rates of urine‐N deposited during grazing offers the potential for considerable N loss through volatilization, leaching, and denitrification. A field experiment was conducted to evaluate the magnitude and pathways of N transformations and losses under warm, moist conditions. Urine was applied to 3‐m2 plots in a ryegrass (Lolium perenne L.)‐white clover (Trifolium repens L.) pasture at 300 or 600 kg N/ha. These rates simulate the urine affected area of sheep and cattle, respectively. Ammonia volatilization, inorganic N in the soil profile, symbiotic nitrogen fixation (by acetylene reduction assay), and herbage N yields were measured over 53 days. Four days after application from 30 to 35% of the added N could not & accounted for as either soil mineral N or volatilized NH3‐N. Ammonia‐N evolution was rapid only for the first 2 days, and I5 to 18% of the N was volatilized as NH3. Profile NH4‐N was completely nitrified by 21 days. Considerable amounts of the applied N was lost from the 0 to 45 cm profile by 53 days. Since rainfall and irrigation exceeded estimated evapotranspiration, leaching of NO3‐N is the probable loss mechanism. The apparent recovery of N in herbage was 37 and 22% of that added. Acetylene reduction was substantially less in the urine‐treated swards. From 30 to 35% of the urine‐N was not accounted for as inorganic N early in the experiment; immobilization and rapid leaching of urine below 45 cm are probable loss mechanisms. The experiment indicated that in a warm, moist environment, typical of late spring‐early summer in New Zealand, more than half of the N in urine voided by sheep or cattle can be lost, and that an intensively grazed grass‐legume system is far from a closed system, or cycle, with respect to N.

The rate of nitrogen fixation (measured by determining the rate of acetylene reduction) by roots of established white clover which had received 80lb/ac of N was 23-30% of the rate for roots from plants which received no fertiliser nitrogen. The rate of acetylene reduction started to decrease within 24 hr of defoliation of the plants and stayed at a low level until the sixth day, after which it increased, reaching levels similar to those of undefoliated plants in 21 days. There was no significant difference in the rate of acetylene reduction between the defoliated and undefoliated plants 34 days after defoliation.

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Contents of Nand nonstructural carbohydrate fractions were measured over a year in herbage from a grazed ryegrass-white clover pasture, which had been fertilised with limeammonium nitrate at an annual rate of 0 (No), 112 (N112), and 448 (N448) kg N/ha. Total N wntent of the mixed herbage was often greater than 4% dry weight, and was highest in earlY spring and lowest in late summer. Levels in many cuts were increased significantly by the N448 treatment; the N112 treatment had little effect. The non-protein N and nitrate N contents were also highest in the N448 herbage, particularly in early spring; the level of nitrate N was also high in autumn. Most of the nitrate N occurred in the grass component. Total watersoluble carbohydrates, and those soluble in hot water but insoluble in cold water, usually comprised less than 12% and 1% dry w¢ight respectively of the mixed herbage. Total water-soluble carbohydrates were negatively and significantly correlated with both total N ;;nd "protein" N contents. The total N/total water-soluble carbohydrate ratio was, with one exception, greater than 0.3. No clear seasonal trends in the carbohydrate fractions were evident. Effects due to N treatments were generally small although sometimes significant for lhe hot-water-soluble fraction. The highest level of fertiliser N brought about high total N/total water-soluble carbohydrate ratios, and high herbage nitrate N contents, namely 0.66% dry weight in early spring and an average of 0.45% in August-January. The implications of these results for animal health are discussed. 231

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