Nitrogen (N) inputs and outputs were measured over 3 years in a trial with four farmlets (each with 16 randomly-allocated 0n4 ha paddocks) on permanent white clover\ryegrass pastures which were grazed throughout the year by dairy cows near Hamilton, New Zealand. Three farmlets were stocked at 3n3 cows\ha and received nominal rates of N fertilizer (urea in 8-10 split applications) of 0, 200 or 400 kg N\ha per year. A fourth farmlet with 4n4 cows\ha received 400 kg N\ha per year and was supplemented with maize grain during the first two years.Nitrogen balances were calculated, with ΣN inputs $ ΣN outputs. Annual inputs from N # fixation were 99-231 kg N\ha in the 0 N farmlet, but declined to 15-44 kg N\ha in the 400 N farmlets. The main N outputs (in kg N\ha per year) were in milk (72-126), nitrate leaching (20-204), and transfer of N via cow excreta from pastures to lanes and milking shed (54-92). Gaseous losses by denitrification (3-34) and volatilization (15-78) were smaller than the other N outputs but increased significantly with N fertilizer application. In the maize-supplemented farmlet, N outputs in milk were 31 % higher than in the corresponding non-supplemented 400 N farmlet, whereas leaching losses averaged 17 % lower during the 2 years of supplementation.In the N-fertilized farmlets, estimated N balances were influenced by inclusion of the transitional N processes of immobilization of fertilizer N into the soil organic N pool (estimated using "&N at 42-94 kg N\ha per year) and the contribution from mineralization of residual clover-fixed N in soil not accounted for in the current estimates of N # fixation (estimated at up to 70 % of measured N # fixation or 46 kg N\ha per year). However, these processes were counteracting and together were calculated to have only a small net effect on total N balances.The output of N in products (milk, meat and feed) relative to the total N input averaged 26 % in the 400 N farmlets, and is compared to that measured for commercial intensively-managed dairy farms in England and the Netherlands (14-20 %). The 0 N farmlet, which was reliant on N # fixation as the sole N input, was relatively very N-efficient with the milk production being 83 % of that in the 400 N farmlet (at 3n3 cows\ha) and the N output in products relative to total N input averaging 52 %.

Nitrogen fixation by nine white clover cultivars in grazed pasture, as affected by nitrogen fertilizationS t e w a r t E L e d g a r d 1, M i k e S. S p r o s e n I and K e i t h W. Steele 2 1AgResearch, AbstractForage 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 N ha-t yr-i. 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-t 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-l, 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-t 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 cuitivars. 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 Kopu than for the other cultivars.

A field lysimeter experiment, using a free-draining pumice soil, was carried out to investigate the effect of different soil nitrogen (N) process inhibitors on the fate of 15N-labelled cow urine. The treatments were a urease inhibitor (Agrotain; N-(n-butyl) thiophosphoric triamide), 2 nitrification inhibitors (dicyandiamide, DCD; 4-methylpyrazole, 4MP), a combination (DCD+Agrotain), a urine control, and a nil urine. The inhibitors were mixed with cow urine, which was then applied in a single application (equivalent to 775 kg N/ha) to lysimeters in autumn and monitored over the following 196 days. DCD and 4MP similarly reduced nitrate leaching by 59%, from 114 to 47 kg N/ha compared with the urine control. Of the DCD applied, 58% of it (8.7 kg/ha) was recovered in leachate, and represented an N loss of 5.8 kg/ha. The presence of Agrotain reduced ammonia (NH3-N) emissions by 64% (equivalent to 70 kg N/ha) over the short term (first 20 days), but led to large leaching losses of urea-N (25 kg N/ha) over the medium term (76 days). The Agrotain-DCD combination resulted in even larger losses of urea-N (45 kg N/ha). The major N component measured in leachate (below 450 mm) was ammonium-N, which constituted about 60% of the average total N (205 kg N/ha) leached. Ammonium-N leaching was rapid and almost entirely driven by macropore flow processes. Further research is required in deeper soil profiles, and in relation to climatic risk of high early rainfall on fresh urine patches, to determine the importance of macropore processes on N loss under typical on-farm soil conditions.

Urine excreted from grazing animals has been identified as the main source of nitrate leaching from grazed pastures. The nitrification inhibitor dicyandiamide (DCD) has the potential to be an important tool for reducing nitrate leaching in grazed pasture systems. A mowing trial near Rotorua examined the effects of different DCD formulations, and the timing of DCD application on plant growth and leaching of urine-N. Treatments were urineonly or urine+DCD (either granular or liquid DCD) applied at a rate of 18 kg DCD ha-1. The effect of timing of DCD application was tested by applying the DCD to plots in May (autumn) or July (winter) immediately after urine was applied. The application rate of the urine was equivalent to 598 kg N ha-1 in the form of artificial urine. There was no significant effect of DCD formulation (granular versus liquid) on any N leaching or yield parameter measured in either the autumn or winter application. On average, the autumn application of DCD significantly (P < 0.001) increased pasture dry matter (DM) from urine plots (34% increase or 21 DM ha-1). Extrapolation of treatments with or without DCD to grazed pasture, with 5% of the area affected by urine at this grazing, would equate to an annual pasture response of approximately 7%. The increased pasture growth resulted in a greater apparent recovery of urine-N in herbage from the urine+DCD treatments compared with the urine-only treatment over the first 98 days of A07178; Online publication date

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