Descripción y análisis del Centro

Annually, the total N leaching loss was similar between catch crops at ~40, ~200 and ~350 kg N ha-1 _{for the control, U350 and U700 treatments, respectively, comprising around 50% of the N}

applied. Nitrate comprised ~75-88% of the total-N leaching loss with ammonium leaching comprising the remainder (12-25%). Frequent irrigation events after the end of the winter-spring period (end of November) largely leached any remaining nitrate present within the lysimeters but it differed significantly between catch crops. Breakthrough curves for the oats saw larger and repeated spikes in nitrate concentration in drainage immediately following the post-oats harvest period, but for the It. ryegrass there was only a single smaller peak in nitrate concentration before declining to baseline. This suggests that while the oats did not take up any more N than the ryegrass, it could lower drainage sufficiently to retain the nitrate within the lysimeter, albeit to lose it in drainage again in the post-harvest period. Consequently, nitrate losses for the It. ryegrass U350 and U700 treatments were largely completed by the end of the winter-spring period but for the oats a further 23-24% of the annual nitrate leached was captured in that subsequent period. Under a winter-forage/catch-crop system, the normal management plan would be to harvest the oats by early-to-mid November and immediately prepare a seed bed for another winter forage crop or new pasture. This suggests that with careful management, much of this N could be captured by a following crop or pasture if excess drainage could be avoided.

The relatively large proportion of ammonium recovered in drainage for both U350 and U700 treatments (13-26% of total-N leached) was surprising but similar to the value of 25% reported by Malcolm et al. (2015) for a urine-N leaching study on the same Balmoral soil. Reports of significant

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ammonium leaching are rare but most N leaching studies are typically conducted under less-free draining and/or deeper soil profiles e.g. Fraser (1992) and Sprosen et al. (2009) and/or at more optimal times for nitrification and plant-N uptake (Di & Cameron 2002b). The significant quantity of ammonium in drainage water can largely be attributed to a combination of preferential leaching and saturation of the soil’s cation exchange complex. Cichota et al. (2016) has recently reported a study examining preferential leaching under a Canterbury stony Lismore soil under two levels of irrigation intensity and reported in both cases that the lower soil water fraction (ft) (boundary) involved in solute transport for a moist soil was ~0.35. The instantaneous winter applications of urine (10 mm at a time) occurring on the U350 (1 application) and U700 (2 applications) lysimeters, a week apart, would not be dissimilar to an intense irrigation event. This means, given the Balmoral soil’s relatively high stone content (around 50% at 20-30 cm) and reduced pore water capacity, a greater potential for rapid solute transport. Although the soil surface on each lysimeter was pugged by the artificial hoof prior to urine application, some preferential channels likely remained open for rapid transfer of the urine to occur. It is also highly likely that with potassium concentrations in urine comparable with those for N (Williams et al. 1990), there was competition between K+ _and

NH4+ ions for the soil’s cation exchange sites, but these diminish rapidly with depth. Once below

the A horizon (>15 cm) there is a reduced presence of nitrifying bacteria and with soil temperatures remaining cool, ammonium concentrations might decline relatively slowly. As shown in the drainage of the U700 treatments, once at depth, this ammonium can apparently persist for some time.

The range of N2O emission factors (EF3 range 1.3-2.1%) for our treatments were relatively high

compared to values of ~1% given by de Klein et al. (2003) but peak daily N2O emissions were

similar to those reported by de Klein (2014) when the urinary-N rates were similar. The high EF3

values arise because of measurements directly from a urine patch involving a single or double urine application to an already compacted wet soil surface, with reduced aeration, minimal plant growth and a high water filled pore space (WFPS) (Ball et al. 2008). Nitrate, under these soil conditions, is highly favoured to undergo denitrification (Bolan et al. 2004) and since these conditions are often prevalent in winter forage grazing, N2O emission factors will likely tend towards the higher end of

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The calculation of the proportion of denitrification contributed from di-nitrogen emissions was partly compromised by the difficulties in obtaining accurate values at low rates of emissions due to the relatively low 15_{N enrichment (~9%) of the labelled urine. It is likely that further} 15_{N2 loss continued}

after the first three months of measurement and the reported 15_{N2 loss of 8-12% is probably an}

underestimate. Other New Zealand studies quantitatively reporting N2 loss directly or indirectly

from pasture-applied 15_{N-labelled urine have suggested values from 23-37% (Clough et al. 2001;}

Di et al. 2002; Buckthought et al. 2015).

Overall, N uptake in oats and It. ryegrass treatments was similar but only increased by about 25% between U350 (24% of total) and U700 (14% of total) treatments. This would appear to indicate a greater proportionate loss from the U700 treatments but this is not reflected in the plant 15_{N fraction}

which was a similarly-sized (3-4%), if considerably smaller, proportion for both crop U350 and U700 treatments. Thus, it is more likely that the larger non-labelled N fraction retained in the plant material is from subsequent SOM mineralisation and/or applied fertiliser, rather than from the urine application directly.