Estructura organizativa

Total 15_{N recovery was as high as 90% (oats control) but generally, recoveries declined with}

increasing urinary-N rate to between 65-70% for the U700 treatments. The lower recoveries probably stem from undetectable N2 denitrification losses below the level of 15N detection. Given

that soil and climatic conditions under winter forage grazing favour an accumulation of nitrate, then prolonged low-level denitrification loss is probably not surprising. The lower N recovery in the It. ryegrass control treatment probably stemmed from the poorer initial establishment producing a similar, albeit smaller, pool of nitrate subject to the same denitrification processes but at a greater enrichment level (98% 15_{N; 35 kg N ha}-1_{). If it is assumed the unaccounted} 15_{N proportion is due}

to dinitrogen loss then the total denitrification loss under winter forage grazing, and as a proportion of urinary-N applied at 350-700 kg N ha-1_{, can probably be estimated at 30-35%. This is similar to}

that reported by Fraser et al. (1994) (~28%) but high compared with the range of 15_{N recoveries}

published for other New Zealand studies where N2 loss was measured or implied by difference

(Clough et al. 2001; Di et al. 2002; Buckthought et al. 2015). However, soil temperatures (>9°C), WFPS, labile-C and nitrate supply conditions in this study were probably near optimal for denitrification in the A horizon by late August/early September (Haynes & Williams 1993; Di et al. 2014).

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Nitrate-15_{N leaching losses for U350 and U700 treatments were about 25% of the urinary-N applied}

but this was only 52-60% of the total nitrate leaching loss although doubling the urinary-N application rate had a largely additive effect on nitrate- and inorganic-15_{N drainage losses. Di et al.}

(2002) and Fraser et al. (1994), who measured 15_{N-nitrate leaching losses under a urine patch in}

two pasture trials on the same deep Templeton silt-on-sandy loam, found 83% and 69% of nitrate leaching losses occurred from autumn- (May; 1000 kg N ha-1_{) and winter-applied (July; 500 kg N}

ha-1₎15_{N-labelled urine applications, respectively. The reason for a relatively lower percentage of} 15_{N recovered in drainage losses in this study is a little unclear but may be due to the history of the}

field site from which the lysimeters were taken. The lysimeters were collected 6 months before the trial began and came out of established pasture but only modest drainage occurred prior to the trial’s start. It is possible some SOM mineralisation occurred over this period, diluting the NO3--15N

pool. Indeed, the It. ryegrass control lysimeters (where establishment problems were encountered) had an annual nitrate loss of 52 kg NO3--N ha-1, but only a small proportion (<12%) of this was

derived from the 15_{N-labelled pool i.e. there was a large background contribution.}

Ammonium-15_{N leaching constituted between 6-10% of the} 15_{N loss for U350 (±DCD) and U700}

treatments of both catch crops, that in turn made up ~40% of the total N leaching loss. These are high values from what has been reported previously in similar lysimeter studies. Di and Cameron (2002b) in a 15_{N-labelled urine (1000 kg N ha}-1_{) leaching experiment on a Lismore stony pasture}

soil reported negligible leaching of NH4+-N but this was a spring (November) application where

nitrification was likely rapid. The 15_{N proportion of ammonia loss from volatilisation was not}

measured directly but was assumed to be the majority (i.e. ~90%) of the 3% and 4% recorded for U350 and U700 treatments, respectively (Lee et al. 2011).

The proportion of the 15_{N-labelled urine retained in the soil fraction was reasonably high}

(approximately 33% and 24% on average for U350 and U700 treatments, respectively, for both catch-crops) but similar to that reported by Di et al. (2002) and Fraser et al. (1994). However, their studies were on pastures whereas soils in this study were essentially fallow at the time of urine application (apart from the harvested kale plant roots) and thus little of the urinary-N applied could be recycled in root or plant residues prior to the establishment of the crops. In addition, the urinary- N rates used in this study were considerably lower in this study and thus the absolute amount of N

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retained from the application ranged between ~115-169 kg N ha-1 _{as opposed to the 200-250 kg N}

ha-1 _{cited in the other two studies. This} 15_{N fraction has presumably been retained by a mix of}

immobilisation, root uptake and ammonium fixation but as there is little other 15_{N recovery data for}

winter urine applications, let alone under winter forage grazing conditions, it is difficult to infer much. Indeed, comparisons can only be made with cropping where the N is usually applied as inorganic fertilisers, but these applications are normally in spring or autumn, not mid-winter. Nevertheless, Gardner and Drinkwater (2009) found in a meta-analysis of cropping 15_{N studies that soil accounted}

for ~29% of the 15_{N applied.}

By the end of the study there was relatively little 15_{N recovered in the roots of the remaining}

vegetation (~1%) and this is similar to Fraser et al. (1992), who found that live roots also only accounted for 1% of the 15_{N recovered. The oats, having been harvested six months prior to soil}

deconstruction, meant whatever 15_{N was retained in the roots would have been recycled back into}

the soil with only a small proportion recovered in the subsequent kale crop. The ryegrass treatments had five harvests over the growing season; thus, little 15_{N remained in the root mass by}

the time of lysimeter deconstruction.