41.
On New Zealand dairy farms, it is the nitrogen excreted directly onto pasture, particularly urine, that drives nitrogen (N)
leaching from the farm. A new framework (UPF: Urine Patch Framework) is presented that post-processes the results of a whole
farm model and runs a mechanistic soil model to simulate the urine patches. Two alternative methods to simulate the spatial
distribution of urine patches were implemented and compared (
Grid: spatially explicit, and
Probabilistic: based on the probability of different temporal urination patterns). This paper describes the implementation of these two
methods in connection with a Whole Farm Model; and compares the N leaching predictions with observed data. Two examples are
provided, one analyzing the impact of urine patch overlap and another, the relative risk of N leaching at different times
of urinary N deposition. The model showed good correlation and predictive ability between simulated annual N leaching results
and observed data [R
2 = 94 %, mean relative prediction error (MRPE) = 10 % for
Grid and R
2 = 72 %, MRPE = 20 % for
Probabilistic]. The two methods produced similar results across an 8-year period for monthly and annual N leaching (R
2 = 96 %, MRPE = 10 % and R
2 = 86 %, MRPE = 8 %; respectively). Only 8 % of the paddock area was covered with multiple urinations during 1 year, but as
much as 39 % of the total urine volume was deposited on overlapped patches. Systematically removing all urinary N for 1 month
in either May or June reduced N leaching by approximately 20 %. Avoiding urinary N deposition during autumn or early winter
could be highly effective in mitigating N leached during the following winter.
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