Finally, soil nitrate supply rates measured in different riparian
vegetation cover types also reveal that allowing livestock access to
riparian woodlots may result in very high NO3 supply rates in soil,
as was the case in the white cedar woodlot at Magog (Fig. 2). At this
site, the very high NO3 supply rates observed in the riparian
woodlot (280 mg/10 cm2/20 d) contrasts sharply with the low
values observed in the nearby herbaceous (19 mg/10 cm2/20 d) and
poplar (10 mg/10 cm2/20 d) buffers, which are fenced to prevent
livestock access. During hot summer days, we observed on many
occasions a herd of cows resting for hours in the shade of the cedar
woodlot. Over the years, this riparian woodlot may have received
high NO3 inputs derived from cow urine and feces. Nitrogen load
representing 1000 kg N/ha have been measured under cattle urine
patches, with NO3 leaching loss being considerable (Di and
Cameron, 2002). Also, the NO3 enrichment of the Magog riparian
woodlot soil by cattle dejections may have been exacerbated by the
fact that white cedar is a slow growing tree species, with a marked
preference for NH4 uptake over NO3 (Farrar, 2006; Gloser et al.,
2009).