The Relative Importance of Denitrification and Nitrate Assimilation in Midcontinental Bogs

Denitrification rates measured in a Minnesota bog and a bog in western Ontario were similarly low $(< 0.20-2.28 \mu g m^-2 h^-1 as N)$. Nitrate addition stimulated denitrification at the Minnesota bog, but $< 1%$ of added $No_3^- - N (0.01-0.1 g m^-2)$ was denitrified in 24 h. In the Ontario bog, denitrification was not stimulated by $NO_3^- - N$ application $(0.08 g m^-2)$. Rates of $NO_3^- -N$ uptake by Sphagnum, measured by nitrate reductase acivity assays and $NO_3^-$ disappearance from cultures, were much higher, $100-24,000 \mu g m^-2 h^-1 (0.57-48.7 \mu g g^-1 h^-1)$, than rates of denitrification at comparable $NO_3^-$ loadings. Measurements of $NO_3^-$ in pore water and moss through-fall confirm that $NO_3^-$ disappears in the top 5-10 cm of moss. Plant uptake appears to be the dominant sink for $NO_3^-$ in midcontinental bogs.