Dissolved Organic Nitrogen Cycling in Hardwood Forests Spanning a Gradient of Nitrogen Fertility.

Dissolved organic nitrogen (DON) has increasingly become recognized as an important component of terrestrial N cycling, sometimes accounting for 60 to over 90% of total N lost from temperate forests, yet we understand little of the processes that regulate those losses. Therefore, in order to gain insight into how DON is cycled across the landscape, we tracked soil solution chemistry at three depths (0, 15, and 100 cm) in five hardwood forests that span a gradient of soil textures, N fertility, and tree species composition in northern Lower Michigan. Our results demonstrated that DON accounted for 10 to over 80% of total N lost from our forests. DON comprised the greatest percentage of total N lost at the low fertility, white oak/black oak forest (80%) where total N losses were low (<0.5 mg/L), although the absolute concentration of DON leaving forests was greatest at intermediate fertility, red oak/red maple and white oak/red maple forests (approx. 0.60mg/L versus 0.25mg/L and 0.20mg/L at low and high fertility sites, respectively). The high fertility, sugar maple/basswood forest lost the most total N of any forest type (approx. 3.0 mg/L), however, this was predominantly as nitrate; DON accounted for <10% of total losses. Based on these results, we concluded DON losses would be greatest from forests where DON production was maximized and its removal minimized, those conditions found in our intermediate fertility forests. DON losses were low from both low and high fertility forests apparently due to low production rates in the former, and high rates of nitrification and soil sorption in the latter.