Influence of moderate phosphate addition on nitrogen retention in an acidic boreal lake

Abstract

Inorganic phosphorus (P) fertilizer was added to a small, acidic lake in southernmost Norway to test the potential of this technique to increase the retention of total inorganic nitrogen (TIN) and thereby mitigate the effects of elevated leaching of atmospherically derived N. The experiment was conducted over 4 years (1 pretreatment year followed by 3 years with weekly P additions during the growing season). To avoid any undesirable eutrophication effects, the total P concentration was increased from 4–5 µg P·L–1 to a moderate level of 10–12 µg P·L–1. Compared with the pretreatment year, the P additions increased the average TIN retention in the experimental lake by nearly 70% (from 53 to 88 mmol·m–2·year–1) during three growing seasons. However, when considering that the reference lake experienced a 55% decrease in TIN retention during the same period, the increase might have been even larger. This underlines the role of P (both natural and anthropogenic) as an important regulator of N retention in upland lake districts that in large parts of southernmost Norway contribute significantly to the N loading on coastal marine areas.