Nitrogen Dynamics Through The Forest Floor Of Two Interior Alaska Black Spruce Ecosystems

Abstract

Nitrogen flow in the forest floor of two interior Alaska black spruce (Picea mariana (Mill.) B.S.P.) ecosystems was investigated and related to environmental constraints unique to the area, specifically temperature, moisture, and organic matter quality (C/N ratio). Pools examined were NH(,4)-N, NO(,3)-N, soluble organic N, total (Kjeldahl) and residual organic N. Low addition levels of high enrichment isotope (< 1% of the total nitrogen pool wth 99 atom percent excess ('15)N) were used to describe nitrogen dynamics through pools of selected forest floor components of permafrost-free and permafrost-dominated black spruce sites. A thick carpet of mosses, made up primarily of the feather moss species Hylocomium splendens (Hedw.) B.S.G. and Pleurozium schreberi (B.S.C.) Mitt. played a vital role in the nitrogen economy of the forest floor. Nitrogen, quickly immobilized in the moss layers (green, brown) and retained there, was released very slowly and sequestered in the fermentation and humus layers (021 + 022) where most of the vascular plant roots were located. Vascular understory ('15)N uptake was minimal as was ('15)N export via the soil solution. Periodic mineralization episodes, more frequent and dynamic at the permafrost-free site (where C/N ratios were lower), were largely restricted to the moss layers since available N pools in deeper forest floor layers incorporated little label over the three year period. It proved difficult to separate the effects of rainfall events from that of forest floor temperature fluctuations upon seasonal nitrogen dynamics. In the lower layers of the forest floor temperature and/or moisture rather than organic matter quality appeared to be the overriding factor controlling N flow. The dominance in pool size of NH(,4)-N over NO(,3)-N is discussed with reference to current theories of ecosystem strategy.