Isotopic Identification of Nitrate Sources and Cycling in Arctic Tundra Active Layer Soils and Permafrost

Abstract:

The effect of nitrogen cycling on release of carbon from tundra ecosystems is being studied as part of the US Department of Energy Next Generation Ecosystem Experiment – Arctic project. Sampling and analysis of active layer soil water at the Barrow Environmental Observatory (Alaska, USA) was performed in ancient drained thaw lake basins (DTLBs), drainages, and in polygonal terrain associated with inter-DTLB tundra. Within active layer soils, nitrate was most commonly found above analytical limits of detection in pore water from the unsaturated centers of high-centered polygons. Nitrate has also been detected, though less frequently, in soil water immediately above the frost table of an ancient (¹⁴C age of 2000 – 5500 BP) DTLB and in a small drainage adjacent to high-centered polygonal terrain.

Nitrate from high-centered polygons had δ¹⁵N ranging from -9.2 to +8.5 ‰ and δ¹⁸O ranging from -8.4 to +1.4 ‰. The δ¹⁵N isotopic range is consistent with microbial mineralization and nitrification of reduced nitrogen sources including ammonium, dissolved organic nitrogen, and soil organic nitrogen. The range in δ¹⁸O of nitrate is also consistent with nitrification based on the δ¹⁸O of site waters. No evidence for an atmospheric nitrate signal, as defined by δ¹⁵N and δ¹⁸O of nitrate in snow and snowmelt, is seen.

In contrast, nitrate in permafrost appears to be a mixture of pre-industrial atmospheric nitrate (with higher δ¹⁵N than modern atmospheric nitrate) and nitrate that is microbial in origin. Massive ice wedges appear to contain larger proportions of snowmelt (based on δ¹⁸O of ice) and atmospheric nitrate, whereas textural ice appears to contain a greater proportion of summer precipitation and microbially-derived nitrate. Nitrate from the ancient DTLB and drainage samples also has isotopic signatures that appear to represent a mixture of pre-industrial atmospheric nitrate and nitrate from microbial nitrification, and may, at least in part, be derived from degraded permafrost.