Spiraling Down Hillslopes: Nutrient Uptake from Water Tracks in a Warming Arctic
Hydrologic flowpaths might propagate biogeochemical signals among connected ecosystems or alter and dampen signals because of reactions or retention occurring during transport. In the Arctic, experimentally warmed terrestrial tundra releases inorganic nitrogen (N), but the fate of this newly released N remains unclear. Nitrogen could be passively transported downslope in flowing water, or retained when flowpaths intercept N-limited ecosystems. We applied nutrient spiraling theory to simultaneously measure reaction and transport of ammonium (NH4+) and phosphate (PO43−), nutrients limiting primary productivity in Arctic ecosystems. Pulse fertilization experiments were focused on flowpaths known as water tracks that hydrologically connect soils to receiving streams and lakes in upland tundra of Alaska. Water tracks typically retained PO43−, but passively transported NH4+, thus potentially propagating NH4+ produced by warming tundra soils to downstream ecosystems. Nutrient uptake was uncorrelated with the relative proportion of downslope transport in transient storage zones, but greater NH4+ uptake occurred as advective hydrologic flux increased relative to dispersion. Phosphate uptake declined as thaw depth increased over the summer season likely because of declining capacity for biotic uptake or sorption in deeper soils. Phosphorus limitation in fluvial ecosystems of the Arctic might result in efficient transport of inorganic N to N-limited lentic and coastal ecosystems, where increasing subsidies furnished by N loss from warming terrestrial tundra could support enhanced primary production.
Keywordsactive layer advection ammonium (NH4+) flowpaths mineralization phosphate (PO43−) saturated soils transient storage transport tundra
We gratefully acknowledge Emily Longano, Sarah Ludwig, Rebecca Risser, Audrey Krehlik, Caitlin Rushlow, Jay Jones, and Kelsey Blake for their contributions to data collection. We thank staff of CH2MHill Polar Services and Toolik Field Station for logistical support. We acknowledge two anonymous reviewers for constructive comments that improved the manuscript. This research was supported by the National Science Foundation (OPP-1108200).
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