One of the overarching conclusions from the recent LINX II stream ¹⁵N tracer experiments was that direct denitrification tends to explain only a minority of nitrate (NO₃^-) loss from the water column (median, 16%: Mulholland et al. 2008, Nature 452: 202). The balance of “retained” NO₃^- (measured as ¹⁵N-NO₃^- lost from stream water) appears to have been assimilated rather than denitrified, and may eventually be released back to the stream.

Phosphorus (P) is often limiting in aquatic ecosystems. The quantity of available P is often determined by sediment binding and release processes. We obtained 32 sediment samples from shallow freshwater ecosystems in Southwest Michigan. Sediment cores were separated into consolidated and flocculent strata for analysis of percent organic matter, total sediment phosphorus (TP), and HCl-extractable iron (HCl~Fe). Sediment TP ranged from 110-3348 ugP/gdw, with an average of 1052 ugP/gdw.

Permafrost plays an important role in shaping the chemistry of streams by restricting subsurface flows through catchments to soils. During the summer thaw of soil, subsurface flows migrate through deeper soil horizons presumably resulting in seasonal shifts in the inputs of carbon and nitrogen to the streams. Within streams, the extent of the hyporheic zone may also shift with seasonal thaw. Hyporheic zones have high mineralization and nitrification rates; thus expansion of the hyporheic zone throughout the season has important implications for stream chemistry.

Rates of biogeochemical cycling in desert ecosystems are inherently constrained by water availability. Water pulses resulting from discrete climate events therefore can significantly alter biogeochemical processes. The McMurdo Dry Valleys of Antarctica, a polar desert region, have experienced discrete warming events that resulted in episodic pulses of water made available through permafrost and snow melt.

Soil C accumulation and turnover are important processes globally: soils contain about 1.5 x 1018 g C, which is 2 3 times that in vegetation. The C flux between soils and the atmosphere is large, with soil respiration representing about 10 times the C flux due to fossil fuel combustion. Thus, any temperature or land-use-induced change in rates of soil C turnover will markedly affect the global C cycle.