The biogeochemistry of dissolved nitrogen, phosphorus, and organic carbon along terrestrial‐aquatic flowpaths of a montane headwater catchment in the Peruvian Amazon

Abstract

AbstractDissolved nitrogen (N), phosphorus (P), and organic carbon (C) were sampled along two transects in a first‐order montane tropical (2414 m.a.s.l.) rainforest catchment of the Peruvian Amazon to investigate spatial and temporal controls on nutrient concentrations from uplands to streams. Surface and subsurface waters along transects were sampled during baseflow conditions and following rainfall events from March 2002 to March 2003. During baseflow conditions, we observed strong terrestrial controls on N, P, and dissolved organic carbon (DOC) concentrations in streams. Median NO3− concentrations were relatively constant during both dry and wet seasons in stream water (dry, 0·8 µM; wet, 1·7 µM) compared to upland soil water (dry, 15·5 µM; wet, 32·5 µM) despite significant seasonal fluctuations of NO3− in the upland, riparian, and hyporheic zones. During the dry season, concentrations of dissolved organic N (DON) also decreased markedly between the upland and the stream. Despite this decrease, DON remained the dominant component of total stream water dissolved N. Dissolved organic P (DOP) and soluble reactive P (SRP) concentrations generally followed a spatial trend inverse to that of N. Low median SRP concentrations were recorded during dry and wet seasons in the upland (dry, 0·11 µM; wet, 0·08 µM) while the highest median SRP concentrations were in stream water (dry, 0·22 µM; wet, 0·20 µM). DOP also dominated total dissolved P concentrations from the upland to the stream. Stoichiometric ratios of dissolved N and P contrasted between the upland (DON:DOP = 734, dissolved inorganic N (DIN):SRP = 166; dry season) and the stream (DON:DOP = 3, DIN:SRP = 12; dry season), indicating a clear divergence of nutrient composition between terrestrial and aquatic systems. Under baseflow conditions, strong mechanisms in the terrestrial environment and at the terrestrial–aquatic interface controlled the nutrient concentrations in the stream and buffered the seasonal fluctuations occurring in the terrestrial environment. In contrast, storm flowpaths may short‐circuit baseflow nutrient controls, thereby exporting a pulse of nutrients to the stream. Understanding the influences of storms in headwater catchments will be a valuable next step in determining the effects of changing precipitation regimes on the nutrient status of montane tropical forests and their receiving waters. Copyright © 2006 John Wiley & Sons, Ltd.