Hydrological controls on chemical export from an undisturbed old-growth Chilean forest

Abstract

Stream water chemical concentrations from an undisturbed, temperate, old-growth Chilean forest were analyzed to gain insights into hydrological controls on watershed-scale losses of different nutrients and chemical elements. Our goal was to understand how hydrological and biogeochemical processes interact to control patterns of export of dissolved chemicals from these types of forested watersheds. The study is unique in that the monitored watershed lies in a part of the world that receives very little anthropogenic influence and this is the first study of its type in this part of the world. A small, densely forested, montane watershed was continuously monitored for stream discharge and precipitation and was sampled biweekly for stream-, precipitation-, and soil-water chemistry. Three different forms of hydrological controls were used to describe how concentrations of different elements changed as a function of increased stream water discharge: dilution of elements due to decreased relative contributions by baseflow sources, increased concentration due to enhanced hydrological access to elements in to shallow soil horizons, and no net effect, i.e. hydrologically constant. A hyperbolic relationship was used to describe element concentration as a function of stream discharge to quantify the variations for major ions, cations, and organic forms of nutrients. In general, this hydro-biogeochemical system showed the most distinct dilution and enhanced hydrological access for chemicals with large concentration differences between deep and shallow soil water. Deep soil water sources were most distinct for Ca2+ and Si, both of which demonstrated dilution. In contrast, variations in H+ concentrations resembled enhanced hydrological access indicating shallow soil sources, coupled with the consumption of acidity by weathering or cation exchange in deeper soil layers. Rainwater sometimes enhanced and sometimes tempered the observed trends. Potassium was the least hydrologically variable element (hydrologically constant) in this study, possibly due to strong biological mechanisms internal to the watershed. The approach used here is a simple method to identify the importance of hydrologic variations in controlling streamwater chemistry relative to internal biogeochemical sources and processes.