Hydrologic, Ecological, and Biogeochemical Drivers of Carbon and Nitrogen Cycling in Forested Headwater Stream Networks

Abstract

Headwater streams serve multiple important biogeochemical, hydrologic, and ecological functions, including: transporting solutes from the terrestrial landscape to downstream fluvial ecosystems; providing a surface for gas evasion to the atmosphere; integrating terrestrial, riparian and aquatic ecosystems, amalgamating surface and groundwater; accumulating and storing sediment; and transforming and retaining solutes. The numerous mechanisms mediating these physical and biological processes remain poorly understood despite their prominent influence on catchment outlet biogeochemical dynamics. In light of this research need, this study sought to determine the influence of hydrologic, ecological, and biogeochemical processes on solute (specifically carbon and nitrogen) concentrations and fluxes in a paired set of headwater stream networks.This research was conducted at the Tenderfoot Creek Experimental Forest in Montana. An empirical, field-based approach that combined observational monitoring using a network of high temporal resolution sensors and experimental solute additions was used to quantify carbon and nitrogen uptake, metabolism, and export across the snowmelt and baseflow recession periods. Based on analysis of this data set, we determined that headwater streams show strong demand for carbon and nitrogen across a range of concentrations from ambient to saturating concentrations; that the variation in uptake kinetics seasonally and between sites is driven by substrate availability; that this retention capacity is linked to the magnitude of metabolic demand; and that through the metabolism of the biological community carbon and nitrogen cycles are coupled. We then demonstrate that these biological processes can have variable roles in mediating carbon and nitrogen export at the catchment scale, but during some periods of the year they can be as influential as physically driven fluxes in mediating watershed export.This study integrates disparate ecological and hydrologic perspectives to address how energy and macronutrients move through headwater stream networks. We believe the findings presented here begin to reconcile the seemingly incompatible paradigms of streams as highly retentive biogeochemical reactors and streams as “passive pipes” that reflect and integrate the terrestrial landscape.