Evidence for pulse-shunt carbon exports from a mixed land-use, restored prairie watershed

Abstract

Watersheds in the Great Plains region of the United States are dominated by agriculture, interspersed with remnant or restored prairie vegetation. The pulse-shunt concept predicts these coexisting land uses likely have opposing effects on seasonal biogeochemical and hydrological controls of organic C (OC) fates in freshwater ecosystems, but few studies have focused on temporal patterns of OC fates in streams that are influenced by agriculture and tallgrass prairie. We estimated stream metabolism and OC spiraling in a stream (Glacier Creek, Omaha, Nebraska, USA) draining a mixed land-use watershed with restored tallgrass prairie and agriculture to answer: 1) Does a mixed land-use stream exhibit seasonal patterns of ecosystem metabolism? and 2) Does the balance between active OC processing vs passive OC transport change across seasons and years as predicted by the pulse-shunt concept? We hypothesized that the stream would be net heterotrophic, rapidly mineralize OC (Vf-OC), and exhibit short spiraling lengths (SOC) at baseflow, and that these functions would be modulated in opposing directions by seasonal patterns of nutrient availability and turbidity. Mean gross primary production was 0.30 g O2 m−2 d−1, mean ecosystem respiration was −1.25 g O2 m−2 d−1, and Glacier Creek was net heterotrophic throughout the study (mean net ecosystem production = −0.94 g O2 m−2 d−1; mean production:respiration = 0.19). Peak gross primary production and ecosystem respiration occurred in the spring driven by discharge and light. High-resolution OC-spiraling estimates revealed a continuum of OC processing and transport consistent with pulse-shunt fluxes. OC-spiraling lengths spanned 2 orders of magnitude (1–934 km), but most SOC ranged between distances of 4 to 15 km at baseflow. SOC was shorter with higher nutrient concentrations and longer with higher turbidity, consistent with the inverse pattern for Vf-OC. Our study confirms that the metabolic regime of a prairie stream was seasonal, while underscoring that factors that are modified by land-use change, such as nutrients and turbidity, can influence OC processing.