Browsing by Subject "Nitrogen cycling"

Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. Microstegium vimineum (Microstegium) is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of Microstegium on both plant diversity and N cycling in a riparian floodplain, paired plots of Microstegium hand-weeded and unweeded were established for three years. Plots without Microstegium increased from 4 to 15 species m-2 and 90% of the newly establishing species were native. The Microstegium community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m-2 year-1, respectively (p=0.05). Decomposition and release of N from Microstegium detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m-2 year-1. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While Microstegium invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (± 11% SE) Microstegium cover and species richness decreased to <8 species m-2.

To learn how Microstegium responds to increased N, we conducted a greenhouse competition experiment between Microstegium and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet Microstegium was most dominant in the high levels of N.

Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without Microstegium in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. Microstegium was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.

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.