Impacts of floodplain restoration on nitrogen transformation pathways on the Cosumnes River, California
Abstract
Throughout the United States levee construction has resulted in the systematic replumbing of river systems. Levee construction results in major reductions in the timing and frequency of floodplain inundation and fundamentally alters the hydrologic, geomorphic, and biogeochemical processes of a riverine system, thus impacting nutrient delivery and transformations in floodplains. Floodplain restoration via levee removal or setback can affect downstream water quality by restoring soil microbial metabolic pathways such as denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), which in turn transform excess nitrogen. However, few studies have quantified the contribution of all three pathways to nitrate retention in restored floodplains. In this study, we address the following questions: (a) how do restored floodplain characteristics, such as time since reconnection, inundation duration, soil structure, and soil chemistry affect nitrogen transformation pathways? and (b) what is the relative contribution of denitrification, anammox, and DNRA to total nitrogen transformation in restored floodplain soils? To address these questions, we simulated flood conditions in soil mesocosms collected from two floodplains where levees were breached in 1985 and 2014 along the Lower Cosumnes River in the San Joaquin Basin of California. River water enriched with K15NO3- tracer was pumped into each mesocosm at a constant rate for a period of 3 months. During this time, samples were collected from the surface water, pore water, and soil for measurements of NO3-, NO2-, NH4+ concentrations, δ15N of dissolved gases (N2 and N2O), and DNA extraction. Results revealed that nutrient concentrations were variable during the first 50 h, followed by a linear increase in organic carbon and a linear decrease in nitrate concentrations over the remainder of the experiment. Further, nutrient concentrations and 15N-NO3 and 15N-NO2 enrichment in the soils differed between the two floodplains. This study quantifies the relevance of anammox and DNRA to total nitrate retention, characterizes the hydrologic conditions most favorable to each pathway, and estimates the overall potential for floodplain restoration to improve nitrogen retention and cycling in the Cosumnes watershed.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.B43B2121H
- Keywords:
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- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 0496 Water quality;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE;
- 1806 Chemistry of fresh water;
- HYDROLOGY