Atmospheric Nitrogen Deposition and Groundwater Nitrate in a Serpentine Grassland Ecosystem: Nutrient Cycling and Hydrogeologic Processes
Abstract
Atmospheric nitrogen deposition from Silicon Valley smog is a major nexus for the Santa Clara Valley Habitat Plan. The Habitat Plan addresses development impacts on 19 covered species, including the Bay checkerspot butterfly, the majority of which occupy nutrient-poor serpentine grasslands. Measuring nitrate levels in springs and seeps appears to be a cost-effective method for long-term monitoring. Late summer baseflow samples from springs contain up to 5 ppm NO3-N, among the highest measured in non-agricultural systems. Nitrate levels follow local and regional gradients described by passive sampler measurements, CMAQ modeling, and Tdep maps from EPA. Δ17O analysis shows that 50-60% of nitrate in summer baseflow is unprocessed atmospheric nitrate. Compilation of N-cycling studies indicate that the low-biomass ecosystem is primed for nitrate leaching early in the Nov-April rainy season; high levels of HNO3 accumulate over the May-October dry season, and are flushed into the soil when plant demand is low and mineralization and nitrification provide more than enough ammonium and nitrate to satisfy that demand. Nitrate generally declined by 50% during the 2017 rainy season, but the atmospheric signal disappeared as high rainfall flushed the shallow groundwater system. The groundwater system has been intensively investigated as part of the development of the Kirby Canyon Landfill, and 92% of the groundwater flow occurs in alluvial/colluvial deposits in stream channels, with the remaining 8% in a 3-15 m mantle of weathered serpentinite. Groundwater residence times in the spring systems appear to be on the order of a few years, so nitrate levels should track deposition trends. Projected reductions in NOx emissions combined with stable/increasing NH3 emissions should reduce the unprocessed nitrate fraction through time as overall nitrate levels decline.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B12A..01W
- Keywords:
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- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 0469 Nitrogen cycling;
- BIOGEOSCIENCESDE: 0478 Pollution: urban;
- regional and global;
- BIOGEOSCIENCESDE: 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE