Extreme nitrogen saturation of serpentine grasslands in Santa Clara County, California: A nexus for conservation action

Stuart Weiss1 and Meredith Hastings2

Serpentine grasslands are nutrient-poor, low biomass ecosystems that harbor high native biodiversity including many listed species such as the Bay checkerspot butterfly.  Research since the 1990s has documented the threat of atmospheric nitrogen deposition from upwind Silicon Valley that allows non-native annual grasses to overrun the dazzling displays of low-statured wildflowers, and that the grass invasions are controlled by managed cattle grazing.  The estimated critical load for serpentine grasslands is 6 kg-N ha-1 year-1. Since 2015, in the high deposition zone (10-20 kg-N ha-1 year-1 ), spring water nitrate up to 25 ppm (as NO3-) were measured in baseflow, levels higher than any reported for non-agricultural sites in California. Nitrate levels in samples from low deposition areas were 1-5 ppm.  Isotopic analyses including Δ17O show that 50-60% of the nitrate in spring and well water is unprocessed atmospheric nitrate.  Passive samplers, CMAQ models, N-cycling measurements, and emissions inventories/projections are used to interpret the findings.  High levels of nitric acid in summer and low ecosystem capacity for N-retention in the fall growing season allow for flushing of accumulated dry NO3 deposition into the fractured bedrock, and eventually into the springs.  Increased N-deposition from highway improvements and development was a major regulatory nexus for the Santa Clara Valley Habitat Plan, a Habitat Conservation Plan/Natural Communities Conservation Plan (HCP/NCCP).  A novel nitrogen fee based on car trips generated is one of the funding mechanisms, driving a need for cost-effective and robust monitoring of N-deposition trends over the 50-year plan and beyond.  A set of sentinel springs for long-term monitoring will be identified and sampled on an interval commensurate with shallow groundwater residence times.


1Creekside Center for Earth Observation, stu@creeksidescience.com
2Brown University, Meredith_Hastings@brown.edu