Estimated Empirical Nitrogen Deposition Critical Loads for Nitrate Export from California Forests and Shrublands
Abstract
Nitrogen deposition loads (i.e., critical loads) leading to elevated streamwater nitrate export, the cardinal symptom of catchment-level N loss, are not well defined for low- and mid-elevation shrublands. Nor have critical loads (CL) been established for nitrate export from springs in montane regions of California. We surveyed streams for nitrate concentrations from small chaparral catchments (generally < 400 ha) in southern California following storm events and we sampled springs in the San Bernardino Mountains. In addition, springs were sampled from serpentine grasslands around the Santa Clara Valley, near San Jose, California and in Sonoma and San Marin counties. Atmospheric deposition inputs were estimated for the chaparral catchments and the serpentine springs from the CMAQ model at a 4-km grid scale. Deposition to the springs in the San Bernardino Mountains were determined with an empirical inferential method. The CL estimated for nitrate loss from chaparral catchments was estimated at 6-8 kg N/ha/yr based on three different surveys. The CL for nitrate export in springs in the San Bernardino Mountains was 15 kg N/ha/yr, very similar to a previously published value of 17 kg N/ha/yr as the CL for streamwater nitrate in mixed conifer forests of California. The estimated CL for the serpentine grassland springs was 5 kg N/ha/yr, but is highly uncertain and may be biased low because of scatter in the data and uncertainty in the deposition estimates. We estimate that 40-50% of the chaparral catchments in the state are at or above the CL for nitrate leaching, largely because they occur in low elevation sites where exposure to pollutants from urban and agricultural emissions are highest. Furthermore, these arid systems may be inherently prone to N loss, particularly during the transition from the dry to wet season.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13N2526F
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES;
- 0469 Nitrogen cycling;
- BIOGEOSCIENCES;
- 0478 Pollution: urban;
- regional and global;
- BIOGEOSCIENCES