Biogeochemistry, V 28, pp 1-31, 1995
Fluxes and transformations of nitrogen in a high-elevation
catchment, Sierra Nevada
Mark W. Williams
Department of Biological Sciences
University of California
Santa Barbara, CA
Roger Bales
Department of Hydrology
University of Arizona
Tucson, AZ
Aaron Brown
Department of Biological Sciences
University of California
Santa Barbara, CA
John Melack
Department of Biological Sciences
University of California
Santa Barbara, CA
Abstract:
The fluxes and transformations of nitrogen (N)
were investigated from 1985 through 1987
at the Emerald Lake watershed (ELW),
a 120 ha high-elevation catchment located in the southern
Sierra Nevada, California, USA.
Up to 90% of annual wet deposition of N was stored
in the seasonal snowpack;
NO3 and NH4 were released from storage
in the form of an ionic pulse,
where the first fraction of
meltwater draining from the snowpack
had concentrations of NO3 and NH4 as high as 28 ueq/L,
compared to bulk concentrations
of < 5 ueq/L in the snowpack.
The soil reservoir of organic N (81 k eq ha) was about ten times
the N storage in litter and biomass (12 k eq ha).
Assimilation of N by vegetation was balanced by the release of
N from soil mineralization, nitrification, and litter decay.
Mineralization and nitrification processes
produced 1.1 k eq ha yr of inorganic N,
about 3 and a half times the loading of N from wet and dry deposition.
Less than 1% of the NH4 in wet and dry deposition
was exported from the basin as NH4.
Biological assimilation was primarily responsible for
retention of NH4 in the basin,
releasing one mole of H+ for every mole of NH4 retained
and neutralizing about 25% of
the annual acid neutralizing capacity produced by mineral weathering
in the basin.
Nitrate concentrations in stream waters reached an annual peak
during the first part of snowmelt runoff,
with maximum concentrations in stream water of
20 ueq/L, more than 4 times the volume-weighted mean
annual concentrations of NO3 in wet deposition.
This annual peak in stream water NO3
was consistent with the release of NO3 from the snowpack
in the form of an ionic pulse;
however soil processes occurring underneath
the winter snowpack were another potential source of this NO3.
Concentrations of stream water @no3@ during the summer growing
season were always near or below detection limits
(0.5 ueq/L).