Characterization of Soil Drainage Classes for the Study of Soil Carbon Storage in Alaska

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Characterization of Soil Drainage Classes for the
Study of Soil Carbon Storage in Alaska
INTRODUCTION Soil drainage as defined by
water-holding capacity, hydraulic conductivity,
and position of seasonal water table is closely
associated with soil carbon storage because of
controls on plant production, decomposition, fire
severity, and fire frequency. As an initial
regional assessment, we have categorized the USDA
Natural Resource Conservation Service (NRCS) 11M
scale maps according to 7 soil drainage classes
using a simple numeric scale and an area
weighting of mapped polygons.
RESULTS Distribution of Soil Types in Alaska.
Most (gt50) of Alaskan landscapes have low
hydraulic conductivity and high (lt1m of surface)
water tables, which in many areas is related to
shallow permafrost. Less than 1/3 of the
landscape (26) is moderately to excessively
drained, and, 4 qualifies as very poorly
drained wetlands.
MATERIALS AND METHODS We ranked 7 NRCS drainage
classes into a numeric system Rank NRCS
Term Explanation 1 excessively drained
very high hydraulic conductivity and low water
holding capacity, water table gt2m 2
somewhat excessively high hydraulic conductivity
and low water holding capacity, water table gt
2m 3 well drained intermediate water
holding capacity with depth to water table gt2m
4 moderately drained low hydraulic
conductivity, wet state high in profile with
depth to water table 1-2m 5 somewhat
poorly layer with low hydraulic conductivity, wet
state high in profile with depth to water table
.3-1m 6 poorly saturated zone or
layer of low hydraulic conductivity and water
table lt25cm 7 very poorly drained
saturated with water table within 30cm
throughout the year. Using MsExcel,
ArcInfo, ArcView, and (ESRI) software, we
computed and plotted area- weighted scores for
each polygon according to the following equation
?(Rank area) / 100 Weighted
Drainage Class for Polygon
Long term additions (NPP), losses (burned,
decomposed), and accumulation rates (NEP) in four
classes of soil drainage, including well-drained
pine, moderately drained black spruce (BS)/
feathermoss, poorly drained black
spruce/sphagnum, and very poorly drained fen
systems. (figure below)
(at right) Estimates are based on decadal
time-step model for 6500 years of NPP,
decomposition, and fire return intervals
determined for these sites from NASA BOREAS
experiments in northern Manitoba (Harden et al,
1997 Trumbore and Harden, 1997 Gower et al,
1997).
DISCUSSION / CONCLUSION Based on the strong
relationship of soil carbon storage to soil
drainage, large amounts of carbon likely reside
in Alaskan soils. Under a changing climatic
regime that includes warming and longer growing
seasons, much of this carbon may become
susceptible to fire, mobilization as DOC, and
decomposition. The relative importance of fire,
mobilization as DOC, and decomposition will
depend largely on climatic cycles of sustained
drought (which could trigger fire) or increased
precipitation, mobilization and decomposition
that accompany warming and changes in growing
season.