Agriculture Reference
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during a storm event did allow meaningful comparisons, and produced
some contrasting results (McTiernan et al ., 1999). The drained plots had
positive logarithmic correlations ( P < 0.001, r 2 = 0.77-0.95, depending on
fertilizer treatment) between drain flow rates and DOC concentrations.
The
undrained
plots,
however,
showed
negative
linear
correlations
( P < 0.05,
r 2 = 0.40-0.44)
between
drain
flow
rates
and
DOC
concentrations.
For the drained plots, there was a direct correlation between the rate
of passage of water through the soil and the concentration of DOC,
tending towards an upper limit. The magnitude of this upper limit
was dependent on fertilizer treatment. Isotopic and other analyses of the
drainage DOM indicated that an increasing amount of OM from the
surface of the plots was carried to the drains as rates of flow increased
(McTiernan et al ., 1999). The size and number of soil channels through
which water can pass limit flow to the drainage system and, as the capacity
of these channels is approached, an increasing amount of water travels
through the topsoil and into surface runoff weirs. As the soils of the
drained plots have lower water contents than the soils of the undrained
plots, the drained soil may have been more aggregated. This may
have resulted in the more extensive formation of macropore channels,
permitting rapid flow of water through the upper soil profile to the drainage
system.
A large proportion of the storm rainfall on the undrained plots flowed
directly over the surface of the soil, giving much higher flow rates than for
the drained plots. As there was a dilution of the DOC content of the runoff
at the higher rates of drain flow, it would appear that the amount of C
available for dissolution was limited, leading to a negative correlation
between flow rate and DOC concentration.
We can speculate that there are three stages in the relationship between
flow rate and DOC concentration in runoff from the undrained plots. At
low rates, most rainfall passes through preferential channels and the soil
matrix. As these soils have a high clay content, matrix flow is slow, so even
small increases in rainfall intensity result in an increasing proportion of
the rain water moving close to and then over the surface. Water moving
through the soil profile loses DOM through sorption, and hence any
increase in the proportion of water movement over the surface will result in
an increase in the concentration of DOC in the discharged water. This
could give a positive correlation between drain flow rates and DOC
concentration. Secondly, as rainfall intensity increases, the additional water
flowing over the surface of the soil removes a proportionate amount of C
and there is no relationship between flow rate and DOC concentration.
Finally, the amount of C that is available for dissolution and transport in
water flowing over the surface of the soil is limited and, as this limit is
exceeded, dilution occurs.
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