The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 260

N application (Morrison et al ., 1980), and increased productivity leads
to greater returns of OM to the soil via faeces, and leaf and root death
(Parsons et al ., 1991), DOC production, and subsequent loss, appear to be
a function of productivity.
Sub-soil drainage
Drainage did not seem to affect exports of DOC from the Zero N plots
(Fig. 4.5.1). However, when values of total DOC export from all plots
were grouped within either drained or undrained treatments, an analysis
of variance showed that there was a significant difference (F = 6.69;
P = 0.032), with drained plots exporting significantly less DOC than the
undrained plots. Either less DOC was produced in the soils of the drained
plots, or DOC was lost between the point of production and the drainage
weirs. One major source of DOC would be from the applied manures, but
both drained and undrained treatments received equal amounts. As no
manures were applied during the present year, residual levels of mobile
C are likely to have been low. The other two major sources of DOC are
fresh organic remains (mainly plant derived) and older SOM. Levels of
productivity are slightly higher on the drained plots (Tyson et al ., 1992) so
there should be at least as much fresh organic remains for DOC formation
as on the undrained plots. Soils from the undrained plots had higher C
concentrations (up to 30% higher) in the top 4 cm (~9% C) compared with
the drained plots (~7% C) although, below 4 cm, differences were not
significant. As DOC export from HNU was nearly three times that from
HND (Fig. 4.5.1), the differences in soil C content could not account for
all of the difference in export.
The inference, therefore, is that a large proportion of DOC is removed
from soil water as it moves into drainage. Several workers have demon-
strated that large amounts of DOM can be sorbed from soil solution to
mineral surfaces (e.g. Greenland, 1971; Jardine et al ., 1989). Evidence
for this occurring on our plots is presented in Fig. 4.5.2, where DOC
concentrations in water that has passed through to the drainage system is
shown to be substantially lower than that of the surface water from the same
plots. This suggests that the DOC has been removed by the soil during
percolation. Additionally, as draining the soil reduces water content, and
hence increases air content, the soil of the drained plots may have been
more oxidized, with more Fe(III) oxides available for DOC sorption than
was the case for the undrained plots. Both sets of values are lower
than those observed at the same time from the HNU plots (Fig. 4.5.2).
However, considered together with the difference in soil C content,
sorption of DOM may explain the differences in DOC export between the
two treatments.