Agriculture Reference
In-Depth Information
Table 4.6.2.
DOC extractability and DOM characteristics in repeated extracts from non-sterile
soil samples during a 12-week incubation at 20
°
C. Significant effects of incubation time on solution
parameters are indicated by different letters following the values in the columns (Duncan test).
Incubation time
(weeks)
DOC
(mmol kg
−
1
)
ε
280
1
(l m
−
1
mmol
−
1
)
Cu complexation
(mmol mol
−
1
DOC)
0
26.1
a
26.8
a
7.7
a
8.0
b
35.5
b
7.9
a
2
4
5.7
c
42.8
d
13.8
b
8
3.3
d
b
39.9
bc
15.9
b
12
4.0
e
b
37.1
cd
20.5
c
1
UV absorption at 280 nm.
DOM degradability were slightly increased by the treatment, as observed
by Powlson and Jenkinson (1976), which may be related to the lower
specific UV absorptivity of these substances since this indicates lower aro-
maticity (Chin
et al
., 1994). As temperature increases, DOM extractability
and some DOM properties are altered in the sterile samples, which is
probably due to the remaining enzyme activity (Powlson and Jenkinson,
1976). However, as the effects are much more pronounced in the
non-sterile samples, certain changes in DOM properties can clearly be
attributed to microbial activity. Contrary to our expectations and reports
from Gödde
et al
. (1996), increasing temperatures did not enhance the
net release of soluble organic matter but decreased it, since DOM present
at the beginning of the incubation was utilized as substrate. Apparently,
the physical effect of rewetting the air-dry samples for the experiments on
DOM release was much more pronounced than subsequent microbial
activity. This effect is commonly observed and attributed to the disruption
of aggregates and the release of otherwise non-available organic matter
(Lundquist
et al
., 1999).
The changes in DOM quality with increasing incubation temperature
show that microorganisms either preferentially degrade certain DOM
fractions or actually release compounds with different properties from
those present initially in the soil. The increase in UV absorptivity in the
remaining DOM fraction indicates an expected accumulation of more
recalcitrant substances (Chin
et al
., 1994). However, this is inconsistent
with the greatly elevated biodegradability of DOM at 20 and 35
C.
Together with the increased Cu complexation ability of DOM at higher
temperatures and longer incubation period, we regard this as evidence for
an increased contribution of microbial metabolic products to the soluble
organic matter fraction. These products could consist of low molecular
weight organic acids which have a high metal complexation ability and are
°
Search WWH ::
Custom Search