Agriculture Reference
In-Depth Information
Most of the increase in LOI following replacement of restored soils,
and of increases with higher inputs on restored soils of similar age, occurred
at the upper sampling depth and was accounted for by changes in the
non-carbohydrate fraction. As a result, carbohydrates made up a markedly
smaller proportion of the organic fraction in restored compared with UD
soils. This trend was more pronounced at the lower depth where carbo-
hydrates accounted for < 4% of organic matter in restored compared with
> 12% in UD soils.
One possible explanation for this difference in organic matter
composition was the low level of macroaggregation in restored soils
(Table 5.4.2), even 21 years after restoration. Several studies (e.g. Eriksen
et al
., 1995) have identified stable aggregates as important sites of physical
protection for labile organic material. In stabilizing aggregates, this organic
matter may itself be protected from microbial degradation.
Without pre-treatment, UD soils had higher stability than restored soils
for both aggregate sizes and depths. There was no general improvement
in aggregate stability in restored soils between years 9 and 21, although
silt + clay stability did increase at the upper sampling depth. Periodate-
sensitive aggregate stability was markedly higher in UD soils, although the
difference from restored soils was less pronounced for clay-sized aggregates.
The contribution of carbohydrates to aggregation was particularly low for
silt + clay aggregation on the 9-year restoration at both depths and on the
21-year restoration at the lower depth. The sludge-treated soil (R9H) had
greater total and periodate-sensitive stability, particularly at depth, than the
comparable (RHL) restored soil, but the older restored soils had generally
similar stability characteristics.
Earthworm surveys (Scullion, 1994) indicated populations decreasing
in order of UD > R21 > R9. These differences may partially explain
Table 5.4.2.
Total and periodate-sensitive clay (Cl) and silt + clay (Z + Cl) aggregate stability in
undisturbed (UD) and restored (R) soils.
0-7.5 cm
7.5-15 cm
%Cl
periodate
stability
%Z + Cl
periodate
stability
%Cl
periodate
stability
%Z + Cl
periodate
stability
%Cl
stability
%Z + Cl
stability
%Cl
stability
%Z + Cl
stability
Sites
UD
91.4
30.5
42.7
25.6
88.9
38.7
36.4
23.9
R21H
84.4
19.8
30.9
11.9
55.0
33.3
13.4
5.8
R21L
74.8
22.9
28.6
12.9
52.8
33.8
13.9
5.9
R9H
78.8
27.7
18.8
1.0
69.1
39.6
12.9
7.3
R9L
72.5
25.2
25.2
1.1
56.0
31.1
8.1
3.1
LSD 5%
3.03
5.66
1.47
6.2
9.94
6.21
5.61
4.97
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