Soil Carbon Fractions Based on their Degree of Oxidation, and the ...

Graeme J. Blair et al.
Results and Discussion
The results of the C fractionation of three cropped and uncropped soils from
northern and central New South Wales, Australia, have been presented by Lefroy
et al. (1993) using three strengths of KMn04 and thus four carbon fractions. The
results showed that reduction in all three fractions was of a similar magnitude
and that the decline in each fraction was greater than in the unoxidized fraction.
Consequently, the two lower concentrations of KMn04, 33 and 167 mM, are no
longer used.
In all three soils, the reduction in CL due to cropping was proportionally
greater than the decline in CNL or CT (Table 1). Averaged over the three soils
the values were -63.3%, -39.3% and -44.9% for CL, CNL and CT respectively.
Incorporation of 2 years lucerne into the cropping rotation at Warialda restored
soil C, with the increases being +58.8, +15.7 and +21.6% for CL, CNL and CT
respectively.
Table 1. Labile and non-labile C and <strong>Carbon</strong> Management Indices for some cropped and
uncropped soils in New South Wales, Australia
Cropping Labile Non-labile Total <strong>Carbon</strong> Lability Lability <strong>Carbon</strong>
or grazing carbon carbon carbon pool of C index management
history index index
(CL) (CNL)~ (CT) (CPI) (L) (LI) (CMI)*
(mg g- )
Nyngan (Solonized Brown Earth, Palexeralfl
Uncropped 4.50~ 13.53 18.03 - 0.332 - -
4 yrs cropping 2.21 10.38 12.59 0-70 0.213 0.64 45
Gunnedah (Black Earth, Pellustert)
Stock route grazing 3.62 16.83 20.45 - 0.256 - -
7 yrs cropping 1.55 9-18 10.73 0.51 0.169 0.66 33
Warialda (Red Earth, Paleustalf)
Lightly grazed 3.99 12.78 16.77 - 0.312 - -
18 yrs cropping 1.02 6.49 7.51 0.45 0.157 0.50 23
16 yrs cropping 1.62 7.50 9.13 0.54 0.215 0-69 38
and 2 yrs lucerne
CT sample
A CMI = CPIxLIx loowhere CPI = , LI
C
=
and L = L.
CT reference L reference CNL
CL values are the means of duplicate samples. The mean CV of these seven values = 2.93%.
These data demonstrate that CL declines faster and is restored faster than
CNL or CT, and hence is a more sensitive indicator of the C dynamics of the
system.
Data from the two soils from Mackay, Queensland (Table 2), provide a contrast.
The soil from Marian had been cropped for 90 'years and has a markedly lower
CL, CNL and CT than the reference (uncropped) soil. By contrast, the Victoria
Plains soil which has been cropped for 15 years has a higher CL, CNL and
CT concentration than the adjacent non-cropped reference soil. Green trash
management has been practiced in this area for some time, replacing the practice
of trash burning. As such, although both sites have had similar periods of green