Jo. Skjemstad et al., CARBON DYNAMICS IN VERTISOLS UNDER SEVERAL CROPS AS ASSESSED BY NATURAL-ABUNDANCE C-13, Australian Journal of Soil Research, 32(2), 1994, pp. 311-321
The effects of three annual crops (oats, C3, Avena sativa; sorghum, C4
, Sorghum bicolor; black gram, C3, Vigna mungo) and a grass pasture (g
reen panic, C4, Panicum maximum) on the content and turnover of organi
c carbon (0-0.15 m) in Vertisols in south-eastern Queensland were meas
ured. Monocultures of the crops and pasture were grown for 11 years in
two blocks (1 and 3) established by cultivating a 44 year old C4 Rhod
es grass (Chloris gayana) pasture. The site was previously under C3 br
igalow (Acacia harpophylla) scrub. In block 1 on Paleustollic Pelluste
rts, the organic carbon in both the < 1.6 Mg m-3 and the > 1.6 Mg m-3
fractions decreased linearly with time in the cultivated plots and inc
reased linearly with time in the green panic pasture. In block 3, the
organic carbon content of the light fraction (< 1.6 Mg m-3) and the he
avy fraction (> 1.6 Mg m-3) in the oats and green panic plots on Typic
Pellusterts also showed a linear relationship with time, decreasing i
n the cultivated plots and increasing under pasture. The organic carbo
n content of the soil fraction > 1.6 Mg m-3 from the black gram and so
rghum plots on Paleustollic Chromusterts showed an initial rapid decli
ne over 2 years with a subsequent slower linear decline. The proportio
ns of organic carbon in the soil due to the original C3 brigalow fores
t and the subsequent C4 Pasture were calculated from the deltaC-13 val
ues of the heavy soil fractions. In block 1, cultivation resulted in a
decline in both the C3 and C4 pools. In block 3, cultivation resulted
in a more rapid decline in the older C3 pool than the C4 pool. The ra
te of decline in block 3 was associated with soil type and, upon culti
vation, the C3 carbon in the Chromusterts declined more rapidly than t
hat in the Pellusterts. Organic carbon which was part of the resistant
pool under pasture therefore contributed significantly to the labile
pool on cultivation. There appeared to be major differences in the mec
hanisms protecting the organic matter from microbial degradation in th
e different soil types. In the Chromusterts, a proportion of the older
C3 pool which may have been physically protected under pasture was ra
pidly mineralized on cultivation. The mechanisms protecting the C3 poo
l in the Pellusterts appeared to be more stable to physical disruption
and therefore more resistant to degradation.