CARBON DYNAMICS IN VERTISOLS UNDER SEVERAL CROPS AS ASSESSED BY NATURAL-ABUNDANCE C-13

The effects of three annual crops (oats, C3, Avena sativa; sorghum, C4, Sorghum bicolor; black gram, C3, Vigna mungo) and a grass pasture (green panic, C4, Panicum maximum) on the content and turnover of organic carbon (0-0.15 m) in Vertisols in south-eastern Queensland were measured. 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 Rhodes grass (Chloris gayana) pasture. The site was previously under C3 brigalow (Acacia harpophylla) scrub. In block 1 on Paleustollic Pellusterts, 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 increased 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 heavy fraction (> 1.6 Mg m-3) in the oats and green panic plots on Typic Pellusterts also showed a linear relationship with time, decreasing in the cultivated plots and increasing under pasture. The organic carbon content of the soil fraction > 1.6 Mg m-3 from the black gram and sorghum plots on Paleustollic Chromusterts showed an initial rapid decline over 2 years with a subsequent slower linear decline. The proportions of organic carbon in the soil due to the original C3 brigalow forest and the subsequent C4 Pasture were calculated from the deltaC-13 values 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 rate of decline in block 3 was associated with soil type and, upon cultivation, the C3 carbon in the Chromusterts declined more rapidly than that 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 mechanisms protecting the organic matter from microbial degradation in the different soil types. In the Chromusterts, a proportion of the older C3 pool which may have been physically protected under pasture was rapidly mineralized on cultivation. The mechanisms protecting the C3 pool in the Pellusterts appeared to be more stable to physical disruption and therefore more resistant to degradation.