KRASNOZEMS - ORGANIC-MATTER

Virgin krasnozems contain about 6% C from 0-15 cm and, while the C con tent is less at depth, the total C reserves may exceed 200 t ha(-1) to about Im depth. Highest organic matter contents occur where the annua l rainfall is 1500+/-200 mm. At higher rainfalls, the C contents are l ess. There is a negative correlation between organic C and N content a nd mean annual temperature. Krasnozems contain more C than other soils in the same climate and are comparatively more fertile. Correlations of C contents with clay contents are confounded by the contents of fre e iron oxides. While data are fragmented, it is evident that the clear ing of rainforests and replacement by pastures or other agricultural a nd horticultural pursuits have led to losses of up to 50% of the organ ic C over several decades. A simple model illustrates that this is cau sed by lower inputs of C to the soil in most situations except perhaps long term productive pastures where inputs may be greater than under rainforests. Studies using natural isotopes of C have shown the persis tence of some C in krasnozems for hundreds of years. Some of this is p articulate organic matter occluded in aggregates but most of it appear s to be associated with clays, particularly at depth. The chemistry of organic matter in krasnozems appears similar to that of other soils, although solid state C-13 nuclear magnetic resonance has shown more al iphatic materials in krasnozems than other soils, but the reason for t his is not clear. There is little information on biota in krasnozems. The deep well structured soils are a good habitat for organisms and th eir high fertility guarantees a good supply of substrate for fauna and flora. It is important to maintain the organic matter contents of kra snozems to maintain cation exchange capacity, mineralization of N and other elements, to decrease phosphate sorption and to stabilize larger aggregates and thus macro porosity.