Krasnozems - organic-matter

Virgin krasnozems contain about 6% C from 0-15 cm and, while the C content 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 annual rainfall is 1500 � 200 mm. At higher rainfalls, the C contents are less. There is a negative correlation between organic C and N content and 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 free iron oxides. While data are fragmented, it is evident that the clearing of rainforests and replacement by pastures or other agricultural and horticultural pursuits have led to losses of up to 50% of the organic C over several decades. A simple model illustrates that this is caused 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 persistence of some C in krasnozems for hundreds of years. Some of this is particulate organic matter occluded in aggregates but most of it appears 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 13C nuclear magnetic resonance has shown more aliphatic materials in krasnozems than other soils, but the reason for this is not clear. There is little information on biota in krasnozems. The deep well structured soils are a good habitat for organisms and their high fertility guarantees a good supply of substrate for fauna and flora. It is important to maintain the organic matter contents of krasnozems to maintain cation exchange capacity, mineralization of N and other elements, to decrease phosphate sorption and to stabilize larger aggregates and thus macro porosity.