ORGANIC-MATTER IN SOIL PARTICLE-SIZE AND DENSITY FRACTIONS FROM MAIZEAND LEGUME CROPPING SYSTEMS

Soil organic matter (SOM) plays an essential role in biologically medi ated nutrient availability. There is, however, a lack of functionally- defined measures of SOM. We analysed C and N in particle-size, density and size-density fractions of SOM in order to identify the measures o f SOM most affected by maize and legume cropping systems. Soil was col lected after the eighth cropping season (4 years) from an experiment o n a Kandic Rhodustalf in Kenya. Total organic C and N in whole soil (W S) were not significantly (P < 0.05) different among seven cropping sy stem; however, the dry wt, N content and C content of several measured SOM fractions differed among these cropping systems. The SOM fraction most affected by cropping systems was the light fraction obtained by flotation of a sand-size fraction in a silica suspension (150-3000 mu m and <1.13 g cm(3)) and referred to as LL. The LL fraction represente d 0.08% of the WS weight and accounted for 1.7 and 1.4% of the WS tota l organic C and N, respectively. The next SOM fraction most affected b y the cropping systems was the light fraction separated by flotation o f WS in NaI (density = 1.7 g cm(-3)) and referred to as NAL. A greater (P < 0.05) dry wt of the LL fraction and greater amounts of C and N i n the LL and NAL fractions resulted from either the rotation of cowpea (Vigna unguiculata) with maize or the seasonal addition of Gliricidia sepium prunings to continuous maize, than from the continuous croppin g of maize with removal of crop residues,The results suggest that the LL and NAL fractions merit further examination as sensitive measures o f differences in SOM.