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Organic matter in soil particle size and density fractions from maize and legume cropping systems

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Soil organic matter (SOM) plays an essential role in biologically-mediated 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 of SOM most affected by maize and legume cropping systems. Soil was collected after the eighth cropping season (4 years) from an experiment on a Kandic Rhodustalf in Kenya. Total organic C and N in whole soil (WS) were not significantly (P < 0.05) different among seven cropping systems; 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 μm and < 1.13 g cm−3) and referred to as LL. The LL fraction represented 0.08% of the WS weight and accounted for 1.7 and 1.4% of the WS total organic C and N respectively. The next SOM fraction most affected by the cropping systems was the light fraction separated by flotation of 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 in 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 cropping of maize with removal of crop residues. The results suggest that the LL and NAL fractions merit further examination as sensitive measures of differences in SOM.

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