INFLUENCE OF 6 CROP SPECIES ON AGGREGATE STABILITY AND SOME LABILE ORGANIC-MATTER FRACTIONS

The effect of the growth of barley, wheat, prairie grass, Italian ryeg rass, white clover and lupin on the aggregate stability and related pr operties of a heavily cropped soil was investigated in a greenhouse ex periment. For the non-leguminous crops, root mass and root length foll owed the order barley = wheat < prairie grass < Italian ryegrass. Less marked, but similar trends were found for microbial biomass C, cold a nd hot water-extractable carbohydrate content and aggregate stability. It was postulated that a higher root mass results in greater rhizodep osition of carbonaceous material and, therefore, a higher microbial bi omass which in turn produces carbohydrate binding agents which increas e aggregate stability. The hot water-extractable carbohydrate fraction was found to have a galactose plus mannose-to-arabinose plus xylose r atio of 2.1 confirming that it was predominantly of microbial origin. In comparison with the non-legumes, growth of white clover and lupin r esulted in an unexpectedly high aggregate stability and to a lesser ex tent microbial biomass C content relative to their rather small root m ass and length. Lupin, for example, had the highest aggregate stabilit y of all the crops, while white clover had an aggregate stability simi lar to that of Italian ryegrass yet the two legumes had the lowest roo t length densities of all the crops studied. It was suggested that the rhizosphere microbial population of leguminous plants differed in som e way to that of non-legumes (possibly due to the higher N content of rhizodeposited material) and that this, contributed to the higher meas ured aggregate stability. A subsidiary experiment showed that fungal h yphal length in aggregates affected by lupin growth was four times tha t under wheat. There Is a need for further research into aggregation i n the rhizosphere of a wider range of legumes. (C) 1997 Elsevier Scien ce Ltd.