Influence of six crop species on aggregate stability and some labile organic matter fractions

Abstract

The effect of the growth of barley, wheat, prairie grass, Italian ryegrass, white clover and lupin on the aggregate stability and related properties of a heavily cropped soil was investigated in a greenhouse experiment. For the non-leguminous crops, root mass and root length followed the order barley = wheat < prairie grass < Italian ryegrass. Less marked, but similar trends were found for microbial biomass C, cold and hot water-extractable carbohydrate content and aggregate stability. It was postulated that a higher root mass results in greater rhizodeposition of carbonaceous material and, therefore, a higher microbial biomass which in turn produces carbohydrate binding agents which increase aggregate stability. The hot water-extractable carbohydrate fraction was found to have a galactose plus mannose-to-arabinose plus xylose ratio of 2.1 confirming that it was predominantly of microbial origin. In comparison with the non-legumes, growth of white clover and lupin resulted in an unexpectedly high aggregate stability and to a lesser extent microbial biomass C content relative to their rather small root mass and length. Lupin, for example, had the highest aggregate stability of all the crops, while white clover had an aggregate stability similar to that of Italian ryegrass yet the two legumes had the lowest root length densities of all the crops studied. It was suggested that the rhizosphere microbial population of leguminous plants differed in some way to that of non-legumes (possibly due to the higher N content of rhizodeposited material) and that this contributed to the higher measured aggregate stability. A subsidiary experiment showed that fungal hyphal length in aggregates affected by lupin growth was four times that under wheat. There is a need for further research into aggregation in the rhizosphere of a wider range of legumes.