Relationship between root length density and soil microorganisms in the rhizospheres of white clover and perennial ryegrass

Abstract Microbial parameters of rhizosphere soil, such as bacterial numbers or microbial activities, depend on the distance that microbes have to the root surface. In this study we show that the number of bacteria found in rhizosphere soil from white clover is highly correlated with root length density of the rhizosphere soil. In contrast, bacterial numbers, microbial activity (measured as fluorescein diacetate hydrolytic activity), and the amount of extractable carbon (C) in the rhizosphere of perennial ryegrass were independent of the amount of soil recovered from the roots. The missing rhizosphere effect in perennial ryegrass soils can be explained by the high rooting density of ryegrass, whereas the low rooting density of white clover results in gradients of microbial numbers and activities in soils. Results of these studies indicate that it is important to express microbial parameters on root length and soil weight bases, especially for less densely rooted soils.