RELATIONSHIP BETWEEN ROOT LENGTH DENSITY AND SOIL-MICROORGANISMS IN THE RHIZOSPHERES OF WHITE CLOVER AND PERENNIAL RYEGRASS

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