Limitations and Potential of in situ Rhizobox Sampling for Assessing Microbial Activity in Fruit Tree Rhizosphere

Controlled in situ sampling of tree rhizosphere is still a major challenge in detailed rhizosphere studies. Rhizoboxes have been widely used to describe the rhizosphere of annual crops, but have been rarely applied to trees. In this manuscript we aimed at assessing pH and microbial activity gradients associated with fruit tree rhizospheres. Rhizoboxes were introduced during the spring season around plum trees in an organic orchard receiving three different fertilization treatments. Relatively thin and incomplete root mats developed within rhizoboxes which had been placed 40 cm from the tree trunk. Roots developed only in a few of the rhizoboxes that had been placed at 150 cm from the trunk. No significant gradients of pH or microbial activity were observed in all fertilization treatments. Absence of gradients could be the result of the incomplete cover of the root mats, or could indicate that the rhizosphere of such trees is very limited in space, such that the degree of slice resolution was insufficient. Yet, the experimental approach taken in this study reveals other potential research applications, by enabling root sampling under controlled conditions of growth period and the medium which is used for rhizobox packing.