According to the United States Department of Agriculture (USDA), on rhizosphere sampling depth and distance for nutrient analysis:

"The rhizosphere is the soil layer directly influenced by the plant roots. It is generally defined as the volume of soil within 1 to 2 mm of a root. To sample the rhizosphere, soil cores are collected from the rooting zone (depth range 0-30 cm) of the tree or stand. For most purposes, a representative sample can be obtained from a distance of about 30 to 60 cm from the tree trunk." (Source: USDA Forest Service, Guidelines for Soil Sampling and Analysis in the National Forest Inventory, p. 3-13)

@ Rajani, actually rhizosphere is the zone of soil surrounding a plant root where the biology and chemistry of the soil are influenced by the root. This zone is about 1 mm wide, but has no distinct edge. To fix the distance for a particular tree rhizosphere we must assured that the volume of soil we are collecting from root zone must be influenced by plant roots and their exudates.

The immediate soil zone in contact with roots of plant is refers to as the Rhizosphere , which is about 1 mm depth within distance of 0 -8mm. When conducting rhizosphere soil sampling the depth distance mentioned must be taken into considerations.

Thank you so much everyone for responses, if anybody have paper related to this plz share