Fig. 3. Photograph of cottonwood roots growing in a mini-rhizotron. The circular white points arrayed across the photograph are the micro-suction cups. Numbers in parentheses are used to identify each sampling point. Values shown are solution P concentrations (mg L^–1) observed after 10 d of root growth. Micro-suction cups near the top of the photograph have been influenced by root uptake for a longer period of time. Lower values near roots emphasize the effect the plant has had in decreasing solution concentration near the root compared with those in bulk solution. The sample volume at Point 2 was not sufficient for analysis of P concentration.