In situ sampling of low molecular weight organic anions from rhizosphere of radiata pine ( Pinus radiata) grown in a rhizotron system

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Collection of root exudates is always a challenge because of difficulties accessing the rhizosphere without damaging plant roots. This paper describes a novel technique for repeated in situ collection of low molecular weight organic anions from plant roots grown in large-scale rhizotrons under controlled environmental conditions. Anion exchange membrane (AEM) was used to collect organic anions from the soil–root interface. The recovery and stability of 12 organic anions (tartarate, quinate, formate, malate, malonate, shikimate, lactate, acetate, maleate, citrate, succinate and fumarate) on AEM was first assessed under controlled conditions using solution and soil media. The AEM strips were shown to be effective for the capture of the range of organic anions from both solution and soil. Contact between AEM impregnated with anions and soil reduced the recovery of organic anions from the AEM by an average of only 13% in comparison with membranes not in contact with soil. The AEM strips were then used for in situ sampling of organic anions from rhizosphere of radiata pine ( Pinus radiata) and bulk soil through horizontal access portals in the rhizotron in two experiments. In Experiment 1, exudates from 24 rhizosphere samples collected at various depths contained up to nine organic anions. Acetate and formate were present in most samples (23 and 20, respectively) at concentrations of 0.3–10.7 μg cm −2 membrane and 0.2–2.6 μg cm −2 membrane, respectively from a 3 h collection period. Various concentrations of shikimate, fumarate and lactate were detected in 8–16 samples, while malate, maleate, tartarate and succinate were detected in 1–5 samples. Only four anions (formate, lactate, acetate and fumarate) were found in 12 bulk soil samples and, with the exception of lactate, these were present at lower concentrations than in adjacent rhizosphere soil. In Experiment 2, exudates were collected repeatedly from two root locations from a depth of 10 cm from 6 am to 10 pm in a series of 2 h collection periods. Eleven anions were detected in the exudate samples and it was evident that the concentrations of most anions changed between collections at different time periods. The anion collection technique was practical for collection of anions from a wide range of root types and for repeated collection of exudates from the same root locations without damage to roots. Considerable variability in quantity and/or composition of organic anions in root exudates was found in both experiments, further highlighting the importance of using in situ techniques for root exudation studies.