Solid-phase root zone extraction (SPRE): a new methodology for measurement of allelochemical dynamics in soil

Root exudates are implicated in the chemical defense of plants, but testing such hypotheses has been hindered by the difficulties of quantifying allelochemical concentrations in soil. Here we describe a new, simple method to quantify the dynamics of non-polar root exudates in soil. Novel soil probes were constructed using stainless steel wire inserted into polydimethylsiloxane (PDMS) tubing. Probes were inserted into soil for 24 h, removed and extracted, and analyzed by HPLC. Lipophilic thiophenes produced by roots of Tagetes and Rudbeckia species were chosen as candidate compounds to test the method. Probes recovered microgram quantities of the highly phytotoxic thiophenes 5-(3-buten-1-ynyl)-2,2'-bithienyl (BBT) and alpha-terthienyl per probe per day from the root zone of Tagetes patula, and distribution of thiophenes beneath plants was spatially and temporally heterogeneous. Flux-proportional sampling of soil provides a means to test hypotheses about the role of root exudates in plant-plant and other interactions.