Stage-dependent border cell and carbon flow from roots to rhizosphere.

Rising CO(2) levels in the atmosphere have drawn attention to the important role of soil in sequestering carbon. This project goal was to quantify soil carbon deposition associated with border cell release and exudation from root growth zones. Carbon was measured with a Carlo Erba C/N analyzer in soil from the rhizosphere of mature grasses and, in separate experiments, in soil collected around root growth zones. Root border cells in "rhizosphere soil" (silica sand) were counted using a compound microscope after soil sonication and extraction with surfactant. For sand-grown Bromus carinatus, Zea mays, and Cucumis sativus, young seedlings (with roots shorter than 2 cm) released thousands of border cells, while older root tips released only hundreds. For a variety of native annual and perennial grasses and invasive annual grasses (Nassella pulchra, B. carinatus, B. diandrus, B. hordeaceus, Vulpia microstachys, Aegilops triuncialis, Lolium multiflorum, Zea mays), the rhizosphere of mature root systems contained between 18 and 32 μg C g(-1) sand more than that of the unplanted controls. Spatial analysis of the rhizosphere around the cucumber growth zone confirmed C enrichment there. The root tip provided C to the rhizosphere: 4.6 μg C in front of the growing tip, with the largest deposition, 20.4 μg C, to the rhizosphere surrounding the apical 3 mm (root cap/meristem). These numbers from laboratory studies represent the maximum C that might be released during flooding in soils. Scaling up from the organ scale to the field requires a growth analysis to quantify root tip distributions in space and time.