Method for C-14-labeling maize field plots and assessment of label uniformity within plots

There is increasing interest in use of isotopic tracers to study nutrient l iberation and transformation in plant tissues and soils. We developed a tec hnique for pulse-labeling plants in the field with C-14. Spatial distributi on of radioactivity was measured in plots of maize (Zea mays L.) plants exp osed to (CO2)-C-14. Two clear polyvinyl chambers measuring 1 m wide x 2 m l ong x 1 m high were used to C-14- label maize plants in conventional tillag e and no-tillage treatments. A closed loop in-line with a pump allowed inje ction of (CO2)-C-14 and unlabeled CO2, and subsampling through an infrared gas analyzer. Cooling and mixing of the air within the chambers was achieve d through the use of a free-standing automobile radiator with fan placed in the center of each plot. The specific activities of leaf tips differed by an order of magnitude among maize plants within the plot. Tillage and time after labeling within the first 48 h had no significant effect on specific activity of maize plants. Plant activity significantly differed by row. The row closest to the inlet and along the edge of the chamber was significant ly lower in several plots. Despite differences among leaf tip specific acti vities, total aboveground activity was uniform within the plot. Plant allom etry and plant sampling immediately after labeling would help in correcting for within chamber variability in future field labeling studies.