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

There is increasing interest in use of isotopic tracers to study nutrient liberation and transformation in plant tissues and soils. We developed a technique for pulse-labeling plants in the field with C-14. Spatial distribution of radioactivity was measured in plots of maize (Zea mays L.) plants exposed to (CO2)-C-14. Two clear polyvinyl chambers measuring 1 m wide x 2 m long x 1 m high were used to C-14- label maize plants in conventional tillage and no-tillage treatments. A closed loop in-line with a pump allowed injection of (CO2)-C-14 and unlabeled CO2, and subsampling through an infrared gas analyzer. Cooling and mixing of the air within the chambers was achieved 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 significantly lower in several plots. Despite differences among leaf tip specific activities, total aboveground activity was uniform within the plot. Plant allometry and plant sampling immediately after labeling would help in correcting for within chamber variability in future field labeling studies.