FIELD VARIABILITY OF CARBON ISOTOPES IN SOIL ORGANIC-CARBON

Free-air CO2 enrichment (FACE) plant-growth experiments conducted in A rizona have allowed a spinoff isotope tracer experiment (tank CO2 is C -14- and C-13-depleted) to follow the input of carbon into soil organi c matter pools. Accurate assessment of the C-14 and C-13 inputs to soi ls by this pathway requires detailed knowledge of the isotopic composi tion of the soils before the experiments. We have examined the variabi lity of delta(13)C in soil organic matter in 8 experimental plots prio r to the beginning of the 1995-96 FACE experiments with wheat. delta(1 3)C variability was higher immediately after harvest of a previous cro p in June, but the plots were much more homogeneous five months later immediately before planting. Intervening field management, including d isking, plowing and installation of irrigation drip tape likely contri buted to mixing the soils.