C-14 PULSE-LABELING OF FIELD-GROWN SPRING WHEAT - AN EVALUATION OF ITS USE IN RHIZOSPHERE CARBON BUDGET ESTIMATIONS

Quantitative data on rhizodeposition under ecologically realistic conditions are scarce. Yet they are necessary to understand various aspects of soil organic matter dynamics. To evaluate the use of C-14 pulse-labelling for rhizosphere carbon budget estimations and to develop a standard labelling procedure, the dynamics of C-14 partitioning and factors affecting the representativity of the assimilated C-14 for the average daily assimilation were investigated. Field-grown spring wheat plants were pulse-labelled with C-14 at five different development stages between elongation and dough ripening. Allocation of C-14 in shoot tissue and soil-root respiration was complete by day 19 after labelling. The distribution of net fixed C-14 was not affected by the time of day when labelling was performed. Therefore, net assimilated C-14 was representative for the average daily net assimilation. The proportion of net fixed C-14 recovered in the shoot increased from 61% at elongation to 85% at dough ripening. In the roots this proportion decreased from 15 to 2% and in soil-root respiration from 14 to 7%, while in the soil organic C the percentage did not change with the development stage. C-14 in roots and soil organic C decreased exponentially with depth. We can conclude that C-14 pulse-labelling of wheat plants with an allocation period of about 3 weeks is a satisfactory method to estimate assimilate distribution at different development stages.