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 cond itions are scarce. Yet they are necessary to understand various aspect s of soil organic matter dynamics. To evaluate the use of C-14 pulse-l abelling for rhizosphere carbon budget estimations and to develop a st andard labelling procedure, the dynamics of C-14 partitioning and fact ors affecting the representativity of the assimilated C-14 for the ave rage daily assimilation were investigated. Field-grown spring wheat pl ants were pulse-labelled with C-14 at five different development stage s between elongation and dough ripening. Allocation of C-14 in shoot t issue 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 d ay when labelling was performed. Therefore, net assimilated C-14 was r epresentative for the average daily net assimilation. The proportion o f net fixed C-14 recovered in the shoot increased from 61% at elongati on to 85% at dough ripening. In the roots this proportion decreased fr om 15 to 2% and in soil-root respiration from 14 to 7%, while in the s oil 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 all ocation period of about 3 weeks is a satisfactory method to estimate a ssimilate distribution at different development stages.