Carbon flow in the rhizosphere of ponderosa pine seedlings

Abstract

The flow of photosynthetically-fixed carbon was followed in microcosms containing ponderosa pine seedlings using a 14CO 2 pulse labeling technique. Plant and soil samples were harvested at the end of labeling and after a 1 week chase period to examine the movement of 14C from different root types into adjacent soil. The total and specific activity (SA) of C was determined in above-ground and below-ground plant fractions and in soil 0–2 mm. 2–5 mm and > 5 mm from coarse, fine or young roots. The SA of mycorrhizal tips and hyphal strands was also assessed. Microbial biomass C and 14C content was determined by the chloroform fumigation incubation technique in the non-rhizosphere soil. The highest SA in the plant was found in the young root tip region demonstrating that this was a strong sink for recent photosynthate. The fine roots with their associated short laterals and mycorrhizae contained > 25% of the 14C recovered in all plant fractions. The highest SA in the soil was found within 2 mm of the young root surface. Fine roots had the highest rate of C loss to the soil per unit C assimilated in the root segment. The large differences in C flow to the rhizosphere from roots of different morphology demonstrates the importance of differentiating root types in studies of C flux. The important role of mycorrhizal hyphae in redistribution of recently-fixed C in soil was illustrated by their high SA and the large percentage ( > 30%) of 14C recovered in non-rhizosphcre soil. The SA of the microbial biomass C in the non-rhizosphere soil decreased by over 50% during 1 week indicating a rapid turnover rate in this active fraction. The loss of recently-fixed C from pine roots by rhizodeposition and hyphal transport was an important energy source for the soil microbial community even in soil not directly adjacent to the root surface.