RELEASE OF ORGANIC-C FROM GROWING ROOTS OF MEADOW FESCUE (FESTUCA-PRATENSIS L)

A new method to estimate the amount of organic C released from roots during plant growth is described. The method is based on decomposing root-derived material left in the soil after plant growth and removal of all visible roots in a long-term incubation. The total amount of organic C released during plant growth is then calculated from the amount of root-derived material remaining in the soil after long-term decomposition and a stabilization factor for this material. An estimate of the total amount of organic C released from roots during plant growth allows the total rhizosphere respiration to be separated into root respiration and microbial respiration. In a previous cultivation of continuously C-14-labelled meadow fescue (Festuca pratensis L.) the distribution of net assimilated C-14 was determined. Root-derived material in root-free soil from the cultivation was allowed to decompose in a long-term incubation (61 weeks). The amount of stabilized root-derived material left in the soil after the incubation, representing a certain proportion of the fresh root-derived material released during growth, was calculated. The stabilization of root-derived material was approximated through comparison with stabilization of known organic materials (glucose, grass shoots and roots). Decomposition of root-derived material resulted in more acid-stable residues than decomposition of glucose but less than shoots and roots. From the relationship, for the known organic materials, between the amount of stabilized organic material remaining after decomposition and the amount of acid-stable residue, the amount of original root-derived material was estimated. This result was combined with data from the previous determination of distribution of net assimilated C-14 in meadow fescue on shoots, roots, soil and rhizosphere respiration. It was found that, during 7 weeks from germination, about 10% of net fixed C-14 was released as organic material from the roots. Microbial respiration, from decomposition of root-derived organic material during growth, was estimated to be 32% of the total rhizosphere respiration.