Earthworm activity and decomposition of 14 C-labelled grass root systems

Abstract

Decomposition of 14C-labelled root systems of the grass species Holcus lanatus and Festuca ovina, representative of mesotrophic and oligotrophic situations, respectively, was monitored during 14 months under field conditions in the presence or absence of earthworms (Lumbricus rubellus). During the experiment respiration was monitored, and after 6 weeks and 3 and 14 months the carbon distribution in the soil was determined in various pools of aggregates and organic matter. Hypotheses tested were: (1) roots of oligotrophic plants decompose more slowly than roots of mesotrophic plants, (2) earthworms prefer mesotrophic organic material, and (3) in worm excrements organic material is protected against rapid decomposition. Initially, Holcus root material decomposed faster than that of Festuca, which was probably connected with the higher amount of rhizodeposits in the former. For both species, decomposition was faster in the presence of earthworms, due to enhanced fragmentation and mixing of the material. It appeared, however, that Festuca material fragmented more easily than that of Holcus and it seemed that earthworms had a higher preference for Festuca material. Consequently, more Festuca material was incorporated in macro- and microaggregates, and these aggregates stayed stable for a longer period than those with Holcus material. Although earthworm activity was markedly decreased after 3 months, their effects remained visible throughout the experimental period. It also appeared that after 3 months the processes taking place with the added root material could no longer be distinguished from those involving older organic material. It is concluded that the first two hypotheses could not be confirmed, and that the third remains likely. Furthermore, it is argued that nutrient cycling in an oligotrophic ecosystem will be decelerated more by earthworm activity than in a mesotrophic system. The preference of worms for Festuca and the probably longer protection of this material within aggregates will accelerate species replacement during succession.