LABORATORY ASSESSMENT OF NUTRIENT RELEASE FROM A PASTURE SOIL RECEIVING GRASS OR CLOVER RESIDUES, IN THE PRESENCE OR ABSENCE OF LUMBRICUS-RUBELLUS OR EISENIA-FOETIDA

Two biotic systems were set up, either with or without large organisms (''macro-organisms''). Decomposition and nutrient release from herbage residues (ryegrass or clover) were examined. Earthworms (L. rubellus Hoff. or E. fetida Savigny) were the test macro-organism and nitrogen the test nutrient. Both soil metabolism and availability of soil N were greater in the presence of earthworms, irrespective of residue type or incubation temperature (15 or 22.5-degrees-C). Oxygen consumption and CO2 evolution increased by 39 and 26% respectively in the presence of earthworms. Mineral N concentrations were 50% higher in soils with earthworms. Microbial biomass was less in the presence of earthworms, and there is an indication that earthworms may have been responsible for about 50% of total respiration in the systems containing mixed populations. Carbon dioxide evolved during incubation was highly correlated (r = 0.84**) with soil mineral N present at the conclusion of the incubation. In a subsequent cropping study, using previously incubated soils as the growth medium and ryegrass as the test plant, this influence of macro-organisms was again apparent. Where soils had previously been influenced by earthworms there was a significant increase in plant growth and N uptake. Again, CO2 evolution during the initial incubation was highly correlated (r = 0.85**) with plant N uptake. The link between elaborated carbon and contained N has long been recognized as providing stability to organic N in soils. In the design of this experiment, other influences of macro-organisms (e.g. mixing or structural influences) were obviated, so one can conclude that enhanced carbon oxidation by earthworms was the main cause of the greater release of N from organic substrates. These results offer a fresh perspective on the balance between mineralization and immobilization in the soil-plant complex and, hence, on the dynamics of nutrients (N, P, S) stabilized in organic matter. Better understanding of these relationships may improve management of the dynamics of soil organic matter in temperate grassland ecosystem.