Earthworms, soil-aggregates and organic matter decomposition in agro-ecosystems in the Netherlands

The relationships between earthworm populations, soil aggregate stability and soil organic matter dynamics were studied at an experimental farm in The Netherlands.

Arable land in general is not favourable for earthworm growth. In the Lovinkhoeve fields under conventional management earthworm populations were brought to the verge of extinction in a few years. Main causes are soil fumigation against nematodes and unfavourable food conditions. Organic matter inputs and N-contents of the organic materials are important aspects of food availability of earthworms, but also bacterial and protozoan biomass play a role. Natural stress, such as summer drought and winter frost, adds to the effects of management. Organic management with reduced pesticide use, however, strongly stimulates population development. Colonisation, especially stimulated in the presence of passive transport mechanisms can help population built up. Population parameters and growth conditions at the invaded plots further influence the speed of colonisation.

Correlation studies, combined with results of manipulation experiments show that earthworms increase resistance against slaking of field macro-aggregates. Fresh earthworm casts are very unstable, but their stability increases during ageing by several mechanisms. Drying makes casts much more stable than dried uningested field aggregates. Under specific conditions, fungal growth on the surface may stabilise wet casts, but the rearrangement of clay around particulate organic material seems to be the most important process for development of stability during ageing. Microbial activity either in the gut or after excretion may play an indirect role in connecting the organic material and the clay.

The organic matter included in stable aggregates formed under the influence of earthworms is much better protected against microbial breakdown during the lifetime of these aggregates than organic matter in stable aggregates formed by other mechanisms. In the presence of earthworms, stable aggregates are either formed at a higher rate or break down at a lower rate. Also, earthworm activity can lead to increased organic matter contents of macro-aggregates. Earthworm activity consequently leads to a concentration of organic matter in the larger aggregate fraction and to a higher organic matter retention capacity of the soil. Grinding soil to fragments smaller than 2 mm, as is often done in standard procedures, will strongly influence organic matter breakdown in incubation experiments in samples from soils with high earthworm activity. Therfore, these standard preteatment methods are unsuitable for fractionation and decomposition studies.