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How dairy farmers manage the interactions between organic fertilizers and earthworm ecotypes and their predators
- Onrust, Jeroen, Piersma, Theunis
- Agriculture, ecosystems & environment 2019 v.273 pp. 80-85
- agricultural land, birds, cattle manure, detritivores, earthworms, ecotypes, farmers, feces, hay, mammals, meadows, organic fertilizers, predators, slurries, soil, straw, trophic levels, urine
- Most Dutch dairy farmland today is fertilized with slurry, a mixture of cattle dung and urine. As a food source for earthworms, this type of organic fertilizer is of lower quality than the traditionally used farmyard manure consisting of dung with some urine and mixed with bedding material such as straw. Earthworms living in dairy farmland belong to two distinct ecotypes, the detritivores and the geophages. Detritivores rely on farmyard manure as a food source more than geophages and therefore the type of organic fertilizer is expected to determine their relative abundances. In turn, this would affect higher trophic levels, as detritivores, when they come to the surface to collect food, are particularly important as prey for birds and mammals. Here we tested the hypothesis that dairy farmland fertilized with slurry will contain fewer detritivorous earthworms (thereby becoming less attractive for earthworm predators) by quantifying the abundance of the two earthworm ecotypes in 45 grasslands fertilized with either slurry, farmyard manure, or both. To determine the availability of detritivores for earthworm predators, we quantified earthworm surface availability by counting surfacing earthworms in the field and compared these numbers with densities belowground. To study the direct effects of different organic fertilizer types on earthworms, we measured their growth rates under controlled constant conditions using either slurry or farmyard manure, with hay as a control. We found that detritivores occurred in the highest densities in grasslands only fertilized with farmyard manure and that they also grew better on farmyard manure than on slurry. These differences were not found in geophages. Detritivores made up 25% of the total earthworm abundance in the soil, but contributed 83% to the surfacing earthworms at night; detritivore earthworms will thus be the main prey for visually hunting earthworm predators. The few dairy farmers using farmyard manure to fertilize their grasslands today will thus encourage the presence and availability of an earthworm ecotype which benefits higher trophic levels such as the endangered meadow birds.