Evaluating the use of gel-based sub-sampling for assessing responses of terrestrial microarthropods (Collembola and Acari) to different slurry applications and organic matter contents

Microarthropods show considerable small-scale spatial variation, which on statistical grounds demands the use of numerous samples per experimental unit to obtain a representative density estimate. To avoid the identification of all individuals in these samples a sub-sampling technique is desirable. Due to various factors, existing sub-sampling methods perform sub-optimally when working with soil microarthropods. As an alternative, this study evaluated a gel-based sub-sampling method. The method was applied in a practical situation comparing effects of two existing methods of slurry manure application (injection of control slurry and the superficial application of slurry to which an additive was added) on soil inhabiting springtails and mites. In the experiment, special attention was paid to the mediating influence of soil organic matter content. The gel-based sub-sampling data showed a Poisson distribution, supporting randomness. Furthermore, none of the identified specimens showed morphological damage, strongly suggesting that neither gel-suspension nor amylase treatment had damaged the integuments of the specimens. Therefore, gel-based sub-sampling seems to offer an efficient and safe tool for sub-sampling microarthropods. Of the two slurry applications studied, the surface application of the slurry manure with the additive resulted in the highest abundances of microarthropods, with an average of 17,890 individuals m−2 in broadcasted fields, compared to 11,486 individuals m−2 in the slit injected fields. A positive correlation between microarthropod abundance and the organic matter content of the soil was found for the fields that were slit injected with unamended slurry. The specially prepared slurry treatment did not show the latter correlation, probably because of the a priori relatively high organic matter content of most fields receiving this treatment. The relative dominance of surface dwelling species in the superficially applied slurry with additive suggests that the microarthropods responded in particular to the method of slurry manure application rather than to slurry manure type.