Comparative study of different earthworm sampling methods.

The species composition and abundance of earthworms are important indicators for the impacts of agricultural management on soil quality. In order to develop a proper method for investigating earthworm population in agricultural soils, the effectivity and accuracy of seven sampling methods including: hand-sorting; formalin expulsion; mustard water suspension expulsion; mustard acetic acid suspension expulsion; and the three corresponding chemical expulsions in combination with hand-sorting, were reported in this paper. The experiment was carried out in the Black soils in northeast China under continuous corn cropping with four replicates. The abundance, biomass and age of the earthworm population obtained with these methods were analyzed synthetically. Results showed that the formalin expulsion without hand-sorting yielded the lowest earthworm abundance and biomass relative to other methods, especially for the juveniles, which indicated that this method is not suitable for earthworm population studies. The mustard suspension expulsion, especially mustard acetic acid suspension, was more efficient than formalin solution, which acquired abundance 22.3% higher than hand-sorting. The earthworm abundance and biomass acquired from formalin expulsion combined with hand-sorting were 19.1 and 9.3 times higher than formalin solution without hand-sorting. However, the proportion of adult earthworms was overestimated by this formalin method compared to other expellants methods studied. The hand-sorting combined with mustard water suspension and mustard acetic acid suspension had 67.8% and 89.1% more earthworms individuals collected, respectively, than those chemicals used independently, and the average of individual weight was 1.8 and 1.3 times higher, respectively. The results indicated that unlike formalin solution, mustard suspension was not likely to kill young or small-sized earthworm species. The mustard suspension without hand-sorting was not efficient to collect adult earthworms and the ones without burrows connected to the soil surface. However, these inefficiencies could be corrected with hand-sorting and the mustard suspension application. Earthworm abundance and biomass acquired using mustard acetic acid suspension in combination with hand-sorting were significantly greater than the other methods (P < 0.05). Using this method, we could collect not only the juvenile earthworms efficiently, but also the endogeic earthworms without burrows connected to soil surface. Accordingly, the method of mustard acetic acid suspension in combination with hand-sorting could be used as a better sampling method compared with other sampling methods for investigation of species composition and abundance of earthworms in agricultural soils.