Comparison of two widely used sampling methods in assessing earthworm community responses to agricultural intensification

To assess whether different sampling protocols provide similar results on earthworm community responses to land use, comparisons across different environments are required. Using an ongoing experiment in France, we assessed whether two protocols, widely used in international projects and global databases, provide similar estimates of earthworm abundance, and detect the same community responses to agricultural intensification. Method A consisted of hand-sorting composite samples of three soil monoliths 35 × 35 × 20 cm each, and applying formalin in the resulting holes. Method B consisted of applying formalin over a 1 m² contiguous area and subsequently hand-sorting a 25 × 25 × 25 cm soil monolith within it. Higher abundance was obtained from Method A than from Method B, but the two methods led to the same ecological conclusions. Firstly, they both showed that earthworm biomass and density decreased with agricultural intensification. Secondly, they showed similar land use effects on earthworm ecological group proportions, age structure, and body size distribution, pointing to a relative loss of large-bodied earthworms with agricultural intensification. These findings suggest that data from the two methods are both suitable to investigate the community response of earthworms, whereas assessments of earthworm abundance per se are more sensitive to the sampling protocol. Merits and drawbacks of the methods in terms of time and labour needed and of statistical variation are discussed.