Altitude, Land Use and Soil Depth Effects on Earthworm Density and its Relationship to Soil Properties in an On-Farm Study

Abstract Purpose Earthworms play a critical role in soil ecosystem functions through the cycling of organic matter and nutrients. However, some land uses or environmental conditions are more favorable habitats for them than others. Thus, the objectives of this study were to evaluate the prevalence of earthworm density and its relationship to land uses and soil properties. Methods The study was conducted in three districts (Wangdue Phodrang, Chhukha and Dagana) in Bhutan in three land uses (organic fields - OrgF, conventional fields - ConF and natural vegetation - NatV) under three altitudes (high-, mid- and low-altitudes) and three soil depths. Results Overall, earthworm density at high-altitude was significantly ( P < 0.001) higher than that at mid- and low-altitude sites. Further, across altitudes and soil depths, OrgF sites had a significantly ( P < 0.001) higher earthworm density (120 earthworms m − 2 ) compared to that in the NatV (56 earthworms m − 2 ) and ConF (43 earthworms m − 2 ) sites, and the density decreased significantly ( P < 0.001) and successively with the increase in soil depth. The coefficient of determination ( R 2 ≥ 0.51; P < 0.001) showed a positive and moderate relationship between the earthworm density with soil organic C and total N in OrgF sites, but the relationship was weak ( R 2 ≤ 0.22) in the ConF or none in the NatV sites. Conclusions Substituting chemical fertilizers with organic manures could increase earthworm density by enhancing soil health through the cycling of organic materials and nutrients in the soil. The findings demonstrate empirical evidence for earthworm prevalence in different land use types across altitudinal gradients and provide valuable decision-making insights to land users and policymakers alike.