Effects of organic amendment on earthworm density and biomass in sugarcane fields with different soil pH

Application of organic amendments to agricultural fields often increases earthworm density and biomass. Soil pH can influence earthworm species composition, density, and biomass. However, the effects of organic amendments on these three characteristics at different soil pH value are not fully understood, especially in tropical agricultural fields. In sugarcane fields on Ishigaki Island, Okinawa Prefecture, Japan, we measured soil properties and earthworm species composition, density, and biomass at three paired sites (with and without filter cake input) in soil with low pH (pH-5 sites; pH < 5.2 without filter cake) and moderate pH (pH-6 sites; pH > 6.0 without filter cake), 12 sites in total. The fields contained Pontoscolex corethrurus (Müller, 1856), Polypheretima elongata (Perrier, 1872), and Dichogaster bolaui (Michaelsen, 1891); P. corethrurus was dominant and mean species richness was 1.2. Filter cake application increased earthworm density and biomass at the pH-5 sites but decreased them at the pH-6 sites. This discrepancy in effect might be due to how pH changed when filter cake was added. At pH near neutral, even a small change in proton content could result in a large change in soil pH. The application of filter cake did not change soil pH at the pH-5 sites, but it significantly decreased soil pH at the pH-6 sites, which might have caused the reduction of earthworm density and biomass. Overall, organic amendment affected soil physicochemical and biological properties over the short term in the subtropics, depending on the prior soil pH. Under certain soil conditions, organic amendment can improve both the physicochemical and biological properties of soil, which could improve soil ecosystem services.