Do earthworms affect phosphorus availability to grass? A pot experiment

•We tested the effect of three earthworm species on soil P dynamics and grass yield.•Dissolved P was strongly increased in casts of all earthworm species.•P uptake by grass was significantly increased in the presence of Lumbricus terrestris.•Earthworms can help to improve utilization of soil P.

The largest part of phosphorus (P) in soil is bound by the soil solid phase; its release to the soil solution therefore often does not meet the demand of plants. Since global P fertilizer reserves are declining, it becomes increasingly important to better utilize soil P. We tested whether earthworm activity can increase P availability to grass (Lolium perenne L.) in a 75-day greenhouse pot experiment in a soil with low P availability. The full factorial design included two factors: P fertilization (control without P; phytate; and inorganic P) and earthworm population (control without earthworms; Lumbricus rubellus Hoffmeister, Lr; Aporrectodea caliginosa Savigny, Ac; and Lumbricus terrestris L., Lt). At four times during the experiment, aboveground plant growth and P uptake were determined. In a separate incubation experiment, earthworm casts and bulk soil were analyzed for inorganic and organic P in water extracts. We observed higher levels of dissolved P pools (p < 0.001) in the water extracts of earthworm casts compared to those of the bulk soil. The magnitude of the difference differed between earthworm species, with the largest levels for Lr: from <0.02 to 8.56 mg L−1 for inorganic P (p = 0.007) and from 0.18 to 1.30 mg L−1 for organic P (p = 0.007). After three harvests, presence of Lt significantly increased P uptake by grass to 44.1 mg per pot compared to 41.8 mg per pot for the control (p = 0.010). Plant growth increased from 15.68 to 16.85 g dry biomass per pot (p < 0.001). We conclude that earthworms casts contain higher levels of plant available P than the bulk soil, and that this might translate into increased plant P uptake. It is well-known that maintaining soil faunal biodiversity is important for a variety of ecosystem services; our results show that these ecosystem services may include improving the utilization of soil P in a world with rapidly declining P stocks.