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.