EXPLORING WHETHER WORMS WEATHER SILICATES IN SOILS

What ecosystem engineer is invading ecosystems around the world? The answer is worms. Earthworms are well-known mediators of soil nutrient cycling, but could they also contribute significantly to mineral weathering? Here we test the hypothesis that earthworm digestion of inorganic soil particles accelerates silicate weathering through a combination of comminution in the earthworm gizzard and microbially-mediated silicate dissolution in the earthworm gut. We start by coupling CAMSIZER grain size analyses with measurements of in situ ¹⁰Be in soils and saprolite across an earthworm and erosion gradient in Puerto Rico's El Yunque rainforest. We observe a sharp shift in median particle grain sizes that corresponds with the observed depth of earthworm burrowing across our field sites and find that quartz grains in intensively worm-bioturbated topsoil horizons undergo a nearly ~50% reduction in volume. We then use in situ ¹⁰Be measurements to establish rates of soil production (mm yr^-1) for El Yunque soils, which we subsequently combine with soil thickness (mm) measurements to calculate average particle residence times (yr). Coupling this substantial volumetric change in quartz and other silicate grains with a 20 kyr mean particle residence time yields worm weathering estimates of around ~0.14 kg Si m^-2 yr^-1 for El Yunque soils. This constitutes roughly ~2% of the total 6.6 ± 3.8 kg Si m^-2 yr^-1 soil weathering rate estimated previously for the Río Icacos watershed, suggesting that worms could accelerate weathering rates at levels comparable to the projected increase in feldspar dissolution rates following a 1–2 °C temperate increase in the tropics. To further validate these observations from Puerto Rico, we also assessed grain size changes across a series of soil profiles spanning earthworm invasion gradients in Alaska, Minnesota, Finland, and Sweden where the timing of earthworm arrival––and hence worm weathering––is more tightly constrained. We again observe appreciable shifts in median particle sizes over periods as short as several decades to centuries, and the observed fining of mineral grains can be directly linked to recent earthworm activities at these historically worm-free sites. Our results highlight that the ongoing and future effects of global w‘o’rming may be on par with global warming in terms of regulating rates of regolith weathering at Earth’s surface.