Following the previous last glacial maximum approximately 22,000 years ago, north-eastern North American forests have developed in the absence of earthworms. European and more recently Asian earthworms have been introduced from human activities, and their influence propagates throughout the entire ecosystem. In forests, the most prolific change is rapid decomposition and redistribution of organic matter, often leading to a complete loss of the organic horizon. We sought to understand the ultimate consequences of earthworm invasion on nutrient dynamics of forest soils. We compared exchangeable and total soil nutrient areal content and concentrations and three native plant species (Acer saccharum, Polygonatum pubescens, Polystichum acrostichoides) in forested plots with a spectrum of earthworm densities in central New York.
Results/Conclusions
Earthworm biomass was related to distribution and concentration of soil nutrients, and was a better predictor of plant nutrition than soil fertility itself. Earthworms were associated with lower total and exchangeable P, but higher Ca, K, Mg and Mn, particularly in the A horizon. Higher soil Ca in earthworm-invaded plots was associated with higher foliar Ca in Acer and Polystichum, and lower soil P with lower foliar P in Polygonatum. Even with higher K, Mg, Mn and S in the A horizon of earthworm-invaded soils, earthworm biomass was associated lower K, Mn and S in plant tissue. Rooting volume in the A horizon was much lower in the presence of earthworms, which corresponds to a build-up of exchangeable nutrients. Taken together, this suggests earthworms are creating stressful rooting conditions for native plants that have evolved to root in deep, undisturbed organic horizons. Conversely, early successional and non-native species that can establish successfully in the absence of an organic horizon may benefit from pools of exchangeable nutrients from mineralization of the organic horizon. This suggests that earthworm invasion is associated with disrupted nutrient cycling and a different set of selection pressures on plant communities.