Effects of Gray-tailed Vole Activity on Soil Properties

Voles are well-known crop pests, especially when peak populations are present, but their role in soil fertility and impacts on agricultural sustainability are not well understood. Five months after the abrupt disappearance of a peak in a gray-tailed vole (Microtus canicaudus) population, we examined burrow structure, determined concentrations of trace elements, carbon and nitrogen in the soil immediately surrounding vole burrows, and compared soil chemical properties to a depth of 90 cm between areas with prior vole activity and areas of no activity. Vole tunneling activity was confined to the top 10 cm of the soil profile and was coincident with the majority of root biomass. Soil NH4 +, NO3 -, extractable organic carbon, and soil organic matter were greater below vole tunnels than above; however, due to small sample sizes, differences were not significant. There were no differences in trace elements with respect to position around vole tunnels. Vole activity was associated with increased soil nitrate concentrations and decreased soil pH to a depth of 90 cm, indicating that nitrification might be enhanced by vole activity, and that this effect continues after vole populations crash. Greater inorganic nitrogen could have long-term effects on ecosystem productivity. The effects voles have on soil processes that influence carbon and nutrient cycle requires further investigation. 1Author to whom correspondence should be addressed. Email: [email protected] Introduction Soil is the product of both biotic and abiotic processes, including the actions of soil-dwelling animals. Although invertebrates make up the greatest biomass of animals in soil, fossorial vertebrates such as gophers, voles and mice have been recognized for their role in soil mixing and nutrient enhancement (Grinnell 1923, Hole 1981, Huntley and Reichman, 1994, Pastor et al. 1996, Reichman and Seabloom 2002, Bakker 2003, Clark et al. 2005). Small herbivorous mammals can play an important role in the nutrient cycling of an ecosystem by increasing the rate of release of carbon and nutrients tied up in plant biomass. The nutrients stored in their fecal matter can turn over rather quickly; the mean residence time of fecal nitrogen of meadow voles and red-backed voles ranged from only 0.7 to 1.5 weeks (Pastor et al. 1996). High densities of burrowing rodents have been associated with high soil nitrate concentrations (Jorgensen 2002) and nitrogen mineralization rates (Bakker 2003). Clark et al. (2005) found that the fecal and urinary nitrogen output of small rodents in perennial grassland represented a flux of 3.7 kg nitrogen ha-1 yr-1 and Bakker (2003) estimated vole fecal outputs to amount to as much as 47 kg nitrogen ha-1 yr-1. These animals can add to local soil fertility not only through their excretion of wastes but also through storage of food caches and nesting materials (Woodmansee 1978). In addition to direct nutrient inputs, they can indirectly affect soil nutrient cycling by changing plant community composition (Canals et al. 2003, Howe et al. 2006) and/or by changing the microclimate of soils (Bakker 2003). Sirotnak and Huntly (2000) showed that although voles increased nitrogen mobility during and right after population peaks, over the long term, nitrogen mineralization decreased on these sites. They found that selective herbivory of plants with high nitrogen content created plant communities with lower carbon-to-nitrogen ratios. The burrowing action of these animals can loosen and increase the aeration status of the soil (Bakker 2003), which can significantly affect the rate of soil processes such as nitrogen mineralization.