The Effects of Large Herbivores on Soil Organic Matter Quality in Terrestrial Ecosystems

Understanding the factors that control the natural fluxes of carbon into and out of terrestrial ecosystems is of paramount importance to forecast and adapt to climate change. Soil respiration – the release of carbon dioxide from roots and soil micro-organisms– is considered to be the largest terrestrial source of carbon dioxide. Microbial respiration, the least understood component of soil respiration, is to a great extent determined by soil organic matter quality, reflecting the relative fractions of labile and recalcitrant soil carbon. The goal of this research was to examine how herbivory affects soil organic matter quality across a wide range of terrestrial ecosystems, including Oregon sagebrush steppe, Colorado short grass prairie, Nebraska mixed-grass prairie, Kansas tall grass prairie, and boreal forests in Isle Royale National Park (Michigan) and Acadia National Park (Maine). Studying the effects of herbivores on soil organic matter quality among such a wide variety of ecosystems has not been previously undertaken. Soil organic matter quality was determined with a long-term, 36-week laboratory incubation experiment on soils collected from paired plots located outside and inside long-term exclosures in each of the ecosystems. For the sagebrush steppe, soils were sampled separately beneath shrubs and interspace because of previous reports of large effects of shrubs on soil properties. Herbivory had varying effects (positive, negative, and neutral) on soil organic matter quality at the sites. Herbivory increased soil organic matter quality at the shortgrass prairie, tallgrass prairie, and Isle Royale boreal forest, and reduced it at one of two boreal forests in Acadia and under shrubs in the sagebrush steppe. Herbivory had no effect on the mixed-grass prairie, one of two boreal forests in Acadia, and under interspace in the sagebrush steppe. The variable effects of herbivores may have been a function of the particular responses of plant species to herbivory among ecosystems and/or potential confounding differences in soil properties between paired plots. Future work should include experiments that investigate how herbivores impact linked plant and soil dynamics of grassland and forest ecosystems. The major implication of these findings is that policy makers will need to develop herbivore management policies on an ecosystem-by-ecosystem basis if they hope to manage soil organic matter quality to maximize soil carbon sequestration.