Soil nutrient losses in an altered ecosystem are associated with native ungulate grazing

1. Protected natural areas often inherit legacies of altered habitat and predator-prey dynamics. Extirpation of top predators and ecosystem engineers modifies the density and distribution of the remaining species, with potentially significant ramifications for ecological condition. Land managers need, among other things, information on how populations of large grazing animals affect their habitat when key components are missing from the ecosystem. 2. Our objective was to examine correlations between intensive grazing by Cervus elaphus (red deer or North American elk) and soil processes in a protected natural area with a history of predator reduction and habitat modification. We used 12 exclosures constructed in 1994 to compare soil carbon and nitrogen, nitrogen cycling, bulk density, soil moisture and soil microbial characteristics in grazed and ungrazed areas. Samples were collected in 1998 and in 2010. 3. Soil carbon (C) concentration was 25% lower in grazed areas than in exclosures; soil nitrogen (N) concentration was 23% lower. From 1998 to 2010, the mean percentage change for soil C and N concentrations was + 14% and + 27%, respectively, in exclosures and -22% and -24%, respectively, in grazed sites. 4. Soil bulk density in grazed areas was 25% greater than in ungrazed areas; soil moisture in grazed areas was 15% lower than in ungrazed areas. 5. Physical soil attributes, soil nutrient stocks and soil microbial biomass were significantly correlated, exhibiting linkages that can accelerate changes in ecosystem function. Plant N concentrations suggest soil nutrient losses were not related to substitutions for N-rich forage species. 6. Synthesis and applications. Intensive grazing was correlated with drier more compacted soils and nutrient loss, suggesting increasing N limitation and reduced plant biodiversity. Recovery of soil nutrients and soil structure appears possible in the studied montane riparian zone if vegetation receives grazing protection. Cervus elaphus density and distribution should be controlled in the short term to restore soil structure, hydraulic conductivity and nutrients. Active management could be obviated in the longer term through restoration of predators and ecosystem engineers.