Biogeochemical cycling in forest soils of the eastern Sierra Nevada Mountains, USA D.W. Johnson a, *, W.W. Miller a , R.B. Susfalk b , J.D. Murphy a , R.A. Dahlgren c , D.W. Glass a a Natural Resources and Environmental Science, University of Nevada, Reno, NV 89557, USA b Desert Research Institute, Reno, NV, USA c University of California, Davis, CA, USA 1. Introduction Conceptual models for nutrient cycling in forest ecosystems are generally oriented toward mesic systems where nutrients cycle steadily, changes in ecosystem nutrient pools are slow, and transport via water is the major mechanism by which nutrients are lost from the ecosystem (Cole et al., 1968; Curlin, 1970; Duvigneaud and Denaeyer-DeSmet, 1970; Johnson and Van Hook, 1989; Likens et al., 1977; Swank and Crossley, 1988; Switzer and Nelson, 1972). Switzer and Nelson (1972) defined three major components to the nutrient cycle: (i) the geochemical cycle, which encompasses inputs and outputs from the ecosystem, most of which are associated with hydrologic fluxes, and chemical weathering of parent material; (ii) the biogeochemical cycle, which encompasses soil–plant relationships and is characterized by fierce competition between microbes and plant roots for limiting nutrients in the O horizon and soil; and (iii) the biochemical cycle, which encompasses the internal translocation of nutrients within the vegetation. This model has worked well for mesic forest ecosystems, but needs modification for application to semi-arid forests of the southwestern US where the hydrologic cycle is dominated by snow, spatial and temporal variation in water and nutrient fluxes are very large, and fire is common (Johnson et al., 1997). In this paper, we review and synthesize results from our studies on nutrient cycling processes in forests of the eastern Sierra Nevada Mountains of western Nevada and eastern California. Over the past few decades, we have determined that several important nutrient cycling processes in these ecosystems differ substantially from those in more mesic and warmer ecosystems and we herein explore the collective implications of these findings. Specifically, we review previous studies on horizontal spatial variability in soil Forest Ecology and Management 258 (2009) 2249–2260 ARTICLE INFO Article history: Received 19 June 2008 Received in revised form 17 December 2008 Accepted 13 January 2009 Keywords: Semi-arid forest Nutrients Decomposition Snow Runoff Leaching ABSTRACT We review some of the unique features of biogeochemical cycling in forests of the eastern Sierra Nevada Mountains, USA. As is the case for most arid and semi-arid ecosystems, spatial and temporal variability in nutrient contents and fluxes are quite high. ‘‘Islands of fertility’’ are common in these forests, a result of spatial variations in both litterfall and decomposition rates. Dry summer conditions greatly inhibit biological activity in the O horizon, and thus most annual litter decomposition takes place beneath the snowpack when moisture is available. Snowmelt duration is shortened near tree boles because of local warming, resulting in earlier drying of the O horizon, significantly lower decomposition rates, and increased O horizon mass. Water and nutrient fluxes vary spatially because of snowdrift in winter and surface runoff over hydrophobic soils in summer and fall. Moisture variability in the vertical as well as the horizontal dimension has significant consequences for nutrient fluxes. Because of the very dry summers, rooting in the O horizons is absent in these forests, and thus competition between microbes and trees for nutrients in that horizon is non-existent. Nutrients mineralized from the O horizon and not taken up by plants enrich runoff through the O horizons over hydrophobic mineral soils, resulting in very high concentrations of inorganic N and P in runoff waters. Substantial temporal variations in water and nutrient fluxes occur on a seasonal (with snowmelt being the dominant hydrologic event of the year), annual, and decadal basis. The most significant temporal variation is due to periodic fire, which we estimate causes annualized N losses that are two orders of magnitude greater than those associated with leaching and runoff. We hypothesize that fire suppression during the 20th century may have contributed to the deterioration of nearby Lake Tahoe by allowing buildups of N and P in O horizons which could subsequently leach from the terrestrial ecosystem to the Lake in runoff. In general, we conclude that biogeochemical cycling in these forests is characterized by greater spatial and temporal variability than in more mesic forest ecosystems. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: dwj@cabnr.unr.edu (D.W. Johnson). Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco 0378-1127/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.01.018