Long-term consequences of disturbance on nitrogen dynamics in an arid ecosystem

Ecology, Jan, 1999 by R.D. Evans, J. Belnap

INTRODUCTION

Ecosystem structure and function are controlled by interactions between climate, resource availability, species composition, and disturbance regime (Chapin et al. 1996). Anthropogenic activity is causing global changes in each of these factors through global warming, alteration of atmospheric chemistry, species invasions and extinction, and land-use change (Vitousek 1994, Chapin et al. 1996). One of the most significant changes in resource dynamics caused by anthropogenic activity is alteration of the nitrogen cycle (Aber et al. 1989, Schulze 1989, Kinzig and Socolow 1994, Vitousek 1994). The increase in atmospheric deposition resulting from anthropogenic N fixation has caused widespread changes in the N dynamics of ecosystems of the northeastern United States and northern Europe (Aber et al. 1989, Schulze 1989, Galloway et al. 1995), and has the potential to alter species composition in formerly N-limited ecosystems (Tilman 1987, Inouye and Tilman 1995). However, anthropogenic activity can also decrease N availability within ecosystems. In contrast to many ecosystems experiencing large increases in N input, many arid ecosystems are experiencing loss or redistribution of nutrients due to land-use change (Schlesinger et al. 1990, 1996, Evans and Ehleringer 1993, 1994, Hulme and Kelly 1993, Milton et al. 1994). The extreme environments and low species diversity of arid ecosystems make them especially susceptible to changes caused by land-use change, especially grazing (Verstraete and Schwartz 1991).

Central to resource dynamics in many arid ecosystems are microbiotic (cryptogamic) crusts, consolidated matrices of cyanobacteria, lichens, moss, green algae, and microfungi that cover the soil surface. The natural absence of both fire and grazing by large mammals has allowed the microbiotic crusts of the Intermountain West of North America to become especially well developed (Mack and Thompson 1982); crusts may be up to 10 cm deep and may approach 100% coverage in plant interspaces (Kleiner and Harper 1972, Harper and Marble 1988). The cyanobacteria, bacteria, and lichens comprising microbiotic crusts are often capable of N fixation, and this has been shown to be the primary source of N input into some arid ecosystems (Evans and Ehleringer 1993). Surface disturbance caused by land-use change can disrupt the microbiotic crust, and this may directly affect plant and soil N dynamics. The greatest impact of disturbance may be to eliminate or greatly reduce N fixation by altering the species composition of the microbiotic crust. The lichen and moss components of the crust are especially susceptible to disturbance and have the longest rates of recovery (Johansen and St. Clair 1986, Beymer and Klopatek 1992, Eldridge and Greene 1994, Belnap 1995), so disturbance may cause a shift in dominance toward cyanobacteria. This is significant because rates of N fixation for lichens can be an order of magnitude greater (on a surface-area basis) than cyanobacteria (Belnap 1991). Therefore, the greatest impact of disturbance may be to eliminate or greatly reduce N fixation by altering the species composition of the microbiotic crust.

The short-term changes in species composition and N fixation following disturbance may have long-term consequences for N dynamics in arid ecosystems. Mineralization, as well as subsequent N loss from volatilization, nitrification, and denitrification, can be rapid following precipitation (Burke 1989, Matson et al. 1991, Peterjohn and Schlesinger 1991, Schlesinger and Peterjohn 1991). Evans and Ehleringer (1993) hypothesized that the short-term loss of N fixation following disturbance, coupled with rapid N loss, may result in net loss of N from the soil. Rates of N mineralization and nitrification depend on substrate availability and microenvironment, so a decrease in soil N could impact soil N transformations (Virginia et al. 1982, Binkley and Hart 1989, Matson et al. 1991, Peterjohn and Schlesinger 1991), causing a decrease in plant-available N and net primary productivity.

The three-dimensional character of the microbiotic crusts makes it difficult to assess changes in species composition simply by quantifying spatial coverage (Belnap 1993). However, changes in species dominance should be apparent in the carbon isotope composition ([[Delta].sup.13]C) of the crust. Species comprising the microbiotic crust have very different [[Delta].sup.13]C. The [[Delta].sup.13]C of lichens can vary with the photobiont associated with the symbiosis, but values are generally lower than -23[per thousand] (Lange 1988, Maguas et al. 1993, 1995). The [[Delta].sup.13] of mosses is similar to that of higher plants that possess the [C.sub.3] photosynthetic pathway, and values are lower than -26[per thousand] (Rundel et al. 1979, Teeri 1981, Proctor et al. 1992). In contrast to lichens and mosses, [[Delta].sup.13]C of cyanobacteria is similar to that of plants with the [C.sub.4] photosynthetic pathway (-12[per thousand]) because they possess a C[O.sub.2]-concentrating mechanism (Palmqvist 1993, Maguas et al. 1995) and photosynthesis is limited by diffusion (Raven 1991, Maguas et al. 1995).