On the Spatial Pattern of Soil Nutrients in Desert Ecosystems

Abstract

We examined the spatial distribution of soil nutrients in desert ecosystems of the southwestern United States to test the hypothesis that the invasion of semiarid grasslands by desert shrubs is associated with the development of "islands of fertility" under shrubs. In grasslands of the Chihuahuan Desert of New Mexico, 35—76% of the variation in soil N was found at distances <20 cm, which may be due to local accumulations of soil N under Bouteloua eriopoda, a perennial bunchgrass. The remaining variance is found over distances extending to 7 m, which is unlikely to be related to nutrient cycling by grasses. In adjacent shrublands, in which Larrea tridentata has replaced these grasses over the last century, soil N is more concentrated under shrubs and autocorrelated over distances extending 1.0—3.0 m, similar to mean shrub size and reflecting local nutrient cycling by shrubs. A similar pattern was seen in the shrublands of the Mojave Desert of California. Soil PO_4, Cl, SO_4, and K also accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces. Changes in the distribution of soil properties may be a useful index of desertification in arid and semiarid grasslands worldwide.

Controls on nitrification in a water-limited ecosystem: experimental inhibition of ammonia-oxidising bacteria in the Patagonian steppe

“…This relatively nitrogen-rich microenvironment showed no significant difference in soil NH 4 þ as a result of the inhibition treatments. Presence of shrubs may account for high soil NH 4 þ concentrations due to high inputs of organic matter in soil (Charley and West, 1975;Schlesinger et al, 1996). Shrub islands not only have high primary production (Soriano et al, 1994) but also trap a large fraction of windblown organic material, which may be produced in other microenvironments, all of which provide organic matter with carbon and nitrogen substrate for microbial growth and nitrogen mineralization.…”

A conceptual framework for understanding semi‐arid land degradation: ecohydrological interactions across multiple‐space and time scales

“…A 10 major form of land degradation in different semi-arid areas is the invasion of productivity, and therefore the sustainability of pastoral, subsistence and commercial 15 livestock grazing (Fisher, 1950). In addition, invasion by shrub species induces a 16 change in surface processes, notably increased runoff and erosion (Abrahams et al, 17 1995; Parsons et al, 1996;Wainwright et al, 2000) and a change in the spatial 18 distribution of soil properties that affect ecological and hydrological processes 19 (Müller et al, in press;Schlesinger et al, 1996). Biophysical and biogeochemical 20 changes that occur during the invasion of grasslands by shrubs may affect land 21 surface-atmospheric interactions, thus potentially affecting ecosystems world-wide 22 due to global biogeochemical feedbacks (Peterjohn and Schlesinger, 1990; 23 Schlesinger et al, 1990).…”

Local facilitation, bistability and transitions in arid ecosystems

“…Vegetation cover plays the role of physical protection that decreases thermal amplitudes, radiation exposures, wind dessication and soil erosion and prevents soil crust formation (Noy-Meir, 1973;Callaway and Walker, 1997;Greene et al, 2001). Moreover, water and/ or nutrient availability are higher under the canopy of certain woody species and perennial grasses as compared to areas deprived of vegetation, leading to the development of 'islands of fertility' (Jackson and Caldwell, 1993;Smith et al, 1994;Schlesinger et al, 1996). Here, we use the term local facilitation to describe this assortment of physical and biological mechanisms that impact the space below and close to the canopy of a plant.…”