Cheatgrass Invasion Alters Soil Morphology and Organic Matter Dynamics in Big Sagebrush- Steppe Rangelands

Cheatgrass (Bromus tectorum L.) is an invasive annual grass that increases wildfire frequency, degrades native ecosystems, and threatens agriculture across vast areas of the Western United States. This research examines how cheatgrass invasion may alter physical and biological properties of soils. Proliferation of very fine roots and high production of low-quality litter by cheatgrass increases porosity and near-surface microbial activity, which may enhance decomposition of soil organic matter (SOM) similar to cultivated systems. This may enlarge active SOM pools (mineral and microbial biomass C and N) at the expense of slow pools and humus. To test this hypothesis, soil properties beneath long-term cheatgrass-invaded areas were compared with carefully matched soils under shrub canopies and grass-covered interspaces at seven undisturbed Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) plant communities in northern Utah and southeastern Idaho. Soils under cheatgrass had (1) higher porosity in surface horizons, (2) higher concentrations of mineral N throughout the soil profiles at the time of sampling, and (3) a larger proportion of mineralizable C and N in total SOM of surface horizons than soil under native shrub-steppe plant communities. These results support our hypothesis and suggest that long-term cheatgrass invasion may alter ecological stability and resilience by depleting slow and passive SOM pools. This research will contribute to improved understanding of fundamental ecosystem processes required for successful ecological restoration. Introduction ___________________ The exotic annual grass cheatgrass (Bromus tectorum L.) has invaded large areas of shrub-steppe ecosystems in the Western United States, and is now dominant on much of the Great Basin, Snake River Plain, and Columbia Plain physiographic provinces (Knapp 1996; Knick 1999; Mack 1981). The area covered by cheatgrass and its ecological impact on shrub-steppe plant communities rivals ecosystem conversion from grasslands to annual crops that took place in Central North American (Samson and Knopf 1994). Numerous investigations of native ecosystem conversion to croplands provide an understanding of agriculture-related environmental change, particularly of nutrient cycling and soil organic matter (SOM) dynamics in both natural and disturbed ecosystems (Parton and others 1987; Schimel 1986). As impacts of invasive plants on ecosystem processes begin to be understood (D’Antonio and Vitousek 1992; Ehrenfeld and Scott 2001), application of concepts from cropland conversion may further this understanding. In this paper, we describe research that compared soils under cheatgrass to those under native shrub-steppe grasslands. We discuss ecological parallels between annual crops and cheatgrass invasion and their implications for restoration of diverse native plant communities. On a worldwide basis, conversion of native ecosystems to cropland results in about a 30-percent long-term decrease in soil organic carbon (SOC). Annually, such conversion accounts for nearly one-fifth of the net carbon (C) transfer from terrestrial ecosystems to the atmosphere from changing land uses (Davidson and Ackerman 1993). In general, cropland conversion is thought to change native ecosystems from net C sinks to important sources of atmospheric CO2 (Schlesinger 1999). In semiarid shortgrass-steppe grasslands, the long-term decrease in SOC from conversion to cropland is often more than 60 percent (Aguilar and others 1988; Schimel 1986), most of which is lost in the first few years of cultivation (Bowman and others 1990). Cultivation also reduces labile C in soils as well as labile C as a proportion of SOC (Bowman and others 1990). Cropland soils typically have less total nitrogen (N), less labile N, and lower labile N:total N than their uncultivated counterparts. However, these N decreases with cultivation are proportionally less than that for SOC, resulting in narrowing C:N ratios in both total and labile fractions after cultivation of grassland soils (Bowman and others 1990). Inorganic N