Reduced Soil Moisture Depletion Rates Associated with Rangeland Plant Species Grown Under Increasing Atmospheric CO_2: Potential Impacts on Soil Moisture Status and Bioremediation Strategies in the Southwestern United States

摘要

Green remediation, or the use of metal-scavenging plants as tools for decontaminating soils, has been proposed as a mechanism for cleaning up mining sites, landfills, and areas contaminated with various toxic metals such as lead, cadmium, zinc, or copper. Ideally, hyperaccumulator plants, or plants which not only tolerate but may selectively accumulate heavy metals in the soil, would incorporate the toxic metals in their aboveground tissues, the plants periodically harvested, and concentrations of metals in the contaminated soils reduced over time. As land management plans are developed which incorporate green remediation strategies, however, it is important that direct influences of environmental parameters on plant growth and productivity and indirect influences on soil biogeochemistry be considered. In this report, it is suggested that increasing atmospheric carbon dioxide concentrations and their influence on rangeland plants and soils be considered as potential hyperaccumulators are evaluated. It has been estimated that atmospheric CO_2 has nearly doubled since the last glacial period (Barnola et al., 1987; Jouzel et al., 1993) and it has been suggested that it may rise to twice the current level within the next one hundred years. Increasing CO_2 concentration usually reduces plant stomatal conductance and transpiration rate, potentially reducing the rate at which valuable soil water is depleted. This effect of CO_2 enrichment may prove of immense importance on desert grasslands where plant growth and species composition must be considered in concert with water availability. In addition, rangeland biogeochemical cycling may be directly impacted by reduced soil moisture depletion rates as mineral nutrients may either (1) be utilized by the plant as they are dissolved in the soil solution and/or (2) lost as they move down the soil column, potentially into groundwater systems. Either alternative may also directly impact the utilization of green remediation systems in rangeland or desert ecosystems in the southwestern USA.