Leaf physiology, production, water use, and nitrogen dynamics of the grassland invader Acacia smallii at elevated CO(2) concentrations

Invasion by woody legumes can alter hydrology, nutrient accumulation and cycling, and carbon sequestration on grasslands. The rate and magnitude of these changes are likely to be sensitive to the effects of atmospheric CO(2) enrichment on growth and water and nitrogen dynamics of leguminous shrubs. To assess potential effects of increased atmospheric CO(2) concentrations on plant growth and acquisition and utilization of water and nitrogen, seedlings of Acacia smallii Isely (huisache) were grown for 13 months at CO(2) concentrations of 385 (ambient), 690, and 980 micro mol mol(-1). Seedlings grown at elevated CO(2) concentrations exhibited parallel declines in leaf N concentration and photosynthetic capacity; however, at the highest CO(2) concentration, biomass production increased more than 2.5-fold as a result of increased leaf photosynthetic rates, leaf area, and N(2) fixation. Measurements of leaf gas exchange and aboveground biomass production and soil water balance indicated that water use efficiency increased in proportion to the increase in atmospheric CO(2) concentration. The effects on transpiration of an accompanying decline in leaf conductance were offset by an increase in leaf area, and total water loss was similar across CO(2) treatments. Plants grown at elevated CO(2) fixed three to four times as much N as plants grown at ambient CO(2) concentration. The increase in N(2) fixation resulted from an increase in fixation per unit of nodule mass in the 690 micro mol mol(-1) CO(2) treatment and from a large increase in the number and mass of nodules in plants in the 980 micro mol mol(-1) CO(2) treatment. Increased symbiotic N(2) fixation by woody invaders in response to CO(2) enrichment may result in increased N deposition in litterfall, and thus increased productivity on many grasslands.