ELEVATED ATMOSPHERIC CARBON-DIOXIDE IN AGROECOSYSTEMS AFFECTS GROUNDWATER QUALITY

Increasing atmospheric carbon dioxide (CO2) concentration has led to c oncerns about global changes to the environment, One area of global ch ange that has not been addressed is the effect of elevated atmospheric CO2 on ground water quality below agroecosystems, Elevated CO2 concen tration alterations of plant growth and C/N ratios may modify C and N cycling in soil and affect nitrate (NO3-) leaching to groundwater. Thi s study was conducted to examine the effects of a legume {soybean [Gly cine max (L.) Merr.]} and a nonlegume {grain sorghum [Sorghum bicolor (L.) Moench]} CO2-enriched agroecosystems on NO3- movement below the r oot zone in a Blanton loamy sand (loamy siliceous, thermic, Grossareni c Paleudults). The study was a split-plot design replicated three time s with plant species (soybean and grain sorghum) as the main plots and CO2 concentration (approximate to 360 and similar to 720 mu L L(-1) C O2) as subplots using open-top held chambers. Fertilizer application w as made with N-15-depleted NH4NO3 to act as a fertilizer tracer, Soil solution samples were collected weekly at 90-cm depth for a 2-yr perio d arid monitored for NO3-N concentrations. Isotope analysis of soil so lution indicated that the decomposition ol organic matter was the prim ary source of NO3--N in soil solution below the root zone through most of the monitoring period. Significant differences were observed for N O3--N concentrations between soybean and grain sorghum, with soybean h aving the higher NO3--N concentrations. Elevated CO2 increased total d ry weight, total N content, and C/N ratio of residue returned to soil in both years. Elevated CO2 significantly decreased NO3--N concentrati ons below the root zone in both soybean and grain sorghum. The results of this study indicate that retention of N in organic pools because o f elevated atmospheric CO2 could reduce the nitrate concentration in g roundwater beneath agroecosystems as indicated by NO3- movement.