Elevated atmospheric carbon dioxide in agroecosystems affects groundwater quality.

Increasing atmospheric carbon dioxide (CO2) concentration has led to concerns about global changes to the environment. One area of global change that has not been addressed is the effect of elevated atmospheric CO2 on groundwater quality below agroecosystems. Elevated CO2 concentration alterations of plant growth and C/N ratios may modify C and N cycling in soil and affect nitrate (NO3(-)) leaching to groundwater. This study was conducted to examine the effects of a legume (soybean [Glycine max (L.) Merr.]) and a nonlegume (grain sorghum [Sorghum bicolor (L.) Moench]) CO2-enriched agroecosystems on NO2 movement below the root zone in a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with plant species (soybean and grain sorghum) as the main plots and CO2 concentration (approximately 360 and approximately 720 microliter L-1 CO2) as subplots using open-top field chambers. Fertilizer application was made with 15N-depleted NH4NO3 to act as a fertilizer tracer. Soil solution samples were collected weekly at 90-cm depth for a 2-yr period and monitored for NO3(-)-N concentrations. Isotope analysis of soil solution indicated that the decomposition of organic matter was the primary source of NO3(-)-N in soil solution below the root zone through most of the monitoring period. Significant differences were observed for NO3(-)-N concentrations between soybean and grain sorghum, with soybean having the higher NO3(-)-N concentrations. Elevated CO2 increased total dry weight, total N content, and C/N ratio of residue returned to soil in both years. Elevated CO2 significantly decreased NO3(-)-N concentrations below the root zone in both soybean and grain sorghum. The results of this study indicate that retention of N in organic pools because of elevated atmospheric CO2 could reduce the nitrate concentration in groundwater beneath agroecosystems as indicated by NO3(-) movement.