Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/1995
Publication Date: N/A
Interpretive Summary: The concentration of CO2 in the atmosphere has been increasing for 150 years and may double sometime in the next century, due to burning of fossil fuels and other human activities. This change in Earth's atmosphere may alter the climate by intensifying the greenhouse effect, changing rainfall patterns and causing temperatures to rise. In addition, higher CO2 levels increase plant growth rates, with implications for agriculture. An aspect of rising CO2 which has not been studied is the possibility that it will affect groundwater quality. We measured the movement of nitrate, a potential health hazard, into soil water where sorghum and soybeans were growing under current and future levels of atmospheric CO2. Less nitrate occurred in soil water under both crops when grown at elevated CO2 levels, probably because more of the nitrogen remained in the crop residue and soil organic matter. Rising CO2 levels do not appear to increase nitrate levels sin groundwater, and may even reduce them.
Technical Abstract: Increasing atmospheric 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 agro-ecosystems. This study was conducted to examine the effects of soybean [Glycine max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench.] CO2- enriched agro-ecosystem on nitrate 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 (375 and 705 muLL**-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 two- year period and monitored for NO3-N concentration. 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 concentration between soybean and grain sorghum, with soybean having the higher NO3-N concentration. Elevated CO2 significantly decreased NO3-N concentration below the root zone in both soybean and grain sorghum. The results of this study indicate that retention of N in organic pools due to elevated atmospheric CO2 could reduce the degradation of groundwater quality beneath agro-ecosystems.