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Elevated Atmospheric Carbon Dioxide in Agroecosystems Affects Groundwater Quality

USDA-ARS, Blackland, Soil and Water Research Lab., 808 East Blackland Rd., Temple, TX 76502;

S. A. Prior and H. H. Rogers

USDA-ARS Natl. Soil Dynamics Lab., Box 3439, Auburn, AL 36831-3439;

W. H. Schlesinger

Dep. of Botany and Geology, Duke Univ., Durham, NC 27706;

G. L. Mullins

Agronomy and Soils Dep., Auburn University, Auburn AL 36849;

G. B. Runion

Dep. of Forestry, Auburn University, Auburn AL 36849.

^* Corresponding author (torbert{at}brcsun0.tamu.edu).

ABSTRACT

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 (NO^–₃) 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 NO^–₃ 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 (360 and 720 µL 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 NO^–₃-N concentrations. Isotope analysis of soil solution indicated that the decomposition of organic matter was the primary source of NO^–₃-N in soil solution below the root zone through most of the monitoring period. Significant differences were observed for NO^–₃-N concentrations between soybean and grain sorghum, with soybean having the higher NO^–₃-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 NO^–₃-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 NO^–₃ movement.

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