Elevated atmospheric CO2 effects on N fertilization in grain sorghum and soybean

Increasing atmospheric CO2 concentration has led to concerns about global changes to the environment. One area of global change that has not been fully addressed is the effect of elevated atmospheric CO2 on agriculture production inputs. Elevated CO2 concentration alterations of plant growth and C:N ratios may modify C and N cycling in soil and N fertility. This study was conducted to examine the effects of legume, soybean (Glycine max (L.) Merr.), and non-legume, grain sorghum (Sorghum bicolor (L.) Moench.) carbon dioxide-enriched agro-ecosystems on N soil fertility in a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with crop species (soybean and grain sorghum) as the main plots and CO2 concentration (ambient and twice ambient) as subplots using open top field chambers. Fertilizer application was made with N-depleted NH4NO3 to act as a fertilizer tracer. Elevated CO2 increased total biomass production in all 3 years of both grain sorghum (average 30%) and soybean (average 40%). With soybean, while no impact on the plant C:N ratio was observed, the total N content was greatly increased (average 29%) due to increased atmospheric N2 fixation with elevated CO2 concentration. With grain sorghum, the total N uptake was not affected, but the C:N ratio was markedly increased (average 31%) by elevated CO2. No impact of elevated CO2 level was observed for fertilizer N in grain sorghum. The results from this study indicated that while elevated CO2 may enhance crop production and change N status in plant tissue, changes to soil N fertilizer application practices may not be needed. # 2003 Elsevier B.V. All rights reserved.