FREE-AIR CO2 ENRICHMENT EFFECTS ON SOIL CARBON AND NITROGEN

Since the onset of the industrial revolution, atmospheric CO2 concentr ation has increased exponentially to the current 370 mumol mol-1 level , and continued increases are expected. Previous research has demonstr ated that elevated atmospheric CO2 results in larger plants returning greater amounts of C to the soil. However, the effects of elevated CO2 on C and N cycling and long-term storage of C in soil have not been e xamined. Soil samples (in 0-50, 50-100, and 100-200 mm depth increment s) were collected after 3 years of cotton (Gossypium hirsutum L.) prod uction under free-air CO2 enrichment (FACE, at 550 mumol CO2 mol-1), i n combination with 2 years of different soil moisture regimes (wet, 10 0% of evapotranspiration replaced, or dry, 75% and 67% of evapotranspi ration replaced in 1990 and 1991, respectively) on a Trix clay loam (f ine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvent) at Ma ricopa, Arizona. Ambient plots (370 mumol CO2 mol-1 (control)), in com bination with the wet and dry soil moisture regimes, were also include d in the study. Soil organic C and N concentrations, potential C and N mineralization, and C turnover were measured. Increased input of cott on plant residues under FACE resulted in treatment differences and tre nds toward increased organic C in all three soil depths. During the fi rst 30 days of laboratory incubation, available N apparently limited p otential C mineralization and C turnover in all treatments. Between 30 and 60 days of incubation, soils from FACE plots had greater potentia l C mineralization with both water regimes, but C turnover increased i n soils from the dry treatment and decreased in soils where cotton was not water stressed. These data indicate that in high-CO2 environments without water stress, increased C storage in soil is likely, but it i s less likely where water stress is a factor. More research is needed before the ability of soil to store additional C in a high-CO2 world c an be determined.