Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil

Soil microbial biomass C (C-mic) is a sensitive indicator of trends in organic matter dynamics in terrestrial ecosystems. This study was conducted to determine the effects of tropospheric CO2 or O-3 enrichments and moisture variations on total soil organic C (C-org), mineralizable C fraction (C-Min), C-mic, maintenance respiratory (qCO(2)) or C-mic death (qD) quotients, and their relationship with basal respiration (BR) rates and field respiration (FR) fluxes in wheat-soybean agroecosystems. Wheat (Triticum aestivum L.) and soybean (Glycine max. L. Merr) plants were grown to maturity in 3-m dia open-top field chambers and exposed to charcoal-filtered (CF) air at 350 mu L CO2 L-1; CF air + 150 mu L CO2 L-1; nonfiltered (NF) air + 35 nL O-3 L-1; and NF air + 35 nL O-3 L-1 + 150 mu L CO2 L-1 at optimum (- 0.05 MPa) and restricted soil moisture (- 1.0 +/- 0.05 MPa) regimes. The + 150 mu L CO2 L-1 additions were 18 h d(-1) and the + 35 nL O-3 L-1 treatments were 7 h d(-1) from April until late October. While C-org did not vary consistently, C-Min, C-mic and C-mic C-org(-1) fractions increased in soils under tropospheric CO2 enrichment (500 mu L CO2 L-1) and decreased under high O-3 exposures (55 +/- 6 nL O-3 L-1 for wheat; 60 +/- 5 nL O-3 L-1 for soybean) compared to the CF treatments (25 +/- 5 nL O-3 L-1). The qCO(2) or qD quotients of C-mic were also significantly decreased in soils under high CO2 but increased under high O-3 exposures compared to the CF control. The BR rates did not vary consistently but they were higher in well-watered soils. The FR fluxes were lower under high O-3 exposures compared to soils under the CF control. An increase in C-mic or C-mic C-org(-1) fractions and decrease in qCO(2) or qD observed under high CO2 treatment suggest that these soils were acting as C sinks whereas, reductions in C-mic or C-mic C-org(-1) fractions and increase in qCO(2) or qD in soils under elevated tropospheric O-3 exposures suggest the soils were serving as a source of CO2.