Ecosystem Warming Affects Carbon Dioxide Flux in An Agricultural Soil.

Global warming seems likely based on present-day climate predictions. Our objective was to characterize and quantify the interactive effects of ecosystem warming (i.e., canopy temperature, T_S), soil moisture content (θ_S) and microbial biomass (B_M: bacteria, fungi) on the intra-row soil CO₂ flux (Φ_S) of an open-field spring wheat (Triticum aestivum L. cv. Yecora Rojo) crop.  A T-FACE (Temperature Free-Air Controlled Enhancement) apparatus utilizing infrared heaters was employed to elevate values of T_S above the wheat crop by 1.5 and 3.0 ^oC during the diurnal and nocturnal periods, respectively. The experimental design was a completely Randomized Latin Square (2x3) consisting of two levels of T_S (i.e., heated, reference with dummy heaters) in three replicates planted on 10 Mar. and 1 Dec. 2008. Values of intra-row Φ_S, T_S (Model LI-8100, LI-COR, Lincoln, NE) and θ_S (gravimetrically) were measured from planting until canopy closure. An extraction of soil DNA enabled real-time quantitative polymerase chain reaction of bacterial and fungal markers to estimate B_M. Under high θ_S a 10% increase in Φ_S was observed in the heated compared with control plots, and Φ_S was positively correlated with an order of magnitude increase in B_M.  In contrast, as θ_S became lower in the heated plots compared with higher θ_S in the control plots, a 10% reduction in Φ_S was observed. Characterization of the response of B_M to T_S under low θ_S is ongoing, but preliminary results suggest that Φ_S depends on the interactive effects of T_S, θ_S and B_M.  Consequently, a more complicated paradigm may be required to elucidate the effect of global climate change on sequestering of C in agricultural soils then previously believed. Knowledge of this interactive effect (i.e., T_Sxθ_SxB_M) will enable more accurate assessments of the sustainability of cereal crop production in a future high-CO₂ world.