Linkages between soil micro-site properties and CO2 and N2O emissions during a simulated thaw for a northern prairie Mollisol

Abstract

Biologically derived emissions of carbon dioxide (CO2) and nitrous oxide (N2O) at 0 °C vary with soil depth during soil thawing. Micro-site soil properties, especially those which influence porosity and substrate availability, also vary with depth and may help explain gas emissions. Intact soil cores collected to a depth of 80 cm from an undisturbed prairie Mollisol in central North Dakota were uniformly subjected to distinct temperature steps during a simulated soil thaw (−15 to 5 °C) and sampled for CO2 and N2O emissions throughout the soil profile. Emission data were fit to a first order exponential equation (E = αeβT). Cores were then analyzed in 10 cm depth increments for micro-site properties including root length and mass, aggregation, and organic substrate availability (available, aggregate-protected and mineral-bound pools). Both CO2 and N2O emissions at 0 °C declined exponentially with depth. Emissions of CO2 and N2O at 0 °C were strongly related to root length (R2 = 0.80 and 0.76, respectively), root mass (R2 = 0.56 and 0.74), large macroaggregate mass (R2 = 0.63 and 0.54), and aggregate-protected organic matter (R2 > 0.57), while available organic matter was related to CO2 (R2 > 0.60) and not N2O. When CO2 and N2O emissions were normalized by available and aggregate-protected carbon pools, respectively, nutrient use efficiency increased significantly with depth. Results suggest CO2 and N2O emissions are (1) positively influenced by the rhizosphere and (2) differentially affected by substrate pool or location. CO2 emissions were more positively affected by available substrate, while N2O emissions were more positively affected by less labile, aggregate-protected substrate.