Numerical representation of soil hot spots and hot moments of carbon dioxide, methane, and nitrous oxide fluxes using microsite probability density functions

Abstract:

Production and consumption of nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) are affected by complex interactions of soil temperature, moisture, and substrate supply, which are further complicated by spatial heterogeneity. Microsite heterogeneity causes non-normal distributions (hot spots and hot moments) of greenhouse gas (GHG) fluxes, which are difficult to reproduce in ecosystem and earth system models that are driven by mean values of soil properties and substrate concentrations.

Here we expanded the Dual Arrhenius and Michaelis-Menten (DAMM) model to apply it consistently for all three GHGs for the biophysical processes of production, consumption, and diffusion within the soil, including the contrasting effects of oxygen (O2) as substrate or inhibitor for each process. Chamber-based measurements of all three GHGs at the Howland Forest (ME, USA) and a corn/soybean farm in Maryland were used to parameterize the model with multiple constraints. Probability density functions (PDF) of soil C and water generated a simulated PDFs of heterotrophic respiration and O2 consumption, which were inputs to simulated PDFs of microsite CH4 and N2O production and consumption. Results demonstrate that it is numerically feasible for net N2O reduction and CH4 oxidation to co-occur under a single chamber. Simultaneous simulation of all three GHGs in a parsimonious modeling framework increases confidence that agreement between simulations and measurements is based on skillful numerical representation of processes across a heterogeneous environment.

Our ultimate goal is to develop a parsimonious module that can simulate hot spots and hot moments of belowground GHG production and consumption for use within larger ecosystem and earth system models.