Simultaneous numerical representation of soil microsite production and consumption of carbon dioxide, methane, and nitrous oxide using probability distribution functions

Abstract

Production and consumption of nitrous oxide (N 2 O), methane (CH 4 ), and carbon dioxide (CO 2 ) are affected by complex interactions of temperature, moisture, and substrate supply, which are further complicated by spatial heterogeneity of the soil matrix. This microsite heterogeneity is often invoked to explain non-normal distributions of greenhouse gas (GHG) fluxes, also known as hot spots and hot moments.To advance numerical simulation of these belowground processes, we expanded the Dual Arrhenius and Michaelis-Menten model, to apply it consistently for all three GHGs with respect to the biophysical processes of production, consumption, and diffusion within the soil, including the contrasting effects of oxygen (O 2 ) as substrate or inhibitor for each process. High-frequency chamber-based measurements of all three GHGs at the Howland Forest (ME, USA) were used to parameterize the model using a multiple constraint approach. The area under a soil chamber is partitioned according to a bivariate log-normal probability distribution function (PDF) of carbon and water content across a range of microsites, which leads to a PDF of heterotrophic respiration and O 2 consumption among microsites. Linking microsite consumption of O 2 with a diffusion model generates a broad range of microsite concentrations of O 2 , which then determines the PDF of microsites that produce or consume CH 4 and N 2 O, such that a range of microsites occurs with both positive and negative signs for net CH 4 and N 2 O flux. Results demonstrate that it is numerically feasible for microsites of N 2 O reduction and CH 4 oxidation to co-occur under a single chamber, thus explaining occasional measurement of simultaneous uptake of both gases. Simultaneous simulation of all three GHGs in a parsimonious modeling framework is challenging, but it increases confidence that agreement between simulations and measurements is based on skillful numerical representation of processes across a heterogeneous environment.

Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

“…The forest composition is relatively diverse, with the mature tabonuco (Dacryodes excelsa Vahl) and sierra palm (Prestoea montana) trees being most dominant (Scatena and Lugo, 1995;Wadsworth, 1951). Sihi et al, 2020a; also see Eq. 14).…”

“…In order to account for the diffusion of gases across the soilair boundary and solutes (e.g., acetate) through soil water films ( Fig. 1), we added the diffusion module of the Dual Arrhenius and Michaelis-Menten (DAMM) model (Davidson et al, 2012;Sihi, 2020;Sihi et al, 2018Sihi et al, , 2020a to the existing microbial functional group model, which we refer to as M3D-DAMM. We calculated initial concentration of gases like O 2, H 2 , CO 2, and CH 4 , [Gas conc ] (unit: V V −1 ), as a function of a unitless diffusion coefficient of gas in air (D gas ), volume fraction of gas in air (V V −1 ), and gas diffusivity (a 4/3 ) as follows:…”

“…To account for the balance of methanotrophy and methanogenesis, separate microbial functional groups for CH 4 production and oxidation would need to be defined. Therefore, a microbial functional group model for CH 4 production and consumption (Xu et al, 2015) was merged with a soil diffusivity module (Davidson et al, 2012;Sihi et al, 2018) to simulate the dynamics of net in situ CH 4 emissions from soil microsites (Sihi et al, 2020a). This module considers three key mechanisms for CH 4 production and consumption: aceticlastic methanogenesis (production from acetate), hydrogenotrophic methanogenesis (production from H 2 and CO 2 ), and aerobic methanotrophy (oxidation of CH 4 and reduction of O 2 ) ( Fig.…”

Twenty‐five years of GCB: Putting the biology into global change

“…The alternatives are starkly presented here in the comprehensive GCB primer by Editor Sage (). GCB will be adding increased focus to the biology of means of offsetting and adapting to change, as represented in this issue (Lavallee, ; Prosser, Hink, Gubry‐Rangin, & Nicol, ; Sihi, Davidson, Savage, & Liang, ; Smith et al, ), and further develop our strong social media presence.…”

Environmental controls of boreal forest soil CO₂ and CH₄ emissions and soil organic carbon accumulation

“…The MM saturation kinetics of respiration also applies to increasing Oxygen under an unconstrained substrate. The Michaelis-Menten type kinetics are characteristic for microbial enzyme models for soil CO2 (Sierra et al 2012, Davidson et al 2012, Moyano et al 2013, Sierra et al 2015, Manzoni et al 2016, Abrahamoff et al 2017, Moyano et al 2018 and CH4 (Davidson et al 2014, Raivonen et al 2017, Sihi et al 2020. In microbial models, Arrhenius temperature kinetics are combined with water limitation through diffusivity of oxygen, and enzymatic transport in the soil pore space.…”