Soil Methanotrophy Model (MeMo v1.0): a process-based model to quantify global uptake of atmospheric methane by soil
Authors: Murguia-Flores Fabiola , Arndt Sandra , Ganesan Anita L. , Murray-Tortarolo Guillermo , Hornibrook Edward R. C. 
Published: June 2018

Abstract. Soil bacteria known as methanotrophs are the sole biological
sink for atmospheric methane (CH4), a potent greenhouse gas that is
responsible for ∼ 20 % of the human-driven increase in radiative
forcing since pre-industrial times. Soil methanotrophy is controlled by a
plethora of factors, including temperature, soil texture, moisture and
nitrogen content, resulting in spatially and temporally heterogeneous rates
of soil methanotrophy. As a consequence, the exact magnitude of the global
soil sink, as well as its temporal and spatial variability, remains poorly
constrained. We developed a process-based model (Methanotrophy Model; MeMo
v1.0) to simulate and quantify the uptake of atmospheric CH4 by soils at
the global scale. MeMo builds on previous models by Ridgwell et al. (1999)
and Curry (2007) by introducing several advances, including (1) a general
analytical solution of the one-dimensional diffusion–reaction equation in
porous media, (2) a refined representation of nitrogen inhibition on soil
methanotrophy, (3) updated factors governing the influence of soil moisture
and temperature on CH4 oxidation rates and (4) the ability to evaluate
the impact of autochthonous soil CH4 sources on uptake of atmospheric
CH4. We show that the improved structural and parametric representation
of key drivers of soil methanotrophy in MeMo results in a better fit to
observational data. A global simulation of soil methanotrophy for the period
1990–2009 using MeMo yielded an average annual sink of
33.5 ± 0.6 Tg CH4 yr−1. Warm and semi-arid regions
(tropical deciduous forest and open shrubland) had the highest CH4
uptake rates of 602 and 518 mg CH4 m−2 yr−1, respectively.
In these regions, favourable annual soil moisture content (∼ 20 %
saturation) and low seasonal temperature variations
(variations < ∼ 6 ∘C) provided optimal conditions for soil
methanotrophy and soil–atmosphere gas exchange. In contrast to previous model
analyses, but in agreement with recent observational data, MeMo predicted low
fluxes in wet tropical regions because of refinements in formulation of the
influence of excess soil moisture on methanotrophy. Tundra and mixed forest
had the lowest simulated CH4 uptake rates of 176 and
182 mg CH4 m−2 yr−1, respectively, due to their marked
seasonality driven by temperature. Global soil uptake of atmospheric CH4
was decreased by 4 % by the effect of nitrogen inputs to the system;
however, the direct addition of fertilizers attenuated the flux by 72 % in
regions with high agricultural intensity (i.e. China, India and Europe) and
by 4–10 % in agriculture areas receiving low rates of N input (e.g.
South America). Globally, nitrogen inputs reduced soil uptake of atmospheric
CH4 by 1.38 Tg yr−1, which is 2–5 times smaller than
reported previously. In addition to improved characterization of the
contemporary soil sink for atmospheric CH4, MeMo provides an opportunity
to quantify more accurately the relative importance of soil methanotrophy in
the global CH4 cycle in the past and its capacity to contribute to
reduction of atmospheric CH4 levels under future global change
scenarios.