Soil structure and greenhouse gas emissions: a synthesis of 20 years of experimentation

Abstract

Soil structure affects microbial activity and thus influences greenhouse gas production and exchange in soil. Structure is variable and increasingly vulnerable to compaction and erosion damage as agriculture intensifies and climate changes. Few studies have specifically related the impact of structure and its variability to greenhouse gas (GHG) emissions over a wide range of soils and management treatments. The objective of this study was to draw from research in Scotland, Japan and New Zealand, which examined how soil structures affected by wheel compaction, animal trampling, tillage and land‐use change influence GHG emissions in order to help identify key controlling properties. Nitrous oxide (N2O) is the main focus, though carbon dioxide (CO2), methane (CH4) and nitric oxide (NO) are included. Gas emissions were measured by using static chambers in the field or incubated intact cores. Poor structure, measured as small relative gas diffusivities and air permeabilities, restricted aeration, resulting in N2O emission or consumption dependent on mineral nitrogen contents. Structural damage (identifiable using the Visual Evaluation of Soil Structure) was especially important near the soil surface where microsites of microbial activity were exposed and aeration was impaired. Moist, well‐aerated soils favoured CH4 oxidation and CO2 exchange. N2O emissions were not necessarily increased in anaerobic soils because of possible N2O consumption and microbial adaptation. Soil matric potential, volumetric water content, relative diffusivity, air permeability and water‐filled pore space are relevant indicators for N2O and CH4 flux and aeration status. As pore continuity and size are so relevant, pore‐scale models are likely to have an increasing role in understanding mechanisms of GHG production, transport and release.

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

“…This is in agreement with recent studies (e.g. Maier et al, 2011;Schmidt et al, 2011;Ball, 2013; also illustrated by Tran et al, 2015) that show that soil physical properties are key to understanding the mechanisms regulating the soil gases emissions. Our study brings new insights by demonstrating the strong linkages between soil physical properties and CO 2 emissions based on in situ and depth-explicit observations.…”

“…Indeed, according to these authors, when a threshold VWC value is exceeded, this: (i) strongly limits the transfer of biotic CO 2 along the soil profile, and (ii) reduces the production of CO 2 in itself due to the lack of oxygen for the microbial community. In both cases, the lower CO 2 emissions at the footslope profile relative to the summit are due to gas diffusion limitations (even indirectly in the case of a lack of oxygen), as also suggested by Ball (2013). This stands in sharp contrast to the summit profile where gas can easily diffuse throughout the year and along the entire soil profile (Fig.…”

“…This CO 2 then accumulates under the soil diffusion barriers. This accumulated CO 2 is then later emitted when VWC decreases under a threshold value which allows a significant gas diffusion, as suggested by Maier et al (2011) and Ball (2013). The main implication of these observations is that if hydrologic regimes change and that footslope soils become drier (reaching moisture conditions favorable for micro-organisms respiration and gas transfer), there is a large amount of potentially easily decomposable OC stored at depth that can suddenly decompose and be emitted to the atmosphere.…”

“…gas diffusion barriers) may also exert an important control on soil microbial activity and soil CO 2 fluxes (e.g. Wiaux et al, 2014b;Ball, 2013;Maier et al, 2011). Furthermore, there is empirical evidence suggesting that physical protection (i.e.…”

“…According to the CO 2 concentration and diffusivity profiles (Fig. 8), the relative contribution of the soil layers to the surface CO 2 flux is more likely governed by soil physical controls (Ball, 2013) rather than by biological production depending on thermal energy and OC substrate. Here, soil gas diffusivity strongly decreases from 10 to 40 cm depth (where diffusivity is null) at the two slope positions, and the profile of CO 2 concentration displays no gradient between 10 and 40 cm depth, particularly at the footslope (Fig.…”