An Evaluation of Root Respiration and of Natural and Agricultural Processes of Soil Aeration.

Soil aeration processes and status are reviewed with regard to different soil, climatic, land-use and crop types and with regard to diffusive and advective flow mechanisms. Factors affecting aeration status and its quantifiers are discussed and active soil aeration (“oxygation”) practices are presented. Movement of oxygen (O₂) from the soil surface into the soil profile and its transport into soil aggregates and toward plant roots is mainly diffusive. In most circumstances root respiration is constrained by vertical O₂ diffusion from the atmosphere to the root zone and by the diffusive resistance of the mucilage layer. Several O₂-diffusive flow models are proposed and discussed with regard to the different geometries, relevant length scales, boundary conditions, and sinks. Soil aeration by advective O₂ flow, driven by barometric pressure changes, may also be significant in dry, coarse-textured soils with no underlying impermeable layers. Respiration in the soil consists mainly of plants root and microbial oxygen uptakes, which is roughly of the same magnitude and is strongly correlated through symbiotic and competitive relationships. The bulk, areal soil respiration rate varies from several to tens of grams of O₂ per square meter per day depending on soil cover: fallow, pasture, forests, non-irrigated and irrigated, cropped fields and orchards (in general increasing order). Soil respiration rate is also affected by climatic conditions, where a temperature difference of 10 ºC increases O₂ consumption (and carbon dioxide (CO₂) production) 2- to 3-fold. The ratio between emitted CO₂ and inspired O₂ (respiration quotient) is not unity (on a molar basis), but rather depends on the types of respiring populations and environmental conditions. Reduced O₂ and elevated CO₂ concentrations negatively affect plant growth and productivity. These conditions are correlated mostly with wet and warm soils, such as intensively irrigated fields with fine-textured soils (high water retention) during the summer. O₂-availability quantifiers such as O₂ concentration, air content, and O₂-diffusion rate (ODR) are superior to other quantifiers such as soil properties (e.g. soil texture, porosity) and redox potential. In the last few decades, several active aeration methods have been proposed and evaluated, such as: adding air/O₂ bubbles or hydrogen peroxide to the irrigation water and air injection into the soil. Although these methods have given mainly positive results, none is widely used in agricultural practice, due mainly to a lack of profitability potential, field-scale proof demonstration and a coherent protocol for field application.