Review and Evaluation of Root Respiration and of Natural and Agricultural Processes of Soil Aeration

Core Ideas Movement of O 2 in the soil is mainly diffusive. Respiration consists of plant root and microbial O 2 uptake, roughly of the same magnitude. Reduced O 2 and elevated CO 2 concentrations negatively affect plant growth and productivity. O 2 concentration, air content, and ODR are good quantifiers for O 2 availability to plant roots. Adding bubbles or H 2 O 2 to irrigation and injecting air to soil showed mostly favorable results. 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 O 2 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 O 2 diffusion from the atmosphere to the root zone and by the diffusive resistance of the mucilage layer. Several O 2 –diffusive flow models are proposed and discussed with regard to the different geometries, relevant length scales, boundary conditions, and sinks. Soil aeration by advective O 2 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 plant root and microbial O 2 uptake, which are roughly of the same magnitude and strongly correlated through symbiotic and competitive relationships. The bulk, areal soil respiration rate varies from several to tens of grams of O 2 per square meter per day depending on soil cover: fallow, pasture, forests, unirrigated 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°C increases O 2 consumption (and CO 2 production) two‐ to threefold. The ratio between emitted CO 2 and inspired O 2 (respiratory quotient) is not unity (on a molar basis) but rather depends on the types of respiring populations and environmental conditions. Reduced O 2 and elevated CO 2 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. Oxygen‐availability quantifiers such as O 2 concentration, air content, and O 2 diffusion rate 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 or O 2 bubbles or H 2 O 2 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.