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

Abstract

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.Soil aeration processes (i.e., air intake and soil gas exchange) and soil aeration status (i.e., soil gas content and composition) are, after water and nutrient availability, the most important factors affecting soil fertility and plant growth. Soil aeration elevates the concentration of O 2 , which is consumed mostly by plant roots and the microbial population, and relieves the high concentrations of CO 2 and toxic gases for...

Bounds to Air‐Flow Patterns during Cyclic Air Injection into Partially Saturated Soils Inferred from Extremum States

“…The O 2 consumption of intensive agriculture soils is about 10 g O 2 m −2 soil d −1 (Friedman and Naftaliev, 2012; Ben‐Noah and Friedman, 2018), while atmospheric air contains about 250 g O 2 m −3 . This means that the gaseous phase of soil, assumed to be fresh, 21% O 2 air, of a root zone of, e.g., 50‐cm depth and a mean volumetric air content of 0.15 contains about 20 g O 2 m −2 soil, i.e., it should be replaced every 2 d. In other words, an air‐discharge rate of 0.04 m 3 m −2 soil d −1 (1.6 L h −1 m −2 soil) is potentially enough to supply the soil O 2 demand.…”

“…This is even more surprising if one considers the relatively simple and inexpensive application of air injection in plots where subsurface irrigation systems already exist, and that air injection was found useful in aerating the rhizosphere (Bhattarai et al, 2006; Ben‐Noah and Friedman, 2016). This reluctance may be due to the lack of a simple and practical tool for the design of an efficient aeration system (Ben‐Noah and Friedman, 2018).…”

Measurement of Gas Diffusion Coefficient in Highly Permeable Porous Media

“…Accounting for gas fluxes within a porous media (PM), such as the upper vadose zone, requires partitioning the flux contribution from two transport mechanisms—diffusion and advection (Hamamoto et al, 2009a; Kuang et al, 2013; Nachshon and Weisbrod, 2015; Ben‐Noah and Friedman, 2018). For purely Fickian diffusion, the key parameter comprising PM properties is the diffusion coefficient.…”

Soil Compaction Effects on Root‐Zone Hydrology and Vegetation in Boreal Forest Clearcuts

“…Air-filled porosity (i.e., total porosity minus volumetric water content [VWC]) is considered to be the main limiting factor for soil aeration and VWC is commonly used for quantifying soil aeration (Ben-Noah and Friedman, 2018). After water and nutrient availability, aeration is the most important soil factor affecting plant growth (Ben-Noah and Friedman, 2018).…”