ABSTRACT
Available water capacity (AWC) is defi ned as the amount of water (cm3 water/100 cm3 soil) retained in the soil between “fi eld capacity” (FC) and the “permanent wilting point” (PWP).[1] FC and PWP are defi ned as the volumetric fraction of water in the soil at soil water potentials of 10 to 33 kPa and 1500 kPa, respectively. One of the paradigms of soil science is that AWC is positively related to soil organic matter (SOM) because SOM raises FC more than PWP[2-4] (Fig. 1). SOM enhances soil water retention because of its hydrophilic nature and its positive infl uence on soil structure.[5,6] Increasing SOM increases soil aggregate formation and aggregate stability[7] (Fig. 2), thereby increasing porosity in the range of pore sizes that retain plant-available water and enhancing infi ltration and water retention throughout the rooting zone. When SOM decreases, soil aggregation and aggregate stability decrease and bulk density increases.[8] These changes in physical properties result in lower infi ltration rates and higher susceptibility to erosion.[9,10] This entry reviews the literature on the sensitivity of AWC to soil organic carbon (SOC) and discusses the environmental implications of changes in SOM and AWC. This entry supersedes and updates a 2003 review[11] with a description of the recent literature on the sensitivity of AWC to SOM and discusses the environmental implications of changes in SOM and AWC.
