Hydric soil characteristics are the product of microorganisms, vegetation, and animal activity in soils saturated with water. Microorganisms are key agents regulating organic matter accumulation and other hydric soil characteristics, such as the formation of redoximorphic features and H

S evolution (Chapter 7). Microbial decomposition of organic matter in combination with plant productivity determines whether and to what extent organic matter accumulates in the soil. The combination of biomass production by plants and reduced microbial decomposition in waterlogged environments leads to accumulation of soil organic matter, producing the surficial organic-rich horizons common to many hydric soils. Both plants and animals contribute to redoximorphic features, such as Fe pore linings (oxidized rhizospheres) as well as reduced soil matrix. This chapter discusses the environmental factors that affect distribution and abundance of the wetland soil organisms that produce physical and chemical characteristics unique to hydric soils. The role of microorganisms, vegetation, and animals in regulating the nature, extent, and intensity of hydric soil characteristics is also discussed.