Water regulation and purification is defined as “the capacity of the soil to remove harmful compounds and the capacity of the soil to receive, store and conduct water for subsequent use and to prevent droughts, flooding and erosion”. In an agricultural system, this refers to rainwater or irrigation water at field scale. There are a numerous biological processes and actors involved in making this function work at is best capacity, these have been categorized as follows:
(1) Infiltration into, and percolation through the soil is dependent on the soil structure, which is made up of a range of aggregates (building blocks) of different shapes and sizes and supported by a network of pores. These aggregates and the associated pore network are define by a continuum of bio-physical processes supported by the soil biota called the ecosystem engineers (earthworms, ants, soil fungal hyphae, plant roots and enchytraieds).
(2) Water storage is also dependent on the size and shape of soil aggregates that form the soil structure in combination with soil organic matter. Supported by the same biota as above plus the soil mesofauna that supports the physical breakdown of soil organic matter.
(3) The removal of harmful substances or purification of the water is supported through Biological retention. Biological retention is generally a passive process whereby soil organisms bio-accumulate toxins within the soil water. These toxins can be in many form of: excess nutrients, metals, metabolic products of pesticides, etc. The bio-accumulation of toxins can be related to different traits within the soil biological community, such as the feeding strategy or body type. Biological retention can also take place by plant roots, as they take up plant nutrients.
In general Water Regulation and Purification plays an important role in agricultural production, ensuring that crops have a plentiful supply of clean water and that through a good soil structure, droughts are not prevalent. This function is also of great importance to the other functions, as excess water can result in anaerobic conditions contributing to N2O and CH4 emissions for climate regulation, can reduce nutrient availability for plants and enhance disease suppression.
 Wall DP et al. 2020. A decision support model for assessing the water regulation and purification potential of agricultural soils across Europe. Frontiers in Sustainable Food Systems.