Читать книгу Forest Ecology - Dan Binkley - Страница 43

The physics of water transport through soils and trees to evaporate into the air depends on water potential. Water potential can be thought of as a gradient, similar to a gradient in elevation. A drop of water sitting at rest in a puddle would have a low potential; it would not be possible to obtain work from movement of the water, and its potential could be defined as zero. But if the water could follow the gravitational gradient into the soil, that movement might have an opportunity to do work (though not much!). In this case, a potential for the water at the soil surface would be zero, and the potential of water deeper in the soil would be less than zero (a negative value). Movement along gradients goes from higher potential to lower potential, and zero is higher than negative numbers.

Water at the soil surface might move into the soil along a potential gradient that does not relate to gravity. A key feature of water molecules is an imbalance in electrical charge from one side of the molecule (slightly positive) to the other side (slightly negative). This polar aspect of water makes molecules line up with each other, providing surface tension to water drops. It also causes water to adsorb (stick) onto surfaces such as soil particles. Indeed, the potential for water being adsorbed onto surfaces of soil particles is very low (a large negative value), which means water in a puddle can be “sucked” into dry soil, faster than movement from gravity alone.

The sizes of mineral soil particles are important for influencing water infiltration into soil, movement through the soil, and storage between wetting events (see Chapter 6). The smallest particles are clay‐sized, meaning <2 μm. One gram of clay has more than 1 m² of surface area to interact with water, so most water molecules are close enough to clay surfaces to slow their mobility. Water molecules that interact with surfaces have a lower (more negative) water potential than free water. The story of potentials also explains how water moves up trees, ascending tens of meters (or even one hundred meters) upward against the pull of gravity. Dry air has a tremendously negative potential compared to the insides of leaves, so water is sucked from leaves into the air, driving a potential gradient that goes against the gradient provided by gravity (Chapter 4).