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Water Potentials of Worms and Soils

We are now ready to begin assembling the pieces of the puzzle into a picture of how essentially aquatic earthworms manage to live in the terrestrial environment of the soil, and how the structures they build help them do it.

Let us begin with what governs the water balance between an earthworm and the soil it inhabits. This is easily understood in light of their water potentials: let us call them and for the worm and soil respectively. The worm's water potential, like that of the soil, comprises the sum of the water potentials arising from the various forces that can move water:

where the subscripts p, g, o, and m refer respectively to the pressure, gravity, osmotic, and matric poten-tials.7

The water inside a worm will have a pressure potential arising from two sources: its blood pressure and the pressure imparted to the body water when its locomotory muscles are active. These have actually been measured: we know the blood pressure of an earthworm varies from +2.5 kPa at rest to about +6.5 kPa when active. Coelomic pressures vary from roughly 600 Pa when resting to as high as 1.5 kPa when active.

Opposing this outward pressure will be the worm's osmotic potential. Worms generally regulate the concentrations of solutes in their body water to between about 100 millimolar (100 millimoles per liter) at the most dilute and 300 millimolar at maximum. This translates into a range of osmotic potentials from -250 kPa to about -750 kPa. Thus, the osmotic and pressure potentials do not balance: the net water potential of the worm will be negative by a few hundred kilo-pascals, and osmotic forces will draw water into the

7. I am ignoring losses of water to evaporation because the humidities of the soil environment are so high that evaporative water loss is virtually nil.

earthworm faster than the worm's internal pressure squeezes it out.

The worm's matric and gravity potentials are best considered in light of the soil's water potential. Like the worm, the soil will have a water potential that is a composite of the water potentials for pressure, osmosis, gravity, and matric forces:

Pressure potentials in soils usually are small, and we can safely estimate these to be less than 1 kPa. Unless the soil is very salty, we can also assume that osmotic potentials of soil waters will be small. Let us assume the water in soil contains solutes at roughly 10 percent the concentration of the body water inside the worms: this gives soil osmotic potentials of roughly - 25 to - 75 kPa. If we use a simple trick, we can safely ignore gravity potentials of both the soil and worm. Gravity potentials are only important for vertical translocations of water. Earthworms are only 2-3 mm in diameter, and any vertical movement of water between a soil and worm will be correspondingly small, with gravity potentials on the order of about 10 Pa, too small to be of any significance. We can similarly ignore the worm's matric potential: most of a worm's pore spaces will be filled with water, and so will contribute little to their matric potentials—let us say they are roughly - 10 Pa.

With these assumptions, we can write a simple equation describing the forces moving water between soil and worm. The important quantity is the net water potential difference, A^ = — This can be expanded to reflect all the components of the worm's and soil's water potential:

Okay, that's not so simple. But if we neglect the small terms, the simple equation we are looking for falls out:

Table 7.2 Water balance and its energetic costs for a hypothetical 500 mg earthworm living in soils of various matric potentials. Water potential difference, AW, is calculated as Ww - Ws. Negative water losses indicate a loss from the worm to soil, and positive "losses" indicate a gain to the worm from soil. Metabolic costs are calculated with respect to an earthworm's average metabolic rate of 35 J g-1 d-1. Soil water is assumed to have an osmotic potential of 10 percent of the earthworm's osmotic potential. Earthworms' urinary losses of water are assumed to decline with soil matric potential, from 60 percent of body weight per day at maximum to 10 percent of body weight per day at minimum. Similarly, osmotic potential of the earthworm's body is assumed to increase with soil matric potential, from -250 kPa at minimum to -750 kPa at maximum.

Table 7.2 Water balance and its energetic costs for a hypothetical 500 mg earthworm living in soils of various matric potentials. Water potential difference, AW, is calculated as Ww - Ws. Negative water losses indicate a loss from the worm to soil, and positive "losses" indicate a gain to the worm from soil. Metabolic costs are calculated with respect to an earthworm's average metabolic rate of 35 J g-1 d-1. Soil water is assumed to have an osmotic potential of 10 percent of the earthworm's osmotic potential. Earthworms' urinary losses of water are assumed to decline with soil matric potential, from 60 percent of body weight per day at maximum to 10 percent of body weight per day at minimum. Similarly, osmotic potential of the earthworm's body is assumed to increase with soil matric potential, from -250 kPa at minimum to -750 kPa at maximum.

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