The temperate grassland biome: climate and soils

Overview

Climate of the temperate grassland biome and process of calcification in a temperate grassland soil.

What you need to know

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Questions from this subtopic

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1. 1

   Describe the climate of the temperate grassland biome.

   Model answer

   It is a continental climate with a very large annual temperature range. Summers are hot, with mean temperatures for the warmest month typically between eighteen and twenty-five degrees Celsius and daytime maxima often above thirty. Winters are cold, with the coldest month below freezing and mean temperatures reaching minus ten to minus twenty degrees Celsius in the interior of the Eurasian steppes, so the annual range can exceed thirty degrees. Precipitation is moderate and unreliable, generally between about two hundred and fifty and seven hundred and fifty millimetres a year, with a maximum in late spring and early summer from convectional storms, and a dry period in late summer and in winter. Winds are strong across the open plains, and drought years are a normal part of the pattern.

2. 2

   Why does temperate grassland develop instead of forest?

   Model answer

   Because there is enough moisture for grasses but not reliably enough for trees. Rainfall totals are modest and, critically, summer evapotranspiration in the hot continental interior can exceed precipitation, so the soil develops a moisture deficit at exactly the time of year when a tree would be transpiring most heavily. Grasses can survive this by dying back above ground and surviving as roots and rhizomes, and they can complete their growth in the moist spring before the deficit develops. The severe winter cold, the strong drying winds and the periodic droughts add further stress that trees cannot tolerate, and natural fires and heavy grazing by large herbivores kill tree seedlings while grasses regrow from their base.

3. 3

   What is a chernozem?

   Model answer

   A chernozem, or black earth, is the deep, dark, extremely fertile soil characteristic of the temperate grassland biome, and it is found most extensively beneath the steppes and prairies. Its defining feature is a very thick surface horizon, sometimes approaching a metre deep, that is almost black with humus, has a crumb structure and is rich in bases. Below this the soil becomes paler and contains white nodules or threads of calcium carbonate. It is neutral to slightly alkaline and is regarded as one of the most naturally fertile soils in the world.

4. 4

   Why is a chernozem so rich in humus?

   Model answer

   Because of the way grass grows and dies. Grassland has an enormous dense mass of fine fibrous roots, most of which are in the upper part of the soil, and a large proportion of this root system dies back each year and is replaced. Organic matter is therefore added throughout the depth of the topsoil rather than only as a layer of litter on the surface, as it is in a woodland. Decomposition is rapid in the warm moist spring and early summer but is halted by winter cold and by summer drought, so humus accumulates faster than it is mineralised. Intense earthworm and burrowing animal activity mixes it thoroughly through the profile, giving the deep, uniformly black horizon and the stable crumb structure.

5. 5

   Explain the process of calcification in a temperate grassland soil.

   Model answer

   Calcification is the accumulation of calcium carbonate within the soil profile in a climate where evapotranspiration broadly balances or exceeds precipitation. During the wetter spring, water moving downwards through the soil dissolves calcium carbonate and carries it down a certain distance, but there is not enough water to leach it right out of the profile. During the hot dry summer, evaporation at the surface draws soil water back upwards by capillary action, and as that water evaporates the dissolved calcium is redeposited. Over time this concentrates calcium carbonate at a characteristic depth, where it appears as white nodules, threads or a distinct horizon, and it keeps the soil neutral to slightly alkaline and high in bases.

6. 6

   Why are temperate grassland soils only lightly leached?

   Model answer

   Because there is rarely enough surplus water moving downwards through the profile to wash the soluble bases out of it. Annual precipitation is modest, and in the hot summer evapotranspiration equals or exceeds it, so downward percolation is limited to the spring and to occasional heavy storms and does not continue for long enough to remove calcium, magnesium and potassium from the soil. The soil therefore retains a high base status and a neutral to slightly alkaline reaction, in contrast to the acidic, heavily leached soils of wetter climates where percolation is continuous.

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