综合自然地理文献翻译.docxVIP

GEOMORPHOLOGICAL AND BIOGEOGRAPHIC AL PROCESSESThe breakdown of rocks and minerals is largerly initiated by plants, animals and bacteria on a scale greater than has been realised to date. Biological weathering operates either physically or chemically. Tree roots and soil animals may penetrate soil and subsoil, mix up the material, expose new surfaces and enable fresh contacts between water, air and minerals, thereby promoting weathering and soil formation. Cast-making earthworms can recycle 50 tons per hectare a year of soil material through their bodies; they may burrow down to 1.5 m into the ground. Their number, quantity and vigour vary generally with soil nutrients and climate. They decline as climate cools，as altitude increases and as soil nutrient levels are lowered. Their relative vigour is directly proportional to the fertility status of the soil. They are abundant in brown earths, relatively rare in podzols and present in variable amounts in gleys, dependent on profile drainage and texture.Moles, more adapted to the temperate latitudes and the more open-textured soils, e.g. brown earths and chernozems, predate on worms in particular, and are also instrumental in the relocation of masses of soil through their burrowing and the creation of mole hills on the ground surface. In the chernozem, the burrows (krotovinas) of the blind mole rat (Spalax typhlnon) (Plate 4.6) and the gopher (Citellus suslyka) are mainly responsible for the disturbances and discolourations in the lighter grey-brown horizon. This lies immediately below the very dark, humose, surface A-horizon, and immediately above the dark-brown or grey -horizon, which is remarkably rich in calcium carbonate nodules. In tropical environments, especially the savannas, termites relocate selective soil material of less than one millimetre in diameter to build their mounds (termitaria) which are often a metre or two high (sometimes as much as four metres high) and usually regularly spaced, due to food requi