【BBC尼罗河】泛滥之谜 19

英语自学网

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The soil here is volcanic. That means it contains a rich cocktail of nutrients. It was soil as fertile as this which nurtured the crops of Ancient Egypt. With the plentiful rain and intense mountain sunshine, the grass here grows fast. This explains why these highlands can support such huge numbers of animals.
The rains are transforming the highlands in another way. Night temperatures could be low enough to freeze any water in the ground. As the ice expands, it breaks up the topsoil. When the ice melts the next morning, the soil is left crumbled and fine. Millions of nest-making rodents loosen the soil further.这里的火山土富含各种养分，同滋养了古埃及农作物的土壤一样肥沃。在丰沛雨水与充足山区阳光的作用下，这里的草长得飞快。这便是这片高原如何能够供养起数量如此之多的野生动物的原因。
雨水还以另一种方式改变着高原。夜间气温有时会低到使土壤中的水汽凝结。结冰扩张时会拱碎表土层。如此一来，隔天清晨结冰融化后土壤就会变得松散而细腻。几百万只拥有地下筑巢习性的啮齿动物则使土壤更加松软。