The tundra consists of two layers: the top layer called the active layer and the second layer referred to as permafrost.

苔原由两层构成，顶层称为活动层，第二层称为冻土层。

You see, most of the soil often stays frozen during peak runoff time and the nutrients stay deep in the active layer.

你看，大部分的土壤在流失顶峰时期保持冻结状态，这样营养物质就留在了活动层深处。

They're not up at the top near the surface, which is the part of the active layer that's affected most by the runoff.

它们不在靠近表面的顶部，这里是活动层受土壤流失最大的部分。

The first reason is that the microbes reside in the top active layer, which holds water that doesn't get into the permafrost.

第一个原因是微生物存在于顶部的活动层，这里面有不进入冻土层的水。

The top active layer stays frozen during the winter and spring but thaws in the summer time when the permafrost layer remains frozen all year long.

顶层活动层在冬季和春季保持冻结，但在夏季会融化，冻土层全年都保持冻结状态。

And because of the cold temperatures, the tundra has two layers: top layer, which is called the active layer, is frozen in the winter and spring, but thaws in the summer.

由于气温较低， 苔原有两层：顶层，称为活性层， 冬季和春季冻结， 夏季解冻。

There're two reasons for this: first, they live in the active layer, which, remember, contains water that doesn't penetrate the permafrost;second, most of the precipitation in the Arctic is in the form of snow.

这有两个原因：首先，它们生活在活动层中，记住，其中含有不会穿透永久冻土的水;其次，北极的大部分降水都是雪的形式。