Sentence:  Even second-generation biofuels, made from crop wastes or wood, are an environmental disaster, either extending the cultivated area or removing the straw and stovers which protect the soil from <WSD>erosion</WSD> and keep carbon and nutrients in the ground. 
Meaning of the word marked with <WSD> tags:  The result of having been worn away or eroded, as by a glacier on rock or the sea on a cliff face.