Most rainforest soils are thin and poor because __

单项选择题

Low-level slash-and-bum farming doesn’t harm rainforest. On the contrary, it helps farmers and improves forest soils. This is the unorthodox view of a German soil scientist who has shown that burnt clearings in the Amazon, dating back more than 1,000 years, helped create patches of rich, fertile soil that farmers still benefit from today.
Most rainforest soils are thin and poor because they lack minerals and because the heat and heavy rainfall destroy most organic matter in the soils within four years of it reaching the forest floor. This means topsoil contains few of the ingredients needed for long-term successful farming.
But Bruno Glaser, a soil scientist of the University of Bayreuth, has studied unexpected patches of fertile soils in the central Amazon. These soils contain lots of organic matter.
Glaser has shown that most of this fertile organic matter comes from " black carbon"—the organic particles from camp fires and charred wood left over from thousands of years of slash-and-bum farming." The soils, known as Terra Preta, contained up to 70 times more black carbon than the surrounding soil," says Glaser.
Unburnt vegetation rots quickly, but black carbon persists in the soil for many centuries. Radiocarbon dating shows that the charred wood in Terra Preta soils is typically more than 1,000 years old.
"Slash-and-burn farming can be good for soils provided it doesn’t completely burn all the vegetation, and leaves behind charred wood," says Glaser. "It can be better than manure." Burning the forest just once can leave behind enough black carbon to keep the soil fertile for thousands of years. And rainforests easily regrow after small- scale clearing. Contrary to the conventional view that human activities damage the environment, Glaser says: "Black carbon combined with human wastes is responsible for the richness of Terra Preta soils."
Terra Preta soils turn up in large patches all over the Amazon, where they are highly prized by farmers. All the patches fall within 500 square kilometers in the central Amazon. Glaser says the widespread presence of pottery confirms the soil’s human origins.
The findings add weight to the theory that large areas of the Amazon have recovered so well from past periods of agricultural use that the regrowth has been mistaken by generations of biologists for "virgin" forest.
During the past decade, researchers have discovered hundreds of large earth works deep in the jungle. They are up to 20 meters high and cover up to a square kilometer. Glaser claims that these earth works, built between AD 400 and 1400, were at the heart of urban civilizations. Now it seems the richness of the Terra Preta soils may explain how such civilizations managed to feed themselves.

Most rainforest soils are thin and poor because ______.

A. the composition of the topsoil is rather unstable
B. black carbon is washed away by heavy rains
C. organic matter is quickly lost due to heat and rain
D. long-term farming has exhausted the ingredients essential to plant growth