BBC News 19 Dec 07;
The benefits of cutting down tropical forests in order to convert the nutrient-rich soil into farmland are only short-lived, scientists suggest.
US researchers studied deforested land in Mexico and found that soil levels of phosphorus, a key nutrient for plants, fell by 44% after three growing cycles.
In the long-term, the land risked becoming so degraded that it would be uneconomic to farm, they added.
The findings appear in the Proceedings of the National Academy of Sciences.
Downward spiral
The researchers from the University of Virginia examined the disruption to the phosphorus (P) cycle in southern Yucatan, where a dry tropical forest had been felled to become farmland.
"After three cultivation-fallow cycles, available soil P declines by 44%, and one-time P inputs from biomass burning decline by 76% from mature forest levels," they wrote.
The team added that the lack of a forest's canopy also resulted in hampering an area's ability to replenish phosphorus levels.
"The decline in new P from atmospheric deposition creates a long-term negative ecosystem balance."
The ongoing decline of the nutrient, which is a key component in the growth of organisms, triggered a "feedback" effect, they explained.
It could affect the growth of plants in the study area, and "may induce a shift to sparser vegetation", they warned.
As well as the area's ecosystem, the researchers added that local farmers were likely to be affected.
"Without financial support to encourage the use of fertilisers, farmers could increase the fallow period, clear new land, or abandon agriculture for off-farm employment," they wrote.
"[The farmers'] response will determine the regional balance between forest loss and forest regrowth."
