Slow convalescence

Seventy years is the minimum amount of time needed to restore forest biomass

In time, tropical rainforests’ deforested areas can reestablish their nitrogen cycle, which plays a key role in the recovery of the forest itself. For quite some time, researchers have been collecting evidence that recovery is feasible. However, for the first time, a group has managed to demonstrate this phenomenon as well as measure the speed of recovery. If the good news is that forests can indeed recover, the bad news is that it takes at least 70 years for them to take significant recovery strides, reveals the article “Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment,”  published in the journal, Nature.

In order to understand this phenomenon, a team of researchers from the Center for Nuclear Energy in Agriculture at the University of São Paulo (CENA-USP), Piracicaba campus, the Emílio Goeldi Museum in Pará and Embrapa in the Eastern Amazonian Region (state of Pará) established what they have called a chronosequence. At  farms in the Pará towns of São Francisco do Pará and of Capitão Poço, in the Amazonian deforestation belt, scientists marked out 12 lots of 1,000 sq. m each within deforested areas, which had started to recover spontaneously after being abandoned by farmers.

With the help of their owners, the group was able to determine how long ago each area had been abandoned. Thus, they identified lots in which the forest had been under recovery for 3, 6, 10, 20, 40 and 70 years, in addition to areas of native vegetation. According to the study’s data, the recovery of the biomass reached 70% or 80% of the original level. “Seventy years is the smallest amount of time for recovery. Had we found locations deforested longer ago, perhaps we would have been able to produce a more precise timeframe for recovery,”  says Luiz Antonio Martinelli, from CENA, one of the study’s authors. “If we leave the system alone, it renovates itself.”

One evidence of recovery was growth of nitrogen emissions in the soil, in the form of nitrous oxide gas (N2). “As soon as the system becomes nitrogen-rich, it allows itself the luxury of losing this element in gas form,”  explains Martinelli. The study also involved Eric Davidson, from the Woods Hole Research Center (USA).

Biodiveristy – It is estimated that some 30% to 50% of the Amazon Region’s deforested area has ceased to be exploited and is now recovering. Tropical rainforests are usually nitrogen-rich. However, agriculture cuts down substantially on the levels of this nutrient in the soil. The research findings are auspicious, but the fact that forests can recover does not mean they will recover their original wealth of biodiversity. The vegetation is never as rich as it was before and the return of animal species will depend on the maintenance of the surrounding areas. “But the recovery of the forest is an indicator that it can once again become inhabitable,”  states Martinelli. The research is also important in terms of signaling ways for speeding up regeneration. The planting of leguminous plants, which are characterized by fixing nitrogen in the soil, is one of the strategies under consideration.

If the deforested forest takes a while to recover nitrogen, agricultural areas face a problem of a different nature. Man currently pours 85 million tons of nitrogen into the soil, largely due to intense use of both natural and chemical fertilizers. As the plants are unable to absorb a fair amount of this extra dose, the outcome is that excess nitrogen enters other environments, polluting rivers and water tables and returning to the atmosphere, where it contributes significantly to global warming, also helping to cause acid rain. “We’re overloading the environment with gigantic and unnecessary amounts of nitrogen. The consequences are terrible and need to be better known and controlled, urgently,”  informs Martinelli.

Contradictions – In 2002, China’s fertilizer consumption reached 25 million tons; in the USA, the figure was almost 11 million tones. At the opposite end of the scale, gross consumption in Namibia and Sierra Leone in 2002 was a meager 100 tons per country. This arena of contradictions will be the target of debates at the International Nitrogen Conference to be held from October 1 to 5 at Costa do Sauípe (state of Bahia), whose committee is under the coordination of Luiz Martinelli. He plans to make the most out of the presence of the more than 500 people from Brazil and abroad expected to attend the event, so as to conduct a discussion in greater depth on how to fight the nitrogen surplus – or deficit. According to him, simple actions, such as reducing the burning off, as well as the crop rotation system, are important. Furthermore, he assures us, any good agronomical engineer knows the balanced amount of fertilizer needed for a given type of soil and crop. “Our chief mission is to maximize the beneficial effects of nitrogen while minimizing its bad consequences,”  he sums up.