Tropical shifting cultivation practise increases soil fertility - farming in a post-oil world

Earlier we reported about the ancient 'terra preta' technique of agriculture which boosts the fertility of poor soils in the tropics. The technique promises to become the basis for biofuel farming in some parts of the South, because not only does it enhance soils and increase yields, it also offers a strategy for carbon sequestration.

Several years ago, a Duke University doctoral student in botany found another indigenous agroforestry technique, a type of shifting cultivation long practiced on the Indonesian island of Borneo, that can boost the fertility of tropical soils. Deborah Lawrence made her findings during five years of research at a remote Dayak village whose inhabitants periodically clear 2 acre patches of land for rice farming or rubber and fruit tree cultivation. In between cultivating these patches, the Dayaks allow them to revert to native shrubs and trees during 20 year long "fallow" periods, with surprising results. This way, the Dayaks succeeded in cultivating plots of land sustainably for more than 200 years, without any loss in fertility.
Since she published her dissertation, Lawrence has been studying the phenomenon in other places (Southern Mexico and Costa Rica) and the results are largely confirmed.

The fallow and active farming sites of the Dayaks are nested within a matrix of "primary" forest land that has never been cleared for farming -- forming a mosaic of differing land uses. Lawrence's analyses of soils and trees focused on seven grown-over fallow sites that had not been cultivated for 10 years, and three other never-disturbed primary forest sites.

The results show that the Dayak method of shifting cultivation actually makes soils more fertile than those in undisturbed primary forest. Those results themselves are "surprising," she said in a recent interview at Duke, because they contradict negative perceptions of "slash and burn" methods that the Dayaks themselves employ to clear land.

In her newest study, Lawrence found that -- especially when she combined all her data from all her sites -- tree biodiversity was "negatively correlated" with soil fertility. In other words, the less carbon, calcium, magnesium, nitrogen, phosphorus and potassium she found in the soil, the more varieties of trees she found growing in a given study plot.

Each of those elements is an essential nutrient, Lawrence said. Carbon is the building block for all organic life, while nitrogen and magnesium both contribute to the photosynthetic process that uses sunlight to create plant sugars. Phosphorus is essential for energy production. Calcium is used to make plant cell walls. And potassium contributes to plant water regulation.

When she separated the fallow site data from that for the primary forest sites, Lawrence found some interesting differences. Not only was there more tree diversity in the never-disturbed woodlands, she said, but to some extent, the relationship between biodiversity and soil fertility was actually reversed when primary forest plots were analyzed alone:
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With increasing fertility, diversity increased in primary forest, whereas it decreased in periodically-cultivated plots. "It's as if cultivation takes the soil over some critical threshold where nutrients increase to the point that they then actually limit diversity in a negative way," said Lawrence, who is specializing in biogeochemistry and community ecology.

Her latest findings offer mixed signals for the prospects of reclaiming former tropical rain forests that have been cleared of their trees. On the one hand, she said, they corroborate a paradoxical finding of other scientists. Tropical rain forests may grow the world's most diverse array of plant life. But conditions are so delicately balanced there that the soils actually contain relatively few nutrients.

That means that subsistence farmers who clear tracts of the rich forests in the hopes of a quick return in bountiful row crops are quickly disappointed. They then must move on to destroy additional rain forest, leaving behind soil too depleted to grow anything but scrub grass.

But Lawrence's findings also suggest that Borneo's Dayaks have found a way to manage rain forest land much more productively by alternating various parcels between rice production, tree gardening, and decades-long resting periods.

"It's a broad mosaic of uses, much richer in species than any other land use system around," she said. "It's certainly richer than tree plantations or oil palm plantations or clove plantations or pulpwood plantations. But, within this land use system itself, there's no doubt that there's a decline in diversity and that it will probably continue."

By improving the soil's fertility, though, Dayak methods may actually create the conditions for the long process of generating the next rain forest, Lawrence added. All that would be needed would be seed from nearby remaining patches of primary forest.

"It means that we don't have to bring fertilizers in to get the forests to come back," she said. "We just have to leave them alone and make sure there are seed sources."

Fluent in Indonesian, Lawrence collected an oral history of the changing uses of each patch of land in her study area by gaining the Dayak's trust over a five-year period. She would ask farmers how many times they had "opened" a given patch for cultivation, knowing that there were intervening fallow periods of approximately 20 years.

She also asked the farmers what they remembered about their parents and grandparents use of various tracts. "I think that by going back three generations you can get a rough but pretty reliable estimate about how many times a patch was used," she said.

Land in her study area around the Dayak village of Kembera has been in continuous though shifting cultivation for at least 200 years.