Slash-and-burn agriculture and tropical cyclone activity in Madagascar: Implication for soil fertility dynamics and corn performance

Alexandre Buttler, Justine Marie Gay des Combes, Thomas Guillaume, Robert Thomas Edmund Mills, Bjorn Jozef Maria Robroek
Elsevier Science Bv, 2017
Journal paper

Abstract

On the western coast of Madagascar, the dry tropical forest of Kirindy may disappear in the next 50 years because of its rapid conversion into agricultural land by slash-and-burn cultivation. The slash-and-burn fields are cultivated during 3 years in average and are further abandoned because of soil depletion, weed invasion and finally lower crop yields. As a consequence, new forest areas are regularly cleared from the primary forest, causing deforestation. In addition, Madagascar is situated in a region with high cyclonic activity. Violent storms hit the western coast every 3-4 years, leading to intense rainfalls and floods. These events may enhance soil physical degradation and nutrient leaching, thereby accentuating the soil depletion by slash-and-burn agriculture and with it, the forest conversion rate. Focusing on the combined effects of historic land management and prevailing climatic conditions, this paper investigates: (1) the temporal evolution of soil fertility along with crop performance from cultivation up to field abandonment, and (2) the relative effects of land use (crop cultivation) and extreme climatic events (heavy rain, cyclonic storms) on soil and crop properties. We used a space-for-time substitution approach in slash-and-burn corn (Zea mays L) fields to describe dynamics of soil fertility and crop performance. We sampled soils and plants during two seasons: (a) a normal rainy season, in 2014, and (b) a cyclonic rainy season, in 2015. We found that under the cyclonic storm, soil becomes not only N and P deficient, but the K concentration also steeply drops. Overall, this leads to a dramatic reduction of corn performance. While a decrease in grain yield due to slash-and-burn agriculture reaches about 37.5% after three years of cultivation on the same field (from 4 to 2.5 t ha(-1)), it decreases up to 75% after a single cyclonic rainy season (from 4 to 1 t ha(-1)). On the sidelines of the study, a locust pest also damaged half of the corn fields in 2014, driving the corn yield down to zero on those particular fields. Given what precedes, the study points out the fragility of traditional agricultural techniques against natural hazards. Along with global Warming, the frequency and intensification of natural disasters are expected to increase, impacting negatively and strongly the livelihoods of rural farmers. This raises the urgent need to increase farmer's awareness to alternative and more sustainable agricultural practices. (C) 2017 Elsevier B.V. All rights reserved.

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Alexandre Buttler, Justine Marie Gay des Combes, Thomas Guillaume, Robert Thomas Edmund Mills, Bjorn Jozef Maria Robroek
Elsevier Science Bv, 2017
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/227841?ln=en

About this result

Related concepts (24)

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Slash-and-burn agriculture is considered as an important driver of deforestation in the tropics. In Madagascar, a biodiversity hotspot, this type of agriculture particularly threatens the tropical forest cover. On the south-western coast of the island, in Central Menabe, the dry tropical forest of Kirindy suffers from an annual deforestation of 2.6%. At such alarming rate, the forest will have entirely disappeared by 2050. The attention of past research programs and conservation actions were mainly on studying the forest or the general interactions between local populations and the forest, with little focus on the slash-and-burn agricultural system itself. Hence, understanding the cultivation practices and assessing alternatives to them is crucial to improve the current forest conservation initiatives. The present thesis answers to the need of an in-depth study of the agricultural practices in Central Menabe. The project aims to (1) analyze the practices of slash-and-burn cultivation in Kirindy forest, (2) explore alternative slash-and-burn practices which may be more sustainable for the forest, (3) determine the implementation perspectives of these alternatives. In a first stage, we have characterized soil productivity along repetitive slash-and-burn cultivation cycles and the farmerâs perception of their agricultural system. Then, we experimented a selective slash-and-burn agriculture (where trees are left intentionally within the cultivated fields), coupled with compost amendment. Finally, we have assessed the implementation potential of that technique through interviews and participating workshops with local farmers. Our findings highlighted that soil depletion due to plant uptake and erosion or leaching by heavy rains, as well as weed invasion were important problems of slash-and-burn cultivation in Central Menabe. Farmers perceive weeding as a work overload. Decrease in grain yield due to slash-and-burn agriculture reaches about 40% after three years of cultivation on the same field (from 4 to 2.5 t ha-1), and decreases up to 75% after a single cyclonic rainy season (from 4 to 1 t ha-1). Further, we demonstrated that combining compost and wood ashes (from the burnings) seems a promising solution to sustain cultivation on the fields. Compost, combined with wood ashes, multiplied corn yield by 5 compared to traditional slash-and-burn agriculture. Growth rates were accelerated and phenology stages of maize plants advanced. Furthermore, this combination also increased soil security by improving the remaining stock of nutrients after a rain simulation. On the other hand, a partial remaining tree cover (selective slash-and-burn agriculture) improved corn yields only when the soil was amended with ashes, not with compost, nor the combination of both. Implementing composting in the forest dwelling communities appears to be a challenging task. Several barriers were highlighted, among them the most important was the water constraints during a consequent period of time related to compost maintenance. Alternative slash-and-burn practices have not been much studied so far because the use of fire was always considered as destructive. Thus, our result will contribute to the development of new forest management strategies which would balance conservation of habitats and rural communities agricultural needs, while minimizing the impact on primary forest.

EPFL2017

Sensitivity and resistance of soil fertility indicators to land-use changes: New concept and examples from conversion of Indonesian rainforest to plantations

Thomas Guillaume

Tropical forest conversion to agricultural land leads to a strong decrease of soil organic carbon (SOC) stocks. While the decrease of the soil C sequestration function is easy to measure, the impacts of SOC losses on soil fertility remain unclear. Especially the assessment of the sensitivity of other fertility indicators as related to ecosystem services suffers from a lack of clear methodology. We developed a new approach to assess the sensitivity of soil fertility indicators and tested it on biological and chemical soil properties affected by rainforest conversion to plantations. The approach is based on (non-)linear regressions between SOC losses and fertility indicators normalized to their level in a natural ecosystem. Biotic indicators (basal respiration, microbial biomass, acid phosphatase), labile SOC pools (dissolved organic carbon and light fraction) and nutrients (total N and available P) were measured in Ah horizons from rainforests, jungle rubber, rubber (Hevea brasiliensis) and oil palm (Elaeis guineensis) plantations located on Sumatra. The negative impact of land-use changes on all measured indicators increased in the following sequence: forest < jungle rubber < rubber < oil palm. The basal respiration, microbial biomass and nutrients were resistant to SOC losses, whereas the light fraction was lost stronger than SOC. Microbial C use efficiency was independent on land use. The resistance of C availability for microorganisms to SOC losses suggests that a decrease of SOC quality was partly compensated by litter input and a relative enrichment by nutrients. However, the relationship between the basal respiration and SOC was non-linear; i.e. negative impact on microbial activity strongly increased with SOC losses. Therefore, a small decrease of C content under oil palm compared to rubber plantations yielded a strong drop in microbial activity. Consequently, management practices mitigating SOC losses in oil palm plantations would strongly increase soil fertility and ecosystem stability. We conclude that the new approach enables quantitatively assessing the sensitivity and resistance of diverse soil functions to land-use changes and can thus be used to assess resilience of agroecosystems with various use intensities. (C) 2016 Elsevier Ltd. All rights reserved.

Elsevier Science Bv2016

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In many tropical regions, slash-and-burn agriculture is considered as a driver of deforestation; the forest is converted into agricultural land by cutting and burning the trees. However, the fields are abandoned after few years because of yield decrease and weed invasion. Consequently, new surfaces are regularly cleared from the primary forest. We propose a reclamation strategy for abandoned fields allowing and sustaining re-cultivation. In the dry region of south-western Madagascar, we tested, according to a split-plot design, an alternative selective slash-and-burn cultivation technique coupled with compost amendment on 30-year-old abandoned fields. Corn plants (Zea mays L.) were grown on four different types of soil amendments: no amendment (control), compost, ashes (as in traditional slash-and-burn cultivation), and compost + ashes additions. Furthermore, two tree cover treatments were applied: 0% tree cover (as in traditional slash-and-burn cultivation) and 50% tree cover (selective slash-and-burn). Both corn growth and soil fertility parameters were monitored during the growing season 2015 up to final harvest. The amendment compost + ashes strongly increased corn yield, which was multiplied by 4-5 in comparison with ashes or compost alone, reaching 1.5 t/ha compared to 0.25 and 0.35 t/ha for ashes and compost, respectively. On control plots, yield was negligible as expected on these degraded soils. Structural equation modeling evidenced that compost and ashes were complementary fertilizing pathways promoting soil fertility through positive effects on soil moisture, pH, organic matter, and microbial activity. Concerning the tree cover treatment, yield was reduced on shaded plots (50% tree cover) compared to sunny plots (0% tree cover) for all soil amendments, except ashes. To conclude, our results provide empirical evidence on the potential of recultivating tropical degraded soils with compost and ashes. This would help mitigating deforestation of the primary forest by increasing lifespan of agricultural lands.