Soil science as part of agroforestry

Highlights:
  • New answers to land degradation problems have been an agroforestry focus for four decades
  • Plot-level experimentation following agronomic traditions proved to be a challenge due to lateral interactions
  • Testing hypotheses at process level and analysing tree–soil–crop interactions led to synthetic simulation models
  • Policy attention to soil-nutrient replenishment in Africa and alternatives to slash-and-burn in humid tropics required more than technical analysis
Introduction:

World Agroforestry (ICRAF) has as its mandate all agricultural land use that involves trees, beyond what is considered to be forest. The latter distinction is rather fluid, both temporally and institutionally, as the example of long-rotation shifting cultivation may show. Agroforestry itself ranges from croplands with a few trees added through to systems where tree crops (considered to be agricultural, such as coffee, cacao or rubber) provide a perennial vegetation layer, augmented with upper canopy layer trees utilized to modify microclimate, yielding economically valuable products. The consequences for soil conditions and functions vary along this range.

Agroforestry research has from its start operated on the active and often contested interface of the need to increase agricultural production, overall and per unit area, and the need to find more sustainable ways of managing natural resources. Agroforestry is typically associated with ‘integrated’ rather than ‘segregated’ solutions to meet the dual imperative, with specific attention to the understanding and management of trade-offs at the scales of farmers, the landscape, (sub-) national governments and the global policy arena. Soils have a key function to both issues of land productivity and environmental effects, and soil research of one type or another has been part of nearly all research activities of ICRAF from its start.

Classifying the research output of ICRAF on the basis of citations to publications grouped by topic (Figure 4.1) shows six identifiable waves. Virtually all literature on agroforestry systems and improvement or ‘tree–soil–crop interactions’ that had been cited by 2013 had been published before 2000; by contrast, publications on agroforestry and environmental services and climate-change mitigation and adaptation started in the mid-1990s and flourished after 2000. Intermediate time patterns (steady progression in time) are found for agroforestry systems in social, policy and economic contexts, and for tree domestication.

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