| Abstract: |
| Soil research has been a prominent part of the agroforestry research agenda from the start of the
current World Agroforestry Centre as ICRAF in 1978 with a focus on new answers to land
degradation problems. Early hopes that, in order to be widely adopted, agroforestry primarily lacked
policy support and effective extension rather than research, proved to be too simple. While policy
attention was drawn to the need for soil replenishment in Africa and for alternatives to slash-and-burn
throughout the humid tropics, the specific ways to achieve these goals in the local context were under
investigation. A research agenda was framed that saw trees on farms and in agricultural landscapes as
ways to conserve and improve soil carbon (C) stocks, add nitrogen (N) by use of N2-fixing trees,
mobilize poorly available phosphorous (P) sources and capture deep soil nutrient stocks and mobile
nutrients on the way out by leaching. Simultaneously the trees should be a source of valuable products
(such as firewood, timber, fodder, fruits, medicinal bark and roots), a regulator of (micro)climate and
watershed functions, and a provider of supportive functions for crops and animals. In doing so the
experience with bottom-up approaches showed that local ecological knowledge of soils included
classifications and functional insights complementary to what formal science had as yet explored. A
phase of research on hypotheses at process level, analyzing the various tree-soil-crop interactions one
by one, was followed by the construction of synthetic simulation models. Meanwhile, the early use of
plot level experimentation, inherited from agronomic traditions, proved to be a challenge as lateral
tree roots were difficult to control unless plot sizes were large and replicated trials huge. Beyond plotlevel
experiments, research shifted from characterizing to managing lateral resource flows and filter
functions, reinterpreting the earlier erosion control emphasis at hill-slope and landscape scales.
Dynamics of soil water led to quantification of soil structure and its dependence on root-based carbon
inputs, old tree root channels and earthworms. Further soil biological work was focused on
mycorrhiza, rhizobia, nematodes and other soil biota. The more fundamental understanding of soil
biology, led to early work on soil carbon dynamics and greenhouse gas emissions from tropical land
use, especially in humid tropical forest margins. Reducing and avoiding below- and aboveground
emissions were combined in the search for alternatives to slash and burn. Understanding the
underlying principles required for sustainable and profitable land use, with or without trees,
contributed to a general trend where promises of packaged technology evolved into supporting farmer
knowledge and decisions. Agroforestry practices aimed at soil fertility improvement were extensively
tested on farms, which led to a better understanding of the risks and benefits under different
conditions. A focus on the diverse realities on farm meant that laboratory methods for soil
characterization had to be scaled up and simplified. The use of soil spectral properties proved to be
efficient in dealing with the spatial diversity of soils in both landscape and farm level applications.
However, at the end of the day, our funders and investors want to see and be assured of pathways to
development impact, and demonstrating that through changes in soil quality over the long-term and
over large extents requires wider application of these methods. The concepts of soil function in
multifunctional landscapes, the interdisciplinary integration of tools and approaches, and the direct
linkage of growing knowledge and increased action will continue to evolve, but can be rooted in a
rich tradition and are on solid ground. |
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