Hotspots of biological activity

Yields are higher and drought-induced crop losses dramatically lower on farms that use this slash-and-mulch system than on farms that don't

Trees play a major role in maintaining and improving soil fertility. Understanding how they do this, and which species affect soil in different ways, is fundamental to designing mixtures of tree species that can make farms more productive.

Soil teems with organisms, from invisible microbes that fix atmospheric nitrogen to earthworms that mix organic materials and develop and maintain soil structure. It is through the activity of a wide range of organisms that fertility is generated and maintained.

In 2011, the World Agroforestry Centre published a review of the impact trees have on soil biodiversity. Led by Edmundo Barrios, who joined the Centre as Soil and Land Management Scientist the previous year, the review shows a consistent pattern: the presence of trees is associated with a greater abundance of soil organisms across a range of sites in agroforestry systems. This is clearly shown in studies that compare the presence of soil organisms in monocrops – for example, a field of maize – with plots where crops are grown together with trees. Earthworms, centipedes and millipedes are more than three, five and six times more abundant respectively under agroforestry systems than they are in fields with annual crops without trees. Beetles, ants and mites are also more plentiful.

Before joining the Centre, Barrios had conducted extensive research on the Quesungual agroforestry system in Central America with colleagues at the International Centre for Tropical Agriculture and collaborators at the University of Western Australia and University of California-Davis. Widely practised in the drought-prone, steeply sloping uplands of western Honduras, the system involves farmers selectively pruning trees interspersed among their annual crops, leaving the green matter as a mulch that eventually decomposes and becomes incorporated into the soil.

"Yields are higher and drought-induced crop losses dramatically lower on farms that use this slash-and-mulch system than on farms that don't," says Barrios. To avoid disruption on the farms, the scientists measured the quantity of worm casts as an index of biological activity. "We found a higher concentration of biological activity, greater amounts of carbon and nutrients, and longer availability of water in soils under the influence of trees, including slash and mulch management, than in fields without trees or soil further from trees," he says.

"We now have a working hypothesis that the perennial nature of trees has a profound impact on soil properties, and on the abundance, diversity and functions of soil organisms," says Barrios. This explains why trees increase and sustain soil fertility. Besides acting as a refuge for soil organisms, trees used in agroforestry systems can increase the supply and availability of nutrients, maintain and improve soil structure and control soil-borne pests and diseases.

"One important area for new research is to identify which tree species and mixtures of species do most to promote biological activity in the soil," explains Fergus Sinclair, who leads the Centre's research on farming systems. "A keystone of our research strategy for improving soil and water productivity is to understand how different types of trees interact with soil organisms so we can develop design principles that enable us to select appropriate mixtures of trees to sustain soil fertility for different sites and circumstances."

AfSIS, the African Soil Information Service, is providing an opportunity for Barrios to work with scientists mapping soil health across the continent. The collection of soil biodiversity data has begun in Tanzania, at one of the 60 AfSIS 'sentinel' sites. Linkages between trees and soil fauna are being explored to identify biological indicators associated with the provision of soilbased ecosystem services. These will become part of the land health surveillance system.

Barrios E, Sileshi GW, Shepherd K and Sinclair F. 2011. Agroforestry and soil health: linking trees, soil biota and ecosystem services. In: Wall DH (Ed.). The Oxford Handbook of Soil Ecology and Ecosystem Services. Oxford, UK: Oxford University Press. Chapter 5.2. (in press).