In July, Basin et al caused a storm by claiming we could capture 25% of the world’s carbon emissions by reforesting unused land. But why wouldn’t you also reforest grazing land, which has 3-4 times this potential? Silvopastoral systems produce livestock on high-quality fodder crops in forest understorey. This is highly productive, using less land by producing the feed in the area occupied by the livestock, and sequestering carbon, much of which is in the soil.
Semi-intensive silvopastoral systems combining trees, shrubs and pasture plants have better soil structure than pasture-only systems. Several studies have demonstrated effects of SPS on the physical, chemical and microbiological properties of the soil. The shrubs and trees in the SPS add layers of vegetation capable of transforming solar energy into biomass, which includes the formation of roots that penetrate to deeper soil layers, from where they extract nutrients and water. The greater number of strata also generates more abundant and heterogeneous biomass that is deposited on the soil in the form of leaves, branches, fruits, resins and exudates with important effects on nutrients, organic matter and biota. Islands of extra soil fertility form around the trees.
Semi-intensive silvopastoral systems with nitrogen-fixing shrubs as well as pasture plants and trees are even better for soil structure. The livestock in the intensive systems are moved to a new area using electric fences every 1-3 days allowing resting periods of more than 40 days for the fodder understorey to recover. Biodiversity is also higher than conventional pasture. Systems in Mexico with ramon trees as well as high protein shrubs and pasture, are an example where trees are used for both shade and as livestock food in drought periods so they are valuable for food provision and soil preservation. The presence of deep tree roots, or relatively deep shrub roots, and the maintenance of complex soil structure, has the consequence that water is retained better by the soil in these systems, improving drought tolerance. A further consequence is a reduction in nutrient leaching to ground water. The deep roots of trees and shrubs are capable of retrieving nitrates and other nutrients that have leached below the rooting zone of herbaceous plants and of eventually recycling these nutrients as litterfall and root turnover in the cropping zone. Silvopastoral systems retain nutrients within plants better than plants in adjacent fertilised pasture with cattle grazing.
Much of the structure of the soil is retained in silvopastoral systems with the consequence that earthworms and other soil invertebrates flourish to a greater extent than on land that includes only pasture plants. A comparison in Colombia of soils from conventional pasture for over 30 years and a semi-intensive silvopastoral system with Leucaena for 3, 8, or 12 years found that while % carbon and % organic matter depended on amount of clay in the soil, microbial biomass and enzimatic activity were higher in older silvopastoral areas than in conventional pasture increasing the availability of nutrients in the system. The presence of readily degraded manure from the livestock on the pasture and of nitrogen fixing plants in the silvopastoral system, is associated with retention of calcium and phosphorus.
Silvopastoral systems can result in better conditions for beneficial insects with consequences for soil composition and diversity of plants in the system. Fertilised pasture and a semi-intensive silvopastoral system with two grasses and the shrub Leucaena were compared using areas of each system on the same farms in Colombia and the numbers of dung beetles were higher in the silvopastoral system. In the silvopastoral system there were more dung beetle species, more dung removed per beetle, more dung removed in total and more seeds deposited by the beetles. Silvopastoral are also beneficial for other biodiversity, while less rich than forest fragments, produce both more richness and abundance than conventional grazing land, and higher productivity creates the potential to spare more land for natural systems. The biodiversity of these systems would be improved with more forest species and structural diversity.
With benefits to soil, water, carbon budget, productivity and biodiversity, semi-intensive silvopastoral systems make a useful contribution to carbon-neutral production systems partly, through sequestration, much of which is in soils, and partly due to reduced enteric emissions due to the high feed quality. In order to be able to report these effects quantitatively in future studies we request those collecting field data to report production data alongside soil measures. This enables soil to be considered on an equal basis to other environmental factors such GHG emissions and water use.
By Prof Donald Broom, Dr Julian Chara, Dr Juan Heberth and Dr Rowan Eisner
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