Agricultural nonpoint source pollution is a significant cause of stream and lake contamination in many regions of industrialized world. A major causative source of this pollution is nutrients such as phosphorus (P) and nitrogen (N) that are lost from soils of fertilized agricultural and forestry operations, particularly in coarse-textured, poorly drained soils where drainage water ultimately mixes with surface water. It has been estimated that beef cattle operations on improved pastures (184 000 ha), unimproved pastures (33 500 ha), and rangeland (47 000 ha) on ranches are a large contributor of P loads to Lake Okeechobee, Florida, one of the largest freshwater lakes in the USA. Recent studies have shown that agroforestry practices such as silvopasture and riparian buffer could be a means of addressing the problem of environmental impact of nonpoint source pollution. The deeper and more extensive tree roots will invariably be able to take up more nutrients from the soil compared to crops with shallower root systems - the so-called 'safety-net' effect that has been affirmed in various agroforestry situations. Consequently, nutrient-leaching rates from soils under agroforestry systems where trees are a major component can be lower than those from treeless systems. Other studies documenting the effects of planted riparian forests on stream quality in temperate agricultural settings have also been reported, for example, in northwestern USA.
The water-quality enhancement resulting from the reduction of nutrient loading could be a substantial environmental benefit of agroforestry in heavily fertilized agricultural landscapes. With increasing realization of the adverse impacts of chemical agriculture and climate change on availability and quality of water in many parts of the world, water is now a critical issue in natural resource management. Time-tested integrated land-use practices such as agroforestry could be appropriate approaches to addressing the problem. However, the science of this is nonexistent and needs to be explored and established.
The connectivity and buffering effect offered by the presence of agroforestry systems on the landscape is another important environmental benefit of agroforestry that is little appreciated. It is common knowledge that the increasing presence of trees of similar or different species in an agricultural setting will have many positive biophysical influences (see the sections, titled 'Soil productivity and protection' and 'Carbon storage'). As the trees age, these influences increase in magnitude; therefore, it is to be expected that when trees are introduced into degraded agricultural landscapes (intercropping) and stream banks (riparian buffer), there will be gradual changes at the system (landscape) level. Long-term studies of this nature are rare, but some convincing examples are available (see the section titled 'Ecological engineering: agroforestry system design'). The fire-protection benefit offered by extensive tree-based systems such as the dehesa in the Mediterranean region (Figure 4) is another benefit that has traditionally been enjoyed but never valued in economic terms.
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