Restoration of semi-natural-habitats, such as grassland or heath on former agricultural land is frequently hampered by high soil nutrient levels. Various techniques have been used in attempts to reduce soil fertility.
Effectiveness of four techniques aimed at reducing soil fertility i.e. straw/stubble burning, topsoil stripping, specific fertiliser additions to increase crop yield and hence increase removal of other nutrients with the crop, and livestock grazing, were reviewed.
Straw/stubble burning - three studies investigating nutrient removal by burning were reviewed. The overall effect of burning (an inexpensive method) is to remove some nitrogen and phosphorus (lost as smoke); this loss will be gradually recouped (over a period of about 12 years) by natural inputs via rainfall. Burning decreases phosphorus availability and increases potassium availability.
Topsoil stripping - whilst visually unattractive for a period, stripping has several conservation benefits including: exposure of less fertile soil or subsoil thus immediately reducing fertility, and topsoil is saleable. However, a large proportion of the seedbank may be lost. Cutting and removing sods for agricultural fertilizer is a traditional Dutch method of managing heaths.
Inorganic fertilizer - several studies suggest some potential for applying nitrogen fertilisers to reduce phosphate levels (and other elements) for conservation purposes.Addition of inorganic nitrogen almost completely exhausted the soil phosphorus store in long-term wheat experiments. The increase in crop (through nitrogen addition) extracts more phosphorus and other elements from the soil than treatments with no fertiliser addition.
Grazing - livestock grazing may or may not remove nutrients from a site. To assess grazing benefits in terms of reducing soil fertility it is necessary to compare removal of nutrients with inputs, and look at other effects on nutrient cycling. Increased efficiency can be achieved in some cases if animals are only allowed to graze during the day and taken off site at night e.g. free-ranging sheep may graze little at night. This approach is used successfully to manage some heathlands in Holland and Germany.
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