Affiliation: | RisøNational Laboratory, Plant Ecosystems and Nutrient Cycling Programme, Building 309, PO Box 49, DK-4000 Roskilde, Denmark; Royal Veterinary and Agriculture University, Department of Economics and Natural Resources, Arboretum, Kirkegårdsvej 3A, DK-2970 Hørsholm, Denmark; Royal Veterinary and Agriculture University, Department of Economics and Natural Resources, Arboretum, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark; Author for correspondence (tel +45 4677 4156;fax +45 4677 4160;e-mail ). |