Decline in metallophytes in tertiary polluted floodplain grasslands in the Netherlands: experimental evidence for metal and nutritional changes in soil as driver factors

Abstract

Metallophyte vegetation along the River Geul has been almost completely replaced by grasses during the last decades. Field investigations indicated that this was accompanied by higher alkalinity and phosphate availability in the soil, related to the closure of the metal industry and intensification of agricultural practices. An experiment with a full factorial design for phosphate and zinc availability indicated that the metallophytes Silene vulgaris and Thlaspi caerulescens did not grow on zinc-poor soils, irrespective of phosphate availability. The grass Holcus lanatus performed well on phosphate-rich soil, irrespective of zinc availability. An experiment with zinc-poor and zinc-rich floodplain soils confirmed the high zinc demand of the metallophytes T. caerulescens and Armeria maritima and the zinc independence of H. lanatus. A third experiment indicated that a reduced zinc availability due to liming affected only the metallophyte T. caerulescens; it had no effect on the growth of the grass Festuca rubra. This means that increasing alkalinity leads to a decrease in zinc availability, limiting the growth of at least some metallophyte species. An increase in phosphate availability stimulates growth in more competitive fast-growing grasses under zinc-rich as well as zinc-poor conditions.