Nutrient availability and limitation in traditionally mown and in abandoned limestone grasslands: a bioassay experiment

Management is necessary for the conservation of limestone grasslands. Howev er, the traditional management of hay-making every year in July is no longe r profitable for farmers. Hence many species-rich grasslands have become ab andoned. The aim of this study is (a) to investigate the consequences of ab andonment (as compared with annual mowing) on the availability of soil nutr ients, and (b) to determine the type of nutrient limitation. The soil was t aken from a long-term experimental site set up 22 years ago in northern Swi tzerland. The availability of soil N and P was assessed in a bioassay where soil from mown and unmown plots was supplied with different nutrient solut ions or deionised water as control. Seedlings of Galium mollugo s.str. L. a nd Raphanus sativus ssp. oleiferus (DC) Metzg. were used as phytometer plan ts. Their growth in the control treatment was limited by N and P almost to the same extent, indicating that the availability of both elements was very low in this soil. P limited plant growth slightly more, but was overcome i n the case of Raphanus by a high P content in the seeds. The N and P availa bility responded differently to management. The availability of N was sligh tly higher in soil from the abandoned plots, whereas the P availability did not differ significantly. Accumulation of nitrogen in the soil after aband onment did presumably not have any decisive effect on the vegetation becaus e the amount was small and because the vegetation seemed primarily P-limite d. The direct effect of mowing or abandonment on plants is therefore likely to be much more important for species composition than the minor changes i n soil nutrients.