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

Management is necessary for the conservation of limestone grasslands. However, the traditional management of hay-making every year in July is no longer profitable for farmers. Hence many species-rich grasslands have become abandoned. The aim of this study is (a) to investigate the consequences of abandonment (as compared with annual mowing) on the availability of soil nutrients, and (b) to determine the type of nutrient limitation. The soil was taken from a long-term experimental site set up 22 years ago in northern Switzerland. The availability of soil N and P was assessed in a bioassay where soil from mown and unmown plots was supplied with different nutrient solutions or deionised water as control. Seedlings of Galium mollugo s.str. L. and Raphanus sativus ssp. oleiferus (DC) Metzg. were used as phytometer plants. 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 in the case of Raphanus by a high P content in the seeds. The N and P availability responded differently to management. The availability of N was slightly higher in soil from the abandoned plots, whereas the P availability did not differ significantly. Accumulation of nitrogen in the soil after abandonment did presumably not have any decisive effect on the vegetation because the amount was small and because the vegetation seemed primarily P-limited. The direct effect of mowing or abandonment on plants is therefore likely to be much more important for species composition than the minor changes in soil nutrients.