Influence of stocking rate and phosphate fertiliser application on the composition of annual legume seedbanks within a Mediterranean grassland

This paper tested the general hypothesis that differences in grazing intensity and phosphate application lead to adjustments in the structure of an annual legume community in natural and diverse grassland at Tel Hadya, Syria, due to changes in competitive relationships between species. The management treatments imposed were a factorial design of 0 or 60 kg/ha of superphosphate fertiliser (applied annually) and relatively low or high continuous sheep stocking intensities. These management treatments were applied for 13 years across three replicate paddocks for each treatment before the annual legume seed banks were sampled. The seed was sorted and representative plants were grown in a common garden in Perth, Australia, to assess reproductive and morphological traits. We found that phosphate application had more influence on species composition than stocking rate and favoured species with relatively competitive reproductive strategies (plants with relatively large seeds, low fecundity and early maturity). Contrary to our expectations, a higher stocking rate did not result in an increase in the proportion of small seeds in the legume seed bank or favour species with other strongly ruderal-type reproductive strategies (high fecundity and high seed dormancy). In paddocks without phosphate application, the high and low stocking rate paddocks were dominated by different species (Trifolium tomentosum and T. campestre), each with a similar reproductive strategy. Plant architecture and/or differences in animal selectivity may account for these differences. The study highlights the importance of phosphate fertiliser for maintaining seedbanks of competitive-type (large seed size, highly productive with good early vigour) annual legume species.