Seasonal Dynamics of Phytomass and Nutrients in Chalk Grassland

Effects of extra N input on above- and below-ground phytomass and on the concentration of N and P in chalk grassland during one growing season were studied, with special attention to the effects on the grass Brachypodium pinnatum compared with other species. N fertilization in spring resulted in a significantly increased peak standing crop. This higher production could be almost completely accounted for by the increase in above-ground dry weight of Brachypodium. The total below-ground phytomass, ca. 2-3 times higher than the above-ground peak standing crop, was not influenced by extra N input. Concentrations of N and P in the below-ground parts decrease in spring, caused by translocation to the developing shoots, and increase again from August to December as a result of withdrawal from the dying shoots. Brachypodium proves to have a highly efficient system of nutrient withdrawal into its well-developed rhizome system. Almost all of the extra N is found in Brachypodium only. As a consequence only dry weight of this species doubled. Phytomass production increases in Brachypodium after extra N application because