Requirements of gap-demanding species in chalk grassland: reduction of root competition versus nutrient-enrichment by animals

1 Arenaria serpyllifolia and Cerastium fontanum (two non-mycorrhizal species of small gaps in chalk grassland) were grown from seed on topsoil (0-5 cm) and subsoil (7-12 cm) from a chalk grassland for 100-105 days in a controlled environment room with and without root competition from Bromus erectus (a matrix-forming perennial grass) and with and without addition of nitrate and phosphate. 2 Neither species grew appreciably without addition of P; there was no interaction of P and N. In the presence of P or P + N, both species showed strong inhibition by root competition from Bromus. 3 In a second experiment the Arenaria ia and the Cerastium were grown for 64 days on four different chalk grassland (CG) soils (pH 6.8-7.6), and on a soil from a ploughed chalk heath grassland (CH; pH 6.4), with and without addition of P and P + N. Both species failed to grow on two of the CG soils, and grew only a little on the other two; both species grew faster with addition of P, but showed no extra effect of P + N. Both species grew rapidly on the CH soil, on which both occur spontaneously in turf 1 cm tall. 4 The mean dry mass of an Arenaria plant in the CH turf was strongly negatively related to the dry mass of grass shoots in a circle of 2.5 cm radius around it. 5 It is concluded that, on most chalk grassland soils, Arenaria and Cerastium are not prevented from invading short turf simply by root competition, but require addition of P via faeces. On soils of lower pH, in which greater quantities of phosphate ions are available, the studied species can grow without nutrient-enrichment by animal activity, but are very sensitive to root competition.