Depth of Root Activity of Coexisting Grassland Species in Relation to N and P Additions, Measured Using Nonradioactive Tracers

1 Root activity of coexisting plant species was assessed by injecting lithium, strontium and caesium at depths of 5 and 15 cm in the soil of an upland grassland to which a factorial combination of N and P additions had been applied. 2 For.each species and each tracer the ratio of the above-ground concentrations after injection at depths of 5 and 15 cm was calculated and the species ranked for each tracer in order of root activity with soil depth; Sr and Cs gave similar results, which in some cases were quite different from those with Li. 3 When Cs and Sr were injected at the depth of 5 cm, a positive correlation was evident between Sr and Ca concentrations for the various species but not for Cs and K. 4 Although root activity was higher at a depth of 5cm for all species, the relative activity with depth differed between species; Agrostis capillaris, the most abundant species, and Thymus sibthorpii had deeper root activity than Carex caryophyllea, the second most abundant species, and Teucrium chamaedrys. 5 When N and/or P were added Agrostis capillaris and Carex caryophyllea tended to become shallower rooted and their above-ground biomass increased; for these species a positive correlation was evident between relative abundance and ratios of concentrations, indicating that the most competitive species tended to become shallower rooted when the limiting nutrients were added. 6 Ranking of species in root activity with depth differed among the various nutrient treatments. Legumes tended to have the deepest root activity when N only was applied and the shallowest root activity when P was applied. 7 Although the ranking of species was influenced by the availability of nutrients, the hypothesis that the less competitive species are forced to deeper rooting was not clearly supported; the legume species Trifolium heldreichianum only tended to become deeper rooted when N was added, i.e. in the treatment in which it was less competitive.