Aims: We investigated whether drought-induced impairment
of grassland species can be explained directly by
plant water deficit or by water-driven limitation of nitrogen
(N) and/or carbohydrate sources.
Methods: In a field experiment, a severe drought treatment
was applied on monocultures of Lolium perenne L.
(cv. Alligator) (Lp) and Trifolium repens L. (cv. Hebe)
(Tr) by using rainout shelters excluding all precipitation,
and effects were compared to a rainfed control. Three
species-fertiliser treatments were set up, crossed with
the drought treatment. The two species were fertilised
equally with N (200 kgN ha−1 year−1), and an additional
high N fertilisation treatment was established for
L. perenne (LphighN, 500 kg N ha−1 year−1).
Results: Severe soil water deficit led to significantly
lower leaf water potentials in all species-fertiliser treatments
(P < 0.001) down to approximately −1.2 MPa
and, on average, to a 79% reduction in living plant
biomass above 7 cm harvest height (P < 0.001), indicating
strong plant water deficits. Under the drought
treatment, living plant biomass above 7 cm did not differ
among species-fertiliser treatments. Plant-available soil
N was 84% lower (P ≤ 0.01) and plant N concentrations
were 24% less (P < 0.001) under the drought than under
the rainfed control treatment, with Lp always being
more N limited than LphighN and Tr. Nitrate concentrations
in water-limited plants were generally very
low (< 0.85 mg g−1 dry matter), whereas non-structural
carbohydrates were distinctly greater under the drought
treatment in Lp (+62%), LphighN (+46%), and Tr (+18%).
Conclusions: Restricted biomass production of these forage
species under severe drought can primarily be explained
by plant water deficits and secondarily by
drought-induced limitation of N supply. However, growth
seems not to be limited by carbohydrate source activity, as
carbohydrates accumulated with water deficiency.