Plant species richness effects on P cycling: a comparison between experimental and established grasslands.

Nina Hacker1, Fabian Alt1, Wolfgang Wilcke2, Yvonne Oelmann1
1 Geographisches Institut - AG Geoökologie, Eberhard Karls Universität Tübingen
2 Institut für Geographie und Geoökologie, Karlsruher Institut für Technologie

O 9.4 in Critical unknowns in the cycling of P in forest, grassland and wetland ecosystems

17.07.2014, 10:00-10:20, H20

Plant species richness was shown to influence ecosystem functioning e.g. by increasing productivity. The latter was associated with increased uptake of nitrogen (N) and other essential elements e.g. phosphorus (P).The up to now published results originate from grassland experiments where plant diversity was manipulated while other environmental conditions did not vary. In established grasslands, however, plant species richness might act both as a variable depending on environmental conditions e.g., soil P availability, and as a driving variable e.g., by uptake of P by the vegetation. The aim of our study was to compare plant species richness effects on a) P stored in aboveground biomass b) P availability in soil, and c) P exploitation by the plant community in experimental (The Jena Experiment, JE) and well-established (Biodiversity Exploratories, BE) grassland ecosystems in Germany.

We assessed plant-available inorganic P (NaHCO3-Pi), total P (TP) concentrations in soil and P concentrations in aboveground biomass in the JE and in the BE (Schwäbische Alb, Schorfheide-Chorin, Hainich-Dün). Phosphorus exploitation was calculated as storage of P in aboveground biomass divided by the total storage of bioavailable P in soil and in aboveground biomass.

Plant species richness increased P storage in aboveground biomass and P exploitation in the JE (ANOVA; explained proportion of variance 43 to 75%; p < 0.001) very likely because of complementary resource use in diverse grassland mixtures. However, NaHCO3-Pi concentrations in soil were not affected by plant species richness (ANOVA, p > 0.05). The decoupling of plant species richness effects above- and belowground can be attributed to the strong physicochemical control of P partitioning in soil which compensates for any biological effect. In the BE, plant species richness increased with decreasing plant-available P concentrations caused by the competitive advantage of dominating species in case of high resource availability (Schwäbische Alb and Hainich-Dün: r = -0.75 and -0.44, respectively). Accordingly, productivity decreased (i) with decreasing plant-available P concentrations in soil, and (ii) with increasing plant species richness. To minimize the confounding effects of P availability in soil on established species richness we calculated P exploitation and used a sequential ANCOVA considering total P concentrations in soil. Phosphorus exploitation increased with increasing plant species richness even if fitted after TP concentrations (ANCOVA; explained proportion of variance 2%; p < 0.05). Therefore, our results demonstrate that plant species richness can simultaneously act as a response and driver variable in the P cycle. Up to now, the difference in the relevance of plant species richness effects on P exploitation between experiments and established ecosystems remains unresolved but may be attributable to the time since establishment of the grasslands.

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last modified 2014-04-03