Plant Functional Traits and Ecosystem Functions in Experimental Grassland Stands
Within the BMBF funded project BIOLOG-Bayreuth (01LC0014) investigations on implications of functional groups on ecosystem functions related to water- nutrient- and carbon cycle were carried out. Three experiments were conceived to test for implications of dominant species traits and phytodiversity on ecosystem functions. I Water, nutrient and DOC fluxes and losses from grasslands were investigated on Experimental Grassland Stands in lysimeters II Two dominant (P. lanceolata / H. lanatus) and two transient species (R. acris / A. odoratum), identified in Experimental Grassland Stands were used for the Rhizodeposit Experiment (RDE) to investigate implications of different Fe acquisition strategies on rhizodeposition of organic compounds in nutrient solution cultures. III The Root Mineralisation Experiment (RME) aimed at evaluating potential implications of the identity of rhizosphere microflora on mineralisation performance of root tissues from H. lanatus and R. acris derived from RDE. Root biomass for the RME was obtained from plants of the RDE Experimental Grassland Stands were sown in lysimeters (1.3×1.3×1.0 m) filled with 70 cm of sub- and 30 cm topsoil of tillered material derived from a Stagnic Cambisol. Precipitation was collected and soil solution was obtained at 15, 30 and 90 cm, seepage at 100 cm depth. KCl-extractable Nmin was determined in June and September. NH4, NO3, DON, DOC, K, Mg and Ca were measured in solution and nutrients in above- and belowground biomass. In Experimental Grassland Stands on lysimeter facilities, herb contribution to grassland stands rather than functional diversity showed implications on ecosystem functions such as nutrient use, use efficiencies, yields and loss with seepage. Differences in ecosystem performance were due to the identity of functional group of dominant and co dominant species (grass/herb). Positive relations between higher nutrient availability and WUEbm of both grass and herb species were indicated for experimental grassland stands. Grassland stands with higher herb contribution favoured a higher nutrient sequestration in biomass and thus played an important role for safety net functions in grassland ecosystems. Slightly higher Nmin concentrations in soil solution likely reflected higher root-turnover rates of grass dominated stands in 2002. Grass species showed higher base cation use efficiency and hence provided considerably growth under low cation supply. Grass dominated stands showed rather low performance in safety net functions for nutrients. Lower base cation yields in biomass of grass dominated stands were not automatically reflected by indicators for use of soil nutrients. In contrast to Nmin, base cation concentrations and losses with seepage did not reflect differences in base cation use by the grassland stands. In the RDE P. lanceolata was identified as a species featuring lower competition ability in concern of biomass building up and Fe acquisition (Cab; WILSON, 1988) under Fe deficiency compared to H. lanatus. For swards containing P. lanceolata complementary was found in concern of individual biomass production and individual Fe stocks. P. lanceolata gained lower biomass than the accompanying species. Inverted patterns of Fe acquisition ability and the ability to build up individual biomass, hint at a trade-off between Fe demand and biomass build up for P. lanceolata. DOC release to rhizodeposit solution was enhanced after the 1st harvest. P. lanceolata showed the highest release of DOC, a higher diversity of carboxylic acids as well as a considerable release of potential Fe chelators (malic, citric and malonic acid), while H. lanatus swards released only small amounts of carboxylic acids. Higher competition ability for species Fe contents after the 1st harvest indicated enhanced competition between P. lanceolata with A. odoratum. This finding was also reflected by higher DOC release and a higher release of potential Fe chelators in these swards. It was found that even transient grasslands species may show a high competition ability for Fe-acquisition under Fe-deficiency. During the RME, the basal respiration of the rhizosphere sand obtained from RDE differed only tendentiously due to its origin (H. lanatus, R. acris, diculture rhizosphere or Ref sand). H. lanatus root tissues increased respiration rates significantly during a 236 h incubation period to a four-fold of basal respiration whereas application of R. acris root tissues did not. Root tissue material was mineralised to the same extent as Corg material within the first 236 hrs. Since no differences of chemical parameters were found, for roots of the two species, enhanced mineralization of H. lanatus roots in the initial phase are likely due to lower root diameters and higher root surface areas.
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