- Assessment of grassland ecosystem functioning: Carbon dioxide exchange and the dynamics of carbon and nutrient pools in temperate and Mediterranean grasslands (2008)
- The objectives of this study were to examine the influence of environmental variables, extreme weather events, management methods and ecosystem heterogeneity on grassland ecosystem functioning in the context of the variables CO2 exchange, nutrient dynamics, production, nitrogen and carbon pools. Two sites were selected in Central Europe, to represent temperate grassland, namely a grassland at Grillenburg and a controlled experimental grassland in the Botanical Garden of the University of Bayreuth, (both located in Germany) and additionally one site at Herdade da Mitra, Portugal, was studied as Mediterranean grassland. In the temperate grassland mowing reduced the leaf area index (LAI) as well as the biomass, with a subsequent impact on the overall assimilatory capacity of the grassland ecosystem. The pattern of root biomass development reflected seasonal variations in temperature and rainfall as well as the growth of the aboveground biomass showing peak root mass between DOY 180 and 210. Cutting increased the foliar N concentration, which came as a result of increased demand for N in the regenerating fresh tissue after the cut. During winter and early spring daily rates of net carbon exchange were low and the balance between net ecosystem CO2 exchange (NEE) and respiration was nearly zero. The gross primary productivity (GPP) at the cut site was higher than at the uncut site due to vegetation recovery at the former and senescing leaves at the latter site. Results from the artificially created grassland showed that drought altered the carbon fluxes in the grassland ecosystems without significantly changing the aboveground biomass production. A possible consequence of drought in these grass species could be an increase in the LAI due to shifts in aboveground carbon allocation from reproductive to vegetative structures. The results showed a crucial role played by species composition in regulating carbon fluxes and ecosystem productivity. The more diverse community exhibited higher potential for carbon uptake as well as increased ecosystem respiration. In the Mediterranean grassland, trees added considerable amounts of nutrients to the soil beneath their canopies, and have the potential to facilitate understory production. Although there was no significant difference in the total biomass accumulation between understory and open locations, analysis of soil N concentration revealed higher soil N under the trees. Although NEE was limited by light intensity in the understory, model projections of GPP showed no difference between the understory and the open locations in their potential assimilatory capacities. Significant differences, however, occurred between the two locations in ecosystem respiration. Depending on the location (open or understory), grazing influenced CO2 exchange processes differently. We found no significant differences in GPP between grazed and ungrazed sites in the open locations, while large differences occurred in the understory, with lower NEE in the grazed as compared to the ungrazed locations. Mean maximum foliar N concentration in the temperate grassland occurred in mid-May, coinciding with early growing season, with 3.4% nitrogen, whereas in the Mediterranean grassland, it reached a peak value in early April which was lower (2.3 %), averaged over the stand biomass. As in the case of the foliar N, the root N concentration was also lower in the Mediterranean grassland (ca. 1 %) as compared to the temperate grassland (1.5 %). In terms of available N in the soil solution, it was low at Mitra (ca. 0.2 micromol g-1 soil) in early spring while it remained near 0.6 micromol g-1 soil throughout the season at Grillenburg. Thus, the development of biomass is sensitive to fluctuations in temperature and radiation during the optimal period for growth, but the total biomass accumulation is at a lower level in the Portuguese grasslands (as well as in the botanical garden) due to greater nutrient limitation. Mediterranean and temperate grasslands were found to differ more strongly than expected due to nutrient availability, which depends on the prevailing higher temperatures, annual changes in water balance and possibly nutrient removal from the ecosystem in Mediterranean regions.