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The dynamics of dissolved organic carbon (DOC) have been intensively studied in mature ecosystems, but little is known about DOC dynamics and the significance of DOC as a substrate for microbial activity in early-successional catchments. We determined the concentration, chemical composition, radiocarbon age, and bioavailability of DOC along the hydrological flow path from soil solution to a downstream pond in a recently constructed catchment (Chicken Creek Catchment, Germany). Soil solution, upwelling ground water, subsurface water in an alluvial fan, and pond water all had high DOC concentrations (averaged 6.0 – 11.6 mg DOC L‐1), despite small carbon stocks in either vegetation or soil of the catchment. The mean 14C age of DOC in upwelling ground water was 2600 to 2800 years. Solid-state CPMAS 13C NMR revealed a higher proportion of aromatic compounds (32%) and a lower proportion of carbohydrates (33%) in upwelling ground water compared to pond water (18% and 45%, respectively). 14C age and 13C NMR spectra suggest that DOC was partly mobilized from charred organic matter inherent in the Quaternary substrate. In a 70-days incubation experiment, 20% of the total DOC was found to be bioavailable, irrespective of the water type. Origin of the microbial communities within the catchment (enriched from soil, stream sediment or pond water) had only marginal effects on overall DOC utilization. Overall, our data suggest that both recent and old DOC support microbial activity during early ecosystem succession, although a large fraction of the DOC is recalcitrant and eventually exported from the catchment once it has been mobilized.
It is widely acknowledged that increasing complexity is a key attribute of ecosystem genesis. This is particularly true for primary succession on homogeneous substrates. However, a mechanistic understanding of spatial colonisation and pattern formation during primary succession has not been achieved yet. Thus, we studied this topic for 7 years within an experimental catchment (6 ha) established in the post-mining landscape of eastern Germany. Equidistant permanent plots (120 plots à 25m²) allowed for autocorrelation analyses, and thus tracing the spatial development of species cover performance. For each species tested, the “cover balance” first increased due to colonization, while decreasing in the course of succession. Drawing a benefit from these temporal trends, we suggested cover balance levels revealed by autocorrelation analyses to best indicate well-defined phases in primary succession at spatial entities. Hence, with the help of this spatial approach, terrestrial succession can be understood now much better at the most general level of the hierarchy provided by Pickett et al. (1987). In accordance with these authors, at subordinated levels of the causal hierarchy differential species performance might get collectively explained by contributing processes or conditions, such as ecophysiological traits, life history strategies, competition and allelopathy of the occurring plant species. Further research is needed to understand their relative contributions to the pattern formation.
Biomass production in agro-‐systems: Is black locust (Robinia pseudoacacia L.) the right choice?
(2013)
Currently, black locust (Robinia pseudoacacia L.) is an important tree species in Central and Eastern Europe for the production of biomass in short-rotation plantations. Being a ruderal species and relatively drought tolerant compared to other broad leaf trees, its use in agroforestry systems will increase. That is particularly true for marginal landscapes characterized by adverse edaphic conditions, where the soil water availability is limited and in summer drought can occur. Several studies have been carried out on the subject of the black locust primary production; however there are still open questions associated with its water consumption and the biomass production, in relation to the soil water availability. In order to investigate the soil-plant-atmosphere system interactions, in our studies the links between the growth rate, water use efficiency and the ecophysiological response have been examined in a two year lysimeter experiment. The two years old black locust plants were selected from a recultivated post-mining area (Welzow Süd, Lusatia, Brandenburg) and established in a 100 L wicked lysimeter, installed under a light transmissive roof to avoid uncontrolled water input. During the first vegetation period, the trees were maintained constantly under three different soil moisture regimes (well-watered, moderate, drought), in relation to the hydrostatic state of the lysimeters. For the second vegetation period instead, the plants were subjected to two different irrigation cycles (long term drought stress, short term drought stress), in relation of the drought stress and recovering time duration. The results were satisfactory. At whole plant level the water use efficiency, the growth rate under different soil moisture conditions and the morphological adaptation to drought condition were assessed. In addition, at leaf level we elucidated the relation between the soil moisture together with the atmospheric evaporative demand and the ecophysiological performance in terms of H2O and CO2 rate variation. From the results emerged that the black locust cultivar may have a positive or negative effect on the ecosystem function, depending on the water availability of the areas in object. The tree can tolerate periods of drought by reducing its transpiration rate, yet it is not to be considered a water saving trees species in a well watered condition.