OPUS 4 | BayCEER-online

6

Carbon dynamics under natural and manipulated meteorological boundary conditions in a forest and a fen ecosystem (2009)

Jan Muhr

Current climate models predict changes in the amount, intensity, frequency and type of precipitation within this century. These changes are likely to result in an increasing frequency of severe drought periods in summer, causing irregular and extreme drought stress in well-drained soils or a lowering of the water table in water-logged soils. Due to rising temperatures precipitation is more likely to occur as rain rather than snow, resulting in reduced snowpacks in winter. In some regions, this can lead to an increasing frequency of soil frost. In summary, changes in the global water cycle can significantly affect boundary conditions within soils. This thesis investigated the impact of extreme meteorological boundary conditions on CO2 fluxes in two ecosystems in South-eastern Germany. Using a combination of field site manipulation and laboratory experiments we investigated the effects of prolonged summer drought and soil frost on soil C dynamics in a Norway spruce forest. In a minerotrophic fen located nearby, the effect of water table lowering (as a result of summer drought) on ecosystem C dynamics was quantified. Additionally, soil C dynamics at both sites were modeled under current meteorological conditions. For the Norway spruce forest, modeling indicated that soil C turnover predominantly occurred within the organic horizons. During the last decades, the soil has acted as a small sink. The possibility of altered C dynamics at the site due to undocumented liming has to be considered when comparing results presented here to results from other sites. For the fen, modeling revealed that soil C turnover was dominated by processes occurring within the uppermost 15 cm of the peat and that root biomass was a very important soil C stock. Most important, modeling indicated that the fen was turned into a net C source during the last decades, presumably because of disturbance of the hydrological conditions. Results from this fen cannot be regarded as representative for undisturbed peatlands. Soil frost was induced at the forest site by removing the snowpack in the winter of 2005/2006. On the snow removal plots, soil frost occurred down to a depth of at least 15 cm and for several weeks, in contrast to the snow-covered control plots where no soil frost occurred. Soil C losses were significantly reduced not only during the soil frost period but also in the summer of 2006. This phenomenon could be explained by changes in the composition of the microbial community due to soil frost, primarily a reduction of fungal biomass. To investigate the effect of drought on soil C dynamics we experimentally induced prolonged drought at the forest-site by excluding throughfall with a transparent roof during the summers of 2006-2008. Additionally, undisturbed soil columns from the site were subjected to drought in the laboratory. In both experiments, drought reduced total soil C losses in comparison to C losses from a control. This reduction was mainly owed to decreased soil respiration rates during the actual drought period, but water repellency also hindered rewetting of the dry soil, thus further prolonging the period of reduced soil respiration rates. In the past, mobilization of stabilized C due to drying-wetting has been repeatedly discussed as a possibility to actually enhance soil C losses. In the studies presented here, no evidence for this assumption was found. Soil C mineralization rates were reduced during drought and recovery was slow, possibly delayed by water repellency and preferential flow. At the fen site we used two approaches: (i) Experimental lowering of water tables to measure resulting C fluxes in comparison to C fluxes under natural conditions (i.e. control plots), and (ii) repeated measurements under varying natural conditions to be able to later statistically identify the main drivers of CO2 fluxes. We included measurements of C uptake and respiration by aboveground vegetation, thus being able to study ecosystem rather than soil C dynamics at the fen site. In summary, the impact of the water table on CO2 fluxes in and out of the fen ecosystem was minimal. Soil respiration was not affected at all by the manipulative lowering of the water table from ca. 15 cm down to more than 60 cm, most likely due to low substrate quality in deeper peat. Measurements of the natural C dynamics indicate that water table could have an impact on soil respiration within the uppermost 0-15 cm of the soil, but predominantly low water tables during summer under current boundary conditions make it unlikely that further lowered water tables due to climate change will markedly affect soil respiration rates at this site. In summary, CO2 fluxes at the site are presumably very resilient towards an increasing frequency of summer drought resulting in lowering of the water table.

5

Vegetation ecology of springs: ecological, spatial and temporal patterns (2009)

Volker Audorff

Acidification is a phenomenon, which affected the forested catchments of the northern hemisphere severely over recent decades. Acidic depositions depleted the buffering capacities of soil and groundwater, what lead to an impairment of forests, headwaters, and lakes. Even though the depositions were reduced considerably since the early 1990s, the recovery of catchments was found to occur time-delayed. The grade of recovery was found to vary significantly between regions. Biomonitoring is an appropriate tool to detect spatial and temporal patterns of ecosystem alterations, such as acidification and recovery. However, to know the interrelationships between organisms and their environment is an indispensable precondition for the identification of indicator species. The complexity of ecosystems and ecological processes hampers this quest oftentimes. Springs provide a natural setting that minimises such constraints. Compared to other habitat types, external factors are less relevant, which makes it easier to relate changes in species abundances to changes in their environment. Studying this species-environment relationship, here the response of plant species to the acidification of the spring waters was of particular interest. In a survey of five regions in Central Europe - taking spatial, hydrophysical as well as hydrochemical parameters of the springs into account - it was clearly shown that the species composition of springs is essentially determined by the spring water chemistry, and more precisely by the gradient of acidity and nutrient availability. This connection was reflected by spatial patterns within and between the regions. These patterns provide useful ecological information about spring water quality and in return about the acidity status of their forested catchments. Including catchment traits - like bedrock, climatic parameters, and forest vegetation - in the analyses, these emerged to be relevant for the species composition of springs, but less than the spring water chemistry. A path analysis showed that the catchments affect the vegetation of springs not directly, but indirectly via the determination of spring water quality. Hence, the catchments are a part of the functional chain, which is driven by the atmospheric depositions. The pH-value was found to represent the gradient of acidity and nutrient availability best. It can serve as a proxy measure that can be related to species occurrence and to species dynamics respectively, aiming to identify indicator species for assessing the status and alterations of spring water quality. With the aim to delineate niche optima and amplitudes, which in return can serve as indicator values, the realised niches of spring-inhabiting species were modelled with respect to pH. The niche attributes were found to be a matter of sampling scale. Larger plot sizes (grain) weakened the species-environment relationship, what consequently resulted in broader niche amplitudes. In contrast, the grain did not influence the species’ pH optima. Monitoring approaches that target to assess processes in time, such as acidification and recovery, are dependent on the response time of indicator species to changes in their environment. Investigating an interval of four consecutive years, inter-annual variability of the species composition could not be attributed to changes in the acidity of the spring waters. Looking at single species, bryophytes did not show a higher sensitivity to the inter-annual variability of the environment than vascular plants. Actually, only a minority of all species featured abundance changes which were significantly correlated to variations in spring water acidity. Our results suggest that the species inertia retards the vegetation dynamics of forest springs. A delayed or long-term integrating response of potential indicator species must be considered when evaluating their indicator suitability. In conclusion, the biomonitoring of spring water acidification or recovery is expedient only for longer time intervals. In a nutshell, the vegetation of springs is closely related to the hydrochemical traits of the spring waters, in particular to a gradient of acidity and nutrient availability. Individual species as well as whole plant communities are suitable indicators which allow for the monitoring of the acidity status of forested catchments. The results of this study contribute to a better understanding of the species-environment-relationships, and in return to an improvement of indicator systems.

4

Instationarität und räumliche Variabilität in Abflusszeitreihen aus Süddeutschland (2009)

Birgit Thies

Die meisten statistischen Methoden zur Auswertung hydrologischer Daten implizieren zumindest asymptotische Stationarität sowohl in den Wahrscheinlichkeitsdichten als auch in der dynamischen Struktur (ergodische Systeme). Vielfach werden außerdem lückenfreie Daten vorausgesetzt. In dieser Arbeit wurde untersucht, wie berechtigt diese Annahmen auf den in der Praxis zur Verfügung stehenden Zeitskalen von bis zu einem Jahrhundert sind. Dazu wurde ein Ensemble von langen, in täglicher Auflösung vorliegenden Abflusszeitreihen aus Süddeutschland analysiert. Die Spektralmethode des Lomb-Scargle-Periodogramms (LSP) – entwickelt für in unregelmäßigen Zeitabständen gemessene astronomische Daten – wurde auf ihre Anwendbarkeit für lückenbehaftete Abflusszeitreihen beurteilt. Die quantitative Fehlerabschätzung erfolgte in Abhängigkeit von Anteil, Zahl und Verteilung der Lücken. Die Methode liefert für einen Lückenanteil von 1 bis 10% verwertbare Ergebnisse und ist hier einfachen Interpolationsmethoden überlegen. Sind die Daten stark saisonal geprägt, sind auch höhere Lückenanteile unproblematisch, die ansonsten die Interpretierbarkeit der Peaks einschränken. Ist bei längeren Lücken eine Datenrekonstruktion aus geeigneten Nachbarpegeln möglich, liefern Fast-Fourier-Analysen der so vorbehandelten Daten bessere Ergebnisse als das LSP. Bei kurzen Lücken (<1%) sind einfache Interpolationsroutinen ausreichend. Der anhand der Modelleffizienz des LSP quantifizierbare maximal zu erwartende Qualitätsverlust kann mit einer für unkorrelierte Daten hergeleiteten Beziehung abgeschätzt und mittels weiterer Kriterien (Anzahl und Position der Lücken, Saisonalität der Daten) präzisiert werden. Das LSP ist damit für die Frequenzraumanalyse von Abflussdaten mit bis zu 10% Lücken eine sinnvolle Alternative zu aufwändigeren Interpolationsverfahren. Die Stationarität von 97 Pegeln im oberen Donauseinzugsgebiet wurde mittels Fenstertechnik auf Zeitskalen von 2 bis 30 Jahren mit verschiedenen Methoden untersucht. Exemplarisch durchgeführte sequentielle parametrische Verteilungsanpassungen waren methodisch unbefriedigend, nicht-parametrische Ansätze standen daher im Fokus. Herkömmliche Charakteristika von Verteilungen wie Perzentile und Momente und die Entwicklung in daraus abgeleiteten Phasenräumen wurden analysiert. Ausgehend von der Testgröße des Kolmogorow-Smirnow-Zweistichprobentests wurden verschiedene Stationaritätsmaße zum sequentiellen Vergleich von Verteilungen entwickelt, um statt einzelner Kennwerte die Veränderungen im gesamten Wertebereich zu berücksichtigen. Die neuen Maße eignen sich hervorragend zur Einschätzung der Stärke der Schwankungen im Pegelvergleich, ihre Darstellung in Matrizenform erlaubt eine detaillierte zeitliche Analyse der Einzelpegel. Der Zeitverlauf der integrativen Stationaritätsmaße ist weniger von einzelnen Episoden oder Extremereignissen geprägt und damit gleichmäßiger als die mit Hilfe einzelner Verteilungsmerkmale dargestellte Dynamik. Die Analyse des oberen Donaueinzugsgebiets zeigt einen fast alle Pegel betreffenden langfristigen Anstieg der Abflussmengen in den letzten drei Jahrzehnten. Hiervon betroffen sind insbesondere niedrige Abflüsse, abgeschwächt der mittlere Bereich der Werteverteilung und in nur geringem Maße die Hochwasserabflüsse. Streuung, Schiefe und Wölbung zeigen keine pegelübergreifenden langfristigen Tendenzen. Auf kürzeren Zeitskalen von 3 bis 6 Jahren ist bei sämtlichen Pegeln eine synchrone Mittelwerts-Schwankung zu beobachten, die sich abgeschwächt auch in Varianz und höheren Momenten zeigt. Aus dieser regional gleichmäßigen Abflussdynamik auf Zeitskalen von mehreren Jahren lassen sich starke räumliche Korrelationen ableiten, die sich über mehrere 100 km und damit über den Bereich des Untersuchungsgebiets hinaus erstrecken. Die Abflussschwankungen sind von Pegel zu Pegel unterschiedlich stark ausgeprägt, wobei die alpenbeeinflussten Abflüsse durch ihre vergleichsweise geringe Variabilität eine Sonderrolle einnehmen. Das pegelspezifische Instationaritätsniveau kann in der Praxis zur Beurteilung der Unsicherheit von aus Verteilungsanpassungen berechneten Bemessungsgrößen mit herangezogen werden.

3

Analysis of flow patterns and flow mechanisms in soils (2009)

Christina Bogner

Matrix flow and preferential flow can occur concurrently in the same soil. Both flow regimes produce typical flow patterns that can be visualised in dye tracer experiments. To extract quantitative information from dye tracer studies a vast variability of approaches exists. One of them is to describe dye patterns by the so called dye coverage function, i.e. the percentage of stained area per soil depth. Based on extreme value statistics the dye coverage function can be reinterpreted as a probability function to find the tracer in a certain depth. Therefore, the two-parametric probability distribution 1 – H, H being the generalised Pareto distribution, can be fitted to the dye coverage function. The form parameter of this distribution serves as a risk index for vertical solute propagation. We did tracer experiments with Brilliant Blue FCF at three study sites: in a Norway spruce forest in southeast Germany, in a tropical mountain rainforest in southern Ecuador and on an agricultural field in southern France. We tested the ability of the risk index to summarise main information obtained in dye tracer studies and characterise flow patterns in different soils. Our results suggest that the risk index is to some degree invariant to changing experimental conditions (such as irrigation rate). The initial soil moisture, however, seems to have a large influence on the risk index. It is difficult to adjust the parameters of the generalised Pareto distribution when the dye coverage function fluctuates or does not decrease monotonically. This might be due to tortuosity of paths, varying flow mechanism or changing soil physical properties. Thus, we restricted the analysis to the lowest part of the profile. Since the theory of the risk index is based on extreme values of vertical solute propagation it is the lowest part of the profile that is the most interesting. We propose to combine the two parameters of the generalized Pareto distribution and to use the complete distribution 1 - H to estimate the risk of vertical solute propagation in soils. Despite a certain resistance to changes of experimental conditions, the risk index is not an intrinsic soil parameter. Since the flow regime in the same soil can be dominated either by preferential flow or by uniform matrix flow, the risk of vertical solute propagation will change. The adjusted parameters of the generalised Pareto distribution will capture the dominant flow regime as reflected by tracer flow patterns. Bearing in mind the boundary conditions of the tracer experiment like irrigation rate, the tracer employed, soil initial moisture or type of vegetation (permanent or seasonal, deep rooted or shallow rooted) it is possible to compare different study sites or to consider the same site at different boundary conditions and to access the risk of vertical solute propagation. Pattern analysis based on the risk index for vertical solute propagation revealed the occurrence of preferential flow at the German study site. To gain insight in flow mechanisms and possible impacts on soil chemistry we analysed soil texture, fine root density, soil bulk density, exchangeable cations, pH and total C and N contents in preferential flow paths and soil matrix. Results from linear mixed-effects models suggested that at this study site roots constituted main preferential flow paths and induced macropore flow, especially in the topsoil. In the subsoil root density decreased and inhomogeneous infiltration from preferential flow paths into the soil matrix caused non-uniform flow. There were no textural differences between the flow domains, but smaller bulk densities in preferential flow paths. This is probably due to a higher soil organic matter content in preferential flow paths. We found smaller pH values, more Ca, more Mg, more C and more N in preferential flow paths. Compared to the adjacent soil matrix, more Al and more Fe (but small absolute amounts) were found in the subsoil where macropore flow along root channels decreases and heterogeneous matrix flow dominates. These distinct chemical properties can be explained by root activity and translocation of solutes and DOC (dissolved organic carbon) via preferential flow paths. During transport along preferential flow paths contact time between DOC and soil is reduced so that DOC is transported to greater depth where it potentially forms organo-mineral associations. If this holds true, preferential flow is a mechanism that promotes C sequestration in subsoil and does not only influence its immediate environment around paths, but also underlying subsoil horizons.

2 (2006)

Biomassebildung und Nährstoffaneignungsvermögen der Wurzeln in experimentellen Grünlandbeständen mit unterschiedlicher Pflanzenartenzusammensetzung (2005)

Andreas Reuter

In der Arbeit wurde der Zusammenhang zwischen der Pflanzenartenzusammensetzung experimenteller Grünlandbestände und für das Nährstoffaneignungsvermögen relevanten morphologischen Wurzeleigenschaften untersucht. Des Weiteren wurde der Einfluss der Pflanzenartenzusammensetzung auf die Biomassebildung und den Nährstoffgehalt der Wurzeln analysiert, um den Eintrag von Kohlenstoff in den Boden und die Zirkulation mineralischer Nährelemente im Boden abzuschätzen. Die Untersuchungen fanden an Grünlandbeständen unterschiedlicher Zusammensetzung von Pflanzenarten (2 bis 8 Arten) und funktionellen Typen von Pflanzen (niedrigwüchsige Gräser, hochwüchsige Gräser, Rosettenpflanzen, hohe stängelbeblätterte Krautige) statt. Des Weiteren wurden auch die Mykorrhizierung und das Stickstoffaufnahmevermögen der Wurzeln untersucht. Die morphologischen Wurzeleigenschaften unterschieden sich deutlich je nach Bestandeszusammensetzung, Untersuchungsjahr, Jahreszeit und Bodentiefe. Wurzellängendichte, durchschnittlicher Wurzeldurchmesser und spezifische Wurzellänge wurden in erster Linie von spezifischen Effekten dominanter Arten bestimmt. Ein Einfluss der Pflanzenartenvielfalt und der funktionellen Typen von Pflanzen konnte nicht nachgewiesen werden. Auch die Mykorrhizierung der Wurzeln in Grünlandbeständen wurde in erster Linie von spezifischen Wurzeleigenschaften der bestandsbildenden Pflanzenarten beeinflusst. Interspezifische Wechselwirkungen scheinen eine untergeordnete Rolle zu spielen. Der Anteil mykorrhizierter Wurzellänge war positiv mit der Wurzellängendichte korreliert. Die Stickstoffaufnahme eines Pflanzenbestandes aus unterschiedlichen Bodentiefen wurde nicht von der Anzahl an Arten oder funktionellen Gruppen bestimmt, sondern vermutlich von den artspezifischen Leistungsfähigkeiten der Wurzeln. Die Stickstoffaufnahme einer bestimmten Art und die Konkurrenzkraft dieser Art bezüglich der Stickstoffaufnahme aus verschiedenen Bodentiefen variierten dabei je nach Wurzelkonkurrenz durch benachbarte Arten. Interspezifische Wechselwirkungen durch unterschiedliche Tiefenverteilung der Wurzeln spielten hier vermutlich eine entscheidende Rolle. Zur Beurteilung der Konkurrenzkraft scheint eine Einteilung der funktionellen Typen auf der Basis von morphologischen Wurzeleigenschaften demnach besser geeignet als die hier vorgenommene Einteilung nach morphologischen Sprosscharakteristika. Die potenzielle Stickstoffaufnahmefähigkeit der Wurzeln wurde ebenfalls in erster Linie durch die spezifischen Eigenschaften der im Bestand vorkommenden Arten beeinflusst. Schnitt der oberirdischen Biomasse und Jahreszeit hatten dagegen keinen Einfluss auf die Stickstoffaufnahmekapazität der Wurzeln. Selbst während der Vegetationspause im Winter blieb das Potenzial zur Nitrataufnahme erhalten. Untersuchungsjahr, Jahreszeit und Bestandeszusammensetzung hatten einen deutlichen Einfluss auf die Bildung pflanzlicher Biomasse. Die Bildung von Spross- und Wurzelbiomasse war im ersten Untersuchungsjahr deutlich höher als im zweiten, was auf witterungsbedingte Änderungen der Abundanz einzelner Arten zurückzuführen war. Dies deutet darauf hin, dass die Biomassebildung und somit der Ertrag von Grünlandsystemen in erster Linie von spezifischen Eigenschaften dominanter Arten bestimmt werden. Wurzelumsatz und Kohlenstoffeintrag in den Boden durch Wurzelstreu waren positiv mit dem Grasanteil in der Sprossbiomasse korreliert. Diese Beziehung fiel jedoch – den gesamten Untersuchungszeitraum betrachtet – deutlicher aus als bei ausschließlicher Betrachtung des letzten Untersuchungsjahres. Da sich die Abundanz der einzelnen Arten jedoch im Verlauf des Experiments änderte, scheinen die untersuchten Parameter in erster Linie durch artspezifische Eigenschaften beeinflusst zu werden. Die Akkumulation von Kohlenstoff im Boden war – über den gesamten Untersuchungszeitraum gerechnet – in den grasdominierten Beständen etwas niedriger als in den von dikotylen Kräutern dominierten Beständen. Eine erhöhte SOM-Zersetzungsrate aufgrund artspezifischer Einflüsse auf die Mikroflora im Boden könnte hierbei eine Rolle spielen. Die Akkumulation von Kalium, Magnesium und Phosphor in den Wurzeln war bei den in der ersten Hälfte des Experiments vom Gras Holcus lanatus dominierten Beständen geringer als in den übrigen Beständen. Die interne Zirkulation dieser Nährelemente wird offenbar von Effekten dominanter Arten bestimmt. In der vorliegenden Arbeit konnten keine konsistenten Zusammenhänge zwischen der Anzahl an Pflanzenarten oder an funktionellen Gruppen in einem Bestand und den untersuchten Parametern nachgewiesen werden. Vielmehr zeigten in erster Linie artspezifische Effekte dominanter Arten einen deutlichen Einfluss auf verschiedene Ökosystemfunktionen. Basis für künftige Untersuchungen zum Einfluss der Pflanzenartenzusammensetzung auf Ökosystemfunktionen sollte deshalb eine gezielte Definition von morphologischen und physiologischen Attributen von Pflanzenarten sein.

1 (2006)

Plant Functional Traits and Ecosystem Functions in Experimental Grassland Stands (2005)

Guido Kossmann

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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