Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Resumen:

In nutrient-poor tropical forests litter decomposition is essential for the supply of nutrients to plants. This process is in turn controlled by nutrient availability. Beside nitrogen (N) and phosphorus (P) other elements may co-limitate litter decomposition. Studies revealed a relationship between sodium (Na) shortage and reduced litter decomposition which can decrease the mineralization of stored carbon (C). Investigations of element fluxes of a Na-poor tropical montane rainforest in southern Ecuador have shown low atmospheric deposition rates since 1998. Additionally, Na was retained in different parts of the ecosystem. A study found that the Na-retention in the canopy was related to the Na demand of the microorganisms in the phyllosphere. Because the Na budget of the organic layer was also positive, it was assumed that decomposers may experience a lack of Na due to low atmospheric deposition. Thus, one main goal was to investigate whether Na fertilization accelerates litter decomposition. Based on the assumption of a higher Na demand of soil fauna than of microorganisms, it was also tested whether the stimulation of soil organisms by Na is limited to the soil fauna. To test these hypotheses, a combined litter decomposition and Na fertilization experiment was conducted in an evergreen premontane rainforest and a montane rainforest in south Ecuador. Fresh litter was incubated in the field and fertilized weekly with different Na concentrations which were similar to the natural deposition rates. Litterbags with different mesh sizes allowed to study the effects of Na fertilization on microbes and mesofauna separately. Additionally, a bait lamina experiment was installed in the organic layer to study the uptake of Na fertilized baits by soil organisms. Differences of the bait loss between the fertilized and the control group should indicate a potential stimulation of the soil organisms by a Na-addition. The statistical analysis confirmed the stimulation of litter decomposition by Na additions. The comparison of both sites revealed a slightly stronger effect of Na additions in the premontane forest than in the montane forest. The accelerated litter decomposition was related to an increased activity of the soil fauna. However, a significant stimulation of microorganisms by Na additions could not be detected. The bait lamina experiment confirmed the results of the litter decomposition experiment and showed a marginally significant increase of the activity of soil organisms by Na fertilization. Sodium can stimulate the decomposition of the organic layer in the studied premontane and montane rainforest, thus affecting ecosystem functioning. Na supply is of higher importance in the ecosystem than usually assumed and its relevance for the C turnover in the organic layer may increase if the atmospheric Na deposition remains on the present low level.

Resumen:

Recent studies raise the hypothesis that Na shortage restricts decomposition and affects the carbon cycle in tropical forests. When Na concentrations in soils are low and the stands are far off-coast, they do not receive substantial Na inputs from the atmosphere. Since terrestrial plants have low concentrations of Na, which is not considered as an essential element, the demand of soil fauna may not be covered. Yet, in contrast to animals, little is known on Na demands of phyllosphere microorganisms. This thesis presents results from a study on Na limitation in a montane forest ecosystem in South Ecuador, which is located on the Eastern cordillera of the Andes, in a microcatchment under an undisturbed lower montane rainforest. The study area is characterized by low Na concentrations because of low deposition rates with incident precipitation and by low Na stocks in in the soils and in the organic layer. Sodium fluxes in rainfall, throughfall, stemflow, litter leachate, litterfall and organic layer have been monitored since 1998. Results reveal overall low Na concentrations in the ecosystem fluxes. Higher Na fluxes with incident rainfall than with throughfall suggest that Na is retained in the canopy. Therefore, this study aims at testing the hypothesis that Na is retained in the canopy because of Na limitation of microorganisms in phyllosphere. To explore the role of the phyllosphere in Na retention, I sampled leaves covered by phyllosphere microorganisms and leaves without phyllosphere cover from 12 tree species belonging to 7 plant families frequently occurring in the study area. The fresh leaves were sprayed with a NaCl solution containing 1 mg L-1 Na, corresponding to the Na concentration in incident rainfall in the study area during La Niña events. Comparison with a control treatment excluded effects by abiotic Na fixation on the surface of the leaves. The results show that increasing phyllosphere cover leads to a significantly enhanced Na retention, which is much higher on understory tree leaves than on leaves of the upper canopy. Leaching of K, Ca and Mg was higher with increasing degree of phyllosphere cover, which can be attributed to increasing element exchange between foliage and phyllosphere with leaf age. These results suggest that Na availability possibly plays a regulating role in the study ecosystem which might even grow in importance if Na deposition from the atmosphere continues to decrease or stabilizes at the current low level.

Resumen:

The current climate change in the tropical Andean rain forests in south Ecuador alters the distribution of rain events with increasing dry and wet phases. The present research focuses on the concentration response of some elements to signicant changes on rainfall distribution. It seeks to determine whether changes in the concentrations of elements in an ecosystem of a rainforest are an eect of dilution by precipitation or other factors that may be aecting these variations, such as microbiological activities. The study examines chloride, ammonium, nitrate, phosphate, total organic carbon (TOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP) in soil solution as well as the ratio of organic nitrogen to organic carbon (C : N) in soil solution samples taken in a tropical rain forest of Ecuador. Soil samples were taken weekly from 1998 to 2007, both below the organic layer and 15 and 30 cm into the mineral layer. Concentrations were measured with a chloride electrode , Continuous Flow Analyzer (CFA for ammonium, nitrate, DON, and DOP) and Total Organic Carbon Analyzer. The results were analyzed with statistical software packages R and SPSS using statistical methods of descriptive statistics and ANOVA. The average weekly precipitation was 38.73 mm and weekly precipitation varied between 0 and 155.2 mm. The variation of chloride concentrations served as reference to detect dilution/concentration effects of the other elements because it is assumed that chloride concentrations behave inversely proportional to the volume of water in soil. Thus, the higher the precipitation the lower is the concentration of chloride in soil solutions. I found that the mineral elements presented similar concentration variations as chloride indicating the strong if not exclusive eect of dilution. The phosphate concentrations were an exception showing irregular variation. Measurement problems due to the low P concentrations, often below the detection limit of the instrument may be the explanation for such irregularities. The variation in chloride-normalized organic components diered from that of chloride. The concentrations of TOC, DON and C : N ratio showed a fairly steep increase with increasing precipitation, especially observable at 15 cm depth in the mineral soil and in some cases also at 30 cm depth. A small TOC consumption by the microbial community during rewetting, a strong microbial TOC production or increased leaching of TOC to the mineral soil are possible explanations for this result. My results demonstrate that the response of inorganic N and P species is mainly driven by concentration/dilution eects while for organic compounds microbial activity in relation to soil moisture was an additional factor controlling the concentrations.

Resumen:

Um das Vorhandensein und den Transport chemischer Elemente auf der Erde zu beschreiben, wird das Konzept der biogeochemischen Kreisläufe angewandt. Ein solches Element ist Phosphor, das wegen seiner Wichtigkeit als limitierendes oder co-limiterendes Element für das Pflanzenwachstum in tropischen Regenwäldern ausgewählt wurde. Am Beispiel eines Einzugsgebietes im tropischen Bergregenwald von Ecuador wird untersucht, wie sich die Phosphorkonzentrationen und Phosphorflüsse entlang des wassergebundenen Kresilaufs vom Freilandniederschlag bis hin zum Oberflächenabfluss sowohl zeitlich wie auch räumlich verhalten. Die untersuchten Daten umfassen die Wasserflüsse und die Konzentrationen von Orthophosphat und totalem gelösten Phosphat zwischen 1998 und 2010 als monatliche Mittel. Die räumliche Verteilung wird mit der Darstellung der Messwerte in Boxplots und der Suche nach räumlichen Mustern analysiert. Um nach saisonalen Prozessen zu suchen, wird einerseits ein lineares Modell, bestehend aus einer Sinus- und einer Cosinus-Funktion gebildet und auf Signfikanz und Aussagekraft, d.h. Erklärung der Variabilität durch das Modell anhang des Anteils an der Gesamtvariation getestet. Anderseits wird die Autokorrelation der Daten auf ein saisonales Muster untersucht. Ein allfälliger langfristiger Trend wird mit dem saisonalen Kendall-Test gesucht. Bei der Analyse der räumlichen Verteilung zeigt sich, dass sich die größten Phosphor-Konzentrationen und -Flüsse zwischen Bestandesniederschlag und organischer Auflage bewegen. Zudem ist der Eintrag durch den Freilandniederschlag größer als der Austrag im Oberflächenabfluss. Bei allen Wasserflüssen wurde eine jährliche Saisonalität festgestellt. Bei den Konzentrationen ist ein antizyklisches Verhalten zu den Wasserflüssen zu erkennen. Der Freilandniederschlag weist keine Saisonalität auf. Die größten Saisonalitäten sind zwischen Krone und organischer Auflage zu finden. Die Konzentrationen in den Saugkerzen und im Oberflächenabfluss sind nur schwach durch saisonale Prozesse geprägt. Bei den Flüssen zeigt nur der Bestandesniederschlag deutlich jährliche Saisonalität. Die Analyse der zeitlichen Trends zeigte keine langfristigen Trends, die auf eine Änderung externer Faktoren zurückzuführen sind. Interne Veränderungen im Pflanzenwachstum könnten für eine Zunahme im Stammabfluss und eine Abnahme der Lysimeterkonzentrationen (unter der organischen Auflage) verantwortlich sein. Bei der Analyse der Feueraktivität als möglicher Quelle für Phosphoreintrag wurden einzelne signifikante Beziehungen mit den Phosphorkonzentrationen gefunden.