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

Abstract:

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.

Abstract:

In einem tropischen Bergregenwaldgebiet in Ecuador, haben verschiedene Forschungsarbeiten gezeigt, dass auch Mikronährelemente einen Einfluss auf die mikrobielle Aktivität haben können. Mittels eines Inkubationsversuches mit einer Streuauflagenprobe aus dem Untersuchungsgebiet, wurde analysiert ob eine multiple Mikronährlimitierung von Mikroorganismen vorliegt. Dabei wurde die Streuprobe in einem sechs Tage dauernden Inkubationsversuch täglich mit verschiedenen Mikronährelementen (Cu, Mo, Mn, Zn, P)gedüngt. Danach wurde die Nährlösung abgesaugt und die daraus entstandenen Eluate mithilfe eines Flammen-Atomabsorptionsspektrometer auf die verbliebenen Metallgehalte analysiert. Zuverlässige Werte konnten nur für Cu, Mn und Zn gemessen werden, die Werte für Mo und P lagen unter der Nachweis- oder Bestimmungsgrenze. Bei der Kupfermessung konnte die höchste Kupferkonzentration in der Cu-gedüngten Probe gemessen werden, doch es wurde schon weniger Cu ausgewaschen, wenn zusätzlich mit einem anderen Mikronährelement gedüngt wurde als in den Kontrolldüngungen. Für Mn wurde in der Kontrolllösung die höchste Mn-Konzentration gemessen, in allen anderen Proben mit zusätzlicher Düngung wurde Mn stärker zurückgehalten. Die gemessene Zn-Konzentration bei Düngung mit Cu, Mn, Mo und P lag fast bei null. Zink wurde sehr stark zurückgehalten. Auch in der Zink-Gedüngten Probe wurde Zn in den ersten drei Tagen stark zurückgehalten und wurde erst danach stärker ausgewaschen als in den Kontrollproben. Aufgrund der Resultate ist zu erkennen, dass auch Mikronährelemente eine limitierende Wirkung auf Mikroorganismen haben können und die Mikroorganismen im untersuchten Gebiet durch verschiedene Mikronährelemente limitiert sind.

Abstract:

The increasing anthropogenic influence on the environment on a global scale has led to a shift in nutrient cycles which are not fully understood yet. These shifts could alter the properties of ecosystems and therefore change habitats and species composition. Therefore it is crucial to understand nutrient cycles and how ecosystems react to these nutrient availability shifts. Especially the phosphorus and nitrogen cycles are highly important because of their direct influence on plant growth as primary macronutrients. Even in some so far undisturbed regions, a human caused rise in atmospheric input of phosphorus-containing particles is expected. One of the reasons therefor is land-use change in rural areas with associated combustions, from which the particles are transported downwind. This study simulates the prognosticated increased nutrient input and focuses on the following fate of the phosphorus in soil. For this, in 2008 a nutrient manipulation experiment was established to fertilize an old-growth tropical montane forest with moderate phosphorus (10 kg P ha-1 yr-1) and/or nitrogen additions (50 kg N ha-1 yr-1). This experimental setup was conducted for the first time at three altitudes (1000m a.s.l., 2000m a.s.l., 3000m a.s.l.) to compare the effects along an altitudinal gradient. After seven years the total phosphorus and bioavailable phosphorus concentrations of these locations were investigated in litterfall, the organic layer and in mineral soil to see if the added phosphorus is retained and enriched. The results showed that the application of the phosphorus fertilizer resulted in increased total and bioavailable phosphorus concentrations mainly in the organic layer. The phosphorus increase in mineral soil was not significant and indicated that the added phosphorus is not leached, but retained in the ecosystems. There were no effects on phosphorus concentrations after nitrogen addition. The altitudinal gradient referred to a pronounced difference between 1000m and the other altitudes (2000m and 3000m), due to the accumulated organic matter at the higher altitudes. Overall this study shows that the sustainable anthropogenic phosphorus increase may cause a change in the ecosystem’s characteristics and nutrient cycles.

Abstract:

Schwermetalle sind im Gegensatz zu Hauptnährelementen wie N, P oder S im Ökosystem gering angereichert, jedoch wirken diese schon bei kleinen Gehalten toxisch für Organismen wie Pflanzen und Tiere. Schwermetalle sind im Boden stabil und werden nicht abgebaut, nur die Verbindungen, die sie enthalten. Es kann auch zur Anreicherung von Schwermetallen in den Pflanzen, vor allem in Wurzeln und Blättern, kommen. Die Motivation der Bachelorarbeit besteht darin, sowohl die Flüsse der Schwermetalle in der Streu als auch deren Umsatzzeiten in der organischen Auflage des Waldbodens zu bestimmen. Es wurden Streuproben des Zeitraums Juli 2012 bis Februar 2013 aufgeschlossen und die Gehalte der Elemente As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V und Zn gemessen. Ausgewertet wurden jedoch die Messresultate der Proben von August 2011 bis Juli 2012. Die benötigten Daten zur Berechnung der Umsatzzeiten wurden von der Gruppe für Bodenkunde zur Verfügung gestellt. Die Schwermetallgehalte in der Streu dienten als Grundlage zur Berechnung der Monats- und Jahresflüsse sowie der Umsetzungsquotienten. Die Gehalte der Schwermetalle in der Streu zeigen kaum eine Variabilität über die Zeit. Die jährlich regelmässigen Schwankungen der Monatsflüsse sind auf die periodisch unterschiedlichen Streufallmengen zurückzuführen, die jeweils 2 Maxima und 1 Minimum aufweisen. Erstaunlich sind sowohl die hohen Kupfergehalte in der Streu als auch die hohen Umsetzungsquotienten für As, V und Fe. Die hohen Umsatzzeiten lassen sich durch Vermischung des Mineralbodenhorizonts mit der organischen Auflage erklären. Die Ursache der hohen Kupfereinträge konnte nicht abschliessend erläutert werden, jedoch wurden mögliche Lösungsansätze formuliert.

Abstract:

The sloping mire landscape of the investigation area, in the southern Andes of Ecuador, is dominated by stagnic soils with thick organic layers. The recursive partitioning algorithm Random Forest was used to predict the spatial water stagnation pattern and the thickness of the organic layer from terrain attributes. Terrain smoothing from 10 to 30m raster resolution was applied in order to obtain the best possible model. For the same purpose, several model tuning parameters were tested and a prepredictor selection with the R-package Boruta was applied. Model versions were evaluated and compared by 100 repetitions of the calculation of the residual mean square error of a five-fold cross-validation. Position specific density functions of the predicted soil parameters were then used to display prediction uncertainty. Prepredictor selection and tuning of the Random Forest algorithm in some cases resulted in an improved model performance.We therefore recommend testing prepredictor selection and tuning to make sure that the best possible model is chosen.This needs particular emphasis in complex tropical mountain soil-landscapes which provide a real challenge to any soil mapping approach but where Random Forest has proven to be successful due to the testing of model tuning and prepredictor selection.

Abstract:

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH4), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH4 consumption (6.2 Tg CH4 year−1), limited data are available on CH4 exchange from tropical montane forests. We present the results of an extensive study on CH4 exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH4 sinks, with decreasing annual uptake rates from 5.9 kg CH4?C ha−1 year−1 at 1,000 m to 0.6 kg CH4?C ha−1 year−1 at 3,000 m. Topography had no effect on soil atmospheric CH4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH4 uptake rates, mineral nitrogen content of the mineral soil and with CO2 emissions indicated that the largest CH4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH4 uptake was N limited instead of inhibited by NH4 +. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH4 budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.

Abstract:

The number of previous hydrological studies concerning water flow paths in tropical montane rainforest is small. However, due to the increasing pressure of deforestation and land use change comprehensive knowledge of these natural ecosystems is needed if sustainable land use strategies should keep negative effects of human impacts on water flow paths as low as possible. In this context, present work addresses the identification, characterisation, and modelling of water flow paths in soils of an undisturbed and landslide affected natural Andean forest ecosystem in the south of Ecuador whose deforestation rate is one of the highest in South America. In an investigation area situated in the Andes of South Ecuador, in gentler slopes and altitudes above 2100 m ASL mainly Stagnosols and Histosols with stagnic colour pattern and low to negligible rock fragment content prevail. With increasing altitude the abundance of these soils increase, while the presence of Cambisols and Regosols is most pronounced below 2100 m ASL and clearly correlated with the slope angle. Therefore, these soils were mainly encountered in steeper, particularly landslide affected sites often resembling a melange of fine soil and high contents of rock fragments. Aside the investigation of the influence of the rock fragment content on soil hydrological- and physical parameters such as the relationship between rock fragment content and saturated hydraulic conductivity of the soil, present study aims particularly to investigate flow paths of water in soils of landslide affected and unaffected hillslopes. Therefore, we employed conventional field- and laboratory methods, dye tracer experiments including an appropriate image processing technique, as well as statistical models. Results show that both rock fragment content and bulk density control significantly, but not largely the saturated hydraulic conductivity of the mineral soils. Dye tracer experiments and soil parameters document a deeper percolation in the landslide affected hillslopes than in the landslide unaffected hillslopes, where we found preferential flow in root channels with low soil matrix interaction as dominant flow mechanism. A surface near quasi impervious layer along the interface between topsoil and subsoil limits percolation of the water giving the prerequisites of a lateral shallow subsurface flow along the interface between topsoil and organic layer. This is in line with previous studies performed in the same investigation area which already proved indirectly the existence of this flow. However, in none of these studies the shallow subsurface flow was assigned to certain slope inclinations or altitudes. Due to a recently published digital soil map and the results we obtained from the landslide unaffected sites, we know that particularly in hillslopes of less than 30 ° above 2100 m ASL prerequisites are given for spatially extended shallow subsurface flow. However, even if these prerequisites are not evident for the landslide affected hillslopes, we cannot exclude the possibility of shallow subsurface flow occurrence here since soil cover of the steep terrain is relatively shallow while rainfall is high throughout the year. Therefore, and given that key parameters such as permeability of subsoil and bedrock, interception and evaporation remain unclear or were investigated exclusively such as the spatial variability of the saturated hydraulic conductivity, we conducted a series of virtual experiments in order to assess the potential occurrence of shallow subsurface flow in Cambisols below 2100 m ASL. In these experiments we also included the organic layer being highly abundant in the investigation area, whose hydraulic parameters were estimated by means of inverse numerical modelling. The virtual experiments were based on a two dimensional finite element model representing a steep forested hillslope transect of ~54 m length. Aside soil properties, evapotranspiration and interception, the model included the spatial variability of the saturated hydraulic conductivity, the pressure head and their spatial trends. The results of virtual experiment series show that a sound evidence of the key parameters aforementioned is obligate if process conceptualisation regarding shallow subsurface flow generation, but also landslide initiation, solute and matter transport is in the spotlight.