9 search hits
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Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem
(2004)
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Thomas Wrzesinsky
- Der nebelgebundene Eintrag von Wasser und Spurenstoffen kann in den Bergwäldern Mitteleuropas eine wichtige Rolle spielen. Die Quantifizierung dieses Eintrags stieß jedoch in der Vergangenheit auf messtechnische Grenzen. Nach der Entwicklung und Erprobung eines Systems aus einem Tropfenspektrometer zur schnellen Messung der Größenverteilung (40 Tropfengrößenklassen zwischen ø1,5 und 50 µm) im Nebel und einem Ultraschallanemometer zur Bestimmung des vertikalen Windes konnten an der Ökosystemmessstation „Waldstein“ von April 2001 bis März 2002 Messungen zur Nebelwasserdeposition durchgeführt werden. Zusätzlich wurden die Sichtweite und die chemische Zusammensetzung (pH, elektrische Leitfähigkeit, Na+, K+, NH4+, Mg2+, Ca2+, Cl–, NO3–, SO42– und PO43–) des Nebelwassers gemessen. Zur Sammlung von Nebelwasser wurde ein aktiver beheizbarer Nebelsammler entwickelt und parallel zu den Wasserflussmessungen eingesetzt. Die Proben wurden automatisch alle acht Stunden genommen. Die Sammelmengen betrugen im Median 249 ml und erlaubten die gewünschten chemischen Analysen. Im Untersuchungszeitraum waren 223 Nebeltage zu verzeichnen. Der Nebelanteil betrug 25,7 %. Für die Qualitätskontrolle der gemessenen Flüsse wurden die Daten auf Stationarität und Turbulenz überprüft und der Datensatz entsprechend angepasst. Die Messung der Nebeldeposition im Untersuchungszeitraum ergab einen Eintrag von 108 kg ha–1 a–1 Wasser für die turbulente Deposition und 17 kg ha–1 a–1 für den Eintrag über Sedimentation. Der turbulente Eintrag dominiert also mit ca. 86 % die Nebeldeposition. Die Summe aus beiden Eintragsarten entspricht einem Nebelniederschlag von 125 mm p. a. Eine klare Saisonalität der Nebelwasserflüsse ist erkennbar. Die höchsten Nebelniederschläge sind im Spätherbst und im Winter zu verzeichnen, monatlich bis zu 24 mm (Januar) wurden gemessen. Die geringste Nebeldeposition wurde im August mit ca. 1 mm gemessen. Die ermittelten Tropfenspektren zeigen bei der Anzahlverteilung Maxima bei 2, 6 und 9 µm sowie ein Maximum von 12 µm in der Massenverteilung. Für die Massengrößenverteilung sind Verteilungen mit Maxima bei 9, 12 und 15 µm häufig. Die gemessenen Flüssigwassergehalte lagen bei einem Median von 156 mg m–3 und erreichten Maxima von 2639 mg m–3 (5-min-Mittel). Den größten Anteil am Fluss hatte die Größenklasse von 14,5 bis 15,5 µm Tropfendurchmesser. Tropfen kleiner 7 µm wurden effektiv emittiert, die größeren deponiert. Die im Untersuchungszeitraum gefallene Menge an Regen und Schnee beträgt 1414 mm. Der Anteil des Nebels am atmosphärischen Eintrag von Wasser beträgt demzufolge etwa 8 %. Für insgesamt 253 Nebelereignisse wurden im Untersuchungszeitraum Proben gewonnen. Außerdem wurden zum Vergleich auf wöchentlicher Basis wet-only-Proben genommen. Die Konzentrationen in Nebel- und Regenwasser sind hoch variabel. Die Mediane liegen im Nebelwasser bei pH 4,14, 621 µeq l–1 für NH4+, 487 µeq l–1 für NO3– und 321 µeq l–1 für SO42–. Diese 3 Hauptionen machen ca. 87 % der Gesamtkonzentration aus. Die Konzentrationen im Nebelwasser sind deutlich gegenüber dem wet-only-Niederschlag erhöht. Die Anreicherungsfaktoren sind 18,1 (NH4+), 13,1 (NO3–) bzw. 11,5 (SO42–). Der nebelgebundene Eintrag der wichtigsten Ionen wurde aus der Konzentration und dem Nebelwasserfluss errechnet. Die eingetragenen Mengen sind 9,8 kg ha–1 für NH4+ (7,9 kg ha–1 für wet-only), 27,9 kg ha–1 für NO3– (25,1) bzw. 14,0 kg ha–1 für SO42– (15,0). Die durch feuchte oder okkulte Deposition eingetragene Menge ist für diese Ionen also im gleichen Größenbereich wie die Menge aus Regen und Schnee. Der Stickstoffeintrag beträgt insgesamt 13,9 kg N ha–1 a–1 (11,8 für wet-only). Der im Unterschungszeitraum durch den Bestandesniederschlag gemessene Eintrag von Stickstoff liegt bei 23,3 kg N ha–1 a–1. Die Differenz aus Bestandesniederschlag einerseits und wet-only und Nebel andererseits liegt mit –0,9 kg N ha–1 a–1 nahe Null. Zusätzliche Einträge sind durch die trockene Deposition (z. B. durch partikuläres Nitrat und Salpetersäure) zu erwarten. Der Umsatz von Stoffen im Kronenraum spielt dann eine wichtige Rolle in der Schließung der Ökosystembilanz für die verschiedenen Stoffe.
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Biomass and Nutrient Studies of Selected Tree Species of Natural and Plantation Forests: Implications for a Sustainable Management of the Munessa-Shashemene Forest, Ethiopia
(2004)
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Asferachew Abate
- Plantation forests with exotic tree species have been introduced to alleviate the problems of deforestation in Ethiopia. In the future, more plantation forests with fast growing species should be grown for coping with the ever-increasing demands for fuelwood and other forest products. However, it is not known whether plantation forests are sustainable or not. For the sustainability of plantation forests with exotic tree species, it is of paramount importance to thoroughly understand their ecological and social attributes through a holistic approach. For this reason, a multidisciplinary project was initiated in the Munessa-Shashemene Forest. Such an approach gives valuable information about the sustainability of plantation forests when the basic ecological features of the natural forests are compared with plantation forests. As an integral part of the multidisciplinary project, the objectives of this study are to: 1) quantify the fine roots and aboveground biomass of selected tree species in both natural and plantation forests; 2) quantify the macronutrient stocks of the fine roots and aboveground components of selected trees species in both natural and plantation forests; and 3) evaluate the implication of the changes in the biomass and macronutrient stocks for a sustainable management of forests. The study focused on four tree species, Podocarpus falcatus (Thunb.) Mirb., Podocarpaceae and Croton macrostachys Hochst. ex Del. Euphorbiaceae, were selected from a natural forest. Cupressus lusitanica Miller, Cupressaceae and Eucalyptus globulus Labill. Myrtaceae were selected from plantation forests. Root architectures of the study trees were studied by excavation. The live fine root biomass (<2 mm in diameter) of the dry and wet seasons was determined from samples collected at the distances of 1, 2 and 3 meters from the bole of the study trees. At each of the distances, root cores were taken at the depth intervals 0-10, 10-35, 35-60, 60-85 and 85-100 cm using a hand auger. Linear regression equations were used to estimate the aboveground biomass on the basis of the relation between DBH and dry weights of the aboveground plant components. Macronutrient concentrations were determined following a standard laboratory procedure. Studies on the root architecture revealed that C. lusitanica has a shallow root and is more susceptible to windthrow compared to E. globulus. With the exception of E. globulus, the dry season live fine root (LFR) biomass was higher for all trees studied. The seasonal variation in the fine root biomass was attributed to the changes in soil moisture of the study area. For all trees investigated, the mean annual LFR biomass was highest at the depth interval 0-10 cm at all distances. The favorable soil texture, pH and organic matter content at the depth interval 0-10 cm might be responsible for higher LFR biomass. The significantly higher LFR biomass of P. falcatus (1.34 kg m-2) coupled with its higher macronutrient stocks compared to C. macrostachys (0.32 kg m-2) suggest the importance of P. falcatus in the sustainability of the natural forest by transferring more macronutrients to the soil through its fine roots. Similarly, the significantly higher total LFR biomass of C. lusitanica (0.88 kg m-2) coupled with its higher macronutrient stock compared to E. globulus (0.27 kg m-2) indicated less depletion of soil nutrients by the former. The stand structure of the natural and plantation forests differed largely. In the natural forest, the density of C. macrostachys was much higher (143 ± 72 trees ha-1) than the density of P. falcatus (73 ± 39 trees ha-1). Generally, the structural change of the natural forest due to selective cutting of P. falcatus was found to have negative implications on the sustainability of the natural forest. The differences in the structure of C. lusitanica and E. globulus, despite their similar densities, resulted in a significantly lower understory ground cover by herbaceous and shrub species in the former. The effect of a poor understory growth on the floor litter thickness and thereby on nutrient capital of the soil may negatively affect the sustainability of C. lusitanica plantation. The harvesting of the stemwood of C. lusitanica and E. globulus removes a substantial amount of nutrients from the plantation sites. Furthermore, the current practice of collecting foliage, twigs and branches for firewood by the local people results in a higher depletion of nutrients. In order to make the plantation forests sustainable, the silvicultural practice in the future should consider on site conservation of foliage and bark. It is recommended that more studies on aboveground and belowground biomass, fine root turnover, and nutrient concentrations of the plantation forests should be carried out in a chronosequence in order to gain more insight on their sustainability.
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Effects of land-use changes on the properties of a Nitisol and hydrological and biogeochemical processes in different forest ecosystems at Munesa, south-eastern Ethiopia
(2004)
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Yeshanew Ashagrie
- The effects of conversion of natural forest into different exotic tree species plantations and crop cultivation were investigated at Munesa, south-eastern Ethiopia with the objectives of (i) determining changes on soil physical and chemical properties, (ii) quantifying water and nutrient fluxes under the different forest ecosystems, and (iii) assessing nutrient dynamics in water flowing through the soil under the different forest ecosystems. Soil samples were taken from the organic layer and at 0-20, 20-40, 40-70, 70-100 cm depths from the mineral soil. Rainfall and throughfall were collected using plastic funnels mounted 1 m above the ground. Soil solutions were collected with zero-tension (organic layer) and tension (mineral soil at the depth of 20, 50 and 100 cm) lysimeters. After 26 years of cultivation, surface (20 cm depth) soil structure was deteriorated and total soil organic carbon (SOC) and N contents both in bulk soil and water stable aggregates were significantly reduced. Below 21 years old Eucalyptus plantation no significant changes on the above mentioned parameters could be identified, but significant reductions in SOC, N and S concentrations associated with the sand and silt separates were evident. There were also significant reductions both in quality and quantity of particulate organic matter (POM) due to cultivation and only in quality of POM due to 21 years Eucalyptus plantation. The organic layer mass under 21 years old Pinus patula, 21 years old Eucalyptus globulus and third rotation Eucalyptus globulus (established 42 yr ago) decreased by 43%, 57% and 15%, respectively, relative to the natural forest. There were also significant reductions in the organic layer C and N stocks (9 to 60% and 25 to 68%, respectively), being highest under Pinus and lowest under third rotation Eucalyptus. In the mineral soil, to 1 m depth, there was a significant (P<0.05) reduction (16 to 20%) in SOC stock after conversion of natural forest into forest plantations. The N stocks under the 21 years old Pinus and third rotation Eucalyptus plantations were significantly reduced amounting 27 and 20% respectively, whereas 21 years old Eucalyptus had nearly an equivalent amount of N as that of the natural forest, probably due to a dense forest floor vegetation, fixing N. The changes in the organic layer and mineral soil S stocks after plantation establishment were not significant. Of the total annual rainfall (1190 mm) recorded during the monitoring period (October 2001 to September 2002), about 47% and 18% were intercepted by the canopies of Cupressus and the natural forest, and Eucalyptus, respectively. Total annual nutrients (Ca, Cl, K, Mg, Na, NH4–N, NO3–N, PO4–P, SO4–S ) deposition by rainfall was 12 kg ha–1yr–1. Throughfall K, Mg, Ca and Cl fluxes were enriched relative to rainfall, whereas Na, NO3–N, NH4–N, PO4–P and SO4–S were depleted. Total annual throughfall nutrient inputs (Ca, Cl, K, Mg, Na, NH4–N, NO3–N, PO4–P, SO4–S) were 14 kg ha–1yr–1 under Cupressus, 21 kg ha–1yr–1 under the natural forest and 24 kg ha–1yr–1 under Eucalyptus. Water passing through the different forest floors differed only in K, Mg and NO3–N concentrations, the latter two being higher under the natural forest and Eucalyptus plantation than Cupressus. Potassium was greater under Eucalyptus than the natural forest and Cupressus. Except for NH4–N in the natural forest, forest floor leachate nutrient concentrations were enriched in all forest types in relation to throughfall. Most nutrient fluxes to the mineral soil decreased in relation to throughfall fluxes, whereas NO3–N fluxes increased by over 50% in all forest types. At all soil depths, the concentrations of most nutrients in the mineral soil solution decreased relative to the concentrations in the forest floor leachate, but Mg, Na and NO3–N at all depths in Cupressus plantation and SO4–S and Na at some soil depths in the natural forest and Eucalyptus plantation had increased. The vertical trends in soil solution nutrient concentrations showed a decreasing trend with depth increments for most of the nutrients, but the concentrations of Cl and Na in all forest types and Ca, Mg and NO3–N in Cupressus increased with increasing soil depth. At 1 m soil depth, the concentrations of Ca, Mg and NO3–N in Cupressus, respectively, were 8, 7 and 23 times higher than in the natural forest and 3, 4 and 81 times higher than in Eucalyptus indicating losses by leaching. Generally, the results of this study emphasize the importance of forest type, species composition and management in affecting carbon and nutrient storage, water and nutrient fluxes and dynamics.
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Detection and Analysis of Coherent Structures within and above Tall-vegetated Canopies
(2005)
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Christoph Thomas
- Coherent structures are an inherent phenomenon of the atmospheric turbulent flow in the proximity to tall-vegetated canopies. Although coherent structures have called increasing attention of the turbulence community during the past decades, the basic mechanisms of their emergence, their contributions to exchange processes and their importance for conventional flux determination methods remain poorly understood or even unknown. In particular, no studies have been published yet dealing with long-term observations rather than exemplary case studies using short-term data. This dissertation aims at the enhanced understanding of the driving mechanisms and statistical properties of coherent structures within and above tall-vegetated canopies through an extensive analysis using long-term observations. Thereby, it places a specific emphasis on the implications of coherent structures for exchange processes and assesses their impact on conventional flux determination methods such as the eddy covariance and relaxed eddy accumulation techniques. Data were mainly obtained using tower-based single-point turbulence measurements and acoustic remote sensing technique (Sodar-Rass) during extensive field campaigns conducted in summer 2003. The developed wavelet software tool for detection and analysis of coherent structures is verified to extract coherent structures objectively under varying environmental conditions and thus allows determining their statistics in long-term datasets. In the proximity to the plant canopy, the temporal scales of coherent structures typically range between 20 s and 35 s. The temporal scales of coherent structures in the horizontal wind velocity, the sonic temperature and the concentration of carbon dioxide and water vapour exceed those of the vertical wind velocity. Within the canopy, the temporal scales of all vector and scalar variables collapse at approx. 24 s to 28 s resulting in an enhanced symmetry. Besides this, coherent structures with temporal scales up to 220 s are evidenced well above the canopy with the aid of acoustic remote sensing. The application of the canopy mixing-layer analogy to the data partially yields departures of the ratio m between the streamwise structures spacing of coherent structures and the canopy shear scale from the prediction m = 7...10. The departures are due to the influence of the terrain affecting the shape of the canopy wind profile and therefore the vertical wind shear. An agreement is found for flows which are forced to reorganise downstream of flow obstacles. The vertical wind shear is identified as the main driving force from which coherent structures emerge close to the canopy. In the layer well above the canopy diabatic processes facilitate the generation of coherent structures of large temporal scales. Clearcuts in a fairly homogeneous canopy cause additional structures in the turbulent flow with large temporal scales. Coherent structures contribute about 16 % to total the momentum transfer and about 26 % to the total fluxes of buoyancy, carbon dioxide and latent heat. A scheme for the qualitative classification of exchange regimes between the atmosphere and the canopy is developed analysing the ejection and sweep phases of coherent structures along the vertical profile in the canopy. The presence of coherent structures causes flux errors in the eddy covariance method below 4 %. The effect of this flux error for long-term observations is negligible as individual flux errors average out. Coherent structures of large temporal scales significantly influence the scalar similarity required for the relaxed eddy accumulation technique. These flow structures are responsible for the diurnal changes of the scalar similarity observed in the traces of acoustic temperature and concentration of carbon dioxide and water vapour.
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Soil-plant dynamics of water, nitrogen and sulfur: A study on indigenous and exotic tree species in Munessa Forest, Ethiopia
(2005)
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Florian Fritzsche
- Forest plantations are necessary to counteract the destruction of tropical montane forests. Sustainable forestry requires comprehensive knowledge of tree effects on site conditions and nutrient cycling, but substantial information is lacking even for widely-planted species. In my work, I aimed at identifying such plant effects on ecosystem dynamics, focussing on water, nitrogen (N) and sulfur (S), which included the development of a stable-isotope methodology for S. Based on a characterization of the soils of the study area at the Main Ethiopian Rift Valley escarpment, experimental plots were set up in neighbouring stands of a natural forest dominated by Podocarpus falcatus and in plantations of Cupressus lusitanica and Eucalyptus globulus. All investigations on the ecology of these trees were conducted on the same single-tree centred plots in a combination of time series of natural parameters with isotope tracer experiments, employing inorganic 15N tracers and litter labelled with 34S. Soils of the region reflected the influence of climate and relief, while the homogeneous bedrock caused no influence throughout the region. Methodological work to improve d34S analysis was a precondition for the ecological study on S dynamics. Technical adjustments to the analytical system including a liquid-nitrogen trap reduced the amount of S required for reliable d34S determination by a factor of six compared to the conventional procedure. Soil-plant water dynamics were strongly related to the root system. P. falcatus with high fine root biomass to below 1 m depth appeared active in redistributing soil water. Its physiological response to changing soil moisture with a marked reduction in transpiration (by a factor of six) at dry conditions had a further balancing effect. P. falcatus and C. lusitanica expanded their root systems substantially in the dry season, shifting to deeper layers. Seasonality was very weakly expressed for root biomass and depth of water uptake under E. globulus. It mainly relied on deep water resources tapped by its low-biomass root system, supporting physiological activity in the dry season, when transpiration was increased by a factor of five. Soil labelling with 15N showed similar patterns of root activity. It also revealed the dominance of C. lusitanica near the surface, with its roots effectively intercepting nutrients. However, this had negative impacts on deeper soil layers by reducing biological transformations and increasing leaching losses. In the natural forest, phosphate-extractable soil N and low natural-abundance d15N indicated an intense, conservative N cycling in the upper 60 cm, which was also evenly exploited by the roots of P. falcatus. Nitrogen uptake by E. globulus was concentrated in the deeper layers. A preferential stabilization of N was observed in the topsoil, while losses were indicated by high natural-abundance d15N values, which probably reflected recent processes. As for the trees, species-specific N uptake strategies were observed for the understorey. Litter for the S mineralization experiment was successfully labelled with 34S, opening a way to elucidate soil processes as well as plant uptake and recirculation. Different regimes of decomposition resulted in increasing extractability of S in the topsoil with depth under P. falcatus and C. lusitanica, whereas a decrease was noticed in the E. globulus stand. Seasonality of both bulk and extractable S were minimal. Isotope labelling showed rapid incorporation of litter into the topsoil of E. globulus, while S from litter of C. lusitanica was susceptible to leaching. Plant uptake by P. falcatus and E. globulus led to a steady increase of d34S values. In contrast, isotope enrichment in C. lusitanica leaves peaked after the first rainy season, thereby indicating recirculation of S. The different approaches of my work complemented one another, revealing a consistent pattern of plant traits. P. falcatus had a balancing influence on the ecosystem and appeared to promote soil life. C. lusitanica confined biological transformations to the topsoil and raised the risk of leaching losses. E. globulus was largely independent of superficial resources, giving space to understory growth. This strategy may lead to depletion of groundwater and structural deterioration of the soil.
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Carbon dynamics under natural and manipulated meteorological boundary conditions in a forest and a fen ecosystem
(2009)
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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.
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Role of Dissolved Organic Nitrogen in the Soil Nitrogen Cycle of Forest Ecosystems
(2010)
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Bettina Schmidt
- In the last years, dissolved organic N (DON) has been shown to be a crucial part of the soil N cycle in forest ecosystems. Despite this, information on its dynamics, sources and fate is still lacking. Especially data from (sub)tropical forest ecosystems are scarce. Therefore, this study investigated (i) the magnitude and drivers of DON fluxes in a subtropical montane forest, (ii) the biodegradability of DON from forest floors, (iii) the abiotic formation mechanism for DON in forest floors as postulated by the Ferrous Wheel Hypothesis and (iv) the link between DON and dissolved organic C (DOC) dynamics. In a field study (2005-2008), average DON fluxes in forest floor percolates and seepage (60 cm) of a subtropical montainous cypress forest (16 and 8 kg N ha-1 yr-1, respectively) were similar to fluxes in other (sub)tropical ecosystems, and dominated total N fluxes. Dissolved organic N concentrations in the soil were independent of the water flux (meaning that no dilution effect was visible). This implies that first, the pool size of potentially soluble DON is variable and second, that this pool is hard to deplete. In contrast, the linear relationship between soil organic solute and water fluxes was positive, showing that precipitation is an important driver for DON losses in this ecosystem. Although this has also been reported from temperate ecosystems, this relationship did not hold when analyzing the combined data from various (sub)tropical and temperate forest ecosystems. The biodegradability of DON was highest in inoculated spruce-Oi water extracts in a 21-day incubation experiment, while in extracts from beech-Oi and Oa horizons, DON concentrations only slightly decreased. Dissolved organic N was recalcitrant in spruce-Oa and cypress-Oa extracts, indicating that this DON could add to the formation of stable soil N pools. As various additions of NO3- never influenced DON biodegradation, it is concluded that microbes do not necessarily prefer mineral N over DON as substrate. Mineralization was always more important than microbial uptake in samples without NO3- additions, and denitrification only played a minor role in spruce-Oi samples (as indicated by a negative balance of all N species after 21 days). Fluorescence excitation-emission spectroscopy and subsequent parallel factor analysis identified four groups of fluorophores in the extracts. The initial concentration of two of these so-called factors was correlated with DON biodegradation, but protein-like fluorescence (which has been suggested as a proxy for dissolved organic matter biodegradation) was shown to be independent of DON biodegradation due to similar excitation-emission-maxima of recalcitrant compounds. Therefore, these factors might not always be suitable to predict DON biodegradation. The abiotic reaction of NO2- with DOC (as postulated by the last step of the Ferrous Wheel Hypothesis) was tested in a second incubation experiment in extracts with varying DOC concentrations and qualities and NO2- additions under oxic conditions. Concentrations of added NO2- never decreased within 60 min, indicating, that no DON formation from added NO2- took place. The results show, that the last step of the Ferrous Wheel Hypothesis (which has been suggested to be fast) is unlikely to occur in forest floors. Dissolved organic N and C fluxes were both highly dependent on precipitation at the cypress site, suggesting a strong link between these two classes of compounds. This assumption was supported by the first incubation experiment, where both DON and DOC biodegradation were not influenced by NO3- additions. Moreover, DOC dynamics closely resembled DON dynamics, which suggests that DON biodegradation could be driven by microbial C demand. Therefore, the often used separation of DON and DOC into functionally different compound classes is not always warranted. In conclusion, this study emphasized the need to include DON in biogeochemical N studies of both temperate and (sub)tropical ecosystems, and provided new and important insights regarding DON biodegradation, possible DON sources in forest floors and the link between DON and DOC dynamics in forest ecosystems.
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Turnover and fluxes of carbon and nitrogen in a spruce forest under natural and extreme meteorological conditions
(2010)
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Kerstin Schulze
- Climate models predict an increase in the intensity and frequency of extreme meteorological climate events like extended summer droughts, heavy rainfall or intensive frost periods with largely unknown effects on microbial activity and pysico-chemical soil properties and their impact on availability of soil organic matter. The influence of drying/rewetting (A/W) and freezing/thawing (G/A) events on solution chemistry and leaching losses of soils is barely known. This thesis aimed to study the effects of A/W and G/A events on soil solution chemistry and solute fluxes, in particular, of dissolved organic carbon (DOC) and inorganic nitrogen (NH4+, NO3-) in a podzol soil under a Norway spruce forest. A field experiment was designed to study the effects of (i) summer drought by exclusion of natural throughfall and subsequent rewetting and of (ii) soil frost by removal of natural snow cover. In complementary laboratory experiments with undisturbed soil columns, (i) drying/rewetting cycles were simulated with different rewetting intensities and (ii) freezing/thawing cycles were induced using different freezing temperatures. In the second part of this work, total C and N stocks as well as radiocarbon signatures of soil organic carbon (SOC) from different soil horizons and density fractions were investigated. A/W increased the DOC concentrations in the organic layer and upper mineral soil. More DOC was released from the organic layer to the mineral soil. However, the effects on total DOC leaching were smaller due to reduced water fluxes. Specific UV absorbance and emission fluorescence detected a switch in the release of easily decomposable DOC to hardly decomposable DOC during the wetting phase. Prolonged summer drought and incomplete rewetting due to hydrophobicity of SOM in the organic layer and upper mineral horizon reduced net N mineralisation as well as concentrations and fluxes of the NH4+ and NO3-. The net nitrification rate in the organic layer was more negatively influenced than net ammonification, indicating that nitrifiers are more sensitive to drought stress than ammonifiers. The effect of soil frost strongly depended on soil freezing temperature. Only soil frost at temperature below -8°C led to short periods of additional DOC production in the organic layer. Spectroscopic properties and ∆14C signatures of DOC implied a disruption of soil aggregates and desorption of older DOC from the mineral associated organic matter fraction of the Oa horizons by G/A events. Severe soil frost below -8°C inhibited the activity of nitrifiers and ammonifiers with decreased NH4+ and NO3- concentrations and fluxes in the mesocosm experiment. A delayed (by 4 months) increase in NO3- concentration in the upper soil horizon by moderate soil frost (-5°C) was attributed to reduced Immobilisation by heterotrophic microorganisms. Summarised, drying and the effect of hydrophobicity led to long-term, severe soil frost to short-term reduction in N mineralisation and N leaching. The effect of increased NO3- concentrations as delayed response to G/A needs further research in case of potentially changes in the N balance. Drying as well as freezing induced changes in the soil structure and properties and led to increased DOC concentrations. Moderate soil temperature had much less effects on C and N in this temperate forest soil. The results of this thesis demonstrated the potential of extreme meteorological events on the quality and availability of dissolved C and N. Both, A/W and G/A cycles decreased C and N mineralisation, increased the sink strength of the soil by the accumulation of SOC and N, considering constant C and N litter input. However, optimal temperature and moisture conditions in other seasons could compensate the sink strength of soils. This work underpins the need for holistic and long-term investigations to understand and model the impact of extreme meteorological conditions on the dynamics of dissolved C and N.
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Heterogeneous chemistry of HONO and surface exchange
(2013)
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Matthias Sörgel
- Nitrous acid (HONO) is an important precursor of OH radicals, which are the key oxidizing species in the atmosphere and are therefore called the detergent of the atmosphere. Despite the importance of HONO for atmospheric chemistry and about 30 years of detailed research the exact formation mechanisms of both day- and night-time formation remain unclear. The main formation pathways discussed to date are heterogeneous reactions with nitrogen dioxide as the HONO precursor or microbiological activity in soil. As the ground surface is a major source of HONO, the vertical distribution of HONO is very sensitive to the extent of vertical mixing. Additionally, some uncertainty in comparing laboratory and field measurements might be caused by the not yet clarified role of relative humidity and surface wetness on HONO formation and deposition, respectively. These influences have been investigated in field measurements in the present study. For HONO measurements, LOng Path Absorption Photometers (LOPAPs) have been deployed.
During the Diel Oxidant Mechanism In relation to Nitrogen Oxides (DOMINO) campaign, HONO and other reactive trace gases were measured above a pine forest in south west Spain. In line with all recently published work, this study also found a substantial daytime formation of HONO. This so-called additional daytime source or unknown source was found to be slightly correlated (r² = 0.16) with actinic flux. Normalizing this unknown source to nitrogen dioxide mixing ratios improved the correlation (r² = 0.38), which indicates an influence of nitrogen dioxide availability. The coefficient of determination improved further to 0.47 by restricting the data to clear days and rejecting data from advection events. Thus, a fraction of the unknown source might be explained by light-induced nitrogen dioxide conversion but other factors have to be taken into account. Two processes of light-induced nitrogen dioxide conversion, proposed by recent laboratory studies, were shown to be negligible for the semirural conditions during our study. HONO photolysis was found to be the most important primary OH-radical source during DOMINO, contributing 20 % more OH than ozone photolysis integrated over the day.
Vertical exchange of HONO was studied at the “Waldstein-Weidenbrunnen” field site of the University of Bayreuth in the Fichtelgebirge Mountains in south east Germany. The simultaneous HONO measurements in and above a forest canopy highlighted the importance of turbulent exchange for the vertical distribution of HONO mixing ratios. The so-called coupling regimes of the forest (with the air layers above) were found to be a very useful micrometeorological concept to study vertical differences of mixing ratios in a forest. They denote which parts of the forest are coupled to the air layer above the canopy and thus take part in turbulent exchange of energy and matter. With this coupling tool it was possible to explain vertical mixing ratio differences by different sources and sinks and the magnitude of the difference by the intensity of vertical exchange. Studying the vertical mixing ratio differences of HONO, an unexpected result was that during late morning and around noon they were close to zero. As the lifetime of HONO below canopy of about 250 to 300 min was a factor of 25 to 30 longer than that above canopy of about 10 min, large mixing ratio differences would have been expected. The lack of these differences could be explained by efficient vertical mixing, which was indicated by a full coupling of the forest or a coupling by sweeps and only intermittent decoupling of the subcanopy during these periods. Around sunset, the whole forest became decoupled from the air layers above. This caused a steep increase in mixing ratio differences up to about 170 ppt due to a faster increase below canopy, indicating local formation below the canopy.
HONO and RH are correlated due to their diurnal cycles which are mainly caused by radiation. This diurnal contribution has to be removed from the respective signals in order to extract correlations on other timescales. Singular System Analysis, a tool for time series analysis, has been applied successfully to remove diurnal variations and long-term trends from the HONO and RH time series. Correlations of the higher frequency contributions of the remaining signals were poor but slightly positive. The HONO mixing ratios increase exponentially with RH from about 25 % RH to about 70 % RH. No clear correlation was found between around 70 and 95 % RH. Above 95 % RH, HONO mixing ratios decreased due to HONO uptake in droplets and liquid films. These features are in line with previously proposed mechanisms for interactions of water and HONO on surfaces.
The study highlighted the need to assess turbulent transport and surface properties in addition to chemistry for understanding the heterogeneous reactions and processes forming HONO.
