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It has been demonstrated that nitrogen fixation is a source of hydrogen (H2) to the ocean and therefore measurements of H2 concentrations may be used as a possible indicator of nitrogen fixation (Moore, Punshon, Mahaffey, & Karl, 2009). However, the limited number and sparse distribution of measurements of dissolved hydrogen and nitrogen fixation rates made in the open ocean in the past have made it difficult to quantify the relationship between them. Toward this end, a new method of equilibrating seawater samples for H2 measurement was employed along the 13,000 km Atlantic Meridional Transect (AMT20) from UK to Chile, allowing H¬2 to be measured from underway samples every 3.5 minutes and thereby considerably increasing the number and resolution of H2 measurements made in the open ocean.These high-resolution measurements reveal two regions with high H¬2 concentrations, one in the North Atlantic and one in the South Atlantic.

Student Number : 0306319G - MSc Research Report - School of Geography, Archaeology and Environmental Studies - Faculty of Science / Combustion of fuel wood, charcoal and non-woody biofuels is a daily practice for about half of the world’s population. Combustion of biofuel is a major source of trace gases, with domestic biomass burning contributing about 17% carbon dioxide (CO2), 13% carbon monoxide (CO) and 6% nitrous oxide (N2O) to the global budget. In Africa, where there is a growing population, domestic biofuel emissions are a particularly important source of trace gases. The most important source of biomass fuels in Africa is wood fuel (wood and charcoal), crop residues and animal dung. In this project, the amount of domestic biofuel used in rural households across southern Africa was measured daily over a nine month period. This data was used to estimate the trace gas production from domestic fires throughout Southern Africa. Results indicate that RSA contributed the most trace gases to the regional budget (9.12 Tg C yr-1 of CO2, 0.89 Tg C yr-1 of CO, 10.77 Gg N yr-1 of NO and 30.25 Gg C yr-1 of CH4) and Botswana the least (0.25 Tg yr-1 of CO2, 0.02 Tg yr-1 of CO, 0.29 Gg yr-1 of NO and 0.83 Gg yr-1 of CH4). Fuel wood is the dominant fuel type used in all southern African countries, whereas charcoal was used in RSA, Mozambique and Zambia and maize residues in Malawi only. CO2 was the gas emitted in the largest quantities, with fuel wood and charcoal in RSA contributing the most (8.71 Tg yr-1 and 0.41 Tg yr-1 respectively), while CO2 from maize residue was highest in Malawi (0.82 Tg yr-1). More trace gases were emitted in the dry season than wet season particularly in Malawi and Mozambique. For the entire region the annual CO2, CO, NO and CH4 emissions produced 23.0 Tg C, 2.2 Tg C, 29.4 Gg N and 81.4 Gg C, respectively.

In this thesis, the role of mid-latitude cyclones in air pollution transport in the Northern Hemisphere is quantified. The storm tracking model, TRACK, is used to study the mechanisms through which pollution, specifically ozone (O3) and carbon monoxide (CO), are vented from the boundary layer to the free troposphere and thus transported over large distances, as well as the introduction of O3 from the stratosphere into the troposphere. The relationship between mid-latitude cyclones and air pollution transport of O3 and CO is explored for the first time using the Monitoring Atmospheric Composition and Climate (MACC) reanalysis, a combined meteorology and composition reanalysis dataset. A comparison between springtime surface ozone measurements at rural background sites on the west coast of Europe and cyclone track frequency in the surrounding regions was used to first establish the correlation between cyclone location and surface air quality. The focus is on spring as it tends to be the season of maximum intercontinental transport of O3. The surface observations were compared to the MACC O3 values at the same locations and case studies of how cyclones can influence surface O3 measurements are described. When cyclones track north of 53°N, there is a significant probability that the surface O3 will be high (> the 75th percentile), due to the close proximity to stratospheric intrusions and the transport at low levels across the North Atlantic Ocean. The most intense spring cyclones (95th percentile) were selected for two regions, the North Atlantic and the North Pacific, for further investigation into the mechanisms which impact O3 and CO concentrations near cyclones. These intense cyclones ( 60 over each region) often tracked over the major emission sources of eastern North America and East Asia. The distributions of MACC O3 and CO within a "typical" intense cyclone are examined by compositing the cyclones together. The cyclone-centered composites were compared to background composites of "average conditions" created by sampling the reanalysis data of the previous year to the cyclone locations. Mid-latitude cyclones are found to redistribute concentrations of O3 and CO horizontally and vertically throughout the cyclone. This is clearly shown to occur through two main mechanisms: (1) vertical lifting of CO-rich and O3-poor air isentropically from near the surface to the mid- to upper-troposphere in the region of the warm conveyor belt; and (2) descent of O3-rich and CO-poor air isentropically in the vicinity of the dry intrusion, from the stratosphere toward the mid-troposphere. This work was expanded to identify the links between teleconnection patterns, mainly the North Atlantic Oscillation (NAO), that affect the major storm track pathways in the North Atlantic sector and the distribution of MACC O3 and CO throughout the troposphere and lower stratosphere. For this analysis, TRACK was used to calculate seasonal weighted-average O3 and CO distribution maps based on the monthly NAO index. During positive NAO phase, the persistence of low pressures over the North Atlantic coupled with the Azores High promotes transport across the North Atlantic throughout the troposphere. During negative NAO phase, blocking high pressure in the eastern North Atlantic are known to occur, which shifts transport pathways to a more southerly zonal flow. This work demonstrates the complex relationship between the horizontal and vertical distribution of pollution, including surface concentrations, and synoptic-scale systems.

Metal oxides in the atmosphere emitted from various natural and anthropogenic processes alter the chemical balance of the Earth's atmosphere due to heterogeneous and photochemical processes with atmospheric trace gases. Therefore, understanding the heterogeneous chemistry and heterogeneous photochemistry of atmospheric trace gases on these oxide surfaces has become vital to precisely predict the effect of mineral dust loading on the Earth's atmosphere. Among the various components of mineral dust, light absorbing oxides play a significantly important role during the daytime. The work reported herein has focused mainly on TiO2 and Α-Fe2O3. These are light adsorbing components found in atmospheric mineral dust. Apart from being a component of mineral dust, TiO2 is heavily used in a number of industrial applications ranging from uses in self-cleaning, water purification to cosmetics. These applications have led to their presence in the atmosphere as anthropogenic dust particles and in contact with the atmosphere as a stationary phase. Iron-containing particles are transferred to the atmosphere mainly from wind and volcanic activities in the form of iron-containing mineral dust and volcanic ash aerosols. Α-Fe2O3 is the most stable iron containing compound found in the Earths' crust which constitutes in significant amounts in mineral dust. The presence of these oxide surfaces in the atmosphere can play a major role in heterogeneous chemistry and photochemistry. In this dissertation research, transmission FTIR spectroscopy and X-ray photoelectron spectroscopy are used to probe the details of heterogeneous chemistry and photochemistry of CO2, SO2, NO2, HCOOH, and HNO3 on titanium dioxide and hematite surfaces. Adsorption sites, surface speciation and surface species stability have been determined from analysis of FTIR and XPS spectra. Isotope labeling experiments were also carried out in order to obtain mechanistic information about the details of surface hydroxyl group reactivity on these oxide particle surfaces. Furthermore, heterogeneous photochemical reactions of adsorbates from atmospheric trace gas adsorption on TiO2 and Α-Fe2O3 were investigated under the conditions pertinent to troposphere. The role of adsorbed water on the stability of adsorbed species that form as a result of heterogeneous reactions and the effect of relative humidity on photochemistry on these oxide particles surfaces has also been investigated due to its important implications in the atmospheric chemistry of oxide surfaces. The research adds to our overall scientific understanding of the molecular level details of heterogeneous chemistry and photochemistry of light absorbing components in the atmosphere.

A high-resolution infrared Fourier Transform Spectrometer (FTS) has been operational at the Toronto Atmospheric Observatory (TAO)since May 2002. An optimal estimation retrieval technique is used to analyse the observed spectra and provide regular total and partial column measurements of trace gases in the troposphere and stratosphere as part of the Network for the Detection of Atmospheric Composition Change. The quality of these results were assessed through two ground-based validation campaigns, comparisons with three satellite instruments, and comparison with a three-dimensional chemical transport model. The two ground-based campaigns involved two lower-resolution FTS instruments: the University of Toronto FTS and the Portable Atmospheric Research Interferometric Spectrometer for the Infrared. The first campaign took place over the course of four months and is the longest side-by-side intercomparison of ground-based FTS instruments, to date. The second campaign was more focused and involved all three instruments measuring over a two-week period. Simultaneous measurements of O3, HCl, N2O, and CH4 were recorded and average total column differences were all < 3.7% in the extended campaign, and < 4.5% in the focused campaign. Satellite-based comparisons were done with the SCanning and Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and the Optical Spectrograph and InfraRed Imager System (OSIRIS). Total column CO, CH4, and N2O compared with SCIAMACHY all had average differences < 10% with results from the TAO-FTS being as good as, or better, than that of other instruments. Validation with the ACE-FTS showed that average partial columns of O3, NO2, N2O, CH4, and HCl were within 10% while observations of CO and NO each had an average bias of about 25%. Comparisons of monthly average partial column O3 and NO2 with OSIRIS were highly correlated (R = 0.82-0.97) with monthly mean differences of < 3.1% for O3 and < 2.6% for NO2. Finally, comparisons with the GEOS-Chem chemical transport model revealed that the model consistently over-estimates tropospheric columns of CO and C2H6 observed at TAO. It was determined that the enhanced CO values were partially due to the North American emissions specified in the model, but more work must be done in the future if the source of this discrepancy is to be fully explained.

A high-resolution infrared Fourier Transform Spectrometer (FTS) has been operational at the Toronto Atmospheric Observatory (TAO)since May 2002. An optimal estimation retrieval technique is used to analyse the observed spectra and provide regular total and partial column measurements of trace gases in the troposphere and stratosphere as part of the Network for the Detection of Atmospheric Composition Change. The quality of these results were assessed through two ground-based validation campaigns, comparisons with three satellite instruments, and comparison with a three-dimensional chemical transport model. The two ground-based campaigns involved two lower-resolution FTS instruments: the University of Toronto FTS and the Portable Atmospheric Research Interferometric Spectrometer for the Infrared. The first campaign took place over the course of four months and is the longest side-by-side intercomparison of ground-based FTS instruments, to date. The second campaign was more focused and involved all three instruments measuring over a two-week period. Simultaneous measurements of O3, HCl, N2O, and CH4 were recorded and average total column differences were all < 3.7% in the extended campaign, and < 4.5% in the focused campaign. Satellite-based comparisons were done with the SCanning and Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and the Optical Spectrograph and InfraRed Imager System (OSIRIS). Total column CO, CH4, and N2O compared with SCIAMACHY all had average differences < 10% with results from the TAO-FTS being as good as, or better, than that of other instruments. Validation with the ACE-FTS showed that average partial columns of O3, NO2, N2O, CH4, and HCl were within 10% while observations of CO and NO each had an average bias of about 25%. Comparisons of monthly average partial column O3 and NO2 with OSIRIS were highly correlated (R = 0.82-0.97) with monthly mean differences of < 3.1% for O3 and < 2.6% for NO2. Finally, comparisons with the GEOS-Chem chemical transport model revealed that the model consistently over-estimates tropospheric columns of CO and C2H6 observed at TAO. It was determined that the enhanced CO values were partially due to the North American emissions specified in the model, but more work must be done in the future if the source of this discrepancy is to be fully explained.

Potentiometrisch betriebene Festelektrolytsensoren auf der Basis von Yttriumoxid-stabilisiertem Zirconium(IV)-oxid als festem Oxidionenleiter weisen einen für elektrochemische Sensoren ungewöhnlich breiten Messbereich von über 30 Zehnerpotenzen sowie eine vergleichsweise hohe chemische, thermische und mechanische Stabilität auf. Dadurch konnten sich diese Sensoren in einem sehr weiten Applikationsbereich etablieren, der hauptsächlich die Abgaskontrolle von Verbrennungsprozessen betrifft. In der vorliegenden Arbeit wird der Frage nachgegangen, ob mit Festelektrolytsensoren (FES) bei coulometrischer oder potentiodynamischer Betriebsweise, die gegenüber dem potentiometrischen Prinzip Vorteile im Hinblick auf Sensitivität beziehungsweise Selektivität bieten, weitere Applikationsfelder erschlossen werden können. Dazu durchgeführte Untersuchungen, Weiterentwicklungen und Optimierungen an coulometrisch betriebenen Festelektrolytsensoren sowie deren Einbindung in ein chromatographisches Messsystem zielten auf die Applikation zur Bestimmung von Spurenbestandteilen in Gasen und Flüssigkeiten ab. Mit den Ergebnissen wird beispielhaft ein neues Anwendungsfeld für FES bei der kontinuierlichen Überwachung von Biogasprozessen eröffnet. Zur Erreichung der Ziele wird zunächst gezeigt, wie der Messbereich potentiostatisch betriebener coulometrischer FES hin zu Spurenkonzentrationen im Vol.-ppb-Bereich erweitert werden kann. Hierfür werden Fehlereinflüsse untersucht, die die Nachweisgrenze dieser Sensoren beeinflussen. Durch die Entwicklung rauscharmer elektronischer Sensoransteuerungen, durch die Optimierung von Betriebsparametern sowie durch die Bestimmung der elektronischen Leitfähigkeit wird die Nachweisgrenze von FES verglichen mit dem bisherigen Forschungsstand um vier Zehnerpotenzen verringert. Als Ergebnis dieser Weiterentwicklungen liegen die Nachweisgrenzen für den FES im Durchflussbetrieb nun bei unter 5 Vol.-ppb für die Analyte H2, O2 und CH4. Zur Steigerung der Selektivität von FES werden zwei Möglichkeiten aufgezeigt. Einerseits werden bei cyclovoltammetrischer Betriebsweise für H2-, O2- oder CO-haltige Messgase im Konzentrationsbereich unterhalb von 10^2 Vol.-ppm lineare Zusammenhänge zwischen den Konzentrationen und den Peakeigenschaften Höhe und Fläche gefunden. Auf diese Weise konnte H2 an einer katalytisch hochaktiven Pt-Elektrode in Anwesenheit eines Überschusses an Sauerstoff mit hoher Selektivität erfasst werden. Andererseits wird die Selektivität potentiostatisch betriebener coulometrischer FES drastisch gesteigert, indem diese einer gaschromatographischen Trenneinheit nachgeschaltet werden. Im Konzentrationsbereich von 10^−1 bis 10^4 Vol.-ppm zeigte sich für H2 und CH4 ein lineares Ansprechverhalten, die Nachweisgrenzen des chromatographischen Messsystems lagen für diese Gase bei 55 bzw. 40 Vol.-ppb. Mit einem neuartigen In-situ-Messsystem, das auf dem Prinzip der kontinuierlichen membranfreien Gasextraktion und anschließender intervallmäßiger chromatographischer Trennung und Detektion mit einem potentiostatisch betriebenen coulometrischen FES basiert, wurden im Gärmedium von Biogasanlagen Spuren von gelöstem H2 und O2 sowie das vielfach höher konzentrierte CH4 parallel erfasst. Es wird gezeigt, dass Instabilitäten im Biogas-Entstehungsprozess, die durch Überfütterung des Fermenters hervorgerufen werden, anhand des Verlaufs des gelösten H2 deutlich früher erkannt werden, als es durch die H2-Bestimmung im Biogas durch kommerziell verfügbare Gassensoren möglich ist. Auf diese Weise ließ sich mit dem FES ein praxistaugliches langzeitstabiles, robustes und wartungsarmes Messsystem für diese Kenngrößen entwickeln. Bei der coulometrischen Bestimmung von Essigsäure mit dem FES kommt es zur Blockierung der Platinelektroden. Infrarotspektroskopische Untersuchungen des Abgases aus dem FES belegen die thermische Zersetzung dieses Analyts bei 750 °C, die mit der Bildung eines Kohlenstofffilms auf der messgasseitigen Pt-Elektrode einhergeht. Diese Blockierung führt zur Peakverbreiterung und verhindert so die Detektion der Carbonsäuren mit zwei bis fünf Kohlenstoffatomen im Molekül. In dieser Arbeit wird gezeigt, dass dieser ungünstige thermische Zerfall durch die Einbringung einer beheizbaren Katalyseeinheit in die Gasleitung zwischen Gaschromatograph und FES verhindert werden kann. Die Säuren zerfallen dann an der Pt-Oberfläche des Katalysators bei 800 °C, so dass nur die gasförmigen Zerfallsprodukte in den FES gelangen, wo sie ohne die Bildung von Pyrolyseprodukten an den Elektroden coulometrisch umgesetzt werden. Mittels Austausch des FES durch einen Flammenionisationsdetektor konnte mit dem In-situ-Messsystem gelöste Essigsäure über einen Zeitraum von achtzehn Tagen im Gärmedium einer Biogasanlage mit hinreichender Langzeitstabilität erfasst werden. Damit werden in der vorliegenden Arbeit wesentliche Beiträge zur Weiterentwicklung von coulometrischen Festelektrolytsensoren im Hinblick auf die Erniedrigung der Nachweisgrenze, die Erhöhung der Selektivität und die Verbreiterung des Anwendungsspektrums geleistet. / Potentiometric solid electrolyte sensors made of the solid oxygen ion conductor 'yttria stabilized zirconia' exhibit a very broad measuring range of more than 30 orders of magnitude as well as comparatively high chemical, thermal and mechanical stability. Therefore, these sensors were established in a large application range which covers mainly the field of exhaust gas control of combustion processes. This work tries to answer the question if it is possible to address new fields of application with coulometrically or potentiodynamically operated solid electrolyte sensors (ses) because of their generally higher sensitivity and selectivity compared to potentiometrically operated ses. Investigations, advancements and optimizations executed for this aim on coulometrically operated ses as well as the integration of these sensors into a chromatographic measuring system were directed on the detection of traces of analytes in gas mixtures and liquids. The results of this work unlock a new field of application for ses in the continuous monitoring of biogas processes. For the achievement of these goals it is firstly demonstrated how the measuring range of potentiostatically operated coulometric ses can be expanded in the direction of trace concentrations in the range of some vol.-ppb. Therefore, error sources influencing the detection limit are investigated. Compared to the current state of research, this limit is decreased by four orders of magnitute by developing low-noise sensor controllers, by optimizing operation conditions and by determining the electronic conductivity of the solid electrolyte material. As a result, the detection limits of the sensor operating in continuous flow-through mode range now below 5 vol.-ppb for the analytes H2, O2 and CH4. Furthermore, two approaches for the increasement of the selectivity of ses are presented. One of them concerns an optimized cyclovoltammetric operation of these sensors, resulting in a linear increase of peak height and peak area with increasing concentrations up to 10^2 vol.-ppm for H2, O2 or CO in nitrogen based gas mixtures. Thus, hydrogen could be detected on a Pt electrode with high catalytic activity in presence of an excess of oxygen in the measuring gas. The second approach is directed on the significant improvement of selectivity by operating coulometric ses in potentiostatic mode downstream of a gas chromatographic separation unit. For H2 and CH4 this chromatographic measuring system exhibited linear operation in the concentration range from 10^-1 - 10^4 vol.-ppm and offered detection limits of 55 and 40 vol.-ppb respectively. A novel in-situ measuring system is based on continuous membrane-free extraction, followed by periodic chromatographic separation and subsequent coulometric detection by a potentiostatically operated coulometric ses. With this measuring system, traces of H2 and O2 as well as the much larger amount of generated CH4 were determined simultaneously in the digestion medium of biogas plants. It is shown that instabilities in the microbial biogas process which are caused by fermenter overfeeding can be realized on the basis of the course of dissolved hydrogen. The novel measuring system indicates these instabilities much earlier than commercially available hydrogen sensors positioned in the biogas stream. Thus, a practicable longterm-stable, robust and low-maintenance measuring system could be developed for these parameters with the use of ses. The ses equipped with platinum electrodes shows electrode blocking during the coulometric measurement of acetic acid. Infrared spectrometric investigations of the ses exhaust gas clearly indicate thermal decomposition of this analyte at 750 °C, which is accompanied with carbon film formation on the Pt electrode surface. This blockage leads to peak broadening and therefore prevents appropriate detection of carboxylic acids containing between 2 and 5 carbon atoms. It could be demonstated in this work that this detrimental thermal decomposition on the ses electrodes could be circumvented by integrating a heated Pt catalyst between separation column and ses detector. The acids decompose then at the Pt surfaces of the catalyst at 800 °C and the decomposition products are detected by ses immediatly without formation of pyrolysis products on the electrodes. By replacing the ses in the measuring system with a flame ionization detector, acetic acid could be measured with appropriate long-term stability in the digestion medium of a biogas plant over a period of eighteen days. In summary this work presents substancial contributions to the advancement of coulometric solid electrolyte sensors by lowering their detection limits, increasing their selectivity and thus broadening their application spectrum significantly.

Los suelos de los bosques tropicales desempeñan un papel importante en el clima de la Tierra mediante el intercambio con la atmosfera de grandes cantidades de gases de efecto invernadero (GEI). Sin embargo, esta importante función podría ser alterada por las actividades humanas causando el aumento en la deposición de nutrientes en los ecosistemas terrestres, especialmente en las regiones tropicales. Las causas de cómo el incremento de las cantidades de nutrientes está afectando los flujos de suelo de los GEI de los bosques tropicales es relativamente poco conocida, por ello los monitoreos de nutrientes in situ de los bosques montanos tropicales (BHT) son aún menos comprendidos. Ya que los BHT representan alrededor del 11-21% de la superficie forestal tropical, es de vital importancia predecir y cuantificar los cambios en los flujos de GEI del suelo en respuesta a la adición de nutrientes ya que podrían favorecer la retroalimentación a otros cambios globales. Esta tesis tiene como objetivo cuantificar el impacto de adición moderada de nitrógeno (N) y/o fósforo (P) en los flujos de tres GEI en suelo: dióxido de carbono (CO2), óxido nitroso (N2O) y el metano (CH4), a lo largo de un gradiente altitudinal (1000 m, 2000 m, 3000 m) de los BHT primarios en el sur de Ecuador. Desde hace más de cinco años, se ha medido los flujos de GEI del suelo en un experimento de manipulación de nutrientes (‘NUMEX’, por sus siglas en inglés), con replicas para control, y la adición de N (50 kg N ha-1 año-1), P (10 kg P ha-1 año-1) y N+P. Las mediciones in situ se realizaron mensualmente utilizando cámaras ventiladas estáticas, seguido por un análisis de cromatografía de gases para conseguir una perspectiva más profunda sobre los procesos implicados en el intercambio suelo-atmósfera de GEI. Se realizaron nuevas investigaciones incluyendo el monitoreo de factores básicos de control (i.e. temperatura del suelo, humedad y las concentraciones del N mineral), los diferentes componentes de los flujos de CO2 del suelo, tasas de reciclaje netos de N y fuentes de los flujos de N2O del suelo. Con este propósito, se utilizó la extracción de hojarasca y técnicas de excavación de zanjas (trenching technique), incubación de las muestras in situ (buried bag method) y el etiquetaje de 15N de corto plazo. Los flujos de GEI del suelo en los bosques que estudiados se mostraron en el rango de aceptado de los flujos de gases de otras BHT en elevaciones comparables, excepto para el N2O. Los flujos de N2O, que se derivan principalmente de la des nitrificación, fueron bajos para un TMF lo que se puede atribuir a los ciclos conservativos de N del suelo en nuestros sitios de estudios. Los suelos fueron fuentes de CO2 y N2O (la intensidad del recurso disminuye al aumentar la altitud) y en todas las elevaciones el CH4 es bajo. Encontramos efectos de los nutrientes en todos los flujos de GEI medidos en cada elevación. Las respuestas de los flujos de CO2 del suelo cambian con la duración y el tipo de nutrientes adicionado. En 1000 m, la adición del N no afecta los flujos de CO2 del suelo, mientras que las adiciones de P y N+P disminuyeron los flujos en el primer y cuarto a quinto año. En 2000 m., la adición de N y N+P incrementa los flujos de CO2 en el primer año; a partir de entonces, la adición del N disminuye los flujos mientras que la adición de N + P no mostro ningún efecto la adición de P carece de efectos. En 3000 m, la adición de N además incrementó los flujos de CO2 constantemente; la adición de P y N+P aumentaron los flujos sólo en el primer año a partir de entonces no existió ningún efecto. Los efectos diferenciales de los nutrientes estuvieron relacionados a un estatus del N y P y respuestas variadas de los componentes de la respiración del suelo. Las respuestas de los flujos de N2O y CH4 a la adición de nutrientes mostraron gran variabilidad entre años. Los flujos de N2O no se vieron afectados por la adición de tres a cinco años de N a pesar de las diferencias significativas observadas durante los dos primeros años del mismo experimento. Atribuimos la ausencia de las respuestas en años mas tardíos debido a los contenidos bajos de humedad del suelo en nuestro periodo de monitoreo 2010-2012. En todo el gradiente altitudinal, la adición de P disminuyó los flujos de N2O y las concentraciones de N mineral, presumiblemente debido a que alivió de la limitación del P en la producción primaria neta, lo que aumentó la captación de N a través de las plantas. La adición de N+P además mostró tendencias similares las respuestas a la adición de N solamente, pero con efectos menos fuertes debido a los efectos contrapuestos de la adición de P. Durante los dos primeros años de la adición de nutrientes, los flujos de CH4 no se vieron afectados en ninguna elevación, lo cual atribuimos a la combinación de cantidades moderadas de nutrientes añadidos, la fuerte inmovilización de nutrientes, y la separación de la más alta capacidad de absorción de CH4 en el subsuelo de la superficie del suelo donde se añaden fertilizantes. En el tercer a quinto año, la adición de nutrientes del suelo aumentaron la captación de CH4, aunque los efectos de N y P variaron a lo largo del gradiente altitudinal: en 1000 m, la adición de N y N+P aumentó la captación anual de CH4 a 20-60%; en 2000 m P y N+P incrementaron la captación a 21-50%; y en 3000 m la adición de P y N+P incrementó la captación de CH4 a 34-40%. Estos efectos diferenciales de la adición de nutrientes pueden estar relacionados con el estatus inicial de del suelo y respuesta diferenciales de otros componentes del ecosistema a la adición de nutrientes en cada elevación. Demostramos que los flujos de GEI del suelo y consecuentemente la red potencial de calentamiento global del suelo pueden cambiar considerablemente a lo largo de un gradiente de elevación, siguiendo una tendencia general de disminución con el aumento de la elevación. Los resultados indican además que la elevada deposición de N y P puede afectar los flujos de GEI del suelo en los BHT Andinos, pero las respuestas a los flujos de GEI a la adición de nutrientes depende del estatus inicial de los nutrientes del suelo, la duración de la adición de nutrientes y la variabilidad inter-anual de las condiciones climáticas. Puesto que los efectos de la adición de nutrientes fueron no lineares con el tiempo de exposición y a la par existen complejas interacciones con otros componentes del ecosistema, aún quedan muchas incertidumbres en la predicción exacta de los efectos de la deposición de nutrientes en los flujos de GEI. Sin embargo, ofrecemos los primeros datos sobre los efectos de nutrientes a medio plazo de N, P y N+P en los flujos de los tres principales gases de efecto invernadero del suelo a lo largo de un gradiente altitudinal de los BHT Andina. Nuestros resultados sugieren que la red potencial de calentamiento global de los suelos en todo el gradiente altitudinal podría aumentar ligeramente con la entrada contribución de N, mientras que podría disminuir con el aumento de la contribución de P y N+P.