Nitrous oxide (N2O) is a potent greenhouse gas (GHG) which is released naturally into the atmosphere as a by-product of the microbial processes of nitrification and denitrification. Agricultural activities are believed to account for up to 80% of anthropogenic N2O emissions at a global scale; however, these estimates are prone to large uncertainties due to the large temporal and spatial variability associated with flux measurements. This thesis contains five studies which aimed to improve the ability to measure and predict N2O emissions from agricultural activities. A closed loop dynamic chamber was developed using a quantum cascade laser (QCL). This method provided high precision chamber measurements of N2O flux from soils with a detection limit below 4 μg N2O-N m-2 h-1. Using the dynamic chamber method allowed for a detailed investigation of uncertainties in individual measurements including contributions from regression fitting, temperature and pressure. The lack of negative fluxes measured that were outwith the detection limits of the methodology (0.3% of all measurements) highlighted that the uptake of N2O reported in some previous literature is likely to have been the result of detection limits of measurement methods applied. Spatial variability of N2O flux was investigated at the plot, field and farm scale. Fluxes were measured from a grassland field plot before and after a tillage event. These measurements highlighted the large spatially variability present in N2O fluxes from agricultural soils. Fluxes varied by up to three orders of magnitude over distances less than 5 metres after the tillage event. A field scale experiment carried out on grazed grassland investigated relationships between soil properties and N2O flux. This study found that N2O emissions correlated strongly with available nitrogen content in the soil and that animal waste was likely responsible for the spatial variability of N2O flux observed at the field scale. A farm scale inventory of N2O emissions was carried out investigating several large point sources of N2O and emissions from the wider field coverage. The inventory estimates that nitrogen fertiliser application is the single largest N2O source from the livestock farm accounting for 49% of annual emissions.

Degradation of permafrost in peatlands can convert forested peat plateaus to inundated collapse bogs. Due to increased unfrozen soil carbon stocks and more saturated conditions, collapse bogs can potentially be large emitters of methane. Using a network of bubble traps permanently installed in peat, I tested several hypotheses about controls on ebullition in collapse bogs with varying time since thaw in interior Alaska. Ebullition increased during the growing season, likely due to increased substrate availability and warmer soils. Bubbles were found primarily in shallow peat layers, and were dominated by modern carbon. Ebullition hot spots were associated with high sedge density throughout the collapse sites. Episodic ebullition occurred during atmospheric pressure changes. Overall, my study demonstrated that permafrost thaw in peatlands will result in methane emissions through ebullition that include both young and old carbon, contradictory to the generally accepted paradigm; that ebullition in peatlands is solely a surface process.

Kleine, flache Gewässer gelten als sogenannte „hotspots“ der Primärproduktion und Kohlenstoffbindung. Diese Doktorarbeit zielt darauf ab, die Primärproduktion verschiedener kleiner Gewässer zu quantifizieren sowie die Mechanismen, die den Kohlenstoffkreislauf dieser Systeme beeinflussen, zu analysieren. Der Fokus liegt dabei auf dem Einfluss globaler Veränderungen, die diese Mechanismen verändern können Im ersten Abschnitt wurde die Primärproduktion (PP) in kleinen, temporären Söllen untersucht, die sehr anfällig für Störungen sind. Ich konnte zeigen, dass die PP der Sölle im Sommer außergewöhnlich hoch ist, was hauptsächlich auf eine hohe Makrophytenproduktion zurückzuführen ist Im zweiten Teil analysiere ich die Ergebnisse eines Experiments zum Einfluss erhöhter Temperaturen auf die benthische PP kleiner Gewässer im Frühjahr. Acht Mesokosmen wurden bei normalen und um 4°C erhöhten Wassertemperaturen gemäßigter Breiten betrieben. In der ersten Hälfte des Experiments konnte ich eine erhöhte benthische PP in den erwärmten Mesokosmen feststellen, die auf direkte Temperatureffekte und indirekte Auswirkungen einer höheren Nährstoffverfügbarkeit zurückzuführen war. Anfang Juni stieg jedoch der Einfluss der Makroinvertebraten auf das Periphyton in den erwärmten Mesokosmen, so dass keine Unterschiede in der PP mehr auftraten. Schließlich, untersuche ich die Resilienz eines Sees gegenüber einem plötzlichen Eintrag gelösten organischen Kohlenstoffs (DOC) aus dem terrestrischen Umland, der zu einer starken Braunfärbung des Wassers führte. Der Fokus liegt dabei auf Veränderungen der Wasserqualität und der aquatischen PP des Sees, nachdem sich die DOC-Konzentration verfünffacht hatte. Drei Jahren nach Erreichen der maximalen DOC- und Gesamt-Phosphor im See sanken diese signifikant, lagen jedoch noch immer 1,5- bzw. 2-fach oberhalb der Ausgangskonzentrationen vor dem DOC-Eintrag. Die benthische PP zeigte eine teilweise Erholung, erreichte jedoch ebenfalls nicht die Ausgangswerte. / Small, shallow freshwater ecosystems are now considered hotspots of primary production & carbon sequestration. Yet till recently they’ve been mostly neglected. This thesis aims at explaining the underlying mechanisms affecting carbon cycling in these systems, particularly focusing on how contemporary global changes alter ecological equilibria. In the first section, using a compartmental approach, I study primary production in small, temporary ponds (kettle holes) within agricultural fields that are highly susceptible to environmental & anthropogenic disturbances. I show that summertime gross primary production (GPP) in kettle holes is exceptionally high, mostly driven by a strong macrophyte production. In winter, periphyton contributes to the majority of the systems’ GPP. High summertime deposition, correlated to GPP, and low sediment mineralization rates, signified a high potential for carbon burial. In the second experiment, I test the impact of increased temperatures on periphyton production during spring. I use eight mesocosms running at normal & +4°C temperatures. Initially, I recorded elevated periphyton GPP in the warmed treatment driven by direct temperature effects & indirect effects of higher nutrient availability. By late spring, the trend is reversed due to increased grazing pressure in the warm treatment. In the third study, I investigate a lake’s resilience to a sudden brownification event: A 5-fold increase in dissolved organic carbon (DOC) concentrations. Within three years after peak brownification, the lake DOC & total phosphorous concentrations dropped significantly but seem to have plateaued at 1.5 & 2-fold their pre-brownification levels, respectively. Consequently, benthic GPP, which had collapsed due to light limitation at peak brownification, marked only a partial recovery, while phytoplankton (& whole-lake) GPP remained higher than pre-brownification levels. Phytoplankton & periphyton exhibited an inverse response to DOC & TP concentrations.