Gross N2O fluxes across soil-atmosphere interface and stem N2O emissions from temperate forests
von Yuan Wen
Datum der mündl. Prüfung:2017-04-07
Erschienen:2017-05-22
Betreuer:Prof. Dr. Edzo Veldkamp
Gutachter:Prof. Dr. Edzo Veldkamp
Gutachter:Prof. Dr. Heinz Flessa
Gutachter:PD Dr. Reinhard Well
Dateien
Name:Yuan Wen_PhD thesis_2017.pdf
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Description:PhD thesis
Zusammenfassung
Englisch
Although nitrous oxide (N2O) is a minor constituent of the atmosphere, it is still of great concern. This is because N2O can significantly affect the physics and chemistry of the atmosphere and thus influence the climate on Earth. Soil is a major source of N2O, and microbial nitrification and denitrification are the dominant N2O producing processes. Soil N2O fluxes usually exhibit significantly spatio-temporal variability since the microbial processes of N2O production and consumption are both affected by the substrate availability, redox potential and temperature. Moreover, plants can influence soil N2O fluxes through altering soil properties and microbial communities and through serving as additional conduits for transport of soil-generated N2O. However, we are still struggling to fully understand the complexity of N2O production, consumption and transportation processes in soil, and the links to abiotic (e.g. soil climate, physics and chemistry) and biotic (e.g. microbial–plant–soil interactions) factors. The difficulty of measuring gross N2O production and consumption in soil impedes our ability to predict N2O dynamics across the soil-atmosphere interface. The aim of the first study was to disentangle gross N2O production and consumption in soil by comparing 15N2O pool dilution (PD) and gas-flow soil core (GFSC) measurements. Intact soil cores were taken from grassland, cropland, beech and pine forests, covering different vegetation, soil types and climatic conditions. Across sites, gross N2O production and consumption measured by 15N2OPD were only 10% and 6%, respectively, of those measured by GFSC. Hence, we proposed to use different terminologies for the two methods. ‘Gross N2O emission and uptake’ are appropriate for 15N2OPD, which encompasses gas exchange within the 15N2O-labelled, soil air-filled pores; while ‘gross N2O production and consumption’ can be used for GFSC, which includes N2O directly reduced to N2 in anaerobic microsites. Although the 15N2OPD could measure only part of gross N2O production in soil, it is the only method that can be used under field conditions to quantify atmospheric N2O uptake, an important process commonly unquantified in many ecosystems. The aim of the second study was to quantify temporal variability and environmental controls of gross N2O fluxes. We measured gross N2O emission and gross N2O uptake using the 15N2OPD technique that we validated in the first study. Asymbiotic N2 fixation was also measured to infer the gaseous N balance. This experiment was conducted in adjacent spruce and beech forests in central Germany. Our results showed that the beech stand had higher soil gross and net N2O emissions and asymbiotic N2 fixation than the spruce stand. Seasonal variation of gross N2O emission was mainly controlled by soil NO3- concentration; gross N2O uptake was largely influenced by soil extractable organic C; and asymbiotic N2 fixation was correlated with soil extractable organic C and temperature. Asymbiotic N2 fixation was an order of magnitude lower than gross N2O uptake in these highly acidic, N-enriched forest soils. The aim of the third study was to determine tree-mediated N2O fluxes under field conditions as well as their contributions to total forest N2O fluxes. Here, we quantified in situ stem N2O fluxes from mature alder trees on poorly-drained soil and mature beech and spruce trees on well-drained soils in central Germany. Alder, beech and spruce consistently emitted N2O via stems and all displayed higher emission rates in summer than in spring and autumn. Stem N2O fluxes from alder were higher than beech and spruce due to the presence of aerenchyma and lenticels as well as higher soil water content and soil C and N availability in the alder stand. Stem N2O fluxes represented 8-11% of the total (soil + stem) N2O fluxes in the spruce and beech stands, whereas in the alder stand with its large soil N2O emission stem emission contributed only 1% of the total flux. Overall, this research provides new insights into gross N2O fluxes and their environmental factors, and also provides an estimate of tree-mediate N2O fluxes which can improve N budgets of forest ecosystems. Our findings show that the 15N2O PD technique was a valuable tool to separate the net N2O flux into gross N2O emission and gross N2O uptake in the gas phase of the soils, but probably did not allow measuring gross N2O production and consumption in anaerobic microsites. Gross N2O emission played an important role in controlling the direction and magnitude of net N2O flux. And the regression relationships between gross N2O emission and net N2O fluxes also open the possibility of making estimates of soil gross N2O emissions based on measured soil net N2O emissions. Tree species had a large influence on gross N2O emission, net N2O flux and asymbiotic N2 fixation, and thus large-scale field quantification under similar soil types and climatic conditions can be based on tree-species stratification as a promising basis to scale up these rates. Lastly, both wetland trees and upland trees act as important conduits for soil-generated N2O and the relative contribution of tree-mediated N2O fluxes to the total N2O fluxes is more important in upland trees than in wetland trees.
Keywords: 15N2O pool dilution; gross N2O emission; gross N2O uptake; temperate forests; stem N2O emission; denitrification