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Dynamics and underlying processes of N2O and NO soil-atmosphere exchange under extreme meteorological boundary conditions
(2009)
- Climate models predict an increasing frequency and intensity of summer drought periods with subsequent heavy rainfall or soil frost and thaw events in mountain regions of Central Europe. These indirect effects of global warming may considerably influence soil microbial processes and in consequence emissions of climate-relevant trace gases. Regarding the nitrogen cycle, N2O and NO emissions are of concern, since they are involved in climate warming and soils represent a main source for these two gases. In spite of a growing number of studies on this subject, knowledge on effects of climate change on soil N2O and NO emissions is still scarce. This is mainly due to a hitherto poor mechanistic understanding of underlying processes within soil. In this thesis, the impact of extreme meteorological boundary conditions on N2O and NO fluxes in a Norway spruce forest and an acidic fen in the Fichtelgebirge area was investigated. The summer drought period and precipitation were experimentally increased in the forest and the fen over a 2-year span. Soil frost was induced in the forest by removal of the natural snow cover. The experiments were run in three replicates each and non-manipulated plots served as controls. Throughout the experiments, N2O and NO fluxes were recorded in weekly to monthly intervals. In addition, N2O concentrations and isotope signatures in soil air were measured along soil profiles to identify and localise the underlying biogenic production and consumption processes. Prolonged drought continuously reduced the N2O emission from the forest soil and even turned the soil temporarily into a sink for atmospheric N2O. Soil freezing and thawing caused a burst of N2O release contributing 84 % of the annual emission. Soil air N2O concentration and stable isotope profiles provide a new mechanistic explanation tool for all of these findings. N2O concentration in the soil air decreased in most cases exponentially from the subsoil to the soil surface. This observation identifies microbial activity in the subsoil (at >= 70 cm soil depth) as an additional source for N2O and diffusion to the soil surface along a concentration gradient. Furthermore, isotope abundance analysis identified simultaneous microbial N2O consumption (reduction to N2). Drought reduced the source strength of the organic layers for N2O while simultaneously the sink function of the mineral soil for N2O remained active. This resulted in a net N2O sink function of the forest soil under severe drought. Frost in the topsoil was the only exception for these trends in N2O concentration and isotope signature along soil profiles. Under conditions of soil frost the topsoil served no longer as a sink for N2O, thus leading to the observed burst in N2O emission. NO emissions from the forest soil exceeded the N2O emissions by up to two orders of magnitude. Prolonged drought in- or decreased NO emissions depending on the soil moisture content of the organic layers. Wetting after long-lasting drought periods – which turned out to be of less importance regarding N2O fluxes – strongly increased biogenic NO emissions and contributed 44 % to the annual loss. In contrast to the forest soil, NO fluxes from the fen were always one to two orders of magnitude lower than the N2O fluxes. These results support earlier findings that this highly reactive gas is either only marginally produced in the fen soil or undergoes chemical conversion before escaping from the soil surface. Nevertheless, water table reduction resulted in significantly increased net NO emission. Regarding N2O, this thesis suggests that summer drought periods may drastically increase emissions from minerotrophic fens depending on the reduction of water table height. Furthermore, heavy rainfall following drought periods caused short lived, but strong N2O peaks having significant impact on the annual N2O loss, that have not been reported so far. N-15 and O-18 isotope data provide evidence that these N2O peaks are due to newly produced N2O in the upper soil. This thesis documents the huge impact of extreme weather events on soil N2O and NO emissions and provides so far scarcely considered mechanistic explanations for these observations. A major outcome of this work is the finding of a hitherto unconsidered sink function of forest soils for atmospheric N2O, when soil net N2O production is compensated for by net consumption during long-lasting droughts. This work underlines the importance of investigating the fate of N2O within soil profiles next to flux measurements to improve the current knowledge on the complex interactions between meteorological boundary conditions and soil biogenic processes and thus help further upgrading global N2O balances.
