The impact of experimentally intensified summer drought and precipitation on N 2 O and NO turnover and fluxes was investigated in a minerotrophic fen over a 2-year period. On three treatment plots, drought was induced for 6 and 10 weeks by means of roofs and drainage and decreased water table levels by 0.1-0.3 m compared with three nonmanipulated control plots. When averaged over the three treatment plots, both N 2 O and NO emission showed only little response to the drought. On the single plot scale, however, a clear impact of the treatment on N 2 O and NO fluxes could be identified. On the plot with the weakest water table reduction hardly any response could be observed, while on the plot with the greatest drainage effect, N 2 O and NO fluxes increased by 530% and 270%, respectively. Rewetting reduced NO emissions to background levels (0.05-0.15 lmol m À2 h À1 ), but heavily enhanced N 2 O emission (18-36 lmol m À2 h À1 ) for several days in the plots with largest water table reduction. These peaks contributed up to 40% to the cumulative N 2 O fluxes and were caused by rapid N 2 O production according to isotope abundance data. According to N 2 O concentrations and isotope abundance analysis N 2 O was mostly produced at depths between 0.3 and 0.5 m. During water table reduction net N 2 O production in 0.1 m depth steadily increased in the most effectively dried plot from 2 up to 44 pmol cm À3 day À1 . Rewetting immediately increased net N 2 O production in the topsoil of the drought plots, showing rates of 18-174 pmol cm À3 day À1 . This study demonstrates that drought and rewetting can temporarily increase N 2 O emission to levels that have to date only been reported from nutrient rich and degraded fens that have been drained for agricultural purposes.
Based on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N 2 O on annual scales. To investigate the influence of drying/rewetting events on N 2 O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N 2 O sink. This was accompanied by subambient N 2 O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N 2 O sink strength. Rewetting quickly turned the soil into a source for atmospheric N 2 O again, but it took almost 4 months to turn the cumulative soil N 2 O fluxes from negative (sink) to positive (source) values. N 2 O concentration and isotope analyses along soil profiles revealed that N 2 O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N 2 O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N 2 O sinks. Accumulation of more field-scale data on soil N 2 O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N 2 O budget.
Prolonged summer droughts due to climate change are expected for this century, but little is known about the effects of drying and wetting on biogenic trace‐gas fluxes of forest soils. Here, the response of CO2, N2O, NO, and CH4 fluxes from temperate forest soils towards drying–wetting events has been investigated, using undisturbed soil columns from a Norway spruce forest in the “Fichtelgebirge”, Germany. Two different types of soil columns have been used for this study to quantify the contribution of organic and mineral horizons to the total fluxes: (1) organic horizons (O) and (2) organic and mineral soil horizons (O+M). Three drying–wetting treatments with different rewetting intensities (8, 20, and 50 mm of irrigation d–1) have been compared to a constantly moist control to estimate the influence of rainfall intensity under identical drying conditions and constant temperature (+15°C). Drought significantly reduced CO2, N2O, and NO fluxes in most cycles. Following rewetting, CO2 fluxes quickly recovered back to control level in the O columns but remained significantly reduced in the O+M columns with total CO2 fluxes from the drying–wetting treatment ranging approx. 80% of control fluxes. Fluxes of N2O and NO remained significantly reduced in both O and O+M columns even after rewetting, with cumulative fluxes from drying–wetting treatments ranging between 20% and 90% of the control fluxes, depending on gas and cycle. Fluxes of CH4 were small in all treatments and seem to play no significant role in this soil. No evidence for the release of additional gas fluxes due to drying–wetting was found. The intensity of rewetting had no significant effect on the CO2, N2O, NO, and CH4 fluxes, suggesting that the length of the drought period is more important for the emission of these gases. We can therefore not confirm earlier findings that fluxes of CO2, N2O, and NO during wetting of dry soil exceed the fluxes of constantly moist soil.
Nitrous oxide is an important greenhouse gas and its origin and fate are thus of broad interest. Most studies on emissions of nitrous oxide from soils focused on fluxes between soil and atmosphere and hence represent an integration of physical and biological processes at different depths of a soil profile. Analysis of N(2)O concentration and isotope signature along soil profiles was suggested to improve the localisation of sources and sinks in soils as well as underlying processes and could therefore extend our knowledge on processes affecting surface N(2)O fluxes. Such a mechanistic understanding would be desirable to improve N(2)O mitigation strategies and global N(2)O budgets. To investigate N(2)O dynamics within soil profiles of two contrasting (semi)natural ecosystem types (a temperate acidic fen and a Norway spruce forest), soil gas samplers were constructed to meet the different requirements of a water-saturated and an unsaturated soil, respectively. The samplers were installed in three replicates and allowed soil gas sampling from six different soil depths. We analysed soil air for N(2)O concentration and isotope composition and calculated N(2)O net turnover using a mass balance approach and considering diffusive fluxes. At the fen site, N(2)O was mainly produced in 30-50 cm soil depth. Diffusion to adjacent layers above and below indicated N(2)O consumption. Values of delta(15)N and delta(18)O of N(2)O in the fen soil were always linearly correlated and their qualitative changes within the profile corresponded with the calculated turnover processes, suggesting further reduction of N(2)O. In the spruce forest, highest N(2)O production occurred in the topsoil, but there was also notable production occurring in the subsoil at a depth of 70 cm. Changes in N(2)O isotope composition as to be expected from local production and consumption processes within the soil profile did hardly occur, though. This was presumably caused by high diffusive fluxes and comparatively low net turnover, as isotope signatures approached values measured for ambient N(2)O towards the topsoil. Our results demonstrate a highly variable influence of diffusive versus production/consumption processes on N(2)O concentration and isotope composition, depending on the type of ecosystem. This finding indicates the necessity of further N(2)O concentration and isotope profile investigations in different types of natural and anthropogenic ecosystems in order to generalise our mechanistic understanding of N(2)O exchange between soil and atmosphere.
In mountain regions of Central Europe an increase of soil frost periods is predicted for this century due to reduced snow fall. To investigate the effects of freezing and thawing on soil N 2
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