One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder (Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N 2 O and CH 4 turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N 2 O and CH 4 during the flooding. Contrary to N 2 O, the increase in CH 4 fluxes delayed in response to flooding. Stem N 2 O fluxes were lower although stem CH 4 emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N 2 O fluxes. The stems contributed up to 88% of CH 4 emissions to the stem-soil continuum during the investigated period but soil N 2 O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH 4 and N 2 O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.Greenhouse gases (GHG), in particular, methane (CH 4 ) and nitrous oxide (N 2 O) contribute 16% and 6% to global warming, respectively 1 . In addition, N 2 O is a dangerous stratospheric O 3 layer depleting agent 2 . Due to the increasing emissions, both gases have high radiative forcing potential. In principle, terrestrial biosphere may be seen as a net source of GHG to the atmosphere 3 . Temperate as well as tropical forest soils (in general) seem to be a central natural emitting source of N 2 O, on the one hand, a natural sink of CH 4 on the other 4-9 . Flux estimations of N 2 O and CH 4 in forest systems are mainly based on studies of forest soil measurements, usually excluding exchange potential of vegetation 5,7,10 . Nevertheless, investigations on GHG fluxes from plants in wetland or riparian ecosystems show that plants, especially trees, can be essential sources of CH 4 and N 2 O 9,11-13 . However, recent studies uncover the relevance of tree stem surfaces playing an important role in understanding GHG dynamics in different forest ecosystems 8,9,14 .Grey alder (Alnus incana (L.) Moench)) is a fast-growing, pioneer tree species with excellent potential for short-rotation forestry in the Northern hemisphere [15][16][17][18] . Due to the symbiotic Frankia bacteria which fix atmospheric nitrogen, alder forests are important nitrogen sequestering ecosystems 19,20 . Decomposition of nutrient-rich alder litter improves soil properti...

Riparian forests are known as hot spots of nitrogen cycling in landscapes. Climate warming speeds up the cycle. Here we present results from a multi-annual high temporal-frequency study of soil, stem, and ecosystem (eddy covariance) fluxes of N2O from a typical riparian forest in Europe. Hot moments (extreme events of N2O emission) lasted a quarter of the study period but contributed more than half of soil fluxes. We demonstrate that high soil emissions of N2O do not escape the ecosystem but are processed in the canopy. Rapid water content change across intermediate soil moisture was a major determinant of elevated soil emissions in spring. The freeze-thaw period is another hot moment. However, according to the eddy covariance measurements, the riparian forest is a modest source of N2O. We propose photochemical reactions and dissolution in canopy-space water as reduction mechanisms.

The decomposition of fresh crop residues added to soil for agricultural purposes is complex. This is due to different factors that influence the decomposition process. In field conditions, the incorporation of crop residues into soil does not always have a positive effect on aggregate stability. The aim of this study was to investigate the decomposition effects of residues from two different cover crops (Brassica napus var. oleifera and Secale cereale) and one main crop (wheat straw) on soil aggregate stability. A 105-day incubation experiment was conducted in which crop residues were mixed with sandy loam soil at a rate of 6 g C kg−1 of soil. During the incubation, there were five water additions. The decomposition effects of organic matter on soil conditions during incubation were evaluated by determining the soil functional groups; carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) emissions; soil microbial biomass carbon (MBC); and water-stable aggregates (WSA). The functional groups of the plant residues and the soil were analyzed using Fourier transform infrared spectroscopy (FTIR) and a double exponential model was used to estimate the decomposition rates. The results show that the decomposition rate of fresh organic materials was correlated with the soil functional groups and the C/N ratio. Oilseed rape and rye, with lower C/N ratios than wheat straw residues, had faster decomposition rates and higher CO2 and N2O emissions than wheat straw. The CO2 and N2O flush at the start of the experiment corresponded to a decrease of soil aggregate stability (from Day 3 to Day 10 for CO2 and from Day 19 to Day 28 for N2O emissions), which was linked to higher decomposition rates of the labile fraction. The lower decomposition rates contributed to higher remaining C (carbon) and higher soil aggregate stability. The results also show that changes in the soil functional groups due to crop residue incorporation did not significantly influence aggregate stability. Soil moisture (SM) negatively influenced the aggregate stability and greenhouse gas emissions (GHG) in all treatments (oilseed rape, rye, wheat straw, and control). Irrespective of the water addition procedure, rye and wheat straw residues had a positive effect on water-stable aggregates more frequently than oilseed rape during the incubation period. The results presented here may contribute to a better understanding of decomposition processes after the incorporation of fresh crop residues from cover crops. A future field study investigating the influence of incorporation rates of different crop residues on soil aggregate stability would be of great interest.

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Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH4) and sources of nitrous oxide (N2O). However, further attention is needed on CH4 and N2O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH4 and N2O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH4 and N2O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH4 and N2O concentrations were measured using an automated dynamic soil chamber system. 
Tree stems were a net source of CH4 and N2O during the winter period. The origin of stem CH4 emissions was unclear, as stem and soil CH4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N2O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N2O emissions and environmental parameters. Soil was a net sink for CH4, largely determined by changes in soil temperature. Soil N2O dynamics were characterised by hot moments – short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH4 sink by 14% and added 2% to forest floor N2O emissions. Therefore, CH4 and N2O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.