Canopy mitigates CO2, CH4 and N2O fluxes from soil and tree stems in a riparian deciduous forest

Riparian forests are known as hot spots of carbon and nitrogen cycling in landscapes. Climate warming speeds up the cycle. Thus, riparian forests are increasingly important hot spots of greenhouse gas (GHG) fluxes in landscapes. Here we analyse data from a 2.5-year (September 2017-December 2019) continuous high-frequent study of soil emissions (automated chambers) and ecosystem (eddy-covariance, EC) fluxes of CO2, CH4 and N2O in a riparian grey alder forest in Estonia. Also, we analyse tree stem fluxes measured by manual chambers. Canopy impact is calculated as difference between the EC, soil and stem fluxes. Based on EC data of all three gases, the forest is a significant sink of CO2 (-7,893 kg CO2 ha-1 y-1), almost neutral regarding CH4 fluxes CH4 (-0.24 kg CH4-C ha-1 y-1 or -9.0 kg CO2eq ha-1 y-1), and slight emitter of N2O (0.26 kg N2O-N ha-1 y-1 or 120 kg CO2eq ha-1 y-1). At ecosystem level we could not define any remarkable hot moments (HM; extreme emission events) for all GHGs, whereas for N2O three remarkable HM for soil fluxes (wet period is autumn, beginning of dry period in spring and freeze-thaw period in winter) were observed. Regarding N2O, HMs lasted a quarter of the whole study but contributed more than a half of soil fluxes. For CH4 a remarkable HM was the wet period when 83% of CH4 was emitted from the tree stems. In the dry period, CH4 was substantially consumed in the soil whereas stem emissions were very low. Forest floor (soil+plants within the chambers) and tree stems emitted 534 and 341 kg CO2 ha-1 y-1, respectively, and 10% of their sum was assimilated within the canopy. The rest was accumulated in biomass. Throughout the whole study, canopy decreased about 80 % of soil+stem fluxes of N2O whereas during the HMs the difference was the largest. The horizontal advective fluxes and decoupling of gas fluxes between the layers, UV-induced photodissociation, and potential N2O dissolution in the canopy airspace are the possible mechanisms for this discrepancy. A significant difference between the EC CH4 fluxes and the sum of soil and stem CH4 fluxes during the dry HM is most likely caused by chemical processes-induced methane emissions from the canopy. In conclusion, the riparian alder forest is a major GHG sink mostly owing to CO2-C assimilation in trees whereas the canopy airspace mitigates the soil N2O fluxes.