Gaseous nitrogen and carbon fluxes in riparian alder stands

Riparian buffer zones are considered to be important elements of agricultural watersheds, in that they control nutrient and carbon fluxes. Although the water purification effect of riparian ecosystems has been well studied, there is little knowledge of their internal cycling, especially in the area of gaseous emissions. We measured fluxes of nitrous oxide (N 2 O), dinitrogen (N 2 ), methane (CH 4 ), and carbon dioxide (CO 2 ) in riparian grey alder stands in southern Estonia and black alder stands in Brandenburg, Germany. Dinitrogen emission was the most important component in N retention in the studied riparian grey alder forests. In 2001-2003, the median values of N 2 emission in the grey alder sites significantly exceeded the N 2 emission rates in the black alder sites, varying from 700 to 1200 and from 360 to 400 kg N 2 -N ha -1 year -1 respectively. In contrast, the median values of N 2 O flux were higher in the black alder sites than in the grey alder sites, i.e., 0.9-2.6 and 0.2-0.7 kg N 2 O-N ha -1 year', respectively. The black alder sites acted as a sink for CH 4 , whereas the grey alder sites emitted a small amount of CH 4 . The CO 2 -C flux was higher in the black alder stands. The estimated N 2 -N emission in the grey alder stands for 1994-1995 was 51.2, whereas N 2 O-N emission was 0.5 kg N ha-' year'. The significant increase in N 2 emission from 1994-1995 to 2001-2003 can be related to changes in microbial activity during the succession of the pioneer grey alder stand into a more stable mixed forest community. Due to CO 2 fluxes and N 2 O fluxes from sites with altered water regime, the estimated GWP of the studied riparian alder stands was relatively high. Further investigations should concentrate on the factors that regulate rates of N 2 O and N 2 emission from riparian buffer zones.