Dynamics of gaseous nitrogen and carbon fluxes in riparian alder forests

Abstract

We studied greenhouse gas (GHG) fluxes in two differently loaded riparian Alnus incana-dominated forests in agricultural landscapes of southern Estonia: a 33-year-old stand in Porijõgi, in which the uphill agricultural activities had been abandoned since the middle of the 1990s, and a 50-year-old stand in Viiratsi, which still receives polluted lateral flow from uphill fields fertilized with pig slurry. In Porijõgi, closed-chamber based sampling lasted from October 2001 to October 2009, whereas in Viiratsi the sampling period was from November 2003 to October 2009. Both temporal and spatial variations in all GHG gas fluxes were remarkable. Local differences in GHG fluxes between micro-sites (“Edge”, “Dry” and “Wet” in Porijõgi, and “Wet”, “Slope” and “Dry” in Viiratsi) were sometimes greater than those between sites. Median values of GHG fluxes from both sites over the whole study period and all microsites did not differ significantly, being 45 and 42 mg CO 2–C m −2 h −1, 8 and 0.5 μg CH 4–C m −2 h −1, 1.0 and 2.1 mg N 2–N m −2 h −1, and 5 and 9 μg N 2O–N m −2 h −1, in Porijõgi and Viiratsi, respectively. The N 2:N 2O ratio in Viiratsi (40–1200) was lower than in Porijõgi (10–7600). The median values-based estimation of the Global Warming Potential of CH 4 and N 2O was 19 and 185 kg CO 2 equivalents (eq) ha −1 yr −1 in Porijõgi and −14 and 336 kg CO 2 eq ha −1 yr −1 in Viiratsi, respectively. A significant Spearman rank correlation was found between the mean monthly air temperature and CO 2, CH 4 and N 2 fluxes in Porijõgi, and N 2O flux in Viiratsi, and between the monthly precipitation and CH 4 fluxes in both study sites. Higher groundwater level significantly increases CH 4 emission and decreases CO 2 and N 2O emission, whereas higher soil temperature significantly increases N 2O, CH 4 and N 2 emission values. In Porijõgi, GHG emissions did not display any discernable trend, whereas in Viiratsi a significant increase in CO 2, N 2, and N 2O emissions has been found. This may be a result of the age of the grey alder stand, but may also be caused by the long-term nutrient load of this riparian alder stand, which indicates a need for the management of similar heavily loaded riparian alder stands.