Soil solution, foliar concentrations and tree growth response to 8 years of ammonium-nitrate additions in two boreal forests of Quebec, Canada.

Highlights • Boreal forests are known to be strongly nitrogen limited. • Chronic ammonium-nitrate additions were made for 8 years at two boreal forest sites in Canada. • The two sites showed a strong resilience to N addition in terms of soil response. • Only the site receiving the lowest natural N deposition showed a response to N addition. • The sites receiving low N deposition are prone to respond in terms of nutrient status and growth. Abstract The boreal forest is recognised as one of the world's most N-limited ecosystems. Increased N deposition is thought to stimulate boreal forest growth and carbon sequestration. However, evidence is lacking about the impact of increasing N supply over long time scales. Moreover, N deposition in forest ecosystems may cause N saturation, which leads to soil cation depletion and, ultimately, affects tree health. Ammonium nitrate (NH 4 NO 3) was applied during an 8-year period in two stands in the boreal forest in Québec (Canada): one composed of balsam fir (Abies balsamea [Linné] Miller), and the other, of black spruce (Picea mariana [Mill.] B.S.P.). Application rates were 3 and 10 times the atmospheric nitrogen (N) deposition rate measured at each site, which was 6 kg ha⁻¹ year⁻¹ at the balsam fir site and 3 kg ha⁻¹ year⁻¹ at the black spruce site. Soil solution (at depths of 30 and 60 cm), mosses, soil and foliar chemistry as well as tree growth were analyzed. After 8 years of N additions, no signs of N saturation were observed in the soil solution of either site, and no treatment response was observed at the balsam fir site. By contrast, in the treated plots at the black spruce site, N concentration increased in mosses and tree needles. Tree basal areal increment also increased by 21% compared with the control. The different responses between the two forest types could be ascribed to the fact that the black spruce site is located further north and receives less N deposition than the balsam fir site. The tree growth increase at the black spruce site (concomitant with foliar N increase) suggests that C sequestration in tree biomass is more likely to increase in areas having soils with high C:N ratio and receiving low N deposition. [ABSTRACT FROM AUTHOR]