Forest Biogeochemical Response to Nitrogen and Sulfur Additions.

Patterns of atmospheric deposition of nitrogen and sulfur often covary, making it difficult to isolate their roles as nutrient and acidifier or to predict recovery responses as deposition declines. In 2011, we initiated a N x S (N x pH) experiment in Central New York, in six mixed hardwood stands. At each stand, four treatments were applied: 50 N ha^-1 yr^-1 as NaNO₃ (pH increase) or (NH₄)₂SO₄ (pH decrease); elemental S (pH decrease); and control. After five years, treatments shifted surface soil (0-10 cm) pH in the intended directions to 5.0 in NaNO₃, 4.2 in (NH₄)₂SO₄, 4.1 in S, and 4.6 in controls. Acidified treatments had lower extractable DOC concentrations and aromaticity (SUVA₂₅₄) and greater DOC bioavailability. Litter production did not vary by treatment, but N and S additions enriched litter N and S concentrations, respectively. Wood production increased in response to both forms of N addition by 1 t C ha^-1 yr^-1 (20 kg C/kg N) over six years, and was unaffected by S addition. Sulfur addition significantly suppressed litter decomposition; (NH₄)₂SO₄ significantly suppressed late stages of decomposition and NaNO₃ trended toward suppression. Soil respiration followed the same relative treatments responses as litter decomposition. Emissions of N₂O increased in response to N addition and decreased with S addition. Bacterial and fungal communities differed with both stand age and N x S treatment. Results show that N addition increases wood production, but both N and acidification suppress decomposition, if perhaps for different reasons. Distinguishing why is important for projecting ecosystem recovery to changing N and S inputs. This experiment indicates that forests recovering from chronic N and S deposition should experience increased DOC loss, decomposition, and denitrification, while possibly experiencing more N limitation; these responses may be easily mis-attributed as responses to climate change and rising atmospheric CO₂.