Chemistry of soil and foliage in declining sugar maple stands over 13 years of nitrogen addition

Cumulative effects of nitrogen (N) deposition are a matter of concern in temperate forests of northeastern North America. Increased N deposition may stimulate forest growth and carbon sequestration while also causing N saturation, which may lead to soil cation depletion, particularly in base-poor systems. We applied ammonium nitrate for 13 years in a sugar maple (Acer saccharum; SM) stand at the rates of 3 (Low N, LN) and 10 times (High N, HN) the ambient N deposition rate. N concentrations in soil layers were similar among treatments, except for a nitrate increase in the top B horizon in the HN treatment. Exchangeable Ca2+, Mg2+, and K+ were significantly lower than for the Control, at least in one of the top organic soil layers in the HN treatment. The temporal dynamics of foliar chemistry revealed that N:P ratio increased and both Ca and Mn decreased in the early years following treatment initiation, but that they did not change much thereafter. Foliar Ca values in the HN treat-ment are the lowest reported in the literature. This probably explains why the proportion of crown dieback steadily increased over time, reaching values as high as 80% in the HN treatment in 2017.