Tree-ring N isotopic ratio increased with increasing latitude and decreasing N availability in pine stands across Finland

Since nitrogen (N) is the main forest-growth limiting nutrient in the boreal region, atmospheric N deposition may be an important source of available soil N. The objective of the study was to determine whether the variation in N deposition is reflected in the stem wood N and in its isotopic signatures (variation of 15N/14N ratios relative to atmospheric N2, reported as δ15N values), as well as in current soil properties with a special focus on N cycling. The study material consisted of twelve Scots pine (Pinus sylvestris L.) stands located along the N deposition gradient from south to north in Finland, representing similar site types with relatively unfertile and N-limited soils. Tree-ring N from 5-year segments, spanning a period of 70 years (1951–2020), were examined. Based on the modelled open-place deposition amounts, annual N deposition increased until around 1990 up to 9 kg/ha/year at the southernmost site and 1 kg/ha/year in the north but decreased substantially thereafter. The deposition of N totaled 472 kg/ha at the southernmost and 123 kg/ha at the northernmost site during the 70-year period. δ15N values in tree rings varied between −5.8 and + 1.3 and were higher at the northern than at the southern sites. Tree-ring δ15N ratio correlated negatively with total N, nitrate-N and ammonium-N depositions. The negative correlation still existed when stand age was used as a controlling factor. The correlation also remained negative when the dataset was divided into periods of generally increasing (1951–1990) and decreasing (1991–2020) deposition, or over 30-year age classes, except for the oldest age class (>90-year-old stands). Humus layer pH did not vary much between the sites, but slight signs of decreasing N status existed from south to north. The temporal development of tree-ring δ15N ratio and the amount of N deposition coincided only on some sites after the effect of stand age was controlled. In conclusion, although climate effects cannot be totally excluded, given the same tree species, mycorrhizal type (ectomycorrhiza), site and soil factors, the N isotopic ratios in tree rings distinguished the highest and the lowest N deposition levels. Within small differences in N deposition other factors affecting the ratio may dominate and tracing of the N input from the past is questionable.