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

• Published in: Ecological indicators 2023-10, Vol.154, p.110604, Article 110604
• Main Authors: Smolander, Aino, Henttonen, Helena M., Hänninen, Risto, Nöjd, Pekka, Taylor, Stephen, Sofiev, Mikhail, Mäkinen, Harri
• Format: Article
• Language: English
• Subjects: Forest soil; N cycling; N deposition; N isotopic ratio; Organic matter; Scots pine; Tree rings
• ISSN: 1470-160X
• DOI: 10.1016/j.ecolind.2023.110604

•N deposition in 1951–2020 totaled 120–470 kg/ha, i.e., annual average 1.8–6.7 kg/ha.•Soils in pine stands were similar, except decreasing N status with the lowest deposition.•Tree ring δ15N was different between the lowest and highest N deposition sites.•δ15N in 1951–2020 correlated negatively with total-, ammonium- and nitrate-N depositions.•Temporal development of δ15N and N deposition coincided only on some sites. 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 so