Potential use of δ¹³C, δ¹⁵N, N concentration, and Ca/Al of Pinus densiflora tree rings in estimating historical precipitation pH

Purpose: The chemistry of annual tree growth rings is affected by precipitation pH, and tree rings store information on environmental conditions at the time of ring formation. The purpose of this study was to evaluate the potential use of tree ring chemistry data in estimating historical precipitation pH using the relationship between precipitation pH and tree ring chemistry. Materials and methods: Red pine (Pinus densiflora) stem disks were collected from a forest in southern Korea and analyzed for C isotope ratio (¹³C/¹²C, expressed as δ¹³C), N concentration, N isotope ratio (¹⁵N/¹⁴N, expressed as δ¹⁵N), and molar Ca/Al in annual growth rings. Correlation analysis was conducted for the relationship between tree ring chemistry and precipitation pH between 1992 and 2005 for which such data were available. A regression equation between precipitation pH and tree ring chemistry was then developed using stepwise multiple regression analysis. Results and discussion: We found that between 1992 and 2005, δ¹⁵N (-0.4‰ to -2.7‰) and Ca/Al (11.6 to 7.3) decreased in the growth rings (all significant at α = 0.05) with precipitation pH decreased from 5.7 to 5.0, while the amount of NO_x emission increased from 12.7 to 22.1 thousand tons between 1999 and 2005 in the study area. The decreased δ¹⁵N and Ca/Al in the tree rings associated with the decreasing precipitation pH seemed to reflect increased N deposition originated from NO_x emission that is known to be depleted in ¹⁵N relative to the soil mineral N and reduction in Ca availability due to soil acidification. The regression model indicated that mean annual precipitation pH values before the late 1990s at the study site fluctuated between 5.5 and 6.8; thereafter, it showed a decreasing pattern below 5. 4 (the lowest measured and estimated pHs were 5.2 and 5.0, respectively) in the mid-2000s with increasing NO_x emission. Conclusions: The significant correlation of precipitation pH with δ¹⁵N and Ca/Al in tree rings suggested that it may be possible to use tree ring chemistry to estimate historical precipitation pH. However, δ¹³C and N concentration of tree rings were not useful as indicators of acid precipitation in this study, as δ¹³C is an integrator of many environmental factors that affect gas exchange and inter-ring movement of N towards the outermost ring may mask time-related information regarding N availability. Further research is needed to validate the model in regions where long-term precipitation pH records are available.