To test the possible use of tree ring chemical properties as proxies for precipitation acidity ([H + ]), we investigated the relationships between tree ring chemistry (δ 13 C, δ 15 N, Ca-to-Al ratio, and N

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We measured soil nitrogen (N) mineralization along an N fertilization gradient (control; irrigation only (I + 0 N); irrigation with 56 (I + 56 N), 112 (I + 112 N), and 224 (I + 224 N) kg N·ha–1·year–1, respectively) in 7-year-old cottonwood (Populus deltoides Marsh.), cherrybark oak (Quercus falcata Michx. var. pagodifolia Ell.), American sycamore (Platanus occidentalis L.), and loblolly pine (Pinus taeda L.) plantations established on a well-drained Redbay sandy loam (a fine loamy, siliceous, thermic Rhodic Paleudult), in Florida, USA. Nitrogen mineralization was measured monthly for 1 year, beginning in April 2001, with the buried bag incubation technique. Irrigation alone or fertigation (irrigation + N) affected annual net N mineralization rates under hardwood species, but no effect was found under loblolly pine. Overall, the rates were higher under cherrybark oak (108 kg N·ha–1·year–1) and cottonwood (101 kg N·ha–1·year–1) than under sycamore (82 kg N·ha–1·year–1) and loblolly pine (75 kg N·ha–1·year–1). Significant correlations were observed between N mineralization and stem volume in all species but loblolly pine. These results suggest that N mineralization response to irrigation or fertigation (irrigation + N) is heavily dependent on species-specific feedback mechanisms. Our results also support the hypothesis that the N mineralization versus productivity relationship is a fundamental feature of forests, resulting from the impact of N availability on productivity and the long-term feedback effects of vegetation on N availability.

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 ( 13 C/ 12 C, expressed as δ 13 C), N concentration, N isotope ratio ( 15 N/ 14 N, expressed as δ 15 N), 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, δ 15 N (−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 δ 15 N 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 15 N 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 mid2000s with increasing NO x emission. Conclusions The significant correlation of precipitation pH with δ 15 N and Ca/Al in tree rings suggested that it may be possible to use tree ring chemistry to estimate historical precipitation pH. However, δ 13 C and N concentration of tree rings were not useful as indicators of acid precipitation in this study, as δ 13 C 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.

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