Impacts of enhanced nitrogen deposition and soil acidification on biomass production and nitrogen leaching in Chinese fir plantations

: Atmospheric pollution levels in China are increasing quickly. Experience from other polluted regions shows that tree growth could be affected, but long-term effects of N deposition and soil acidification on Chinese forests remain mostly unknown. Soil acidification and N deposition were simulated for Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations managed for three consecutive 20-year rotations in southeastern China. A factorial experiment combined four rain pH levels (2.5, 4.0, 5.6, and 7.0), four N deposition rates (1, 7.5, 15, and 30 kg N * [ha.sup.-1] * [year.sup.-1]), and two site qualities (poor and rich). Results indicate that atmospheric pollution effects are not immediate, but after one to two rotations, soil acidification effects could reduce ecosystem C pools significantly (-25% and -11% in poor and rich sites, respectively). N deposition rates above 15 kg N * [ha.sup.-1] * [year.sup.-1] could offset some of the negative effects of soil acidification and lead to more ecosystem C (19 and 28 Mg C * [ha.sup.-1] more in poor and rich sites, respectively, than in low N deposition). However, at high N deposition rates (> 15 kg N * [ha.sup.-1] * [year.sup.-1]), N leaching losses could greatly increase, reaching 75 kg N * [ha.sup.-1] * [year.sup.-1]. Moderate N deposition could increase tree biomass production and soil organic mass, resulting in increased ecosystem C, but these gains could be associated with important N leaching. Atmospheric pollution could also result in the long term in nutrient imbalances and additional ecological issues (i.e., biodiversity loss, eutrophication, etc.) not studied here. Resume: Les niveaux de pollution atmospherique augmentent rapidement en Chine. L'experience d'autres regions aux prises avec la pollution montre que la croissance des arbres pourrait etre ralentie mais les effets a long terme des depots azotes (N) et de l'acidification des sols dans les forets chinoises sont pratiquement inconnus. L'acidification du sol et les depots de N ont ete simules dans des plantations de sapin de Chine (Cunninghamia lanceolata (Lamb.) Hook.) amenagees pendant trois rotations consecutives d'une duree de 20 ans dans le sudest de la Chine. Une experience factorielle a combine quatre niveaux de pluie acide (pH 2,5, 4,0, 5,6 et 7,0), quatre taux de deposition de N (1, 7,5, 15 et 30 kg N * [ha.sup.-1] * [year.sup.-1]) et deux qualites de station (pauvre et riche). Les resultats montrent que les effets de la pollution atmospherique ne sont pas imme-diats, mais qu'apres une a deux rotations, les effets de l'acidification du sol pouvaient reduire de facon significative les reserves de carbone (C) de l'ecosysteme (-25 et -11 % respectivement dans les stations pauvres et riches). Les taux de deposition de N plus eleves que 15 kg N * [ha.sup.-1] * [year.sup.-1] pouvaient attenuer certains des effets negatifs de l'acidification du sol et ont mene a plus de C de l'ecosysteme (19 et 28 Mg C * [ha.sup.-1] de plus respectivement dans les stations pauvres et riches qu'a-vec un faible taux de deposition de N). Cependant, les pertes de N par lessivage pouvaient augmenter de facon importante, jusqu'a 75 kg N * [ha.sup.-1] * [an.sup.-1], lorsque le taux de deposition de N etait eleve (> 15 kg N * [ha.sup.-1] * [year.sup.-1]). Un taux de deposition de N modere pouvait augmenter la production de biomasse des arbres et la masse de sol organique, ce qui a entraine une augmentation du C de l'ecosysteme, mais ces gains pouvaient etre associes a un lessivage important de N. La pollution atmosphe-rique pourrait aussi entrainer des desequilibres nutritifs a long terme et d'autres problemes d'ordre ecologique (c.-a-d. perte de biodiversite, eutrophisation, etc.) qui n'ont pas ete etudies ici. [Traduit par la Redaction]
Introduction Concern about the possible effects of acid rain in China emerged in the late 1970s (Fan and Wang 2000), with the first reports appearing in the international literature in [...]