Anthropogenically elevated nitrogen (N) deposition has induced soil acidification and eutrophication, resulting in nutrient imbalance in many natural and semi-natural ecosystems. In N-limited ecosystems, N deposition can benefit plant growth and increase net primary productivity. With progressive N saturation, excess N inputs will negatively affect plant growth through ecosystem nutrient imbalance. However, this assertion has been less tested in naturally N-rich forest ecosystem. Here we conducted long-term N fertilization experiments (0-150 kg N ha-1 yr-1) in two N-rich tropical forests: Dinghushan mature forest of Southern China and Hawaiian montane rainforest at Kokee Forest Reserve. We selected six dominant tree species in Dinghushan mature forest, and one dominant tree species and one dominant understory species in Hawaiian montane rainforest.
Results/Conclusions
In Dinghushan mature forest, we found that long-term (6-10 years) N additions significantly accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soil, but plants maintained foliar nutrient supply without significant changes in foliar nutrients such as N,P, K, Ca, Na, and Mg. In Hawaiian montane rainforest, where soil organic carbon and base saturation were significantly higher than Dinghushan mature forest, 26 years of N fertilization (100 kg N ha-1 yr-1) also did not change foliar nutrient status, although soil pH showed a decrease. Accordingly, foliar N/P ratios showed no responses in two forests. Considering no responses of plant growth to long-term N fertilization, these findings may go beyond the popular “Nitrogen saturation hypothesis” developed in temperate forests. We suggest that plants can acclimate to excess nitrogen inputs through two alternative ways in N-rich ecosystems: (1) in soils with low base saturation (e.g., <10%), plant species can increase aboveground transpiration to maintain nutrients supply; and (2) in soils with high base saturation (e.g., >40%), plant species can keep a constant underground nutrient uptake, in spite of soil acidification.