Nitrogen saturation in humid tropical forests after 6 years of nitrogen and phosphorus addition: hypothesis testing

Nitrogen (N) saturation hypothesis suggests that when an ecosystem reaches N‐saturation, continued N input will cause increased N leaching, nitrous oxide (N₂O) emission, and N mineralization and nitrification rates. It also suggests that a different element will become the main limiting factor when N saturation has been reached. Although this hypothesis has been tested in temperate forests, whether they can be directly applied to N‐saturated tropical forests remain poorly addressed. To test this hypothesis, soil inorganic N, soil N mineralization and nitrification rate, soil N₂O emission rate and nitrate (NO3−) leaching rate were measured in an N‐saturated old‐growth tropical forest in southern China, after 6 years of N and P addition. We hypothesized that N addition would stimulate further N saturation, but P addition might alleviate N saturation. As expected, our results showed that six continuous years of experimental N addition did cause further N saturation, which was indicated by significant increases in soil inorganic N concentration, N₂O emission and nitrate (NO3−) leaching. However, in contrast to our expectations, N addition significantly decreased in situ rates of net N mineralization and nitrification, which could be related to associated changes in enzyme activity and microbial community composition. On the other hand, P addition mitigated N saturation, as expected. Soil inorganic N concentration, N₂O emission and NO3− leaching decreased significantly after P addition, but the net rates of N mineralization and nitrification were significantly increased. Our results provide a new understanding of the N saturation hypothesis, suggesting that the effects of long‐term N deposition on net N mineralization and nitrification rates in N‐saturated tropical forests can be negative and that P addition can alleviate N saturation in such tropical systems.