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

Summary Nitrogen ( N ) saturation hypothesis suggests that when an ecosystem reaches N ‐saturation, continued N input will cause increased N leaching, nitrous oxide ( N 2 O ) 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 N 2 O emission rate and nitrate ( NO 3 − ) leaching rate were measured in an N ‐saturated old‐growth tropical forest in southern C hina, 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, N 2 O emission and nitrate ( NO 3 − ) 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, N 2 O emission and NO 3 − 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.