Data from: 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 (inline image) 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 (inline image) 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 inline image 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.

氮饱和假说（Nitrogen saturation hypothesis）提出，当生态系统达到氮饱和状态时，持续的氮输入会导致氮淋失、一氧化二氮（N₂O）排放以及氮矿化和硝化速率的提升。该假说同时指出，当生态系统达到氮饱和后，其他元素将成为主要限制因子。尽管这一假说已在温带森林中得到验证，但将其直接应用于氮饱和热带森林的相关研究仍较为匮乏。为验证该假说，研究团队在中国南部一处氮饱和的热带老龄林中，开展了为期6年的氮、磷添加实验，并测定了土壤无机氮、土壤氮矿化与硝化速率、土壤N₂O排放速率以及硝酸盐（inline image）淋失速率。我们提出如下研究假设：氮添加会进一步加剧系统的氮饱和状态，而磷添加则可缓解氮饱和。正如预期，实验结果显示，连续6年的人工氮添加确实加剧了氮饱和状态，具体表现为土壤无机氮浓度、N₂O排放与硝酸盐（inline image）淋失量显著升高。但与预期相悖的是，氮添加显著降低了原位净氮矿化与硝化速率，这一现象可能与酶活性及微生物群落组成的相关变化密切相关。另一方面，磷添加如预期般缓解了氮饱和状态：磷添加后，土壤无机氮浓度、N₂O排放及inline image淋失量显著降低，而净氮矿化与硝化速率则显著提升。本研究结果为氮饱和假说提供了新的认知，表明长期氮沉降对氮饱和热带森林的净氮矿化与硝化速率可产生负面影响，且磷添加能够缓解此类热带生态系统的氮饱和状态。