Nitrogen deposition and its fate in N-rich tropical forests of Southern China

Background/Question/Methods

Elevated atmospheric nitrogen (N) deposition has greatly accelerated terrestrial N cycling at global scale, leading to N accumulation and even N saturation in many naturally N-limited temperate/boreal ecosystems. However, there remains unclear on the fates of elevated N deposition in tropical ecosystems, which are often regarded as N-rich with the lack of N limitation to NPP. To address this question, we expanded on a long-term N addition experiment in a representative N-rich tropical forest, with varied N addition levels: 0, 50, 100 and 150 kg N ha^-1 yr^-1, respectively. Specially, we focused on the following questions: (1) What is the dominant form of N leached out from this N-rich forest? (2) Does elevated N addition increase N sequestration in both plants and soils? (3) Can δ¹⁵N be used to assess ecosystem N status? Here we measured soil and plant N status, and N leaching dynamics in water solution after 6-10 years of N addition.

Results/Conclusions

Our results showed that long-term N addition significantly increased N leaching loss from soils during the whole year, and NO₃^- dominated N leaching dynamics (~80%) in soil solutions, indicating ecosystem N saturation. However, calculations from N budget balances indicated that this N-saturated forest could still accumulate quite an amount of added N inputs. Notably, there were no changes in N contents of different plant pools of dominant species, and there were no significant plant growth response (e.g., no change in monthly littlerfall) under N treatments. Hence, N-addition induced N accumulation mainly occurred in soils, which was proved well by significant increases in total soil N with elevated N additions. There were tightly positive relationships between soil total N and total C contents across all plots. Further results showed that long-term N addition did not affect water extractable soil N, indicating that the added N could react with soil organic matter and be turned into recalcitrant compounds, which were not easy to leach out. Lastly, we found that δ¹⁵N values of plant pools were sensitive to elevated N inputs, and increased greatly over the last 10 years.

In conclusion, our findings suggest that: (1) Long-term N deposition can accelerate N leaching losses mainly as NO₃^- from N-rich tropical forests; (2) N-rich forests can still sequestrate excess N input, which mainly ended in soils, though they have been N-saturated; and (3) measurement of ecosystem δ¹⁵N can be used well to assess ecosystem N status.