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