Nitrogen deposition contributes to soil acidification in tropical ecosystems

Background/Question/Methods

Elevated anthropogenic nitrogen (N) deposition has greatly changed terrestrial ecosystem N cycling, and threatened ecosystem health including acidification and eutrophication. Atmospheric N deposition has been shifted from a temperate zone concern to a global issue with dramatic increases predicted in the tropics over the next several decades due to intensifying human activities. However, responses of soil acidification to N deposition are often ignored in the humid tropics, because soil acidification is recognized as a major pedogenetic process that occurs in climates where precipitation exceeds evapotranspiration, and widely exists in these areas. Also our understanding mainly comes from originally N-limited temperate ecosystems. The object of our study is to explore how long-term N deposition affects soil acidification process and pattern in tropical forests. We hypothesize that tropical forests are vulnerable to N addition, and elevated N addition will decrease soil buffering capacity and accelerate soil acidification. 

In 2002, we established long-term N deposition research plots in typical N-rich lowland tropical forests of Southern China. Experimental N addition was administered at the following levels: 0, 50, 100 and 150 kg N ha^-1 yr^-1, respectively. Here we measured soil acidification status (0-40 cm soil layers) and element leaching dynamics in water solution after 7 years of N addition.

 

Results/Conclusions

Our results showed that the studied forest was a tropical- type of acidification and was acid-sensitive ecosystem with high acidification, richness in aluminum ion and low base cation contents, which was inherently different from that of temperate ecosystems. We found that long-term high N addition significantly increased the mobility and leaching of inorganic N (especially for NO₃⁻-N), leading to the exhausted soil base cations (e.g. Ca and Mg) and the increased net acid loads in soils. Thus, soil base saturation (BC) and acid neutralizing capacity (ANC) decreased greatly, which further accelerated soil acidification, and decreased soil buffering capacity. Meanwhile, we found that long-term N addition did not affected soil exchangeable Al, which was on the contrary to the traditional finding. Our findings suggested that the shortage of Ca and Mg should be paid more attention to than Al toxicity in tropical forests with the elevated N deposition in the future.