氮沉降加剧热带森林土壤酸化

记者从中科院华南植物园获悉，该园科学家近期发现氮沉降可能加剧热带/亚热带森林土壤酸化。相关成果发表于《全球变化生物学》杂志。

研究人员在鼎湖山国家级自然保护区“富氮”的南亚热带成熟林建立了长期氮沉降研究样地，并对氮沉降增加如何影响该区森林土壤酸化进程，进行了长达6年的持续跟踪研究。

研究发现，长期氮沉降加剧热带/亚热带森林土壤酸化。科学家研究的这片树林正在经历严重的土壤酸化过程。不过，有关土壤酸化的格局挑战了源于温带生态系统的发现。例如，长期氮沉降增加并未改变土壤可交换铝，但显著增加了土壤离子库中可交换氢的比例，同时提高了土壤阳离子交换量。

上述现象表明，随着大气氮沉降的增加，热带/亚热带森林土壤钙、镁盐基离子的缺乏，比土壤铝毒问题更应受到关注。（来源：中国科学报 李洁尉 周飞 鲁显楷）

Nitrogen deposition contributes to soil acidification in tropical ecosystems

Abstract Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long-term N deposition on soil acidification processes in tropical forests. We have established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China since 2002 with N addition as NH4NO3 of 0, 50, 100 and 150 kg N ha−1 yr−1. We measured soil acidification status and element leaching in soil drainage solution after 6-year N addition.

Results showed that our study site has been experiencing serious soil acidification and was quite acid-sensitive showing high acidification (pH(H2O)<4.0), negative water-extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long-term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al3+, but increased cation exchange capacity (CEC). Nitrogen addition-induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20cm depth, but not at 40cm. Furthermore, there was no evidence that Al3+ was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H+ dominating changes of soil cation pool, an exhausted base cation pool, N-addition stimulating SOC production, and N saturation. Our results suggest that long-term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.

原文链接：http://onlinelibrary.wiley.com/doi/10.1111/gcb.12665/pdf