Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China.
Glob Chang Biol. 2014 Dec;20(12):3790-801. doi: 10.1111/gcb.12665. Epub 2014 Jul 16.
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 NH4 NO3 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 Al(3+) , 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 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al(3+) 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.
人为氮（N）沉降的增加极大地改变了陆地生态系统的功能，通过酸沉降和富营养化对北半球的温带和北方森林的生态系统健康造成了威胁。然而，与其他森林类型相比，在湿润热带地区，森林土壤酸化对 N 沉降的响应研究较少。本研究旨在探索长期 N 沉降对热带森林土壤酸化过程的影响。自 2002 年以来，我们在中国南方的一个富氮低地热带森林中建立了一个长期 N 沉降实验，N 添加方式为 NH4NO3，添加量分别为 0、50、100 和 150 kg N ha-1 yr-1。我们测量了经过 6 年 N 添加后土壤渗滤液中的土壤酸化状况和元素淋失情况。结果表明，研究地点一直处于严重的土壤酸化状态，对酸化非常敏感，表现出高酸化（pH（H2O）<4.0）、负水提取酸中和能力（ANC）和低基础饱和度（BS，<8%）。长期 N 添加显著加速了土壤酸化，导致基础阳离子枯竭和 BS 降低，进一步降低了 ANC。然而，N 添加并没有改变可交换的 Al（3+），但增加了阳离子交换能力（CEC）。增加的 SOC 可能是由于更高的 CEC 和更低的 pH 值导致的。我们进一步发现，增加的 N 添加大大降低了 20 cm 深处的土壤溶液 pH 值，但对 40 cm 深处没有影响。此外，没有证据表明 Al（3+）从更深的土壤中淋出。这些在热带气候下的独特反应可能是由于：交换性 H（+）主导土壤阳离子库的变化、基础阳离子库枯竭、N 添加刺激 SOC 产生以及 N 饱和。我们的研究结果表明，长期 N 添加会对土壤酸化产生可测量的影响，在未来富含 N 的热带森林中，应更加关注土壤交换性 Al 以外的 Ca 和 Mg 等短缺问题。
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