植物叶片同位素15N自然丰度值（δ15N）受氮循环的多个过程及相互作用的影响，能够综合反映生态系统氮循环的特征，如开放程度和可利用N状态。大量研究发现，热带和亚热带相对于温带和北方森林氮更为富足，其生态系统氮周转和循环较快。热带亚热带森林植物叶片和土壤δ15N比温带森林的较高的研究结果也支持这一普通现象。因此，比较不同森林生态系统植物叶δ15N可以判断生态系统氮状态。然而，最近几项发现，我国亚热带森林叶片δ15N多为负值，远远低于全球热带亚热带森林的平均值，而与温带森林的平均值接近。
为了研究我国南方森林叶片δ15N较低是不是普遍的现象，中国科学院沈阳应用生态研究所稳定同位素生态学研究组方运霆研究员和林业科学院热带林林业研究所合作，对海南尖峰岭自然保护区4个典型热带森林进行了研究。研究发现，无论是原始林还是次生林，植物叶δ15N值都明显偏负。研究所采集的39个植物种中有35个的δ15N为负值（图1）。所获得的结果与其它东亚地区热带和亚热带森林植物叶片δ15N值相近，却明显低于世界其它区域热带森林（图2）。结合土壤有效氮含量和土壤氮矿化硝化速率的数据，植物叶片δ15N的结果说明，我国南方森林可能都是氮限制的生态系统，氮沉降增加可能会促进植物生长而增加对大气CO2的吸存。但这一推测需要更多研究来验证。此外，该研究还比较了不同植物种间的δ15N的差异，比较了原始林和次生林间的差异，并探索了为什么所研究的森林的植物叶片δ15N偏低的原因。
该研究有助于了解我国热带森林生态系统氮循环过程、氮循环状态、植物氮吸收的来源，为该区域森林生态系统的结构和功能等相关研究提供参考。该文章已被生态学领域期刊Oecologia（IF 3.011）正式在线发表（生物谷Bioon.com）。
生物谷推荐的英文摘要
Oecologia DOI: 10.1007/s00442-013-2778-5
Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China
Ang Wang, Yun-Ting Fang, De-Xiang Chen, Keisuke Koba, Akiko Makabe,Yi-De Li, Tu-Shou Luo, Muneoki Yoh
The foliar stable N isotope ratio (δ15N) can provide integrated information on ecosystem N cycling. Here we present the δ15N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ15N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ15N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more 15N enriched. Our results show that foliar δ15N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO3? had low δ15N (?11.4 to ?3.2 ‰) and plant NO3? uptake could not explain the negative foliar δ15N values (NH4+ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH4+ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH3/NH4+. The variation in foliar δ15N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ15N and the enrichment factor (foliar δ15N minus soil δ15N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ15N between primary and secondary forests.
