植物葉片同位素15N自然豐度值（δ15N）受氮循環的多個過程及相互作用的影響，能夠綜合反映生態系統氮循環的特徵，如開放程度和可利用N狀態。大量研究發現，熱帶和亞熱帶相對於溫帶和北方森林氮更為富足，其生態系統氮周轉和循環較快。熱帶亞熱帶森林植物葉片和土壤δ15N比溫帶森林的較高的研究結果也支持這一普通現象。因此，比較不同森林生態系統植物葉δ15N可以判斷生態系統氮狀態。然而，最近幾項發現，我國亞熱帶森林葉片δ15N多為負值，遠遠低於全球熱帶亞熱帶森林的平均值，而與溫帶森林的平均值接近。
為了研究我國南方森林葉片δ15N較低是不是普遍的現象，中國科學院瀋陽應用生態研究所穩定同位素生態學研究組方運霆研究員和林業科學院熱帶林林業研究所合作，對海南尖峰嶺自然保護區4個典型熱帶森林進行了研究。研究發現，無論是原始林還是次生林，植物葉δ15N值都明顯偏負。研究所採集的39個植物種中有35個的δ15N為負值（圖1）。所獲得的結果與其它東亞地區熱帶和亞熱帶森林植物葉片δ15N值相近，卻明顯低於世界其它區域熱帶森林（圖2）。結合土壤有效氮含量和土壤氮礦化硝化速率的數據，植物葉片δ15N的結果說明，我國南方森林可能都是氮限制的生態系統，氮沉降增加可能會促進植物生長而增加對大氣CO2的吸存。但這一推測需要更多研究來驗證。此外，該研究還比較了不同植物種間的δ15N的差異，比較了原始林和次生林間的差異，並探索了為什麼所研究的森林的植物葉片δ15N偏低的原因。
該研究有助於了解我國熱帶森林生態系統氮循環過程、氮循環狀態、植物氮吸收的來源，為該區域森林生態系統的結構和功能等相關研究提供參考。該文章已被生態學領域期刊Oecologia（IF 3.011）正式在線發表（生物谷Bioon.com）。
生物谷推薦的英文摘要
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.