Bai Edith, Boutton Thomas W, Liu Feng, Wu X Ben, Archer Steven R, Hallmark C Thomas
Department of Ecosystem Science and Management, Texas A&M University, College Station, TX 77843-2138, USA.
Oecologia. 2009 Mar;159(3):493-503. doi: 10.1007/s00442-008-1246-0. Epub 2008 Dec 16.
Variation in the stable N isotope ratio (delta15N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf delta15N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long x 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and delta15N were also measured along the transect. At the landscape scale, leaf delta15N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil delta15N, total N, and GWC appeared to contribute to this spatial pattern of leaf delta15N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil delta15N and leaf [N] were positively correlated with leaf delta15N of non-N2 fixing plants. Leaf delta15N of P. glandulosa, an N2-fixing legume, did not correlate with leaf [N]; the delta15N of P. glandulosa's leaves were closer to atmospheric N2 and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf delta15N of C. hookeri and Z. fagara, indicating the N2-fixing P. glandulosa may be important to the N nutrition of nearby non-N2-fixing species. Our results indicate plant 15N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales.
植物和土壤中稳定氮同位素比率（δ¹⁵N）注）的变化常常反映环境对氮循环的影响。2005年4月和8月，我们在美国得克萨斯州南部亚热带稀树草原生态系统中，对一条长308米、宽12米的带状样带内的所有腺牧豆树（落叶豆科乔木）、胡氏苦槛蓝（常绿灌木）和墨西哥花椒（常绿灌木）个体，测量了叶片δ¹⁵N和氮浓度（[N]）。同时还沿着样带测量了土壤质地、重量含水量（GWC）、总氮和δ¹⁵N。在景观尺度上，沿着这条地形 - 土壤序列，这三个物种的叶片δ¹⁵N均与海拔呈负相关。土壤δ¹⁵N、总氮和GWC的变化似乎促成了叶片δ¹⁵N的这种空间格局。在景观的较低部分，较高的土壤氮有效性和GWC与较高的氮矿化率和硝化率相关。土壤δ¹⁵N和叶片[氮]均与非固氮植物的叶片δ¹⁵N呈正相关。固氮豆科植物腺牧豆树的叶片δ¹⁵N与叶片[氮]不相关；腺牧豆树叶片的δ¹⁵N更接近大气中的N₂，且显著低于胡氏苦槛蓝和墨西哥花椒的叶片δ¹⁵N。此外，在较小的空间尺度上，一个邻近指数（反映周围腺牧豆树的密度和距离）与胡氏苦槛蓝和墨西哥花椒的叶片δ¹⁵N呈负相关，这表明固氮的腺牧豆树可能对附近非固氮物种的氮营养很重要。我们的结果表明，植物¹⁵N自然丰度可以反映氮保留程度，并有助于我们更好地理解生态系统和景观尺度上的氮动态以及植物 - 土壤相互作用。
