Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna

Variation in the stable N isotope ratio (delta N-15) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf delta N-15 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 delta N-15 were also measured along the transect. At the landscape scale, leaf delta N-15 was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil delta N-15, total N, and GWC appeared to contribute to this spatial pattern of leaf delta N-15. 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 delta N-15 and leaf [N] were positively correlated with leaf delta N-15 of non-N-2 fixing plants. Leaf delta N-15 of P. glandulosa, an N-2-fixing legume, did not correlate with leaf [N]; the delta N-15 of P. glandulosa's leaves were closer to atmospheric N-2 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 delta N-15 of C. hookeri and Z. fagara, indicating the N-2-fixing P. glandulosa may be important to the N nutrition of nearby non-N-2-fixing species. Our results indicate plant N-15 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.