Variability of plant nitrogen and water use in a 100-m transect of a subdesertic depression of the Ebro valley (Spain) characterized by leaf δ13C and δ15N

We studied carbon and nitrogen isotopic composition (delta(13)C and delta(15)N) in sunlit leaves of four dominant species (Rosmarinus officinalis L., Stipa parviflora L., Juniperus thurifera I,. and Pinus halepensis L.) in a characteristic gradient of water and nitrogen availability produced by relief and micrometeorology in a subdesertic valley of central-NE Spain. Minimum values of delta(13)C were found at the foothills, and higher values were found both in the valley and on the top of the hill where water availability was lower. However, different species (functional groups) presented different delta(13)C values in the same valley. The lowest values of delta(15)N were found on the top of the hill and the highest ones in the valley, where N losses would thus be higher. In general, when growing together, trees showed 2 parts per thousand higher values for delta(13)C as well as for delta(15)N than shrubs and grasses. The specific responses show that they use different available water and nitrogen resources within small catchments. For this ecosystem type, C and N isotope analyses are sensitive enough to resolve fine spatial and functional patterns even over a very short distance (100 m), where topography generates great gradients in microclimate, hydrology, soil physical conditions, vegetation and biogeochemistry. (C) Elsevier, Paris.