Variability of plant nitrogen and water use in a 100-m transect of a subdesertic depression of the Ebro valley (Spain) characterized by leaf delta C-13 and delta N-15

We studied carbon and nitrogen isotopic composition (delta(13)C and delta(1 5)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 reli ef and micrometeorology in a subdesertic valley of central-NE Spain. Minimu m 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 availabilit y was lower. However, different species (functional groups) presented diffe rent 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 sh owed 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 di fferent available water and nitrogen resources within small catchments. For this ecosystem type, C and N isotope analyses are sensitive enough to reso lve fine spatial and functional patterns even over a very short distance (1 00 m), where topography generates great gradients in microclimate, hydrolog y, soil physical conditions, vegetation and biogeochemistry. (C) Elsevier, Paris.