Stable isotope signatures in bulk samples from two soils with contrasting characteristics. What do they tell about ongoing pedogenic processes?

Abstract

Isotopic ratio mass spectrometry (IRMS) has been proven as a promising tool for the monitoring of biogeochemical processes in soil. In this work, stable isotope signatures of light elements ¿15N, ¿13C, ¿18O and ¿D were determined for two soils with contrasting characteristics in terms of climate, vegetation, land use and management. The studied soils were a Cambisol from a subtropical area (Paraná region, South Brazil) and an Arenosol from a Mediterranean climate (Andalusia, South Spain). A Flash 2000 HT (N, C, S, H and O) elemental analyzer (Thermo Scientific) coupled to a Delta V Advantage IRMS (Thermo Scientific) was used. Isotopic ratios are reported as parts per thousand (¿,) deviations from appropriate standards recognized by the international atomic energy agency (IAEA). In a first approach we took advantage of the well-known different ¿13C signature between plants using either the C4 or C3 carbon fixation pathway (O¿Leary, 1981). The Arenosol (Spain) revealed a ¿13C signature which is clearly in the range of C3 plants (-26 to -30 ¿,). Different plant canopies (tree, shrubs or ferns) caused only slight variations ¿13C (STD= 0.98). In contrast, the Cambisol (Brazil) showed less depletion of the heavier carbon isotope corresponding to C4 predominant vegetation. In addition an increase from -19 ¿ in the soil surface (0 ¿ 5 cm) to -16 ¿ in the subsoil (20 ¿ 30 cm) was observed in line with a recent (2 years old) shift of the land use from the predominant C4 grassland to eucalypt (C3) cultivation. Crossplots of ¿15N vs. ¿18O may provide information about nitrate (NO3-) sources and N cycling (Kendall, 1998). In the Mediterranean Arenosol this signal (¿18O = 30¿, ¿15N = 2¿,) was found compatible with a predominant nitrate atmospheric deposition, whereas the signal in the Brazilian Cambisol pointed to the use of a mineral N fertilization with signs of denitrification processes (¿18O = 13¿, ¿15N = 9¿,). No conclusive results could be obtained from the ¿D isotopic signature probably due to overlapping of the ¿D signals from the organic and the mineral fractions. For a more detailed analysis steps allowing their separation are necessary (Ruppenthal et al. (2013) and references therein).