C-13 and N-15 distributions in three spodic dystric cambisols under beech and spruce

The study of natural isotopic abundance signatures is useful to gain further insights in the processes resulting in depthwise changes in the composition of soil organic matter (SOM). Objectives were to describe the delta(13)C and delta(15)N abundances of SOM with depth in soils from a 153-year old beech (B1), a 119-year old spruce (F1) and a 61-year old spruce (F2) stand at Selling, north-west Germany, and to study, how podzolisation affects the isotopic abundances of C-13 and N-15 in the SOM. The degree of podzolisation decreased in the order F1 > B1 > F2. At the surface of the humus layer of all three sites, delta(13)C values are approximately 1 to 4 parts per thousand higher than in the leaves and needles, probably mainly due to the discrimination of C-13 by microbial decomposition. C-13 abundances in the organic layers of Fl and F2 increased only slightly from -27.6 parts per thousand PDB (B1, L) to -27.2 parts per thousand PDB (B1, Oh) and from -26.3 parts per thousand PDB (F2, L) to -25.9 parts per thousand PDB (F2, Oh), suggesting that biotic activity resulted in mixing of organic matter. At Fl, however, C-13 abundance increased from -27.5 parts per thousand PDB (L) to -26.0 parts per thousand PDB (Oh) which reflects the lack of mixing by animals. In the upper 2-4 cm of the mineral soil, i.e., in the eluvial horizons Aeh, C-13 values showed a minimum at the spruce sites which was presumably related to a translocation of C-13 enriched fulvic acids. Depthwise changes in delta(15)N values were not related to podzolisation processes. At all three sites, a N-15 enrichment with depth occurred in the mineral soil which is the result of the discrimination of N-15 by microbial decomposition.