Land use impacts on the isotopic signature (C-13, C-14, N-15) of water-soluble fulvic acids in a German fen area

Little is known regarding the impact of land use on the isotopic signature of dissolved organic matter (DOM), although such impacts should occur first in this most mobile part of soil organic matter. The aim of the present st udy was to investigate the effects of both land use and change on the isoto pic composition of DOM, We extracted water-soluble fulvic acids (FA) as the main fraction of DOM from the topsoil, groundwater, and surface water of s ix sites differing in land use in a natural fen area, and we analyzed C-13, C-14, and N-15. Long-term intensive land use (of at least 50 years) result ed in the enrichment of water-soluble FAs with C-13. Enhanced peat decompos ition caused by conventional crop fanning resulted in the highest C-14 age of water-soluble FAs, The radiocarbon age of the water-soluble FAs in the t opsoil and surface water was as much as 1700 years, much older than previou sly assumed. Long-term agricultural land use with high fertilizer input res ulted in N-14 enrichment of water-soluble FAs, probably because of the inco rporation of light N isotopes derived from mineral fertilizers. C-14 and N- 15 isotopes indicated a distinct change of C and N turnover after changing from conventional crop farming to unimproved pasture. The N and C isotopes of water-soluble FAs are a useful tool to indicate changing C and N cycles caused by longterm (50 to 200 years) differences in land use. Short-term ch anges in land use (<5 years) are reflected only in the C-14 and N-15 signat ure of water-soluble FAs.