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 study was to investigate the effects of both land use and change on the isotopic composition of DOM, We extracted water-soluble fulvic acids (FA) as the main fraction of DOM from the topsoil, groundwater, and surface water of six 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) resulted in the enrichment of water-soluble FAs with C-13. Enhanced peat decomposition 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 topsoil and surface water was as much as 1700 years, much older than previously assumed. Long-term agricultural land use with high fertilizer input resulted in N-14 enrichment of water-soluble FAs, probably because of the incorporation 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 changes in land use (<5 years) are reflected only in the C-14 and N-15 signature of water-soluble FAs.