Which are important soil parameters influencing the spatial heterogeneity of ¹⁴C in soil organic matter?

Abstract. Radiocarbon (¹⁴C) analysis is an important tool that can provide information on the dynamics of organic matter in soils. Radiocarbon concentrations of soil organic matter (SOM) however, reflect the heterogeneous mixture of various organic compounds and are affected by different chemical, biological, and physical soil parameters. These parameters can vary strongly in soil profiles and thus affect the spatial distribution of the apparent ¹⁴C age of SOM considerably. The heterogeneity of SOM and its ¹⁴C signature may be even larger in subsoil horizons, which are thought to receive organic carbon inputs following preferential pathways. This will bias conclusions drawn from ¹⁴C analyses of individual soil profiles considerably. We thus investigated important soil parameters, which may influence the ¹⁴C distribution of SOM as well as the spatial heterogeneity of ¹⁴C distributions in soil profiles. The suspected strong heterogeneity and spatial variability, respectively of bulk SOM is confirmed by the variable ¹⁴C distribution in three 185 cm deep profiles in a Dystric Cambisol. The ¹⁴C contents are most variable in the C horizons because of large differences in the abundance of roots there. The distribution of root biomass and necromass and its organic carbon input is the most important factor affecting the ¹⁴C distribution of bulk SOM. The distance of the soil profiles to a beech did not influence the horizontal and vertical distribution of roots and ¹⁴C concentrations. Other parameters were found to be of minor importance including microbial biomass-derived carbon and soil texture. The microbial biomass however, may promote a faster turnover of SOM at hot spots resulting in lower ¹⁴C concentration there. Soil texture had no statistically significant influence on the spatial ¹⁴C distribution of bulk SOM. However, SOM in fine silt and clay sized particles (< 6.3 µm) yields slightly higher ¹⁴C concentrations than bulk SOM particularly at greater soil depth, which is in contrast to previous studies where silt and clay fractions contained older SOM stabilized by organo-mineral interaction. ¹⁴C contents of fine silt and clay correlate with the microbial biomass-derived carbon suggesting a considerable contribution of microbial-derived organic carbon. In conclusion, ¹⁴C analyses of bulk SOM mainly reflect the spatial distribution of roots, which is strongly variable even on a small spatial scale of few meters. This finding should be considered when using ¹⁴C analysis to determine SOM.