Changes in stable isotopic signatures of soil nitrogen and carbon during 40 years of forest development

Understanding what governs patterns of soil delta15N and delta13C is limited by the absence of these data assembled throughout the development of individual ecosystems. These patterns are important because stable isotopes of soil organic N and C are integrative indicators of biogeochemical processing of soil organic matter. We examined delta15N of soil organic matter (delta15NSOM) and delta13CSOM of archived soil samples across four decades from four depths of an aggrading forest in southeastern USA. The site supports an old-field pine forest in which the N cycle is affected by former agricultural fertilization, massive accumulation of soil N by aggrading trees over four decades, and small to insignificant fluxes of N via NH3 volatilization, nitrification, and denitrification. We examine isotopic data and the N and C dynamics of this ecosystem to evaluate mechanisms driving isotopic shifts over time. With forest development, delta13CSOM became depth-dependent. This trend resulted from a decline of approximately 2 per thousand in the surficial 15 cm of mineral soil to -26.0 per thousand, due to organic matter inputs from forest vegetation. Deeper layers exhibited relatively little trend in delta13CSOM with time. In contrast, delta15NSOM was most dynamic in deeper layers. During the four decades of forest development, the deepest layer (35-60 cm) reached a maximum delta15N value of 9.1 per thousand, increasing by 7.6 per thousand. The transfer of > 800 kg ha(-1) of soil organic N into aggrading vegetation and the forest floor and the apparent large proportion of ectomycorrhizal (ECM) fungi in these soils suggest that fractionation via microbial transformations must be the major process changing delta15N in these soils. Accretion of isotopically enriched compounds derived from microbial cells (i.e., ECM fungi) likely promote isotopic enrichment of soils over time. The work indicates the rapid rate at which ecosystem development can impart delta15NSOM and delta13CSOM signatures associated with undisturbed soil profiles.