Airborne contaminants in the refractory organic carbon fraction of arable soils in highly industrialized areas

Airborne carbonaceous particles produced by industrial activity have been recorded in soils in proximity to contamination sources that produce coal dust, coke and other residues from fossil fuel combustion. The present study assesses the contribution of airborne contaminants to the refractory organic carbon pool of three soils (a sandy Albic Luvisol, sandy Orthic Luvisol and a loamy Haplic Chemozem) in C-depleted and conventionally managed plots from European long-term experiments located in industrialized regions. The C-depleted plots (unmanured plots and bare fallow) are depleted in labile C and thus relatively enriched in refractory C as compared with the conventionally managed plots. Bulk soils and particle-size fraction were analyzed for soil organic carbon (SOC) structure by cross-polarization magic-angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy. Coarse-intermediate separates were subjected to a morphological characterization of organic matter (OM) composition by coal petrography. The C-depleted plots show unusually high proportions of aromatic C in nearly all separates, most pronounced in coarse-intermediate fractions. Coal petrographic analysis indicates that high aromaticity in separates 2000-20 μm can be explained by the presence of air-borne contaminants that are identified as coal, coke and combustion residues from fossil fuels. For the two sandy sites, aromatic C in the fine separates (< 6 μm) is dominated by protonated aryl C as revealed by dipolar dephasing (DD) NMR experiments. For the loamy soil, aromatic C was comparatively high and about half of the aryl signal was attributable to non-protonated/C-substituted carbons. These results indicate a contribution of airborne contaminants to the refractory C pool for all investigated sites. For the two sandy soils, this contribution appears to be limited to coarse-intermediate separates; in the loamy soil, a contamination is also present in fine separates. From the OC balance of the C-depleted plots, it was estimated that the contribution from contaminants to the refractory C pool is still smaller than that from native OM, which is found mainly within fine particle-size separates. The presence of refractory contaminants will strongly affect the estimation of the long-term turnover of OC at such long-term experiments. A careful examination of the SOM composition and the SOM fractions at such sites is necessary to avoid biased results on SOM turnover.