Source and depth translocation of combustion residues in Chinese agroecosystems determined from parallel polycyclic aromatic hydrocarbon (PAH) and black carbon (BC) analysis

Rice straw burning in agroecosystems delivers BC (black carbon) to soil/sediment but its contribution relative to other BC sources is unspecified. We have differentiated for the first time combustion residues (CRs) derived from autochthonous and pyrogenic agricultural vs. allochthonous and biogenic riverine input by way of complementary analysis of BC and PAH (polycyclic aromatic hydrocarbon) abundance and composition. Both CR markers in four upland and six periodically flooded paddy soils having different cultivation times were analyzed. BC was the quantitatively dominant CR fraction at 16-512 mg/g SOC (soil organic carbon), whereas PAHs yielded trace amounts of eleven EPA-PAHs at 1.5-47.0 μg/g SOC. CR concentrations in paddy soils exceeded those in upland topsoil. The lowest BC and PAH concentration occurred in subsoil ⩽ 700 yr old, independent of paddy or upland use, but BC content increased in paddy subsoil ⩾ 700 yr old. Exceptional BC enrichment and composition of benzene polycarboxylic acid (BPCA) products revealed dark buried horizons (700 and 1000 yr sites) as former topsoil. Relative distributions of 5-ring PAHs differentiated CRs from biogenic residues in substrate, topsoil and subsoil. Perylene was enriched in tidal wetland substrate, indicating allochthonous input from soil/sediment erosion in the Yangtze River catchment. An identical pattern was observed in subsoil. Present and former topsoil yielded a distinctive distribution of rice ash PAHs. Although PAHs occurred in traces, their wider compositional variation improved source discrimination power exceeding that of BPCAs. The former are recommended for source identification and the latter for quantifying CR input to soil/sediment. Parallel application of both methodological approaches is advocated for investigating combustion processes in ecosystems.