Anthropogenic compound inputs decouple particulate and mineral-associated carbon pools in soils of a tropical urban agroecosystem

Urban soils accumulate diverse anthropogenic compounds (hereafter ACs). Yet, AC accumulation and its influence on soil organic carbon (SOC) dynamics remain poorly constrained. Here, we investigate how urbanisation-derived ACs affect SOC pools and composition in 618 arable topsoils (0–10 cm) in Kumasi, Ghana, using a ‘space-for-time’ substitution with a ‘distance-to-road’ approach to derive a gradient of urbanisation influence. We assessed AC occurrence in coarse soil fraction (≥2 mm), and quantified total SOC in fine fraction (<2 mm) that was separated into particulate (POC) and mineral-associated (MAOC) pools, each partitioned for pyrogenic OC (PyOC), and characterised by mid-infrared spectroscopy. We classified ACs into six types: biogenic litter (plant residues, textiles), pyrogenic (charcoal), plastic, shell/bone, concrete, and inorganic artefacts (metals, ceramics, glass). Biogenic litter and pyrogenic ACs were highly prevalent (>80% of samples) but declined with urbanisation. This decline, together with a non-significant increase in total SOC contents and significant increases in POC and PyOC contents, suggests both ACs progressively degrade and are incorporated into the fine fraction. The remaining durable ACs increased with urbanisation, paralleling rising POC but invariant MAOC, resulting in POC dominance within SOC. These contrasting responses suggest decoupled pool dynamics, whereby the recalcitrant nature of the durable ACs likely retain C in POC rather than in MAOC. Consistently, POC was enriched in reduced aliphatic and condensed aromatic compounds indicative of plastic and pyrogenic ACs. ACs appear to sustain SOC in tropical urban soils through POC enrichment, suggesting an input-driven pathway that may operate widely across urbanising tropical soils.