Lower residue decomposition in historically charcoal-enriched soils is related to increased adsorption of organic matter

- Mineralization of added maize straw is lowered in charcoal-enriched soils.
- Charcoal-enriched and adjacent soils have similar microbial biomass concentration.
- Recovery of DOC is reduced with increasing soil charcoal concentration.
- We suggest carbon sorption onto charcoal as main stabilization mechanism.

Previous field observations have shown increased soil carbon (C) sequestration in charcoal amended soils due to an accumulation of non-charcoal derived soil C. This study was set up to compare and analyse mineralization of non-charcoal derived C between soils that were either or not historically enriched in charcoal. Maize straw (13C-enriched) was added to samples of arable soil collected under historical charcoal kilns and corresponding adjacent control soil. The respiration was monitored in laboratory conditions for 227 days. Charcoal in soil significantly lowered total soil respiration (1905 vs. 1984 mg C kg−1). Mineralization of 13C-enriched added maize C (AMC) was unaffected by charcoal in the initial weeks after maize straw addition, however differences became significant at longer incubations yielding a markedly lower mineralized fraction in the charcoal enriched samples after 227 days (70 versus 62%, P < 0.05). A two fraction mineralization model revealed no charcoal effect on the labile fraction and its degradation but that the stable fraction of AMC was larger and degraded slower in the presence of charcoal. A soil drying-rewetting event (35 days) increased respiration to the same extent in charcoal-enriched as in adjacent soils. In contrast, the dissolved organic carbon (DOC) in pore water was significantly lower in charcoal-enriched than in adjacent soils. Microbial biomass-C (MBC) determined by fumigation-centrifugation was not significantly different between charcoal-enriched and adjacent soil (309 mg vs. 266 mg MBC kg−1). A soil adsorption experiment with DOC, extracted from a grassland soil, revealed larger DOC sorption with increasing soil charcoal concentration. This study shows that reduced C mineralization of non-charcoal C in charcoal-enriched soil is most likely related to enhanced sorption of more recalcitrant organic matter, rather than to lower MBC.