Soil carbon characterization in a subtropical drained peatland

•Organic matter and organic carbon decrease as soils get shallower.•The dominant carbon pool in the studied drained peatland is recalcitrant carbon.•Recalcitrant carbon is enriched in aromatic and depleted in O-alkyl C compounds.•Recalcitrant carbon is vulnerable to soil subsidence.•The addition of fresh biomass could minimize subsidence and labile carbon losses. Peatlands store a significant portion of the global carbon (C) budget. Peatland drainage triggers soil subsidence and is a threat to soil conservation. Differences in C composition as soils get shallower in drained peatlands are not clearly understood. Soil C quality has been considered as one of the possible causes for changes in soil subsidence rates in drained peatlands. The purpose of this research was to characterize soil C in drained peats experiencing soils subsidence by using a fractionation procedure and 13C Nuclear Magnetic Resonance (13C NMR) spectroscopy. The study was done in the Everglades Agricultural Area (EAA), located south of Lake Okeechobee in south Florida, US. The EAA is a subtropical peatland that has been drained for more than 100 years. Subsidence rates at the EAA have shown a reduction in recent decades possibly due to higher water tables and increases in inorganic and recalcitrant C (Shih et al., 1998). Analyses were performed for the top and bottom of sampled peat soils collected from shallow and deep soil areas within the EAA. Hot water (80 °C) and acid extractions were used to determine operational C fractions with residues characterized by magic angle spinning (MAS) 13C NMR spectroscopy before and after extraction to quantify C compound classes. Recalcitrant C is the dominant C pool in EAA soils with 85% of total carbon (TC) on average, while acid extracted C, hot water extracted C, and inorganic C comprise 19%, 1%, and 0.7% of TC, respectively. Shallow soils had significantly less organic matter (OM), hot water extracted C, and recalcitrant C, and showed a reduction of OM and C fractions deeper in the soil profile. The 13C NMR spectra analyses indicate that average C compounds in EAA peats are O-alkyl C (36.9%), followed by alkyl C (28.2%), aromatic C (24%), and carboxyl C (10.8%), with shallow and deep soils having a similar distribution of these C compounds. Extracted recalcitrant C fractions of both shallow and deep soils showed an increase in aromatic C (34.3%) and a reduction in O-alkyl C (26.6%). Subsidence reduced OM in shallow soils and can reduce soil C, in