C-rich sandy Ap horizons of specific historical land-use contain large fractions of refractory organic matter

The study focuses on quantitatively important fractions of very stable organic matter in sandy arable soils in which clay and silt, commonly assumed to be the major factors for organic carbon (OC) stabilization, are practically missing. We characterized 23 Ap horizons plus one subsoil with respect to total OC, total N, chemical resistance of OC, microbial biomass, respiration in laboratory incubation and solubility of OC in water (WSOC). The soils varied with respect to historical land-use (heathland, plaggen-manuring, woodland) and soil genetic factors (podzolization, groundwater). Data from long-term experiments on sand and sandy loam were also considered. ils revealed a wide range of OC content from below 10 to about 50 mgg−1. C to N ratios varied between 10 and 30 and were positively linked to total OC (R2=0.87). Graphitized C (‘charcoal’, as H2O2-resistant fraction) was of minor importance, with just about 0.5 mg C g−1 soil in most samples. Only some plaggen and heath soils had charcoal C contents around 1 and 1.5 mg g−1 but this was still less than 5% of the total OC. The HCl-resistant C fraction, on the other hand, was extremely large, amounting to 50–90% of the total OC. Its percentage was closely related to total OC (R2=0.84). Biomass C contents (fumigation–extraction) were not correlated with total OC. The C-rich soils had low biomass C to total OC ratios (<0.005). The activities of the enzymes catalase and dehydrogenase showed similar relationships. This strongly indicated that major proportions of the organic material do not support a microbial population and are not in a process of net-decomposition. The average respiration rates per unit of OC after 100 days of incubation at 20 °C were lowest in the C-rich samples. Water soluble organic carbon (WSOC) increased slightly with total OC. On an average, a sum of 379±80 μg C g−1 soil was obtained over seven sequential extractions. This was about the same amount as respired in 140 days (373±107 μg CO2-C g−1) but there was no significant correlation between the respired CO2-C and the extracted WSOC. The steady-state rate of WSOC released (steps 3–7) per unit of total OC was lowest in the C-rich soils, again indicating the high stability of OC. We concluded that old sandy Ap horizons soils may have high levels of OC with high proportions of very stable (refractory) constituents amounting to more than 50% of total OC or 10–30 mg C g−1 soil.