Precipitation of dissolved organic matter by aluminum stabilizes carbon in acidic forest soils

Dissolved organic matter (DOM) is often neglected as a factor in the formation of stable soil organic matter (OM). Precipitation of DOM by dissolved Al could contribute substantially to C retention in acidic forest soils; however, no information is available on the stability of precipitated OM against microbial decay. We investigated the stability of Al-OM precipitates against microbial decay as related to (i) DOM composition, (ii) Al speciation, and (iii) the dissolved Al/C ratio. We produced Al-OM precipitates by adding AlCl3 (molar Al/C ratios: 0.05-0.3) at pH values of 3.8 and 4.5 to DOM solutions derived from Oi and Oa horizons, from either beech (Fagus sylvatica L.) or spruce [Picea abies (L.) Karst.] litter. Between 13 and 84% of the C was precipitated, depending on pH, Al/C ratio, and the type of DOM. Precipitates were found to be enriched in aromatic C and mostly depleted in N when compared with DOM. Only 0.5 to 7.7% of precipitated C was mineralized during 7 wk of incubation. Mineralization of Al-OM precipitates was up to 28 times less than that of the respective DOM solutions. The extent of mineralization of Al-OM precipitates formed at pH 3.8 was reduced by 50 to 75% when compared with those formed at pH 4.5. The stability of precipitates against microbial decay increased with larger aromatic C content and larger C/N ratios. Our study clearly demonstrated that a large fraction of DOM can be precipitated and is thereby substantially stabilized against microbial decay.