Project code: 33176663
Contract period: 01.01.2006
Purpose of research: Basic research
From soil solutions of acidic forest soils, up to 90% of organic matter (OM) can be precipitated by Al. Precipitation of OM contributes to its stabilization against microbial decay. Thus, this process should be of major relevance for C cycling and sequestration, but little is known about the properties and the composition of precipitated OM. Furthermore, it is still unclear whether Al reduces C mineralization by direct toxicity to microorganisms or by decreased bioavailability of OM because dissolved organic matter (DOM) is precipitated by Al. For our experiments we used DOM from organic horizons of two acidic forest soils. Organic matter was precipitated by addition of Al at concentrations typical for acidic forest soils at pH values of 3.8 and 4.5. We carried out incubation experiments, investigated the sizes of precipitated OM flocs and determined the chemical components which were preferentially precipitated. We determined different Al species including the potentially toxic Al3+, by Diffusive Gradients in Thin Films (DGT). A large fraction of DOM can be precipitated by Al and is thereby substantially stabilized against microbial decay. When precipitated OM remained in solution total C degradation was reduced by up to 65%. This total stabilization was strongly dependent on the amount of OM precipitated. The C stabilization upon Al addition did not result from toxic effects, but was caused by reduced bioavailability of OM after its precipitation. This was related to the preferential precipitation of aromatic compounds with low N contents and a large number of functional groups being complexed by Al. The type of bonds between OM functional groups and the Al cations was independent of pH, Al/C ratios and solution composition. However, we observed larger Al/C ratios in OM precipitated at pH 4.5 than at 3.8, which could be related to the amount of Al(OH)2+ cations present in solution or the formation of small Al-hydroxide primary particles. The diameters of precipitated OM flocs ranged from 3 to 110 μm with smaller sizes at pH 3.8 (average: 16.6 μm) than at pH 4.5 (average: 27.6 μm). We observed that the Al cations formed on average 2 to 3 bonds to the organic molecules. As a consequence, the spatial accessibility of the OM should be considerably reduced in these flocs leading to additional stabilization. Also the large amounts of non-precipitated, colloidal Al-OM complexes which remained in solution could contribute to OM stabilization. We conclude that precipitation of DOM is an important pathway for long-term carbon stabilization in mineral horizons of acidic forest soils. Stabilization is very efficiently because all known stabilization mechanisms are involved; preferential precipitation of recalcitrant compounds, formation of strong chemical bonds between Al and organic matter (i.e. interactions with the mineral soil phase) and limited spatial accessibility of organic matter in the precipitated flocs.