Properties of organic matter precipitated from acidic forest soil solutions

Abstract

From solutions of acidic forest soils, up to 90% of the organic matter (OM) can be precipitated by Al. Precipitation of the OM contributes to its resistance to microbial decay, so the process should be of relevance for C cycling and sequestration. However, little is known about the properties and the composition of the precipitated OM. Therefore, we investigated the size of precipitated OM flocs and determined the chemical components which were preferentially precipitated. We used four different OM solutions and precipitated OM at different Al/C ratios and pH values. Results of 13C NMR (nuclear magnetic resonance), 1H NMR and UV/vis spectroscopy clearly showed preferential precipitation of aromatic components with a high content of carboxylic groups at all pH and Al/C ratios. This selective precipitation was supported by FTIR measurements which additionally revealed that binding mechanisms between OM and Al were independent of pH and Al/C values. However, we observed greater Al/C ratios for OM precipitated at pH 4.5 than at 3.8, which could be related to the amount of Al(OH) 2+ cations in solution or the formation of small Al-hydroxide primary particles. The diameter of precipitated OM flocs ranged from 3 to 110 μm, with smaller size at pH 3.8 (ave. 16.6 μm) than at pH 4.5 (ave. 27.6 μm). We observed that the Al cations formed on average 2–3 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, besides the intrinsic stability of the precipitated organic compounds. We conclude that the fate of precipitated OM might be similar in many soils, regardless of the parent dissolved organic matter (DOM) because of equalization of OM properties upon precipitation.