A basis for organic matter accumulation in soils under anaerobiosis

The disappearance of cellulose from flooded Honeoye silt loam was initially rapid at depths of 0 to 1.5, 7 to 8, and 14 to 15 cm, but the decomposition subsequently stopped at the two lower depths though not in the surface layer. Glucose accumulated as the cellulose was decomposed at all three depths, but the glucose concentration continued to rise in the surface layer, although little or none was formed at the two lower depths after 28d. Flooding the Honeoye soil increased the concentration of extractable Al and Mn, and adding cellulose to this soil increased the levels of extractable sulfide, Mn, and Fe and reduced the pH. We conducted tests at a depth of 14 to 15 cm in Honeoye soil that had been flooded for 60d to determine why cellulose was no longer being decomposed. The addition of nitrate or a brief period of aeration did not enhance cellulose decomposition appreciably, although sulfate additions were stimulatory and caused the pH to rise. Acetic, butyric, and formic acids accumulated in the celluloseamended flooded soil, and the addition of sulfate reduced the butyric acid and often the acetic acid levels at this depth. In studies to determine which substances were toxic in Honeoye soil incubated anaerobically, added sulfide delayed glucose decomposition, an aqueous extract of peat and added ammonium reduced the rate, and a mixture of acetic, butyric, formic, and propionic acids stopped CO2 formation from glucose. The inhibition of glucose mineralization in flooded soil by individual fatty acids and by an aqueous extract of peat was slight at pH 7.0, greater at pH 6.5, and almost complete at pH 6.0. A mixture of the fatty acids had only a modest deleterious effect at pH 7.0 on the conversion of cellulose to water-soluble products in anaerobic Honeoye soil, but almost completely prevented the conversion at pH 5.0. Acetic and butyric acids were present in high concentrations and formic and propionic acids in low concentrations in naturally flooded soils in the field, and their levels were much higher at depths at 6 cm or greater than at the surface. The data suggest that the inhibitory effect of organic acids at low pH values is the reason for the accumulation of organic matter in some flooded soils.