INFLUENCE OF LIQUID CATTLE MANURE ON REDUCTION PROCESSES IN SOIL

Reduction processes in cultivated soil during the decomposition of liquid cattle manure were studied in laboratory experiments. Anoxic slurries of soil and manure in different proportions or alone were followed over 5 weeks. Upon NO3- depletion, ferrous Fe appeared in solution in soil-manure slurries. Sulfide levels were generally low in the presence of both soil and manure, probably because of precipitation by Fe2+; Fe thereby counteracted the production of free hydrogen sulfide. Methane formation was the quantitatively most important electron sink under prolonged anoxic conditions. When extra sulfate was added to soil-manure slurries, the concentration of Fe2+ remained low and less methane was produced. The detection of Fe2+ was examined in a model system with a gel-stabilized soil-manure mixture sandwiched between two soil phases with a water content near field capacity. Large methodological problems made it impossible to quantify Fe2+ in this oxic-anoxic environment, but the results indicated that reducing conditions were maintained in the manure-saturated zone for the 20-day period of this experiment.