Denitrification and fermentation in plant-residue-amended soil

Summary Nitrous oxide production (denitrification) during anaerobic incubation of ground-alfalfa-, red-clover-, wheat-straw-, and cornstover-amended soil was positively related to the initial water-soluble C content of the residue- amended soil. The water-soluble C concentration decreased in all treatments during the first 2 days, then increased in the alfalfa-, red-clover-, and wheat-straw-amended soil until the end of the experiment at 15 days. An accumulation of acetate, propionate, and butyrate was partly responsible for the increased water-soluble C concentration. Denitrification rates were much higher in the alfalfa-and red-clover-amended soil, but NO₃^–was not fully recovered as N₂O in these treatments. Supported by earlier experiments in our laboratory, we conclude that some of the NO₃^–was reduced to NH₄⁺through fermentative NO₃^–reduction, otherwise known as dissimilatory NO₃^–reduction to NH₄⁺. Acetate, the primary product of anaerobic fermentation, accumulated in the alfalfa- and red-clover-amended soil in the presence of NO₃^–, supporting previous observations that the processes of denitrification and fermentation occur simultaneously in C-amended soil. The partitioning of NO₃^–between denitrification and fermentative NO₃^– reduction to NH₄⁺depends on the activity of the denitrifying and fermentative bacterial populations. NO₂ concentration may be a key in the partitioning of NO₃^–between these two processes.