Denitrification vs dissimilatory nitrate reduction in soil with alfalfa, straw, glucose and sulfide treatments

The effects of available C from alfalfa and straw on denitrification rates in incubated soil were investigated. Denitrification rates were compared with ground alfalfa and wheat straw at concentrations of 1.0, 10 and 100 g kg −1 soil using the acetylene block method. Similar rates of denitrification were observed with those treatment pairs known to produce similar soluble C concentrations in soil; specifically 1.0 g alfalfa and 10 g straw, or 10 g alfalfa and 100 g straw kg −1 soil. Less than half the applied NO − 3 was recovered as N 2 O with the 100 g kg −1 alfalfa treatment. A subsequent experiment using K 15 No 3 showed that about one half of the added 15 N appeared as 15 NH + 4 with the 100 g kg −1 alfalfa treatment, thus suggesting dissimilatory nitrate reduction (DNR). DNR did not occur when a comparable amount of glucose C was added to the soil. However, DNR occurred when sulfide and glucose C were added together to soil at concentrations comparable to those with the 100 g kg −1 alfalfa treatment. It is suggested that sulfide, under highly reduced conditions, may inhibit denitrification and thereby promote DNR by microorganisms. It appears that DNR becomes more competitive with denitrification as the reducing state of the soil increases.