Concept:Nitrogen gain in soil means adding nitrogen compounds to the soil; nitrogen loss means removing them.
Explanation:Denitrification converts soil nitrates into gaseous nitrogen, so it removes nitrogen from the soil.
Organic manure releases nitrogen compounds into the soil as it decomposes, so it adds nitrogen.
Electrical discharge fixes atmospheric nitrogen into nitrates that fall to the soil in rain, so it adds nitrogen.
Symbiotic nitrogen fixation adds nitrogen to the soil through bacteria in legume root nodules.
Thus, the only option that does not help the soil gain nitrogen is denitrification.
Answer:A. Denitrification.