Denitrification is the conversion of nitrate ions (NO3−) in the soil into nitrogen gas (N2), which returns to the atmosphere. It is carried out by denitrifying bacteria.Denitrifying bacteria are active in anaerobic conditions, for example in waterlogged or compacted soils where the air spaces are filled with water. In the absence of oxygen, they use nitrate in place of oxygen in their respiration, converting it to nitrogen gas.Denitrification completes the nitrogen cycle by returning fixed nitrogen to the air. For a farmer, however, it is a problem: it removes nitrate that crops could have used, so it reduces soil fertility and lowers yield.
Farmers reduce denitrification by keeping soil aerobic:• ploughing, which breaks up compacted soil and lets air in;
• draining waterlogged fields;
• improving soil structure by adding organic matter.These conditions also favour nitrifying bacteria, which need oxygen, so nitrate is made faster and lost more slowly.Example: After a very wet winter, a poorly drained part of a field has yellow, stunted wheat. The soil there was waterlogged and anaerobic, so denitrifying bacteria converted much of the nitrate to nitrogen gas, and nitrification slowed. The plants were short of nitrogen for amino acids and chlorophyll. Better drainage would help prevent this.Compare the two groups: nitrifying bacteria need oxygen and make nitrate; denitrifying bacteria work in anaerobic conditions and remove it.
Farmers reduce denitrification by keeping soil aerobic:• ploughing, which breaks up compacted soil and lets air in;• draining waterlogged fields;
• improving soil structure by adding organic matter.These conditions also favour nitrifying bacteria, which need oxygen, so nitrate is made faster and lost more slowly.Example: After a very wet winter, a poorly drained part of a field has yellow, stunted wheat. The soil there was waterlogged and anaerobic, so denitrifying bacteria converted much of the nitrate to nitrogen gas, and nitrification slowed. The plants were short of nitrogen for amino acids and chlorophyll. Better drainage would help prevent this.Compare the two groups: nitrifying bacteria need oxygen and make nitrate; denitrifying bacteria work in anaerobic conditions and remove it.