Concept:Soil nitrogen is increased only by processes that add nitrogen compounds; the exception is a process that removes nitrogen from the soil.
Explanation:Organic manure contains nitrogen-rich remains that decompose and release nitrates into the soil.
Electrical discharge, such as lightning, fixes atmospheric nitrogen into nitrates that dissolve in rainwater and enter the soil.
Symbiotic nitrogen fixation occurs when Rhizobium bacteria in root nodules convert atmospheric
N2 into nitrogen compounds usable by plants.
Denitrification is the opposite process: denitrifying bacteria convert soil nitrates (
NO3−) into gaseous nitrogen (
N2), which escapes into the atmosphere.
Therefore, the soil loses nitrogen through denitrification rather than gaining it.
Answer:A. Denitrification