Which is the product of that reaction

nitrogen fixation is the process of fixing gaseous nitrogen to usable forms while in decomposition the source of nitrogen is organic waste.

fixation

nitrogen in its gaseous form (n2) can’t be used by most living things. it has to be converted or ‘fixed’ to a more usable form through a process called fixation.

biologically: nitrogen gas (n2) diffuses into the soil from the atmosphere, and species of bacteria convert this nitrogen to ammonium ions (nh4+), which can be used by plants.

decomposition:

plants take up nitrogen compounds through their roots. animals obtain these compounds when they eat the plants. when plants and animals die or when animals excrete wastes, the nitrogen compounds in the organic matter re-enter the soil where they are broken down by microorganisms, known as decomposers. this decomposition produces ammonia, which can then go through the nitrification process.