Stikstoffixers

Stikstoffixers, or nitrogen fixers, are organisms and processes that convert atmospheric nitrogen (N2) into ammonia (NH3) or other nitrogen compounds usable by living organisms. Biological nitrogen fixation is a central part of the global nitrogen cycle, supporting plant nutrition in natural ecosystems and in sustainable agriculture.

Biological nitrogen fixers include symbiotic bacteria living in plant roots, free-living bacteria, and some cyanobacteria. In many legumes, rhizobia such as Rhizobium, Bradyrhorizobium, and Sinorhizobium form root nodules and provide nitrogen to the plant. Actinorhizal plants host Frankia in nodules. Free-living nitrogen fixers such as Azotobacter and Azospirillum occur in soils, while cyanobacteria like Anabaena and Nostoc fix nitrogen in aquatic and terrestrial habitats, often using heterocysts to protect nitrogenase from oxygen.

Nitrogen fixation is carried out by the enzyme nitrogenase, which reduces N2 to NH3 at the cost of ATP and electrons. The enzyme is highly sensitive to oxygen, so nodules create low-oxygen conditions via leghemoglobin, and heterocysts provide microaerobic environments in cyanobacteria.

Ecological and agricultural significance: biological fixation enriches soil nitrogen, reducing dependence on synthetic fertilizers in crop rotations and pasture systems. Inoculants introduce effective rhizobial strains to improve nodulation and yields.

Industrial fixation by the Haber-Bosch process converts atmospheric nitrogen to ammonia under high pressure and temperature, forming most commercial fertilizers. Biological fixation operates at ambient conditions and can be more energy-efficient, though it is limited by ecological and physiological factors.

Genetic aspects: nif genes encode nitrogenase components; nifH is commonly used as a marker. Fixation rates vary with plant and microbial genotype, soil properties, moisture, temperature and oxygen availability.