Nitrogen fixation
Nitrogen fixation in soils is a biological process mediated by microbes. Through this process, atmospheric nitrogen is transformed into a form of nitrogen that can be utilized by soil life and taken up by plants. Rhizobium bacteria carry out this conversion in symbiosis with leguminous plants, where they are hosted in root nodules. This symbiotic form of nitrogen fixation is especially important to agricultural systems, yet free-living bacteria and archaea can also fix nitrogen.
Nitrogen fixation is included in the Nutrient cycling model constituting one of the most important inputs of nitrogen to soil in agroecosystems.
The process of nitrogen fixation can be measured using a variety of methods. Enzyme activity of the nitrogenase enzyme[1] or its associated functional gene using molecular methods[2] could be quantified from soil or plant roots. Alternatively, a 15N tracer method could also be used[3]. Indirectly, symbiotic nitrogen fixation could be assessed by counting the number of nodules of sampled root systems.
[1] Hardy RW et al. 1968. The Acetylene-Ethylene Assay for N2 Fixation: Laboratory and Field Evaluation. Plant Physiology 43: 1185–1207.
[2] Köberl M, et al. 2016. Comparisons of diazotrophic communities in native and agricultural desert ecosystems reveal plants as important drivers in diversity. FEMS Microbiology Ecology 92.
[3] Saiz E et al. 2019. Biological nitrogen fixation in peatlands: Comparison between acetylene reduction assay and 15N2 assimilation methods. Soil Biology and Biochemistry 131: 157–165.