Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems

Abstract

The demand for nitrogen (N) for crop production increased rapidly from the middle of the twentieth century and is predicted to at least double by 2050 to satisfy the on-going improvements in productivity of major food crops such as wheat, rice and maize that underpin the staple diet of most of the world's population. The increased demand will need to be fulfilled by the two main sources of N supply - biological nitrogen (gas) (N₂) fixation (BNF) and fertilizer N supplied through the Haber-Bosch processes. BNF provides many functional benefits for agroecosystems. It is a vital mechanism for replenishing the reservoirs of soil organic N and improving the availability of soil N to support crop growth while also assisting in efforts to lower negative environmental externalities than fertilizer N. In cereal-based cropping systems, legumes in symbiosis with rhizobia contribute the largest BNF input; however, diazotrophs involved in non-symbiotic associations with plants or present as free-living N₂-fixers are ubiquitous and also provide an additional source of fixed N. This review presents the current knowledge of BNF by free-living, non-symbiotic and symbiotic diazotrophs in the global N cycle, examines global and regional estimates of contributions of BNF, and discusses possible strategies to enhance BNF for the prospective benefit of cereal N nutrition. We conclude by considering the challenges of introducing in planta BNF into cereals and reflect on the potential for BNF in both conventional and alternative crop management systems to encourage the ecological intensification of cereal and legume production.

Keywords: Crop nitrogen nutrition; Diazotrophs; Legumes; Nitrogen cycle; Non-symbiotic nitrogen fixation; Symbiotic nitrogen fixation.

Fig. 1

Trends in measures of total soil N (to a depth of 0.5 m) in a long-term continuous rice-rice system experiment at the IRRI farm, Philippines comparing a nil-N treatment with the inputs of synthetic N (137 kg N ha^-1 per year), and the inclusion of sources of BNF via azolla or sesbania (adapted from Ladha et al., 2000). Different letters shown between treatments indicate statistical significance (α =0.05) according to Duncan’s multiple range test. * and ** indicate significance at the 0.05 and 0.01 levels, respectively

Fig. 2

Each bar shows additional crop area measured in hectares and as the percentage of global single cropping area (=1.02 billion ha) under different scenarios, including (a) with a two-or four-month difference between potential and actual growing season, and (b) for all cropland and for cropland with low frost and drought risk (adapted from Waha et al., 2020). Potential for increasing cropping intensity on current global croplands.

Fig. 3

Key genetic elements participating in common symbiotic signaling pathway and specialized cellular processes involved in the development of mycorrhizal (left) and root nodule (right) symbioses (adapted from Reddy et al., 2013).