Table 12.

Commonly reported range in estimates of BNF (kg N ha^-1 crop^-1), proposed potential levels achievable, and the prospective outlook for improving BNF inputs from various N₂-fixing systems^a.

BNF system	Common range of reported BNF	Theoretical maximum BNF potential			Advantage	Constraint	Status of adoption by farmers	Outlook
		Proposed	Assumptions
Total non-symbiotic N2 fixation	rice: 18–51 wheat: 3–40 maize: 13–26	60–80 in rice, wheat, and maize		Prolific populations of endophytic and rhizospheric N2-fixing bacteria All C input (2 t crop-1) is used by N2 fixers 40 mg N is fixed g C-1	Inherent to the system	Prone to N loss Improvement is difficult	Widely used by default	Potential to improve through agronomic (including straw) management as part of soil health agenda
Cyanobacteria in rice cultivation	0–80	70		Photosynthetic aquatic biomass is composed of exclusively of N2-fixing BGA (C:N = 7) Primary production is 0.5 t C ha-1 crop-1	Inherent to the system	Requires continuous standing water Inhibited by combine N in flood water Grazer inhibits growth of cyanobacteria	Widely used by default	Low potential because of difficulty in managing the algal bloom as inoculations do not work
Azolla in rice cultivation	20–150	225		Two Azolla crops grown and incorporated per rice crop	High (>80%) %Ndfa and large amounts of N produced Improves SOM Reduces N volatilization loss Reduces weed pressure	Requires continuous standing water on soil surface Labor intensive Difficult in maintaining inoculum supply	Use by the farmers has declined, and currently negligible	Low or negligible potential
Aquatic legume green manure in rice cultivation	20–260	260 in 55 days		Fast-growing species such as Sesbania rostrata is used as green manure	High %Ndfa (80–90%) and large amount of N production Improves soil organic matter	Farmers prefer legumes with economic value Labor Intensive	Use by the farmers declined, currently insignificant use	Modest potential in single rice cropping system in Africa and some parts of Asia
Grain legumes in cereal rotations	57 kg total N fixed ha-1 (common bean) to 212 kg total N fixed ha-1 (faba bean)b	245–290 kg total N fixed ha-1		Legume crops other than common bean 10–12 t shoot dry matter ha-1 (3.5–4 t grain ha-1) %Ndfa of 85%, 20 kg N fixed per t shoot dry matter accumulated Nodulated roots represent 30% of total crop N	Inherent to the system Provide multiple rotational benefits that improve cereal productivity	Dominant cereals restrict legume cultivation Cereals have larger markets and are easier to grow than legumes Grain economic value is highly volatile Many pulses are susceptible to disease and insect pests	Widely adopted by the farmers, but their inclusion in farming systems driven by fluctuations in market demand and value	High potential to enhance yield and improve consistency of legume productivity through agronomic management and breeding

^aAdapted from Herridge and Bergersen (1988), Chalk (1991), Roger and Ladha (1992), Ladha and Reddy (1995), Peoples et al. (1995a), Gupta et al. (2006), Roper and Gupta (2016).

^bGlobal average amounts of shoot N fixed presented for common bean and faba bean in Table 9 adjusted to include assumed below-ground contributions of fixed N associated with nodulated roots represented ~30% of total plant fixed N (Herridge et al., 2022).