Properties of selected micro-organisms involved in biological N2 fixation in agriculture and natural ecosystems†.
There is a consensus within the scientific community that nitrogenous fertilizers are almost indispensable in today’s agriculture. However, the geometric increase in nitrogenous fertilizer applications and the associated environmental concerns call for focus on more sustainable alternatives. Biological dinitrogen (N2) fixation (BNF) is one of the most sustainable approaches to meeting crop nitrogen (N) demands. The BNF is, especially, important in low value crops (e.g., forages) and in developing economies. However, just like synthetic N fertilizers, BNF has issues of its own. Among the issues of great importance is the low and highly variable proportion of fixed N2 transferred to non-N2-fixing plants. The proportion of transfer ranges from as low as 0% to as high as 70%, depending on a myriad of factors. Most of the factors (e.g., N fertilizer application, species, and cultivar selection) are management related and can, therefore, be controlled for improved N2 fixation and transfer. In this chapter, we discuss current trends in BNF in selected legume crops, the global economics of BNF, and recent reports on N2 transfer in agricultural production systems. Additionally, factors affecting N2 transfer and management considerations for improving N2 fixation and transfer are discussed.
- biological nitrogen fixation
- nitrogen transfer
- grass-legume mixtures
Plants require N in relatively large quantities to grow and reproduce. In fact, N is the third most important factor in the growth and development of crop plants . This made N one of the most important nutrients in agricultural production systems. The important role N plays in global food production is evident in the ever-increasing amounts of N fertilizers applied annually. It has been estimated that approximately 100 Tg of synthetic N fertilizers were applied in 2009 . The geometric increase in N fertilizer use worldwide is in part, attributable to the need to produce enough food to feed the over 7 billion people currently living on earth. Although there is a consensus within the scientific community that N fertilizers are almost indispensable in today’s agriculture, there are great concerns with the use of N fertilizers. Some of these include pollution of surface and underground waters, greenhouse gas (e.g., nitrous oxide: N2O) emissions, and low N use efficiency (NUE). There is, therefore, a multi-pronged approach to N management in global food production. While N fertilizers are being increasingly applied to crops to increase crop productivity, there are calls for more sustainable approaches to meeting N demand of crops such as climate-smart agriculture and sustainable intensification.
The BNF, the process whereby micro-organisms use nitrogenase enzyme to convert atmospheric inert N2 to plant usable forms [3, 4], was the main source of N prior to the industrial revolution . It is generally agreed that BNF is one of the most sustainable approaches to meeting crop N demands. For example, it has been estimated that NUE increases exponentially with increasing levels of biologically fixed N2 in soils while NUE decreases linearly with increasing levels of applied synthetic N fertilizers . There are concerns about the best approach for quantifying inputs of fixed N2. Conservative estimates based on harvested areas and yields from 2005 Food and Agricultural Organization (FAO) database on world crop production (FAOSTAT) showed that 2.95 and 18.5 Tg N was fixed annually by pulses and oilseed crops, respectively . Soybean (
Just like synthetic fertilizers, BNF has issues of its own. Among the issues of great importance is the transfer of fixed N2 to non-N2-fixing plants. The proportion of biologically fixed N2 transferred to neighboring plants can range from as low as 0% to as high as 73%, depending on a myriad of factors . The biology, chemistry, and processes involved in BNF have been extensively described in the literature [7, 8, 9, 10, 11, 12]. Therefore, in this chapter, we discuss briefly the organisms involved in BNF and then proceed to current trends in global N2 fixation and value of BNF transfer in agricultural production systems with special emphasis on N2 fixation from
2. Biological dinitrogen fixation: importance and economics
Several micro-organisms can convert inert atmospheric N2 to plant usable forms. These organisms may exist in association and symbiosis with host plants or independent of a host plant (Table 1). Organisms relying solely on atmospheric N2 as their N source for growth are referred to as diazotrophs . Biological N2 fixation is a significant source of N in agricultural and natural ecosystems. The N input from BNF is particularly important in low value crops (e.g., forages) and developing economies, where farmers either have limited access to synthetic N fertilizers or are unable to afford N fertilizers. In fact, forage accumulation and profitability from grass-legume mixtures have been reported to be equal or greater than N-fertilized grass monocultures [13, 14, 15]. Aside direct N input from BNF, N from BNF reduces the amount of synthetic N fertilizers applied in agriculture and natural ecosystems. This, in turn, reduces cost of production, greenhouse gas (GHG) emissions, and pollution of surface and underground waters. Low NUE and N recovery are major issues associated with use of N fertilizers [16, 17]. In a comprehensive analysis, Lassaletta et al.  showed that the efficiency of N use of biologically fixed N2 is greater than synthetic N. Among the micro-organisms involved in BNF, N2 fixation from
|Micro-organism||Properties and importance|
|Rhizobia||Symbiosis with roots of legumes (nodules); important source of N for legumes; proper Rhizobia strains required for effective nodulation and N2 fixation|
|Frankia (Actinomycetes)||Symbiosis with non-legume angiosperms (e.g., |
|Anabaena||Autotrophic; mostly aquatic but can be terrestrial; symbiosis with non-legumes (e.g., |
|Bradyrhizobium||Aerobic, heterotrophic, free-living N2-fixer|
|Azospirillum||Microaerophilic; heterotrophic; free-living N2-fixer or in association with grass roots; can be important source of N for non-legumes|
|Acetobacter||Heterotrophic; endophytic, can be important source of N for sugarcane (|
|Azotobacter||Aerobic; heterotrophic; free-living N2-fixer|
|Cyanobacteria||Autotrophic; free-living N2-fixer (e.g., |
2.1. Amount and value of N2 fixed by legumes
The amount of N2 fixed from
The economic value of N2 fixation is extraordinarily large. Of course, the value of biologically fixed N2 is directly related to the amount N2 fixed. Using estimates of N2 fixation from Figure 1 and cost of urea N fertilizer from the World Bank , it is estimated that in 2014, the value of N fixed by these eight crops is about 18.5 billion US dollars (Figure 2). Of this amount, about 14.9 billion (81%) is contributed by soybeans.
3. Management considerations for improving biological dinitrogen fixation
There are several management practices that influence BNF in agricultural production systems. These include but not limited to N-fertilization , species , genotype and cultivar , and seeding ratios (intercropping systems). Adopting best management practices can, therefore, improve N2 fixation. In mixed swards, perennial ryegrass (
The strain of
4. Transfer of biologically fixed nitrogen in agricultural production systems
Biologically fixed N2 satisfies the immediate N needs of the host plants. However, the fixed N2 can be transferred to other crops in the cropping system, especially non-N2-fixing plants. The transfer is accomplished through three main routes, viz.: decomposition of nodules and secondary roots that are not thickened, exudates of soluble N compounds, and transfer mediated by mycorrhizal fungi [1, 41, 42, 43]. The transfer of N through nodule and root decomposition and exudation of N compounds is termed as rhizodeposition . The proportion of biologically fixed N2 transferred to neighboring or succeeding crop plants is highly variable . This can range from as low as 0% to as high as 73%, depending on a myriad of factors . In an extensive review, rhizodeposition was reported to vary from 4 to 71% . Review of literature from 2015 to 2017 on transfer of N in selected crops has shown that N transfer ranged from 0 to 70% (Table 2). Among the three main N transfer routes, rhizodeposition through decomposition of the nodules and roots represents the main pathway of N transfer.
|Crop(s)||Amount of N transferred (% of fixed N)||Reference(s)|
|Caragana (||38–45 kg ha−1 (60–70)§|||
|Alfalfa-tall fescue (||0–650 kg ha−1 (0–12)†|||
|White clover-perennial ryegrass||0–340 kg ha−1 (0–47)†|||
|Mung bean-oat||12.8 mg plant−1 (9.7)|||
|Soybean-maize||7.84 mg pot−1 (7.57)|||
|Soybean-maize||10.77–13.72 mg pot−1 (1.26–2.17)|||
|Faba bean-wheat||0.17 mg plant shoot−1 (14.9)|||
|Red clover-bluegrass (||35.85 mg plant−1 (1.5)|||
|Pigeon pea (||21.8 g kg−1 (na)|||
|Crotalaria-coffee||13.5 g kg−1 (na)|||
|Velvet bean (||19.7 g kg−1 (na)|||
|Red clover-perennial ryegrass and forbs||25–58 kg ha−1 (9.5–15)|||
Nitrogen transfer from signal grass (
5. Factors affecting nitrogen transfer
It has long been acknowledged that since plant N composition is partitioned into various plant organs or parts, not all the N2 fixed by plants will be transferred to neighboring plants or succeeding plants in cropping systems . However, there are a number of biotic and abiotic factors influencing N transfer in agricultural production systems . Environmental factors such as water, temperature, and light have direct and indirect effects on N transfer in cropping systems. Soil moisture has a great influence on decomposition and it is required for the uptake of N. Thus, moisture stress affects both the mineralization of fixed N2 and uptake of mineralized N by plants. However, moisture stress promotes nodule senescence, implying that more nodule biomass will be available for mineralization during moisture stress conditions . Nitrogen is highly soluble. Thus, excess water can result in N leaching out of the rooting zone of plants making it unavailable for uptake. Flooding (e.g., low land rice production systems) results in anaerobic conditions, and thus could result in gaseous N losses in the form of N2O . Optimum light conditions (quality, quantity, and duration) and temperature have a direct effect on photosynthesis and hence, promote both N2 fixation and transfer. For example, nodule activity and N exudation from roots of soybean and sesbania (
A common practice in agricultural production systems is intercropping N2-fixing legumes with non-N2-fixing crops (Figure 3) . This is particularly important in low value crops (e.g., forages) and in developing countries. In intercropping systems, the proximity of the N2-fixing crop to the non-N2-fixing determines the amount of N transferred. The concentration of N in the rhizosphere is the greatest closer to the root surface . Therefore, N transfer predominantly occurs in upper soil layers . Since N uptake is along with concentration gradients , close proximity between N2-fixing legumes and non-N2-fixing crops reduces the distance of travel for dissolved N compounds . Close proximity is achieved either through direct root contact or mycorrhizal hyphae connections . However, Issah et al.  reported that maximum oat productivity was obtained when grown 4 m from caragana shelterbelt compared to 2 m from the shelterbelt.
Aside proximity, species (Table 2) of N2-fixing legumes as well as the non-N2-fixing crops (when grown in mixtures) influence the amount of N2 fixed and transferred to neighboring crops. The amount of N transferred to Arabian coffee (
Other factors such as age or stage of growth , season or year [69, 70, 71], proportion of N-fixing species , compatibility , and stand persistence  affect N transfer in cropping systems. For example, N in naked oats (
It is generally agreed that BNF is one of the most sustainable sources of N in agricultural production systems. The BNF is especially important in low value crops (e.g., forages) and in developing economies. Estimated N2 fixation from selected crops showed that the contribution of N2 fixation to the global N budget is enormous. Though N2fixation from peas, lentils, common bean, faba bean, cowpea, chickpeas, and groundnut is dwarfed by soybean (because of the larger area planted to soybean) based on these estimates, the contribution of N2 fixation from these crops (e.g., cowpea) to farmers in developing countries is substantial. Unlike forages, grains from grain legumes are harvested and removed from the field. Thus, grain legumes usually remove more soil N than forages. There are, however, several issues related to BNF that are of concern to the scientific community. Among the issues of great importance is the low and highly variable proportion of fixed N2 transferred to non-N2-fixing plants. Proportion of fixed N2 transferred to non-N2-fixing plants ranges from as low as 0% to as high as 70%, depending on a myriad of factors. This was not different than the range of values reported from previous reviews. However, most of the factors (e.g., N fertilizer application, species, and cultivar selection) are management related and can, therefore, be controlled for improved N2 fixation and transfer. Most