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Application of Free Living N-fixers in Agriculture SOIL311 Literature Review University of New England School of Environmental and Rural Science Jesse L. Fenn March, 2015 Introduction Nitrogen (N) is essential to maintaining complex lifecycles spanning all living organisms. Biological N fixation is the process by which some 97% of new N inputs are incorporated into unmanaged terrestrial ecosystems (Vitousek et al., 2002; Galloway et al., 2004, as cited in Reed, Cleveland, & Townsend, 2011, p. 490). Plants require N for growth and reproduction. In non-legume species, it is the most important mineral nutrient and in all plants the most abundant, comprising of 2-4% of plant dry matter (Blair & Sale, 1996, p. 35). While N is abundant in the atmosphere in various forms (Webb, 2015, para 2), in the soil system N levels fluctuate and are often low enough to limit production due to the cyclic nature of its various forms, mobility and numerous loss channels (McLaren & Cameron, 1996, p. 192-193). Producers attempt to meet the plant’s requirements for N through the application of synthetic fertilizers, recycled organic wastes (manures, etc) and the use and promotion of biological Nfixers (Orr, James, Leifert, Cooper, & Cummings, 2011, p. 911). As agriculture depends so heavily on N, interest in these biological N-fixers is intensifying. Furthermore, with markets demanding “organic” options, some producers are moving away from conventional synthetic fertilizers altogether. This has initiated numerous studies on biological N-fixers seeking to answer questions of quantity of N fixed, ecological interactions and management considerations. While these questions are being debated in the research realm, various products exist on the market to inoculate soil systems with these diazotrophic microorganisms to boost populations for the purpose of N-fixation. Rhizobium bacteria are known to form symbiotic relationships with the root systems of leguminous species and their use is a commonly accepted method of increasing N availability to plants under crop rotation systems. Less utilized are the freeliving bacteria capable of N-fixation, which have previously been viewed as largely noneffective for agriculture (Reid & Wong, 2005). The use of Free-living N-fixing bacteria (FLNFB) in agriculture as well as management considerations found in the scientific literature for the promotion of FLNFB will be reviewed. Free Living N-Fixing Bacteria The term “free-living” in relation to N-fixing bacteria refer to all bacteria that do not live in a direct symbiotic association with vascular plants, both the true FLNFB and autotrophic and heterotrophic organisms that fit this description (Cleveland, et al., 1999, p. 624). FLNFB are present in nearly all terrestrial ecosystems and are often the primary pathway for the fixation of atmospheric N into a plant available form (Reed et al., 2011, p. 489). A number of genera have been identified as FLNFB (Stewart, 1969, p. 367), with strains capable of fixing N to differing degrees. The most well known include Azotobacter, Biejerinckia and Clostridium, all found in different climatic zones with preferences for specific soil conditions (acidic, anaerobic etc.) (McLaren & Cameron, 1996, p. 195). Orr et al. (2011, p. 911) cite studies where FLNFB fixation rates of up to 60 kg N ha-1 year-1 have been recorded. Free Living N-Fixing Bacteria in Agriculture The ecosystems where FLNFB are found are vast and varied, and Zechmeister-Boltenstern & Kinzel (1990, p. 1079) state that the rates of fixation in forests and arable fields are relatively low compared to saline and peat soils with studies documenting remarkable N2 fixation rates under the latter conditions. The contribution of the FLNFB is dependant on physiochemical properties where only a proportion of the microbes capable of fixing N will be active under given conditions (Wakelin, Gupta, & Forrester, 2010, p. 395, 397). In addition, overall microbial activity plays a role, as shown in Patra et al. (2007, p. 155), where contribution of FLNFB was increased when soil microbial activity decreased. Thus it can be deduced that FLNFB are less competitive in the rhizosphere. Still, studies have been undertaken to quantify the amount of N fixed by these bacteria in agricultural systems. Herridge, Peoples, & Boddey (2008, p. 11) cited studies where it was estimated that FLNFB fixed 25 kg N ha-1 year-1 in sugar cane crops and <5 kg N ha-1 year-1 in crop lands other than used for legumes and rice; their conclusions on both studies was that large variations were evident in the data or that suffient data was not available for decisive values. While models exist to calculate potential N-fixation rates, measuring and predicitng actual rates is extremely difficult. These values are underwheling compared to the ~100 kg N ha-1 year-1 of N fertilizer applied to many modern agriculture systems (Reed et al. 2011, p. 490-491). Using this rate as a reference point, we are beginning to quantify FLNFB’s impact and contribution to an agricultural production system. Herridge et al. (2008, p. 11) then show in stark contrast the N-fixaton rates that occur in leguminous species with the symbiotic relationship of the rhizobium bacteria. Not only are the models used to predict fixation rates based on sound historical crop yield data, actual fixaton rates are easier to measure in both cropping and pasture/fodder systems (cropping more so) (p. 5). Herridge et al. offer the follow table summarizing the differences in fixation rates (shown in Table 1): Table 1 Summary of estimates of N fixed annually in agricultural systems comparing the agent by which it occurs (Herridge et al., 2008, p. 11) Agent Agricultural system Legume rhizobia - Crop (pulse and oilseed legumes) Rate of N2 fixation (kg N ha-1 year-1) 115 Legume rhizobia - Pasture and fodder legumes 110-227 12-25 Free-living bacteria Crop lands (other than legumes and rice) <5 <4 Free-living bacteria Extensive grazing <10 <14 savannas used for Crop N fixed (Tg year-1) 21 It is worthwhile noting however, that symbiotic associations cost the plant significant photosynthates (De Mita, 2012, p. 885) and reduce the total yields of legumes. Despite the lesser amount of N that FLNFB actually fix when comapred to common fertilizer rates and symbiotic associations, products exist to help boost the numbers of these micoorganisms to play their part (Webb, 2015, p. 4). While inoculating the soil with mass numbers of these FLNFB may increase the amount of N they fix, it would seem unlikely that it would ever match the amount of N required of other inputs for agricultural systems. Still, increasing numbers and managing the soils to promote their activity can have beneficial affects, as Wakelin et al., (2010, p. 391) show. While acknowledging that the majority of N-fixation occurs in symbiosis with leguminous plants, Wakelin et al. state that FLNFB play an important role between crop cycles, contributing the N needed to decompose crop residues which generaly have wide C:N ratios. So while their contribution to the N needed in agricultural production systems may not be their primary role, they clearly contribute to overall soil fertility and nutrient cycling and thus should not be ignored. Management Considerations Agricultural soils differ infinitely in the intricate physical and chemical properties where microbial communities are found. These factors cause the spatial and temporal variation seen in microbial populations (Gupta & Roper, 2010, p. 50) and introduce difficulties in sampling for quantification studies (Orr et al. 2011, p. 918). Despite these differences, Wakelin et al. (2010, p. 391) found that the effects of management practices often had a greater impact on FLNFB than between-soil differences. This highlights the importance of management of soil systems for continued productivity. In the research conducted by Wakelin et al (2010, p.391-399), factors affecting diversity, abundance and activity of FLNFB in Australian agricultural soils was studied. Based on the previous conclusions regarding the role of FLNFB in agricultural systems, their findings are helpful when considering practices to promote FLNFB. Experiment 1 looked at the nifH gene (a component of the nitrogenase complex) levels in 20 agricultural sites of different management practices and soil types. The results showed of the properties analyzed within these soils, particulate organic carbon (POC) levels were the most correlated (ρ=0.502; P=0.001) with nifH structures. Conversely, Total C (or N) levels were not strongly correlated, highlighting FLNFB's affinity for a type of C in the soil, not just the total level. From a management perspective, POC is the component of soil organic matter (SOM) that consists of recently added and semi-decomposed material, hence Wakelin et al.'s previous conclusion of the role FLNFB play in nutrient cycling of crop residues. Experiment 2 focused on heavily textured, high input agricultural soils with varying management practices, details of which are presented in Table 2. Table 2 Sites from Experiment 2 focusing on management practices and the affect on FLNFB (Wakelin et al. 2010, p. 392393) Site "Border Check" Rotation 5 year; three cereals, canola and faba bean Management Practice Stubble burnt, incorportated or left on surface "Contour" Rice Stubble burnt, incorportated or left on surface "Pivot" 3 year; two cereals and one canola Stubble incorporated It was found that for the Border Check site, incorporating stubble increased N2-fixation alomst four times over that of stubble burnt or left on surface. In contrast, there was no difference in N2-fixation between management practice evident at the Contour site. This difference could be attributed to the cultivation system (rice over cereal cropping) than management practice. Finally, in the Pivot site, N2-fixation was more than 3.5 times higher (P = 0.005) in the high input (irrigated) cropping area when compared with an adjacent nonirrigated fallow area when analysied by the acetylene reduction technique. From this study, the management implications for broadacre cropping would be to incorporate crop residue into the soil as the amount of C entering the system is directly related to the activity and abundance of FLNFB (Wakelin et al., 2010, p. 397-398). Orr et al., (2011, p. 911) and Wakelin et al. (2010, p. 397) cite studies showing that external stress events, such as fire, salinity, liming and application of chemcial herbicides and pesticides, can all have a negative affect soil microbe populations, including FLNFB. Gupta and Roper (2010, p. 50) further investigate this understanding looking at factors that increase the physical protection of FLNFB in the soil matrix from these common practices. From a soil physics perspective, they found that with increased clay content, soil aggregation was increased. Gupta and Roper (2010, p. 53) cite several studies which have found that within these micro-habitats, FLNFB thrive and find protection from predation, environmental extremes and exposure to toxic or biocidal compounds. Roper and Smith (1991, as cited in Gupta & Roper, 2010, p. 53) found that clay soils supported greater nitrogenase activity than sandy soils. The management implications are: for clay soils, every effort should be taken to preserve the integrity of the soil; avoiding compaction of susceptible clays, avoiding exposing sodic clays and minimizing erosion wherever possible for sandy soils, SOM can still act as a suitable micro-habitat for FLNFB, hence on these soil types, the incorporation of stubble becomes paramount to the promotion of microbial activity. This practice also increases the possibility of maintaining sufficient moisture in the soil to support the soil microbes which itself presents as a significant challenge where long-dry periods are encountered. This conclusion support the findings of Lamb et al. (1987, as cited in ZechmeisterBoltenstern & Kinzel, 1990, p. 1081) where C2H2 reduction (a measure of microbial activity) was greater in no-till systems than in conventional wheat-fallow systems due to higher soil moisture and the presence of SOM in the no-till soils. Finally, from a soil nutrients perspective, Reed, Cleveland, and Townsend (2013, p. 136) cite several studies that show that P additions stimulate N-fixation, and thus there may be a P limitation to N-fixation. This is particuraly important for Australian soils that are known to be low in available P (Moody & Bollard, 1999, p. 187 - 188). Similarly, in the study conducted by Orr et al. (2011, p. 911) it was found that FLNFB were more active in conventionally fertilized plots over organically fertilized plots, however the effects are often subtle and short lived (918). Finally, Reed et al. (2011, p. 501) cite Barron et al. (2008) in stating P fertilizers coated and/or contaminated with Mo can confound interpretations showing Mo may also play a necessary role and that responses vary significantly within and between ecosystems. The management implications in regards to soil nutrients is to maintain the addition of suitable fertilization or other appropriate soil amendments in order to maintain soil vitality. This will differ from location to location and depend on crop/pasture composition, rotation, past fertilizer regime and specific enterprise requirements. Soil tests are a valuable tool in assessing possible nutrient limitations and when used skillfully, can save producers significant money in amendment application. Conclusion It has been shown from the amount of N FLNFB actually fix that their role should not be over emphasised, however their important contribution to soil fertility should not be ignored. Assessing return from and contribution of applying FLNFB to a soil system would be difficult in the field. Until more accurate and accessible methods for determining contribution from FLNFB are developed, investing in this practice would be inadvisable. Nevertheless, acknowledging the existence of, and managing for the promotion of FLNFB should contribute to overall soil fertility leading to an increase in productivity. References Blair, G., & Sale, P. (1996). Plant Nutrition. Armidale, Australia: University of New England. Cleveland, C., Townsend, A., Schimel, D. … Wasson, M. (1999). Global patterns of terrestrial biological nitrogen (N2) fixation in natural ecosystems. Global Biological Cycles, 13, 623-645. De Mita, S. (2012). For better or for worse: cooperation and competition in the legumerhizobium symbiosis. New Phytologist, 194, 885-887. Gupta, V., & Roper, M. (2010). Protection of free-living nitrogen-fixing bacteria within the soil matrix. Soil Tillage and Research, 109, 50-54. Herridge, D., Peoples, M., & Boddey, R. (2008). Global inputs of biological nitrogen fixation in agricultural systems. Plant Soil, 311, 1-18. McLaren, R., & Cameron, K. (1996). Soil Science - Sustainable production and environmental protection. South Melbourne, Australia: Oxford University Press. Moody, P.W., & Bollard, M.D.A. (1999). Phosphorus. In D.J. Reuter (Ed), Soil Analysis: an interpretation manual (pp. 187-220). Collingwood, Australia: CSIRO Publishing. Orr, C., James, A., Leifert, C., Cooper, J., & Cummings, S. (2011). Diversity and activity of free-living nitrogen-fixing bacteria and total bacteria in organic and conventionally managed soils. Applied and Environmental Microbiology, 77, 911-919. Patra, A., Le Roux, X., Abbadie, L., Clays-Josserand, A., Poly, F., Loiseau, P., & Louault, F. (2007). Effect of microbial activity and nitrogen mineralization on free-living nitrogen fixation in permanent grassland soils. J. Agronomy & Crop Science, 193, 153-156. Reed, S., Cleveland, C., & Townsend, A. (2011). Functional ecology of free-living nitrogen fixation: a contemporary perspective. Annu. Rev. Ecol. Evol. Syst., 42, 489-512. Reed, S., Cleveland, C., & Townsend, A. (2013). Relationship among phosphorus, molybdenum and free-living nitrogen fixation in tropical rainforests: results from observational and experimental analyses. Biogeochemistry, 114, 135-147. Reid, G., & Wong, P. (2005). Soil Biology Basics. Retrieved from www.dpi.nsw.gov.au/ _data/assets/pdf_file/0017/41642/Soil_bacteria.pdf Stewart, W. (1969). Biological and ecological aspects of nitrogen fixation by free-living micro-organisms. Proc. Roy. Soc., 172, 367-388. Wakelin, S., Gupta, V., & Forrester, S. (2010). Regional and local factors affecting diversity, abundance and activity of free-living, N2 fixing bacteria in Australian agricultural soils. Pedobiologia, 53, 391-399. Webb, G. (2015). Nitrogen fixing bacteria in Agriculture. Retrieved from: www.activesoil.com/files/9513/7080/1105/TwinN_Nitrogen_Fixing_Bacteria_in_Agriculture.pdf Zechmeister-Boltenstern, S., & Kinzel, H. (1990). Non-symbiotic nitrogen fixation associated with temperate soils in relation to soil properties and vegetation. 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