The influence of topdressing nitrogen on Azospirillum spp. inoculation in maize crops through meta-analysis

Douglas Mariani Zeffa Lucas Henrique Fantin Odair José Andrade Pais dos Santos André Luiz Martinez de Oliveira Marcelo Giovanetti Canteri Carlos Alberto Scapim Leandro Simões Azeredo Gonçalves About the authors

ABSTRACT

The Azospirillum is considered one of the most studied plant growth promoter genus. These bacteria are capable of promoting plant growth through several factors. However, the effect of Azospirillum spp. associated to nitrogen fertilization on maize grain yield has brought about controversial results. Thus, the objective of this study was to verify the influence of topdressing nitrogen on the effect of Azospirillum spp. inoculation on maize crops through meta-analysis. Data were collected from articles published in scientific journals, obtained from the Web of Science®, Scopus® and Google Scholar® databases. The bibliographic review included only articles with direct comparisons between maize yield in the presence and absence of Azospirillum spp. under field conditions in Brazil. The meta-analysis of the random effects was carried out using all entries. Subgroup and meta-regression analysis were realized to verify the influence of nitrogen fertilization on the explanation of the possible heterogeneity among effect measures. Maize inoculation with Azospirillum spp. showed an average yield increase of 651.58 kg·ha-1, considering all trials, and of 1034.28 kg·ha-1 considering only the trials without topdressing nitrogen. There was no significant increase in grain yield when inoculation with bacteria from the Azospirillum genus was realized together with nitrogen fertilization, indicating that such technologies are non-additive.

Key words
Zea mays L.; meta-regression; plant growth-promoting bacteria

INTRODUCTION

Among the microorganisms that live in the rhizosphere, a heterogeneous group of bacterial species, also known as plant growth-promoting bacteria (PGPB), are capable of promoting plant growth (Bach et al. 2016Bach, E., Seger, G. D. S., Fernandes, G. C., Lisboa, B. B. and Passaglia, L. M. P. (2016). Evaluation of biological control and rhizosphere competence of plant growth-promoting bacteria. Applied Soil Ecology, 99, 141-149. https://doi.org/10.1016/j.apsoil.2015.11.002.). These bacteria can play an important role in nutrients acquisition by plants, acting as biofertilizers, phytostimulants and biotic and abiotic stress controllers (Lugtenberg and Kamilova 2009Lugtenberg, B., Kamilova, F. (2009). Plant growth-promoting rhizobacteria. Annual Review of Microbiology 63, 541-556. http://dx.doi.org/10.1146/annurev.micro.62.081307.162918.; Pii et al. 2015Pii, Y., Mimmo, T., Tomasi, N., Terzano, R., Cesco, S. and Crecchio, C. (2015). Microbial interactions in the rhizosphere: beneficial influences of plant growth-promoting rhizobacteria on nutrient acquisition process: a review. Biology and Fertility of Soils, 51, 403-415. http://dx.doi.org/10.1007/s00374-015-0996-1.). Several PGPB genera show association with different species of agricultural importance, such as Azospirillum, Arthobacter, Azobacter, Bacillus, Bradyrhizobium, Burkholderia, Clostridium, Gluconacetobacter, Herbaspirillum, Pseudomonas, Rhizobium and Streptomyces (Steenhoudt and Vanderleyden 2000Steenhoudt, O., Vanderleyden, J. (2000). Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects. FEMS Microbiology Reviews, 24, 487-506. https://doi.org/10.1111/j.1574-6976.2000.tb00552.x.; Videira et al. 2012Videira, S. S., Oliveira, D. M., Morais, R. F., Borges, W. L., Baldani, V. L. D. and Baldani, J. I. (2012). Genetic diversity and plant growth promoting traits of diazotrophic bacteria isolated from two Pennisetum purpureum Schum. genotypes grown in the field. Plant and Soil, 356, 51-66. http://dx.doi.org/10.1007/s11104-011-1082-6.).

The Azospirillum (Beijerinck 1925Beijerinck, M. W. (1925). Über ein Spirillum welches freien Stickstof binden kann? Zentralblatt für Bakteriologie. Parasitenkunde, Infektionskrankheiten and Hygiene, 63, 353-359.) genus includes a group of diazotrophic bacteria that can be associated with the plant’s rhizosphere, characterizing an external colonization, or associated endophytically when penetrating the roots intercellular spaces (Van Dommelen and Vanderleyden 2007Van Dommlen, A., Vanderleyden, J. (2007). Associative nitrogen fixation. In H. Bothe, S. J. Ferguson and W. E. Newton (Eds.), Biology of the nitrogen cycle. (p.179-192). Amsterdam: Elsevier. https://doi.org/10.1016/b978-044452857-5.50013-8.). According to the List of Prokaryotic Names with Standing in Nomenclature (LPSN 2017LPSN (2017), List of Prokaryotic Names With Stading in Nomeclature. [Access 20 Mar 2017] http://www.bacterio.net/azospirillum.html.
http://www.bacterio.net/azospirillum.htm...
), 19 species of Azospirillum were described, making it one of the most studied PGPB genera (Cassán and Díaz-Zorita 2016Cassán, F. and Diaz-Zorita, M. (2016). Azospirillum spp. in current agriculture: from the laboratory to the field. Soil Biology and Biochemistry, 103, 117-130. https://doi.org/10.1016/j.soilbio.2016.08.020.). Among the main species are the A. brasilense, A. lipoferum, A. halopraeferens and A. oryzae, widely used as biofertilizers, mainly for cereals. However, Pereg et al. (2016)Pereg, L., De-Bashan, L. E. and Bashan, Y. (2016). Assessment of affinity and specificity of Azospirillum for plants. Plant and soil, 399, 389-414. http://dx.doi.org/10.1007/s11104-015-2778-9. verified that bacteria of the Azospirillum genus has affinities with more than 113 species of plants and 35 botanic families.

Azospirillum-genus bacteria, when associated with the plant’s roots, are capable of promoting plant growth due to the production of aminoacids, indoleacetic acid, giberelines and other polyamines, according to the additive hypothesis (Bashan and Levanony 1990Bashan, Y., Levanony, H. (1990). Current status of Azospirillum inoculation technology: Azospirillum as a challenge for agriculture. Canadian Journal of Microbiology, 36, 591-608. https://doi.org/10.1139/m90-105.), which favors root system growth and, consequently, the absorption of water and nutrients by plants (Bashan and de-Bashan 2010Bashan, Y. and De-Bashan, L. E. (2010). How the plant growth-promoting bacterium Azospirillum promotes plant growth-a critical assessment. Advances in Agronomy, 108, 77-136. https://doi.org/10.1016/S0065-2113(10)08002-8.; Doornbos et al. 2012Doornbos, R. F., Van Loon, L. C. and Bakker, P. A. H. M. (2012). Impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere. A review. Agronomy for Sustainable Development, 32, 227-243. http://10.1007/s13593-011-0028-y.; Tien et al. 2012Tien, T. M., Gaskins, M. H., De-Bashan, L. E., Hernadez, J. P. and Bashan, Y. (2012). The potential contribution of plant growth-promoting bacteria to reduce environmental degradation - a comprehensive evaluation. Applied Soil Ecology, 61, 171-189. https://doi.org/10.1016/j.apsoil.2011.09.003.). Such bacteria also have the capacity to fix atmospheric nitrogen and, for this reason, contribute directly in increasing nitrogen availability to several non-leguminous species (Hungria et al. 2010Hungria, M., Campo, R. J., Souza, E. M. and Pedrosa, F. O. (2010). Inoculation with selected strains of Azospirillum brasilense and A. lipoferum improves yields of maize and wheat in Brazil. Plant and Soil, 331, 413-425. http://dx.doi.org/10.1007/s11104-009-0262-0.; de-Bashan et al. 2012; Ferreira et al. 2013Ferreira, A. S., Pires, R. R., Rabelo, P. G., Oliveira, R. C., Luz, J. M. Q. and Brito, C. H. (2013). Implications of Azospirillum brasilense inoculation and nutrient addition on maize in soils of the Brazilian Cerrado under greenhouse and field conditions. Applied Soil Ecology, 72, 103-108. https://doi.org/10.1016/j.apsoil.2013.05.020.). However, according to Van Dommelen and Vanderleyden (2007)Van Dommlen, A., Vanderleyden, J. (2007). Associative nitrogen fixation. In H. Bothe, S. J. Ferguson and W. E. Newton (Eds.), Biology of the nitrogen cycle. (p.179-192). Amsterdam: Elsevier. https://doi.org/10.1016/b978-044452857-5.50013-8., the amount of ammonia released by these bacteria is limited and, therefore, their contribution has been questioned many times (Steenhoudt and Vanderleyden 2000Steenhoudt, O., Vanderleyden, J. (2000). Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects. FEMS Microbiology Reviews, 24, 487-506. https://doi.org/10.1111/j.1574-6976.2000.tb00552.x.; Bashan and de-Bashan 2010Bashan, Y. and De-Bashan, L. E. (2010). How the plant growth-promoting bacterium Azospirillum promotes plant growth-a critical assessment. Advances in Agronomy, 108, 77-136. https://doi.org/10.1016/S0065-2113(10)08002-8.).

The effect of Azospirillum on maize crop yield (Zea mays L.) has shown contrasting results (Hagh et al. 2010Hagh, E. D., Khoii, F. R., Valizadeh, M. and Khorshidi, M. B. (2010). The role of Azospirillum lipoferum bacteria in sustainable production of maize. International Journal of Food, Agriculture and Environment, 8, 702-704., Hungria et al. 2010Hungria, M., Campo, R. J., Souza, E. M. and Pedrosa, F. O. (2010). Inoculation with selected strains of Azospirillum brasilense and A. lipoferum improves yields of maize and wheat in Brazil. Plant and Soil, 331, 413-425. http://dx.doi.org/10.1007/s11104-009-0262-0., Repke et al. 2013Repke, R. A., Cruz, S. J. S., Silva, C. J., Figueiredo, P. G. and Bicudo, S. J. (2013). Eficiência da Azospirillum brasilense combinada com doses de nitrogênio no desenvolvimento de plantas de milho. Revista Brasileira de Milho e Sorgo, 12, 214-226.; Quadros et al. 2014Quadros, P. D., Roesch, L. F. W., Silva, P. R. F., Vieira, V. M., Roehrs, D. D. and Camargo, F. A. O. (2014). Desempenho agronômico a campo de híbridos de milho inoculados com Azospirillum. Revista Ceres, 61, 209-218. http://dx.doi.org/10.1590/S0034-737X2014000200008.). Repke et al. (2013)Repke, R. A., Cruz, S. J. S., Silva, C. J., Figueiredo, P. G. and Bicudo, S. J. (2013). Eficiência da Azospirillum brasilense combinada com doses de nitrogênio no desenvolvimento de plantas de milho. Revista Brasileira de Milho e Sorgo, 12, 214-226. found that the inoculation of A. brasilense, followed or not by nitrogen dosages, had no interference in maize yield. On the other hand, Hungria et al. (2010)Hungria, M., Campo, R. J., Souza, E. M. and Pedrosa, F. O. (2010). Inoculation with selected strains of Azospirillum brasilense and A. lipoferum improves yields of maize and wheat in Brazil. Plant and Soil, 331, 413-425. http://dx.doi.org/10.1007/s11104-009-0262-0. evaluated the use of different strains of A. brasilense and A. lipoferum in maize and observed an average increase in gran yield by 27%. Hagh et al. (2010)Hagh, E. D., Khoii, F. R., Valizadeh, M. and Khorshidi, M. B. (2010). The role of Azospirillum lipoferum bacteria in sustainable production of maize. International Journal of Food, Agriculture and Environment, 8, 702-704. reported a yield increase of 13% in maize inoculated with Azospirillum lipoferum in relation to the treatment without inoculation plus 70 kg·ha-1 of N in topdress application. However, the same authors found no significant differences for the treatment with A. lipoferum when compared to treatments without inoculation plus 140 and 210 kg·ha-1 of topdress N. Quadros et al. (2014)Quadros, P. D., Roesch, L. F. W., Silva, P. R. F., Vieira, V. M., Roehrs, D. D. and Camargo, F. A. O. (2014). Desempenho agronômico a campo de híbridos de milho inoculados com Azospirillum. Revista Ceres, 61, 209-218. http://dx.doi.org/10.1590/S0034-737X2014000200008. verified a significant interaction among three maize hybrids and the inoculation with Azospirillum spp., showing yield increase only for hybrids, with an average increase of 750 kg·ha-1.

According to Castro-Sowinski et al. (2007)Castro-Sowinski, S., Herschkovitz, Y., Okon, Y. and Jurkevitch, E. (2007). Effects of inoculation with plant growth-promoting rhizobacteria on resident rhizosphere microorganisms. FEMS Microbiology Letters, 276, 1-11. https://doi.org/10.1111/j.1574-6968.2007.00878.x., inoculation response may vary according to the plant genotype, bacterial strain, environment conditions, nitrogen fertilization management and the quality of the PGPB cells used as inoculant. In this sense, in the midst of all evidence inconsistencies on the contribution of the bacteria of the Azospirillum genera associated with nitrogen fertilization in maize grains yield, the statistical technique known as meta-analysis may be fundamental to indicate the effects of this technology (Veresoglou and Menexes 2010Veresoglou, S. D. and Menexes, G. (2010). Impact of inoculation with Azospirillum spp. on growth properties and seed yield of wheat: a meta-analysis of studies in the ISI Web of Science from 1981 to 2008. Plant and soil, 337, 469-480. http://dx.doi.org/10.1007/s11104-010-0543-7.). Therefore, the objective of this work was to verify the influence of nitrogen fertilization under the effect of the Azospirillum spp. inoculation on maize crops through meta-analysis.

MATERIAL AND METHODS

Data were collected from articles published in scientific journals, obtained through a bibliographic review, using the Web of Science®, Scopus® and Google Scholar® database. As search strategy, the following research terms were used: Azospirillum AND (maize OR corn) OR (yield OR productivity). The same terms were searched in the Portuguese language, with no restriction in regard to article publication dates. The search for the articles was done by two independent reviewers, in which only articles dealing with the direct comparison between maize yield in the presence and absence of bacteria of the Azospirillum genus, under field conditions, in Brazil, were included.

The following criteria were used for excluding articles:

  1. Experiments without the following variability measures: environment variation coefficient, mean squared of the residue or standard error of the mean;

  2. Experiments with an interactive effect with other biofertilizers;

  3. Studies with results presented in graphs, preventing their tabulation.

Effect measures (Yi) were estimated using the grain yield variable (kg·ha-1) through the following equation, represented in Eq. 1:

Yi = ln ( Inoculated treatment / Control without inoculation ) (1)

The synthesis produced by the meta-analysis is weighed according to the studies weights, so that each one could contribute, independently, to the meta-analytical result. The inverse variance method was used to attribute weights, as shown in Eq. 2:

Wi = 1 / Vi (2)

where Wi represents the weight attributed to the ith study and Vi the variance of the ith study.

Thus, the lowest the variability, the greater the study weight in the synthesis produced by the meta-analysis.

Heterogeneity among effect measures was verified through the Cochran Q test (Cochran 1954Cochran, W. G. (1954). The combination of estimates from different experiments. Biometrics, 10, 101-29. https://doi.org/10.2307/3001666.), at 1% of significance. The I2 statistics of Higgins and Thompson (2002)Higgins, J., Thompson, S. G. (2002). Quantifying heterogeneity in a meta-analysis. Statistics in Medicine, 21, 1539-1558. https://doi.org/10.1002/sim.1186. was used to indicate the total variability percentage of a set of effect measures, due to the heterogeneity among the real effects, in which percentages of approximately 25, 50 and 75 would indicate low, average and high heterogeneity, respectively. Studies publication bias was verified through the funnel plot and the Egger significance test (Egger et al. 1997Egger, M., Davey, S. G., Schneider, M. and Minder, C. (1997). Bias in meta-analysis detected by a simple, graphical test. BMJ., 315, 629-34. https://doi.org/10.1136/bmj.315.7109.629.).

Effect measures normal distribution were verified by the Shapiro-Wilk test (Shapiro and Wilk 1965Shapiro, S. S. and Wilk, M. B. (1965). An analysis of variance test for normality (complete samples). Biometrika, 52, 3-4. https://doi.org/10.2307/2333709.), at 5% of significance. The meta-analysis was realized by the random effect model to estimate the means and their respective 95% confidence intervals for each effect measure (Gurevitch and Hedges 1999Gurevitch, J. and Hedges, L. V. (1999). Statistical issues in ecological meta-analysis. Ecology, 80, 1142–1149. http://dx.doi.org/10.1890/0012-9658(1999)080[1142:SIIEMA]2.0.CO;2.; Borenstein et al. 2009Borenstein, M., Hedges, L. V., Higgins, J. P. T. and Rothstein, H. R. (2009). Introduction to meta-analysis. Chichester, UK: John Wiley & Sons.).

Subgroup and meta-regression analyses are models that incorporate one or more moderators (study-specific categorical or continuous co-variables) which, in turn, can explain part of the heterogeneity found among real effect measures (Borenstein et al. 2009Borenstein, M., Hedges, L. V., Higgins, J. P. T. and Rothstein, H. R. (2009). Introduction to meta-analysis. Chichester, UK: John Wiley & Sons.). In this sense, the objective of using these models in the present study was to verify the influence of nitrogen fertilization levels on the explanation of the possible heterogeneity among effect measures. To do so, the hypotheses were tested through mixed effects models, being the studies considered of random effect and the moderating variable (nitrogen fertilization level) of fixed effect. For the subgroups analysis, the studies were categorized according to the following topdressing nitrogen: absence of fertilization (0 kg·ha-1 of N), low (> 0 and≤ 50 kg·ha-1 of N), moderate (> 50 and ≤ 100 kg·ha-1 of N) and high (> 100 kg·ha-1 of N).

Statistical analyses were carried out with the help of the software R (R Corel Team, 2016), using the meta (Schwarzer and Schwarzer 2017Schwarzer, G. and Schwarzer, M. G. (2017). Package ‘meta’. Meta-analysis with R, p.2-1.) and metafor (Viechtbauer 2010Viechtbauer, W. (2010). Conducting meta-analyses in R with the metafor package. Journal of Statistical Software, 36, 1-48. https://doi.org/10.18637/jss.v036.i03.) packages.

RESULTS AND DISCUSSION

Initially, the study surveyed 72 scientific articles; however, after the exclusion criteria, 14 articles, forming an aggregate of 26 trials, were considered for the meta-analysis study (Table 1).

Table 1
References and trials number of articles used by the meta-analysis.

The Cochran Q test observed heterogeneity among effect measures (Q = 732.71, DF = 171, p < 0.001). The statistical I2 test also detected the existence of heterogeneity with the magnitude of 85.40%, inferring a high heterogeneity among effect measures. According to Higgins and Thompson (2002)Higgins, J., Thompson, S. G. (2002). Quantifying heterogeneity in a meta-analysis. Statistics in Medicine, 21, 1539-1558. https://doi.org/10.1002/sim.1186., the quantification of the heterogeneity extension among the studies is of extreme importance, since it can influence the meta-analysis conclusions. According to Borenstein et al. (2009)Borenstein, M., Hedges, L. V., Higgins, J. P. T. and Rothstein, H. R. (2009). Introduction to meta-analysis. Chichester, UK: John Wiley & Sons., whenever this heterogeneity is identified, it can be incorporated in the statistical model through the random effect meta-analysis or explained, at least partially, by the subgroups and meta-regression analyses.

The funnel plot of the relationship between the mean standard deviation and the residual values of the effect measures used by the meta-analysis is shown in Fig. 1. In general, data dispersion behavior was relatively symmetrical, suggesting a weak publication bias evidence, also detected by the Egger method (p = 0.89). According to the Shapiro-Wilk test, the effect measures showed a normal distribution (p > 0.05). Therefore, all effect measures were considered during the meta-analysis.

Figure 1
Funnel plot with confidence pseudo-limits of 90, 95 and 99% form the relationship between the mean standard deviation and the residual value of the effect measures used by the meta-analysis.

Random effect meta-analysis showed that maize inoculation with bacteria of the Azospirillum genus showed significant yield increase estimate (p < 0.001), with an average increase of 651.58 kg·ha-1 (95% CI = 510.89 – 792.27 kg·ha-1) in inoculated treatments in relation to non-inoculated treatments. Veresoglou and Menexes (2010)Veresoglou, S. D. and Menexes, G. (2010). Impact of inoculation with Azospirillum spp. on growth properties and seed yield of wheat: a meta-analysis of studies in the ISI Web of Science from 1981 to 2008. Plant and soil, 337, 469-480. http://dx.doi.org/10.1007/s11104-010-0543-7., based on the meta-analysis of studies on the effect of Azospirillum spp. inoculation wheat grain yield observed an average yield increase of 8.9% in inoculated treatments in relation to non-inoculated under field conditions. In this same context, Díaz-Zorita et al. (2015)Díaz-Zorita, M., Canigia, M. V. F., Bravo, O. A., Berger, A. and Satorre, E. H. (2015). Field evaluation of extensive crops inoculated with Azospirillum spp. In F. D. Cassan, Y. Okon and C. M. Creus (Eds.), Handbook for Azospirillum: technical issues and protocols. 435-445. Switzerland: Springer. https://doi.org/10.1007/978-3-319-06542-7_24. analyzed 47 articles published in scientific journals and described 347 cases of grain yield response of different species to inoculation with bacteria of the Azospirillum genus. Among the crops reported, the average grain yield response to inoculation with Azospirillum was 10%, as the highest responses were observed in winter cereals (14%), summer cereals (9.5%) and legumes (6.6%).

In addition to making nitrogen available and offering greater control of abiotic and abiotic stresses (Lugtenberg and Kamilova 2009Lugtenberg, B., Kamilova, F. (2009). Plant growth-promoting rhizobacteria. Annual Review of Microbiology 63, 541-556. http://dx.doi.org/10.1146/annurev.micro.62.081307.162918.; Hungria et al. 2010Hungria, M., Campo, R. J., Souza, E. M. and Pedrosa, F. O. (2010). Inoculation with selected strains of Azospirillum brasilense and A. lipoferum improves yields of maize and wheat in Brazil. Plant and Soil, 331, 413-425. http://dx.doi.org/10.1007/s11104-009-0262-0.; De-Bashan et al. 2012De-Bashan, L. E., Hernandez, J. P. and Bashan, Y. (2012). The potential contribution of plant growth-promoting bacteria to reduce environmental degradation–A comprehensive evaluation. Applied Soil Ecology, 61, 171-189. https://doi.org/10.1016/j.apsoil.2011.09.003.; Pii et al. 2015Pii, Y., Mimmo, T., Tomasi, N., Terzano, R., Cesco, S. and Crecchio, C. (2015). Microbial interactions in the rhizosphere: beneficial influences of plant growth-promoting rhizobacteria on nutrient acquisition process: a review. Biology and Fertility of Soils, 51, 403-415. http://dx.doi.org/10.1007/s00374-015-0996-1.), the bacteria of the Azospirillum genus also promotes plant growth through the production of aminoacids, indoleacetic acid, giberelines and other polyamines. This favors root system growth and, consequently, greater water and nutrients absorption by plants (Bashan and De-Bashan 2010Bashan, Y. and De-Bashan, L. E. (2010). How the plant growth-promoting bacterium Azospirillum promotes plant growth-a critical assessment. Advances in Agronomy, 108, 77-136. https://doi.org/10.1016/S0065-2113(10)08002-8.; Doornbos et al. 2012Doornbos, R. F., Van Loon, L. C. and Bakker, P. A. H. M. (2012). Impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere. A review. Agronomy for Sustainable Development, 32, 227-243. http://10.1007/s13593-011-0028-y.; Tien et al. 2012Tien, T. M., Gaskins, M. H., De-Bashan, L. E., Hernadez, J. P. and Bashan, Y. (2012). The potential contribution of plant growth-promoting bacteria to reduce environmental degradation - a comprehensive evaluation. Applied Soil Ecology, 61, 171-189. https://doi.org/10.1016/j.apsoil.2011.09.003.). Although many mechanisms have been described to explain the plant growth promotion by Azospirillum spp., in this context, a single mechanism, most of the times, is not responsible for the total effect (Cassán and Díaz-Zorita 2016Cassán, F. and Diaz-Zorita, M. (2016). Azospirillum spp. in current agriculture: from the laboratory to the field. Soil Biology and Biochemistry, 103, 117-130. https://doi.org/10.1016/j.soilbio.2016.08.020.). Thus, the action mode of the Azospirillum spp. could be better explained by the additive hypothesis, that considers multiple mechanisms in the successful association of Azospirillum spp. with plants (Bashan and Levanony 1990Bashan, Y., Levanony, H. (1990). Current status of Azospirillum inoculation technology: Azospirillum as a challenge for agriculture. Canadian Journal of Microbiology, 36, 591-608. https://doi.org/10.1139/m90-105.).

The incorporation of the nitrogen fertilization moderating variable in both meta-analytical models was significant (p < 0.001), inferring that the different levels of nitrogen fertilization can influence the response of Azospirillum spp. on maize crops grain yield. Veresoglou and Menexes (2010)Veresoglou, S. D. and Menexes, G. (2010). Impact of inoculation with Azospirillum spp. on growth properties and seed yield of wheat: a meta-analysis of studies in the ISI Web of Science from 1981 to 2008. Plant and soil, 337, 469-480. http://dx.doi.org/10.1007/s11104-010-0543-7. reported that nitrogen fertilization is considered a key factor in determining the efficiency of Azospirillum inoculation in wheat crops.

Matsumura et al. (2015)Matsumura, E. E., Secco, V. A., Moreira, R. S., Santos, O. J. P., Hungria, M., Oliveira, A. L. M. (2015). Composition and activity of endophytic bacterial communities in field-grown maize plants inoculated with Azospirillum brasilense. Annals of Microbiology, 65, 2187-2229. http://dx.doi.org/10.1007/s13213-015-1059-4. evaluated the impact of nitrogen fertilization on the diversity of endophytic bacteria on maize plants inoculated with A. brasilense. The authors found that nitrogen fertilization management affects dominant populations from a metabolically active bacterial community. In addition, under nitrogen regular level, the libraries of the genes 16S rRNA indicate a smaller diversity of such populations in relation to libraries of plants with low nitrogen. The same authors still report that the combination of treatments with nitrogen regular levels and plant inoculation had no additive effect. On the contrary, it shows a tendency to reduce yield.

Through the adjusted meta-regression model (Fig 2), a decreasing linear behavior within nitrogen fertilization levels was detected. Thus, the average yield increase of 651.58 kg·ha-1 (95% CI = 510.89 – 792.27 kg·ha-1) attributed to Azospirillum tends to diminish by 4.36 kg·ha-1 (95% CI = 1.81 – 7.21 kg·ha-1) in response to the addition of 1 kg·ha-1 topdressing nitrogen. According to Hartmann (1988)Hartmann, A. (1988). Ecophysiological aspects of growth and nitrogen fixation in Azospirillum spp. Plant and Soil, 110, 225-238. https://doi.org/10.1007/bf02226803., the presence of high concentrations of nitrogen reduces or even inhibits the efficiency of the nitrogen biological fixation by bacteria of the Azospirillum genus, inhibiting the nitrogenase activity by these bacteria, enzymes responsible for converting atmospheric nitrogen (N2) into ammonium (NH4 +), which are highly digestible by plants.

Figure 2
Bubble-plot of the linear relationship between measures of effect and topdressing nitrogen fertilization. Each bubble area is proportional to the study weight in the meta-analysis.

In addition to the process of controlling and regulating the nitrogenase complex, there are evidences that nitrogen contents in the soil can also regulate bacterial colonization (Carvalho et al. 2014Carvalho, T. L. G., Balsemão-Pires, E., Saraiva, R. M., Ferreira, P. C. G. and Hemerly, A. S. (2014). Nitrogen signalling in plant interactions with associative and endophytic diazotrophic bacteria. Journal of Experimenal Botany, 65, 5631-5642. https://doi.org/10.1093/jxb/eru319.). Coelho et al. (2009)Coelho, M. R., Marriel, I. E., Jenkins, S. N., Lanyon, C. V., Selsin, L. and O’Donnell, A. G. (2009). Molecular detection and quantification of nifH gene sequences in the rhizosphere of sorghum (Sorghum bicolor) sown with two levels of nitrogen fertilizer. Applied Soil Ecology, 42, 48-53. https://doi.org/10.1016/j.apsoil.2009.01.010., for instance, quantified the presence of diazotrophic bacteria through the gene nifH in a rhizosphere and non-rhizosphere soil of two sorghum genotypes (Sorghum bicolor L.) under different nitrogen fertilization dosages. The authors reported a significant reduction in diazothrophic bacteria abundance in the rhizosphere soil for both genotypes, under high dosages of nitrogen (Coelho et al. 2009Coelho, M. R., Marriel, I. E., Jenkins, S. N., Lanyon, C. V., Selsin, L. and O’Donnell, A. G. (2009). Molecular detection and quantification of nifH gene sequences in the rhizosphere of sorghum (Sorghum bicolor) sown with two levels of nitrogen fertilizer. Applied Soil Ecology, 42, 48-53. https://doi.org/10.1016/j.apsoil.2009.01.010.). On the other hand, there was no reduction in diazotrophic bacteria in the non-rhizosphere soil, where these bacteria remained constant, independent from nitrogen dosages.

Based on the subgroups analyses (Table 2), only the Without N subgroup showed a significant grain yield increase (p < 0.001), with an average increase of 1034.28 kg·ha-1 (95% CI = 772.94 – 1295.80 kg·ha-1) in relation to control treatments. Thus, the addition of topdressing nitrogen fertilizers does not promote significant yield increases in the inoculated treatments, suggesting that such technologies are non-additive (Spolaor et al. 2016Spolaor, L. T., Gonçalves, L. S. A., Santos, O. J. A. P. D., Oliveira, A. L. M. D., Scapim, C. A., Bertegna, F. A. B. and Kuki, M. C. (2016). Plant growth-promoting bacteria associated with nitrogen fertilization at topdressing in popcorn agronomic performance. Bragantia, 75, 33-40. http://dx.doi.org/10.1590/1678-4499.330.).

Table 2
Effect of nitrogen fertilization levels on maize yield in response to inoculation with bacteria of the Azospirillum genus.

According to Zhu et al. (2016)Zhu, S., Vivanco, J. M. and Manter, D. K. (2016). Nitrogen fertilizer rate affects root exudation, the rhizosphere microbiome and nitrogen-use-efficiency of maize. Applied Soil Ecology, 107, 324-333. https://doi.org/10.1016/j.apsoil.2016.07.009., the structure of the microbial communities in the rhizosphere is the result of complex interactions between maize and nitrogen fertilization. Furthermore, nitrogen has the capacity to modify the composition and abundance of root exudates and, posteriorly, affect the microbial communities from the rhizosphere. In this sense, the increase in nitrogen fertilization levels tends to recruit greater number of microorganisms for the rhizosphere and, consequently, increasing competition among them, lowering the efficiency of the inoculation process.

Currently, more than 100 biological products with Azospirillum spp. are commercially available only in South America (Cassán and Diaz-Zorita 2016Cassán, F. and Diaz-Zorita, M. (2016). Azospirillum spp. in current agriculture: from the laboratory to the field. Soil Biology and Biochemistry, 103, 117-130. https://doi.org/10.1016/j.soilbio.2016.08.020.). In general, maize inoculation with these bacteria is recommended additionally to topdressing nitrogen fertilization, which is in disagreement with this study’s results. Therefore, the use of inoculants with bacteria of the Azospirillum genus can be considered a promising practice, mainly under limiting cultivation conditions.

CONCLUSION

Maize inoculation with Azospirillum spp. showed an average yield increase of 651.58 kg·ha-1, considering all trials, and of 1034.28 kg·ha-1 considering only trials without topdressing nitrogen fertilization. Topdressing nitrogen fertilization is considered a determining factor in maize inoculation with Azospirillum spp., showing no significant increase in grain yield when realized jointly.

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Publication Dates

  • Publication in this collection
    16 July 2018
  • Date of issue
    Jul-Sep 2018

History

  • Received
    15 Aug 2017
  • Accepted
    14 Nov 2017
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