The Effect of Application of Nitrogen Fixing Bacteria and Plant Residues on Dry Matter Remobilization and Barley Yield under Water Stress after Anthesis

Grain yield of cereals significantly affects by assimilate remobilization in arid zones. Many factors including N fertilizer application affects assimilate remobilization in cereals (Ercoli et al., 2008). Many researches have been done aboat the effects of chemical N fertilizer under normal and water stress conditions on assimilate remobilization in arid environments (Ercoli et al., 2008; Barati and Ghadiri, 2017; Bahrani and Tahmasbi-sarvestani, 2006). However, the effects of other N sources such as crop ressidues and N fixing bacteria alone or in combination with chemical N fertilizers on assimilate remobilization and consequently grain yield was very scarce specially in arid zones. Therefore, there are two main questions; 1. Can using of Azospirillum brasilense as a N fixing bacteria instead of chemical N fertilizers or in combination with them in bio-agriculture boost the assimilate remobilization and consequently grain yield of barley under water stress conditions?, and 2. Can applying plant residues increase Azospirilum activity and consequently increase assimilate remobilization and grain yield in dry areas by improving water storage capacity or another mechanisms? For answering the above questions, the effects of application of N fixing bacteria (Azospirillum brasilense) and wheat residues on dry matter remobilization and barley yield under water stress after anthesis stage was evaluated in an arid zone of Southern Iran (Fars province).

This research was conducted at the experimental farm of the Darab Agricultural College of Shiraz University. A split factorial experiment in a randomized complete block design with three replications were carried out in 2017 - 2018 growing season. Treatments included: two levels of irrigation as the main plots [normal irrigation (IRN): irrigation based on the plant's water requirement up to the physiological maturity and another factor was deficit irrigation (IRDI): irrigation based on the plant's water requirement up to the anthesis stage (cutting of irrigation after anthesis)]. Also, sub plots were two levels of wheat residues [1. without residue, 2. returning 30% of wheat residues to soil] and four fertilizer sources [N0, no nitrogen fertilizer (control); N100, 100 kg N ha-1; Bio + N50, Biofertilizer (Azospirillum brasilense) + 50 kg N ha-1 and Bio, Biofertilizer (Azospirillum brasilense)]. Finally, at the anthesis stage, dry matter of shoot samples were measured. Also, at the end of growing season, grain yield, dry matter of vegetative parts of shoot were measured. Then, assimilate remobilization, assimilate remobilization efficiency and contribution of pre-anthesis assimilate to grain were calculated by Ercoli et al., 2008 methods. Data were analyzed using SAS 9.1 software (SAS Institute, 2004).

Overall, combined N fertilizer (Bio + N50) significantly increased grain yield (53%) in normal irrigation. Similarly, the grain yield was improved up to 12% under water stress conditions by Bio + N50. Water stress increased assimilate remobilization and contribution of pre-anthesis assimilate to grain in Bio + N50 treatment by 37 % and 148%, respectively, as compared with the normal irrigation treatment. The application of plant residues decreased the amount of assimilate remobilization, assimilate remobilization efficiency and contribution of pre-anthesis assimilate to grain in all fertilizer treatments. However, it did not significantly affect the grain yield.

Generally, considering the economic aspects and moving towards the sustainable agriculture, using of combined N fertilizer [Biofertilizer (Azospirillum brasilense) + 50 kg N ha-1] in optimal irrigation conditions is recommended for farmers of Southern Iran. Furthermore, if cutting of irrigation after anthesis is considered because of irrigation water resource shortage, this fertilizer regime is recommended as compared with the other N sources due to the significant increase in assimilate remobilization (37%) and contribution of pre-anthesis assimilate to grain (148%) and lower grain yield reduction as compared with the normal irrigation conditions.