Effects of Different Nitrogen Fertilizer Sources and Crop Residues on Yield and Yield Components of Barley (Hordeum vulgare L.) under Late Season Water Stress

Inappropriate use of chemical inputs damages soil beneficial microorganisms and consumer health. In order to reduce or eventually eliminate chemical inputs, bio-agroecosystems was founded. Azospirillum is one of the most studied genera as nitrogen fixing bacteria in agroecosystems. About 70 percent of the experiments have demonstrated the Azospirilum ability for increasing crop yield. Furthermore, some studies have shown that Azospirilum has a stress-reduction mechanism. Therefore, it seems that the Azospirilum is a suitable microorganism for low yielding conditions (water and N) of southern Iran. On the contrary, with respect to the importance of soil organic matter for microorganisms survival, its deficiency in soils of southern Iran is the biggest challenge for using the microorganisms as biofertilizers. Also, experiments on the interactive effects of crop residue, water deficiency and N sources (biological or chemical) on barley yield are quite scarce in southern Iran. Therefore, the aims of this study were to investigate the effects of crop residue management, different N sources (such as biological and chemical) and water stress conditions on barley yield in arid conditions of southern Iran (Fars province).

This research was conducted at the experimental farm of the Darab Agricultural College of Shiraz University during the 2017-2018 growing season. A split factorial layout based on a randomized complete block design with three replications was used. Treatments included: two irrigation levels as the main plots [1. Normal (IRN): irrigation based on the plant's water requirement up to the physiological maturity and 2. Deficit irrigation (IRDI): irrigation based on the plant's water requirement up to the anthesis stage (cutting of irrigation after anthesis)]. Also, subplots were two levels of plant 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)]. Biological yield and grain yield, yield components, plant height, spike length and chlorophyll content of flag leaf were measured. Then, the harvest index was calculated. Data were analyzed by using SAS 9.1 software and the means were separated using Duncan’s multiple range test at 5% probability level.

The results showed that the cutting of irrigation after anthesis could cause severe water stress in the grain-filling period of the barley life cycle and consequently reduction of 1000-seed weight, the number of seeds per spike and the number of fertile tillers per m2. The interaction effect of irrigation regime × N fertilizer source on grain yield was significant at 1% probability level. The highest grain yield was achieved in IRN and Bio + N50 (4049 kg ha-1). Water stress reduced grain yield at all N fertilizer sources as compared with IRN. However, this reduction was different in N fertilizer sources (24.1%, 46.8%, 44.3% and 22.1% in N0, N100, Bio + N50 and Bio, respectively). With respect to the lowest amount of grain yield reduction due to water stress in Bio and then in N0 treatments and economic considerations, N0 treatment can be recommended for use in the cutting of irrigation after anthesis (IRDI) strategy.

According to the results of this study, the highest barley grain yield achieved by the integrated N fertilizer [Biofertilizer (Azospirillum brasilense) + 50 kg N ha-1] in normal irrigation. Deficit irrigation after anthesis significantly decreased barley grain yield at all N sources. Therefore, this irrigation regime was not recommended for barley farms of southern Iran. But if farmers intend to the cutting of irrigation after anthesis because of water resources deficiency, with respect to the lack of significant difference among N sources in this condition, it is recommended that no N fertilizer is applied.