Uptake and nitrogen remobilization of barley under deficit irrigation in presence of plant residue and Azospirillum bacteria

Grain N content in barley plant is affected by N remobilization from vegetative organs to the grain during the grain filling period. The amount of N remobilization is negatively affected by the severe water stress after anthesis. Using of Azospirillum as a biofertilizer is a way for reduction the adversely effects of the severe water stress in the arid regions. Another way is increase the amount of soil organic matter (such as using of plant residues). Therefore, this study investigated the effects of wheat residues application and different N sources (biological and chemical) on N remobilization and grain N content of barley.

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 replicates 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 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)].

Application of N100 and Bio + N50 treatments significantly increased the N remobilization efficiency by 36% and 34%, respectively, as compared with the control (N0) under IRN. In contrast, under IRDI condition, all N sources decreased the N remobilization efficiency as compared with the control (N0). However, the amount of reduction in Bio + N50 and Bio treatments (10% and 11%, respectively) was lower than N100 (21%). The similar trend of N remobilization efficiency was observed for N harvest index in IRN and IRDI. Nitrogen harvest index showed a positive and linear relationship with N remobilization efficiency under both irrigation regimes. However, the justified variation of the N harvest index by N remobilization efficiency under IRDI was higher than (25%) the IRN conditions (R2 = 0.70 and R2 = 0.45). Also, Grain N content linearly increased with increasing N remobilization under both irrigation regimes. Application of wheat residues significantly reduced the N remobilization efficiency and grain N content in Bio + N50 and Bio treatments as compared to non-applied of residue. Also, there was no significant difference between with and without residue treatments in N remobilization efficiency and grain N content when N100 treatment was applied.

According to the environmental and economic considerations, integrated N fertilizer [Biofertilizer (Azospirillum brasilense) + 50 kg N ha-1] is recommended for normal irrigation conditions in order to maximum grain protein content achievement. In contrast, under cutting of irrigation after anthesis conditions, biofertilizer treatments (using of Azospirillum brasilense alone or using of it with integration to 50 kg N ha-1) reduced N remobilization efficiency as compared with control (N0). However, the decrement values were lower than the sole chemical N fertilizer (100 kg N ha-1). Therefore, in Southern Iran where water stress after anthesis is possible, biofertilizers is recommended. Also, using of wheat residues is not recommended for increasing N remobilization efficiency and consequently grain N content across over all irrigation and N regimes.