3.1 Effects of biofertilizers on N2-fixation efficiency and soil microbial activity
The interactive effects of cyanobacteria and yeast either each inoculated alone or both in combination with different levels of mineral nitrogen fertilization on soil microbial activity (e.g., nitrogenase activity and CO2 evolution) are shown in Table (1). Compared to FRD (100% N), the biological amendment with cyanobacteria significantly increased nitrogenase activity (N-ase) and CO2 evolution in soil rhizosphere of the two wheat genotypes. For instance, N50 + Cy and N75 + Cy significantly increased (P ≤ 0.05) N-ase activity in Sids-14 by 576% and 688%, and in Giza-171 by 1508% and 1427%, respectively, during 1st season. Similarly, N-ase activity was found to considerably increase in Sids-14 (196% and 202%) and in Giza-171 (100% and 93.2%) during 2nd season. Furthermore, the integrated biofertilization using N50 + Y + Cy and N75 + Y + Cy showed the highest increase in N-ase activity in Sids-14 (710%, 1258%) and Giza-171(5047%, 3345%) at the 1st season, and also at 2nd season in Sids-14 (270%, 279%) and Giza-171(141%,124%). N50 + Y + Cy and N75 + Y + Cy significantly increased CO2 evolution in soil rhizosphere of Sids-14 (58%, 56%) and Giza-171(151%, 80%) at the 1st season, and in Sids-14 (71%, 74%) and Giza-171(85%,72%) at the 2nd season. Although cyanobacteria amendment increased the soil microbial activity, however, the integrated application of cyanobacteria with yeast had the highest inductive effects on N2-fixation process and the soil microbial activity of the two wheat genotypes through field experiments.
3.2 Effects of biofertilizer on wheat growth
Nitrogen is a structural element of chlorophyll molecules and affects its accumulation in the plant leaves. Our results indicated that the Chl a,b contents were progressively decreased with reduction in applied mineral nitrogen levels (N 100˃N75˃N50%). The FRD resulted in high pigmentation patterns in Sids-14 and Giza-171 genotypes over the two seasons. Meanwhile, integrated biofertilization (N50 + Y + Cy and N75 + Y + Cy) significantly increased Chl a, b contents in two wheat genotypes when compared with individual Cy or Y treatments. Integrated biofertilization had statistical insignificant results (P ≤ 0.05) compared to FRD, and sometimes Chl a, b values surpassed those obtained by 100% N (e.g., Chl a of Sids-14 during 2nd season, Chl b of Sids-14 during 1st and 2nd and Chl b of Giza-171 at 2nd season). On contrast, the individual biological treatments such as Cy, N50 + Cy, N75 + Cy, N50 + Yand N75 + Y marginally decreased Chl a contents in Sids-14 by 6.3–31.3% at 1st and 2nd season. Similar results were also found in Giza-171 where Cy, N50 + Cy, N75 + Cy, N50 + Y and N75 + Y decreased Chl a contents by 6.3–43.8 at 1st and by 6.25 at 2nd season. At the level of genotype, slight fluctuations were observed in Chl a, b contents of each wheat strain during 1st and 2nd seasons, and both showed comparable pigmentation patterns in response to biofertilization (Table 2).
3.3 Influence of biofertilizer on wheat grain quality
NPK-uptake by wheat grains implies quality of wheat grains. As anticipated, FRD resulted in the highest NPK-uptake (Kg/fed) in the two genotypes over the 1st and 2nd seasons. Results indicated that the NPK-uptake was significantly decreased with decreasing of applied mineral nitrogen levels (N 100%˃ N 75%˃N 50%). Our results also indicated that the treatment with Cy as a sole biofertilizers or yeast with partial mineral nitrogen fertilization (e.g., N50 + Y, N75 + Y) induced remarkable decreases in NPK-uptake of wheat grain in both genotypes. However, the integrated biofertilization of cyanobacteria and yeast with partial mineral nitrogen fertilization showed similar and insignificant values of NPK-uptake obtained by 100% N (Table 3). Among seven biological amendments, N75 + Y + Cy recorded the highest averages of NPK-uptake of wheat grains in Sids-14 (52.8, 7.7 and 15.9 kg/fed) and in Giza-171 (63.9, 11.3 and18.9 kg/fed) during 1st season. The efficiency of N75 + Y + Cy was also confirmed by results of 2nd season where NPK-uptake was 53.1, 7.6 and 15.1 kg/fed in Sids-14 and 64.1, 7.7 and 17.6 kg/fed in Giza-171.
Protein content of wheat grains supported the results of photosynthetic pigments, and indicated a significant positive relationship between protein content and N-fertilization. The highest protein content in wheat grains was detected at 100% N as well as under treatments of integrated biofertilization. Giza-171 showed the highest protein content (10.2%, 10.5%) at the 1st season and (10.0%, 10.3%) at the 2nd season under N50 + Y + Cy and N75 + Y + Cy treatments, respectively.
3.4 Influence of biofertilizers on wheat yield components
The interactive effect of inoculation with cyanobacteria (Cy), yeast (Y) and mineral fertilization on wheat yield components is shown in Tables 4 and 5. Results indicated that crop growth indices during stage of vegetative growth (e.g., heading, plant height and number of spikes/m2 were N-level dependent. Reducing mineral nitrogen fertilization by 75% N significantly decreased plant height, and this reduction was more pronounced at 50% N in Sids-14 (14.5%, 6.4%) and in Giza-171 (13.1%, 11.2%) at 1st and 2nd season. On the other hand, treatment with Cy as a sole biofertilizers or N50 + Y remarkably decreased plant height in Sids-14 (12%, 11%) at 1st season and (10.3%, 7.3%) at 2nd season, and in Giza-171 (9.3%, 14%) and (3.7%, 6.5%) at 1st and 2nd seasons, respectively, when compared with FRD. Contrarily, the integrated biofertilization represented by N50 + Y + Cy and N75 + Y + Cy showed comparable results of plant heights with that of 100% N which, attained the highest plant height.
Results of number of spikes/m2 interpreted the level of soil nitrogen content, and showed similar pattern to previous results. Where 75% N significantly decreased number of spikes/m2 and this reduction was further strengthen by 50% N in Sids-14 (26.8%, 31.2%) and in Giza-171 (20.2%, 14.2%) during 1st and 2nd seasons.
Treatment with Cy as a sole biofertilizers or N50 + Cy and N50 + Y was also not enough for wheat growth and significantly decreased number of spikes/m2 in Sids-14 (23%, 14.5% and 16.2%) in the 1st season and (30%, 16% and 23%) in the 2nd season. Giza-171 exhibited also a comparable reduction patterns of spikes/m2 (15.8%, 6.4% and 13%) at 1st season and (10.2%, 0.7% and 6.1%) at 2nd season. On contrast, the integrated inoculation of N50 + Y + Cy and N75 + Y + Cy significantly enhanced the number of spikes/m2 compared to individual treatments of Cy or Y alone, and showed equivalent results to those of 100% N particularly at 2nd season for both genotypes.
At the harvest stage, results of wheat yield index represented by number of grains/spike, weight of 1000 grains and grains yield (tonne/ fed) were consistent with the photosynthesis and growth indices results, and reflected the positive relationship between wheat yield components and N-fertilization levels (Table 5). Treatment with 100% N recorded the highest number of grains/spike, weight of 1000 grains and the greatest wheat yields for both Sids-14 and Giza-171 over two seasons. However, 75% N treatment decreased number of grains/spike, weight of 1000 grains and wheat yields, this reduction was more pronounced under 50% N level in Sids-14 (38%, 3.8% and 18.6%) at the 1st season and (21.3%, 10.1% and 23.3%) at the 2nd season. Giza-171 also showed a similar reduction patterns (28.7%, 16.7% and 24.9%) at 1st season and (21.2%, 17.4% and 31.5%) at 2nd season. At the level of genotype, Giza-171 was more vulnerable to N-level deficiency and showed higher reduction percentages of number of grains/spike, weight of 1000 grains and wheat yields (tonne/fed) when compared to Sids-14.
On the other hand, treatment with Cy as a sole biofertilizer significantly decreased number of grains/spike, weight of 1000 grains and wheat yield in Sids-14 (29%, 1.8% and 25.7%) during the 1st season and (18.7%, 10.7% and 31.2%) in the 2nd season, and also in Giza-171 (29.9%, 12.2% and 30.1%) at 1st season and (27.1%, 12.3% and 23.5%) at 2nd season. Similarly, N50 + Cy and N50 + Y did not fulfill nitrogen requirement needed for wheat growth and resulted in significant decreases in number of grains/spike, weight of 1000 grains and wheat yields in the two wheat genotypes. The integrated biofertilization of N50 + Y + Cy and N75 + Y + Cy significantly enhanced number of grains/spike, weight of 1000 grains and wheat yields compared to individual treatments of Cy or Y alone. Moreover, both genotypes treated with N50 + Y + Cy and N75 + Y + Cy showed wheat yields (tonne/fed) similar to those of 100% N over two seasons.
At scale of genotype, Sids-14 treated with N50 + Y + Cy showed significant yield components (number of grains/spike, weight of 1000 grains and wheat yields) compared to Giza-171 over 1st and 2nd season.