To evaluate the effect of minimizing N, P and K fertilization levels in the presence of biofertilizers, two field experiments were conducted during the two successive winter season of 2003/04 and 2004/05 at the irrigated research experimental farm of Nubaria. The experimental design was a randomized complete blocks design (RCBD). Main plots consisted of three N, P and K combination levels (low= L, medium= M, and high=H). Meanwhile, the sub-plots consisted of four biological fertilization treatments: control (without inoculation) inoculation with Rizobia (R), infection with Arbuscular Mycorrhiza (AM) and co-inoculation and infection (R+AM).
The results showed that plant height, GP (green pod) yield, and seed yield (SY) of fababean were responded significantly to the mineral fertilization. Inoculation with microorganisms increased biological yield (BY), SY, and staw yield (StY) of faba bean in the two growing season. Nitrogen use efficiency (NUE), phosphorous use efficiency (PUE), and potassium use efficiency (KUE) increased with decreasing fertilizer rate. With high fertilizer level, the coupled inoculation (R+AM) recorded the highest NUE, PUE, and KUE. Soil NH4+ concentration increased significantly with the coupled inoculation (R+AM) while it decreased significantly with (R) and (AM) compared to un-inoculated. In contrary, Soil NO3- increased significantly with (R), (AM), and (R+AM).
Nutrients uptake by faba bean plants increased significantly with different levels of mineral fertilizers. The coupled inoculation had remarkable influences on all nutrients uptake than single biofertilizers. Concerning, the interaction of mineral and biofertilizers, it was noticed that (R) significantly increased N uptake in straw with L and M NPK. Phosphorous uptake was increased significantly in both straw and seeds with (AM) inoculation at all NPK fertilizer levels. For the coupled inoculation it was noticed that (R+AM) increased significantly N uptake in straw at the three fertilizer levels. Nitrogen, P and K uptake in the whole plants was also significantly increased by the interaction between mineral and biofertilizers. The combined treatment (high NPK+ coupled inoculation) produced the highest P and K in plants, while, the L-NPK with coupled inoculation recorded the highest N uptake for the whole plants.