Effect of mineral and biological nitrogen fertilization on Thompson Seedless grape transplants. III. Effect on microbial activity in the soil rhizosphere.

Abstract

A pot experiment was carried out in 2001 and 2002 seasons on Thompson seedless grape transplants to study the effect of two sources of nitrogen fertilizers (mineral and biological) on microbial activity in the rhizosphere of transplants. The experiment included three levels of mineral nitrogen (M1=zero, M2=5 and M3=10 g N/plant/year) and four levels of biological nitrogen (B1=zero, B2=50, B3=100 and B4=200 ml) of liquid culture of free living nitrogen fixation bacteria Azotobacter chroococcum. The study is a factorial experiment in a randomized complete block design. The activity of Azotobacter expressed as counts of bacteria and CO2 evolution in the rhizosphere were determined after 45, 90, 135 and 180 after inoculation time. Results proved that the levels of mineral nitrogen showed more or less similar values. On the contrary, the levels of biological nitrogen increased Azotobacter activity and the highest values were obtained by levels B3 and B4. Regarding the interaction between mineral and biological nitrogen, fertilizing with mineral nitrogen alone had no promising effect on both the count and CO2 evolution. This is excepted because the soil of these treatments was not inoculated with Azotobacter. On the contrary, fertilizing with biological nitrogen alone increased counts of bacteria and CO2 evolution compared with those of unfertilized plants. The promising biological nitrogen treatment was (M1 × B4) in the two seasons. However, other combinations (treatments (M2 × B3), (M2 × B4) and (M3 × B4)) gave the highest values of bacterial counts and CO2 evolution. In other words, combining mineral with biological nitrogen showed more increase in the activity of the bacteria in the rhizosphere of transplants than adding any form alone and the recommended treatment was M2 × B3.