Jabbar F. al-Maadhidi. “Effect of Exudates Excreted by Trichoderma Harzianum on the Nitrogen Fixation (C2H2- Reduction) Rate of the Cyanobacteria Anabaena Variabilis ”. Journal of Madenat Alelem University College, Vol. 1, no. 1, June 2009, pp. 21-28, http://116.203.176.164/index.php/JMAUC/article/view/290.