Abstract:
The effect of sewage sludge application to a desert soil on nodulation, nitrogen fixation and plant growth of faba bean (Vicia faba cv. Giza 1), soyabeans (Glycine max cv. Clark) and lupin (Lupinus albus cv. Marita) was investigated in a pot experiment under greenhouse conditions. Nodulation, nitrogenase activity and plant growth of faba bean, soyabeans and lupin were significantly increased by moderate application rates of sewage sludge (20 and 30%, w/w). The decisive effect caused by application of sewage sludge on legume growth was the promotion of nodulation and subsequent nitrogen fixation. At an application level of 30% (w/w), nodulation was increased by 55%, 96%, and 171% for faba bean, soyabeans and lupin, respectively, compared with controls. Conversely, at high application rates (40 and 50%) sewage sludge significantly inhibited nodulation, nitrogen fixation, dry matter accumulation and nitrogen yields of faba bean and soyabeans. Again, inhibition of nodulation by high amounts of sludge was the decisive process responsible for low nitrogen fixation and dry matter production of legume plants. At 50% application rate, sewage sludge decreased nodulation by 62%, 79% and 29% for faba bean, soyabeans and lupin, respectively, compared with controls. Plant analysis indicated that the inhibitory effect of sewage sludge at high application rates was most probably due to a toxic (noxious) effect of heavy metals (Cu and Zn) on the microsymbiont rather than on host plants. It is concluded that sewage sludge at low application rates may significantly improve legume growth on desert soils.