Effects of sewage sludge application on nodulation, nitrogen fixation, andplant growth of faba bean, soybean, and lupin

The effect of sewage sludge application to a desert soil on nodulation, nit rogen fixation, and plant growth of faba bean (Vicia faba I,.), soybean (Gl ycine,max L.), and lupin (Lupinus albus L.) was investigated in a pot exper iment under greenhouse conditions. Nodulation, nitrogenase activity, and pl ant growth of faba bean, soybean, and lupin were significantly increased by moderate application rates of sewage sludge (20 and 30%, w/w). The decisiv e effect caused by application of sewage sludge on legume growth was the pr omotion of nodulation and subsequent nitrogen fixation. At an application l evel of 30% (w/w), compared with control, nodulation was increased by 55%, 96%, and 171% for faba bean, soybean, and lupin, respectively Conversely, a t high application rates (40 and 50%) sewage sludge significantly inhibited nodulation, nitrogen fixation, dry matter accumulation and nitrogen yields of faba bean and soybean. Again, inhibition of nodulation by high amounts of sludge was the decisive process responsible for low nitrogen fixation an d dry matter production of legume plants. At 50% application rate, compared with control, sewage sludge caused significant nodulation decrease by 62%, 79%, and 29% for faba bean, soybean and lupin, respectively. Data of plant analysis indicates that the inhibitory effect of sewage sludge at high app lication rates was most probably due to toxic (noxious) effect of heavy met als (Cu and Zn) on the microsymbiont rather than on host plants. Our result s demonstrated that sewage sludge at low application rates may significantl y improve legume growth on desert soils.