EFFECT OF SEWAGE-SLUDGE ON NODULATION AND N-2-FIXATION IN ALFALFA GROWN ON CALCAREOUS LOAMY SOILS

The influence of different rates of sludge applications to calcareous loamy soils of Saudi Arabia, on nodulation and symbiotic N-2-fixation in alfalfa plants (Medicago sativa L.) was studied in a pot experiment . The effect of heavy metals accumulation in soil due to continuous ir rigation of the test soil with sewage water was also investigated. App lication of up to 80 g sludge per pot enhanced nodulation, nitrogenase activity, dry matter yield and N-contents of alfalfa plants growing i n loamy soils either previously irrigated with sewage water or well wa ter. However, sludge applied at the rate of 160-200 g pot(-1) inhibite d the nodulation, N accumulation and dry matter yield of alfalfa. The response of alfalfa to sludge was dependent on the rhizobial strain us ed. Our results also showed that accumulation of heavy metals due to c ontinuous irrigation of a calcareous sandy loam soil with sewage water , for more than 10 years, didn't inhibit N-2-fixation in alfalfa plant s, but enhanced it. Microelements in alfalfa plants increased with inc reased in the rate of sludge application. Although high rates of sludg e application affected nodulation and N-2-fixation of alfalfa, dry mat ter and the nitrogen contents of the plants were not highly affected. Therefore, the inhibitory effect of high rates of sludge was most prob ably due to the toxic effect of heavy metals on the microsymbiont rath er than on the plants.