Nitrogen metabolism is related to improved water-use efficiency of nodulated alfalfa grown with sewage sludge under drought

Leguminous plants grown in sewage sludge-amended soils can acquire nitrogen by assimilation of nitrate and ammonium from the soil solution or from atmospheric-dinitrogen (N-2) fixation through association with N-2-fixing bacteria. We proposed that operation of both metabolic processes could contribute to alleviate the impact of drought in sludge-treated plants. A greenhouse experiment was conducted to evaluate the involvement of nodule metabolism in the use efficiency of water and N in sludge-treated plants. Treatments comprised (1) plants inoculated with rhizobia and amended with sewage sludge; (2) plants inoculated with rhizobia without any amendment; and (3) noninoculated plants supplied with ammonium nitrate, each under well-watered and drought conditions. Under drought, sludge-treated plants had increased plant growth and higher photosynthetic and water-use efficiencies than untreated plants. Drought stimulated nitrate reductase and GS/GOGAT activities but did not affect the activities of phosphoenolpyruvate carboxylase and malate dehydrogenase or the leghemoglobin concentration. The results suggest that under drought conditions, both N-2 fixation and nitrate assimilation in nodules of sludge-treated plants contributed to improve plant N supply and to increase the drought tolerance of alfalfa.