Irrigation of Secondary Sewage Effluent: Salinity and Nitrogen Effects on Growth and Nitrogen Fixation of Nodulated and Non-nodulated Soybeans

Salinity and nitrogenous components are the most critical water qualities in secondary sewage effluent (SSE) when used as an alternative resource for agricultural irrigation water. In this study a pot experiment was conducted to investigate the effects of salinity and inorganic nitrogen in the irrigation water on the growth and nitrogen fixation of soybean (Glycine max (L.) Merrill) isoline T201 and T202. Nitrogen in the irrigation water as the plant nutrient contributed slightly to dry matter production because the total amount of nitrogen applied into a pot was small compared to the plant's demand for nitrogen or to the dosage of applied fertilizer nitrogen. Nitrogen in the irrigation water, however, alleviated the toxic effect of salinity on nitrogen fixation of soybean. Moderate salinity that is similar to the salinity of SSE from the city sewage disposal plant slightly affected nitrogen fixation and reduced dry matter production. Sever salinity water with electroconductivity of 270mS/m greatly reduced both growth and nitrogen fixation. We should pay more attention to salinity as the water quality of SSE rather than nitrogen components when SSE is reclaimed and used as irrigation water in upland fields.