Wastewaters are increasingly used for irrigation of cropping systems in Tunisia. However, to develop environmentally sound practices the contribution of wastewater to crop N nutrition needs to be clarified, especially in cropping systems already receiving mineral fertilizers. For a better understanding of the interaction between fertilizer N and N originating from wastewater, experiments using 15 N were conducted. 15 N-labeled fertilizer was applied at different rates (0, 60, 100 and 140 kg N•ha-1) and with different water irrigation qualities (tap water or treated wastewater) to sorghum grown in lysimeters during 1998 and 1999. Recovery of 15 N-labeled fertilizer in the above-ground crop at final harvest in treated wastewater irrigation was higher at the lowest rate of fertilizer application (54%), with the amount recovered in the crop increasing as the rate of 15 N-labeled fertilizer application increased up to the rate of 100 kg N•ha-1. Nevertheless, in spite of this increase in 15 N-labeled fertilizer in the crop, total plant N uptake did not differ between rates. Treated wastewater irrigation had no negative effect on the recovery of 15 N-labeled fertilizer. About 62 and 55% of 15 N-labeled fertilizer was removed by Sudangrass in either tap water or treated wastewater. Neither fertilizer N rate nor water quality had an effect on the 15 N-labeled fertilizer remaining in the soil at final harvest. On average 20% in the wastewater treatment (19-24%) and 30% in the tap water treatment (26-31%) of the 15 N fertilizer applied were in the 0-60 cm layer of soil at final harvest in 1998 and 1999, respectively, and mostly present in the 0-20 cm layer. The proportion of applied 15 Nlabeled fertilizer remaining in the soil at final harvest increased with increasing N rates. About 60, 69 and 72% of 15 N left in the soil at final harvest was in the surface 0-20 cm layer. Residual 15 N was greatly higher in soil following the first harvest than after the final harvest, with the greatest value (38%) measured at the lowest rate of 15 N-labeled fertilizer (30 kg N•ha-1). Losses of 15 N-labeled fertilizer increased with application rate, but were unaffected by water quality irrigation. Approximately 13% of the applied 15 N fertilizer was lost following the application of 100 kg N•ha-1 with either treated wastewater or tap water irrigation.

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Abstract-

A pot experiment was conducted to assess the fate and recovery of urea-N applied to sorghum (Sorghum sudanense) both in the presence and in the absence of sewage sludge. For a better understanding of the interaction between urea N and N originating from sewage sludge, 15N isotope technique was used. 15N-labeled urea was added to soil at 0, 60, 100 kg N.ha-1, and unlabeled sewage sludge was added at 0 and 45 kg N.ha-1. In the absence of sewage sludge, 15N recovery (15NR) was 34% of the 15N- Urea applied at 60 kg N.ha-1. It increased to 55% as the urea N rate increased to 100 kg N.ha-1. Co-application of sewage sludge with the highest dose of urea led to a decrease of urea 15NR by 11% as compared to that in the absence of sewage sludge. Application of sewage sludge significantly improved the immobilization of urea-derived N in soil, from 13 to 42 % and from 24 to 31% of 60 and 100 kg N urea applied respectively. Thus, sewage sludge ensured prolonged and continued availability of fertilizer N to plants thereby leading to reduce N loss and to higher fertilizer use efficiency.