Eect of fertilizer rate and water irrigation quality on the recovery of 15N-labeled fertilizer applied to Sudangrass

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 N-labeled 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.