Interaction effects of irrigation by municipal wastewater and inorganic fertilisers on wheat cultivation in Bangladesh

The interaction effects of five irrigation water qualities formed with municipal wastewater (hereafter called wastewater) and two inorganic fertiliser levels on the growth and yield of wheat (Triticum aestivum L. cv Shatabdi) were explored in three consecutive growing seasons during 2007–2010. The experiment was set in a split-plot design with two factors and three replications. The five irrigation treatments – I1: freshwater (groundwater extracted by tubewell) as control, I2–I4: diluted wastewater (having wastewater fraction of 0.25, 0.50 and 0.75, respectively) and I5: raw/undiluted wastewater – were allocated to the main plots, and the two fertiliser levels – F0: no fertiliser and F1: recommended standard dose – were allocated to the sub-plots. The wastewater contained nitrogen (N), phosphorus (P) and potassium (K) at concentrations of 17.5, 3.7 and 10.3 mg L−1, respectively, which correspondingly contributed 4.8–19.1, 3.8–15.1 and 5.4–21.7% of the recommended N, P and K. Wastewater exerted a significant (α = 0.05) positive impact on most growth and yield variables and grain and biomass yields of wheat irrespective of the applied fertiliser dose. The highest values of the crop variables and yields were generally obtained in I4F1 (irrigation water containing 75% wastewater under the fertilised condition) and I5F0 (irrigation by raw wastewater under the non-fertilised condition). On average, over three years, the treatments that received 50–100% wastewater in irrigation (I3F1–I5F1) provided statistically identical results. Although wastewater always promoted leaf growth, its effectiveness in improving the leaf area index (LAI) decreased with its elevated quantity in irrigation. Raw wastewater promoted the highest leaf growth at 65 days after sowing (DAS) and provided the maximum LAI under both fertility treatments. The significantly different above-ground dry matters (ADM) among the irrigation treatments obtained at the later growth stages (95–110 DAS) exposed a delayed effectiveness of wastewater in the accumulation of shoot biomass. The raw wastewater contributed to producing the highest ADM under both fertility treatments. The most extensive root proliferation was in I4F1, which produced the highest grain and biomass yields; the grain yield increased in this treatment by 14% over I1F1. Both the grain and biomass yields in I3F1–I5F1 treatments were statistically similar. The highest water use efficiencies obtained in I4F1 and I5F0 depicted the most effective utilisation of irrigation water in these treatments.