Evaluation of fertigation applied to furrow and overhead irrigated cotton grown in a Black Vertosol in Southern Queensland, Australia

Evaluation of fertigation applied to furrow and overhead irrigated cotton grown in a Black Vertosol in Southern Queensland, Australia

Field trials were conducted at gated pipe surface and overhead irrigation sites established to cotton (Gossypium hirsutum L.) to evaluate irrigation and fertigation management using a model-based control system. The control strategies determined the timing and volume of irrigation, and the rate of fertilizer-N to apply through fertigation. For this, nitrogen (N) was applied in-crop season using urea ammonium nitrate (UAN, 30% N solution) at a rate of 40 kg ha-1 N. At the furrows site, the uniformity of distribution of fertilizer-N applied through fertigation was satisfactory, which was achieved both at distance (600 m) and depth (0-600 mm). Applying fertilizer-N through fertigation, at the rate used in this study, showed relatively small (≤8%) improvements in cotton yield, which was explained by relatively high N rates (180 kg ha-1 N) applied before planting. Given current price ratios (fertilizer-to-cotton), application of N through fertigation appears to be economical in both systems, but relative agronomic efficiencies and economic return from the fertilizer applied were lower in furrow compared with overhead (P<0.05). Fertigation may be recommended when pre-season N application rates are low (e.g., <100 kg ha-1 N), particularly in overhead irrigation as significantly higher efficiencies both in terms of water and N use can be achieved with this system. This would enable some of the operational constraints associated with application of N in-crop season to be overcome; thereby, reducing the need for high rates of N applied up-front. For the overhead system, there were also advantages compared with the furrow system in terms of reduced potential for N2O emissions after irrigation or fertigation. Overall, short-term (30-day period) soil emissions of N2O were approximately eight times higher in furrow compared with overhead. Emissions from non-fertigated crops were approximately two times higher in furrow compared with overhead. Emissions from the fertigated crop under the overhead system were comparable to the non-fertigated crop of the furrow system (P>0.05). In both systems, fluxes were highest within five days of irrigation or fertigation, but they decreased significantly after that time as soil moisture content (water-filled pore space) and soil nitrate levels decreased due to crop uptake. Nitrous oxide fluxes were similar in furrow and overhead 15 days after the irrigation or fertigation event. Areas that warrant further investigation are presented and discussed, including the need for improved timing of fertilizer delivery during the irrigation cycle to ensure that N losses through leaching or gaseous evolution (e.g., N2O, N2) are not economically or environmentally significant.

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