Solutes in runoff under simulated rainfall on fertilised sugarcane (Saccharum sp.) beds: Measurements and results

Abstract

Wet Tropics region is ranked as the highest relative risk to Great Barrier Reef water quality with improved nitrogen management as priority. Sugarcane is a major crop in Wet Tropics and contributes 84 % of estimated anthropogenic dissolved inorganic nitrogen (DIN). Experiments were undertaken using a rainfall simulator (rate ≈ 90 mm h−1, depth 48–67 mm) to measure solute amounts lost in runoff from raised beds planted with sugarcane on bare soil. Eighteen experimental plots, were constructed with a thin metal walls. Runoff was collected from 18 plots (1 m wide x 1.7 m long), plots were covered with shelters between simulations. Six plots had surface-applied fertiliser, six plots buried (subsurface) fertiliser in a band at 50–150 mm depth along the middle of plots, and a further six plots received no fertiliser. Fertiliser (Nitrophoska® Special) was applied at an equivalent rate of 46 kg-N ha−1 (ammonium at 19 kg-N ha−1 and nitrate at 27 kg-N ha−1), 20 kg-P ha−1 phosphate, 54 kg-K ha−1 potassium and 31 kg-S ha−1 sulfate. Two plots from each fertiliser treatment rained upon 7, 20 and 55 days after fertiliser was applied (DAF). Two fertilised treatment plots were re-rained upon on 20 and 55 DAF and another two fertilised plots were re-rained upon a third time on 55 DAF. No effect on runoff volume occurred when all treatments are included in analysis. On plots with repeated rainfall simulations, time to commencement of runoff decreased from rainfall simulation 1–2, no further decrease occurred. Runoff nitrogen load was dominated by particulate nitrogen with: > 75 % for all fertiliser treatments and > 90 % for no fertiliser treatment. DIN concentration was dominated by nitrate and nitrite (NOx-N) except for the first rainfall simulation on surface-applied fertiliser plots. Ammonia comprised more of the DIN in surface compared to subsurface treatment. Evaporative concentration resulted in no difference in NOx-N load between fertiliser treatments. Subsurface fertiliser placement did reduce ammonium concentration by 74 % and 90 % compared to surface applied fertiliser. Phosphorus in runoff was dominated by particulate phosphorus being > 95 %. Subsurface application of fertiliser reduced phosphorus load in runoff from 0.9 to 0.14 kg ha−1. Eighty-one% of applied surface sulfate was lost in runoff compared to 20 % of subsurface-applied fertiliser. Results highlighted the importance of particulate nitrogen and phosphorus in runoff and suggested that evaporative concentration of nitrogen can occur at the surface between rainfall events.