Effects of Irrigation Efficiency, Phosphorus Rate, and Tillage On Phosphorus Runoff Losses In a Furrow Irigated Cotton Field.

Phosphorus (P) additions are vital for profitable crop and livestock production. However, excess losses of P from agricultural lands to watersheds can contribute to eutrophication problems. Therefore, implementation of management practices that minimize erosion and runoff plays a crucial role in P transport. This field study was conducted to investigate the effects of P rates (0, 90 and 135 kg ha^-1), tillages (conventional-till (CT), and Chisel-till (ChT)), and irrigation efficiencies (40% and 60%) on P losses (total P (TP), Bioavailable P (BAP), and water soluble P (WSP)) in runoff water from furrow irrigated cotton fields at two different locations (Eyyubiye and Koruklu). Sediment losses, runoff volume and P were summed over the growing season. Significant interaction between tillage and irrigation efficiency for runoff volume in 2009 were found in both locations. Runoff volume was generally highest with low irrigation efficiency (40%) and ChT. Similar interaction was also found for sediment losses in 2009 at Koruklu location as well as in 2010 at Eyyubiye location.  Generally the highest sediment losses were with CT and lower irrigation efficiency, whereas the lowest ones were with ChT and higher irrigation efficiency (60%). Phosphorus losses significantly increased with increasing P rate and decreasing irrigation efficiency. For example, P losses in 2010 for Eyyubiye location were ranged from 132.4 to 1298.6 g ha^-1 for TP, 60.4 to 293.2 g ha^-1 for BAP, 83.7 to 716.2 g ha^-1 for WSP. The highest P losses were with low irrigation efficiency and CT, whereas the lowest ones were with high irrigation efficiency and ChT.  Overall, the results suggested that increasing P rate increased P losses but no effects on sediment losses and runoff volume which were accelerated by CT and low irrigation efficiency.