Benefits of aeration of subsurface drip irrigation water: Field evidence on CQ highlands vertosols

Irrigation of heavy soils can result in temporary anoxic conditions in the rhizosphere. The aeration of the subsurface drip irrigation water was calculated to deliver an amount of oxygen that is significant to root respiration (equivalent to some hours of respiration need). Broadacre cotton (Gossypium hirsutum L.) irrigated with aerated water developed significantly more extensive and heavier (17.3%) root systems, canopies that enabled greater light interception (2.6% ), and produced significantly higher lint yields ( 16%) due to more bolls per plant (15%) in the 05/06 season. The increased yields of aerated plants were associated with improved gross production water use index (GPWUI), irrigation water use index (IWUI) and crop water use index (CWUI) by 16.9, 16.8 and 15.5% respectively. These improvements in yields and water use efficiencies in response to aeration were ascribed to enhanced root development and performance. Although trends were similar across two seasons, the differences were not significantly different in the first (04/05) season, a result attributed to inadequate replication in the trials of that year. A chickpea ( Cicer arietinum L.) crop grown in rotation to cotton produced significantly higher yields (26.9%; 359 vs. 283 g m-2), IWUI (27%, 2400 vs. 1890 g m-3) and GPWUI (26.8%; 1750 vs. 1380 g m-3) under aerated SDI, compared to nonaerated SDI. The aeration effect was more pronounced when drippers were placed at depth (25 and 35 em, compared to 5 and 15 em) (chickpea pot experiment). Aerated chickpea irrigated at the deepest emitter depth produced significantly higher yields (by 1 0.9%) and GPWUI (21 %) than their non-aerated counterparts. A field sorghum (Sorghum bicolor L.) crop grown following the second chickpea crop did not respond to the aeration treatment in terms of yield or water use efficiency (WUE), a result attributable to either the effect of a relatively wet season where the contribution of irrigation to total water use was less than 50%, or to a relative insensitivity of sorghum to soil anoxia. Aeration of SDI water improved cotton plant growth (larger root systems, heavier above-ground biomass and higher yield) for irrigation water with an electrical conductivity of0.78 dSm-1, but not at higher levels. The ratios of cotton leaf K+:Na+, Ca2+:Na+ and Mg2+:Na+ were increased (by 45.7%, 31.7% and 34.9%, respectively) by the aeration treatment, compared to the control, suggesting a higher level of salt exclusion by the root in the aerated treatments. This result is interpreted as due to maintenance of membrane integrity under aerobic conditions. Aeration of SDI has the potential to make a major contribution to sustainable water use for irrigated agriculture on heavy soils. The benefits of aeration of SDI in terms of both crop yield and water use efficiency improve the economic feasibility of this irrigation method.