Sustainable Irrigation to Balance Supply of Soil Water, Oxygen, Nutrients and Agro-Chemicals

Abstract

The socio-economic pressure for improvements in irrigation efficiencies is increasing due to intense competition for water between agricultural, domestic and industrial users as well as demands for compliance with environmental regulations. Precision irrigation technology involving less irrigation water and uniform application across the field is therefore important. In the context of declining water allocation for irrigation and the variations in weather and drought patterns attributed to global climate change, efficient and precise applications are necessity. Traditional irrigation methods such as furrow, flood and sprinkler are neither efficient nor environmentally benign. Precision irrigation methods such as drip and subsurface drip irrigation are advocated because they are more water use efficient and because they offer a possible approach to meet projected food demand, a doubling by the 2050. In spite of greater water use efficiency afforded by minimal soil surface evaporation and deep drainage, and ease of automation, wide scale adoption of surface and subsurface drip irrigation technology is limited. This is due to their high investment cost for installation. They often lack a significant yield benefit when compared to conventional irrigation practice. Reasons are probably linked to a sustained wetting front around emitters. These emitters impose a condition of low oxygen content in the root-dense rhizosphere surrounding emitters that impede root respiration, and negatively impact on plant uptake of water and nutrients, leading to constrained yield performance. Here we review aspects of soil oxygen dynamics during irrigation and present evidence for sustained hypoxia in the wetting fronts associated with drip and subsurface drip irrigation. This condition of low oxygen content in the rhizosphere conditioned by the drip irrigation we term as the irrigation paradox. At dissolved oxygen concentrations one half of that at saturation root respiration and function start to decline. The pros and cons of different approaches to maintain aeration of the crop root zone are reviewed and we suggest that the best approach to overcome hypoxia is through aeration of the irrigation water stream, a practice known as oxygation. This conclusion is based on the evidence derived from a number of controlled environment experiments and field trials on crops where consistent increases of 1030% in yield and water use efficiency were reported with aerated irrigation water. Aerated drip delivery of irrigation also allows for the simultaneous application of agro-chemicals directly into the crop root zone. We define this approach of delivering multiple agro-chemicals with aerated drip irrigation as multigation. Delivering nutrients and other chemicals with irrigation water according to the crop requirements reduces the cost of application and improves input use efficiencies and becomes increasingly practical with adoption of automation using dosing equipment. The dynamics and fate of agrochemicals in the soil with aerated water irrigation differs from those of non-aerated water irrigation, e.g. oxygen provided greater salt exclusion by roots compared to non-aerated treatments. Opportunities also exists for improving the use of treated effluent water for irrigation with multigation as treated effluent water has high biological oxygen demand and often record poor dissolved oxygen concentration causing hypoxia upon crop irrigation. Review of previous research and modelling suggests that the behaviour of multiphase components in aerated irrigation water streams, and subsequently in the soil, and their interaction with plant roots influences the dynamics of salt, nutrients and pesticides and therefore provides opportunity for the sustainable management of these inputs for irrigated agriculture.