The physiology of water use efficiency of crops subjected to subsurface drip irrigation, oxygation and salinity in a heavy clay soil\r

Abstract

Furrow is the dominant irrigation method for agriculture throughout the world. However, due to water use inefficiencies, only about 50% of the water that reaches the field is used by the crop, the remainder at times negatively impacting the environment. This research explored the potential of subsurface drip irrigation (SDI) to raise water use efficiency (WUE) of cotton produced on a heavy clay soil, and to minimize the negative environmental impact of irrigation water. SDI cotton irrigated at the rate of 75 % daily crop evapo-transpirational demand (ETc) produced yield equivalent to, or greater than, that with SDI at 120/105 ETc in both seasons in Emerald, Queensland. Lint yield of SDI 75% ETc was comparable to the conventional furrow method in 2001/02 season and less by 18% in 2002/03 season, but used only approximately half of the water input (52%). The SDI crop irrigated at 50% ETc showed enhanced crop earliness and WUE, but had lower lint yield, whereas higher SDI irrigation levels (> 90% ETc) delayed crop maturity without benefits for yield or WUE. Furrow registered the highest drainage and runoff over the two seasons (114 mm and 224 mm) compared with the SDI at 105/120 ETc (65 mm and 32 mm) whereas SDI 90% ETc had no runoff but leakage of 17 mm and neither drainage nor runoff were observed at SDI at 50 and 75% ETc. It was hypothesized that at higher irrigation rates, SDI crops experience lack of oxygen in the root zone, which becomes a limiting factor for improving WUE at higher irrigation rates at and above 90% ETc for cotton in heavy clay soil. The potential of subsurface oxygation (irrigating oxygen-rich water to plants through drip tape- the details about oxygation approach, mechanism and terminology are presented in Bhattarai et al., 2005c) using aerated irrigation water (mixing 12% air by volume of water using Mazzei model venturi for in-line air injection) or hydrogen peroxide solution (at 0.5 ml H2O2 L-1 of irrigation water throughout the irrigation cycle) viii was therefore investigated at a range of soil moisture levels in the glasshouse, screenhouse and outside at Rockhampton, Queensland. Following irrigation events soil O2 declined by 45% in non-aerated plots while in aerated plots soil O2 decreased by only 25%. Oxygen measurements in the rhizosphere over a 72-hour period during the flowering stage revealed greater oxygen concentration with aerated treatments compared with the control at both field capacity (8.1 vs 7.1 mg L-1) and deficit (9.2 vs 8.1 mg L-1) soil moisture conditions. Yield was increased on average by 86, 20 and 21% for soybean, cotton and tomato, respectively, due to aeration across soil moisture levels and types of aeration.Such increase in yield was associated with greater number of pods for soybean, bolls and their individual weight for cotton and fruit size in tomato. Aeration treatments also increased water use by plants and were associated with greater WUE in all experiments. The effect of aeration was significant on the rate of net photosynthesis per unit leaf area when pots were aerated, but instantaneous leaf stomatal conductance and unit leaf transpiration rates were not affected. However, higher stem sap flow rates indicated greater canopy transpiration over longer time intervals in aerated treatments. Higher root weight and soil respiration were observed in aerated treatments compared with the control. Hence, aeration-induced root functioning was arguably responsible for greater fruit set and yield in all three crops, while in vegetable soybean greater canopy interception of radiation and greater total vegetative biomass were also responsible for additional yield benefits, and in tomato the effect was due to higher leaf area, chlorophyll content, and bigger fruit. Salinity is a major environmental threat in many parts of the world. Salinity in clay soils is often associated with sodicity, which reduces the porosity in the soil thereby reducing soil oxygen concentration. The effect of oxygation (with aerated water) for SDI crops in a range of salinities (tomato: 2.0, 4.0, 8.6,10.0; cotton and ix soybean: 2.0, 8.0, 14.0, 20.0 dS m-1 ECe) in heavy clay soils was evaluated. Oxygation on average increased yield of tomato, vegetable soybean and cotton by 38, 12 and 18 percent respectively, but yields decreased significantly with increasing salinity levels. Aeration of saline soil increased WUE for fruit and biomass in all three species but not the instantaneous WUE, measured as μmol CO2 fixed per mmol H2O transpired, with the exception of cotton. Aeration increased, and salinity decreased, cumulative transpiration as determined by stem sap flow over a two week period during flowering in vegetable soybean. Plants in aerated treatments showed increased stem diameter, improved membrane permeability expressed by reduced relative leakage ratio and possibly enhanced ion regulation as revealed by greater sodium exclusion and intact root membrane as revealed in the TS of aerated roots in the saline soils. The increase in yield in tomato and cotton was also accompanied by increased harvest index, greater fruit size, higher fruit number, shoot: root ratio, and lower water stress index. The rate of net leaf photosynthesis increased with aeration and decreased with salinity in cotton and soybean; however, in tomato the aeration effect on photosynthesis was not significant although salinity did significantly reduce net leaf photosynthesis. Aeration improved selective membrane permeability as evidenced by reduced electrolyte leakage. Hence it is suggested that aeration helps exclude the ingress of salts into the plants and increases uptake of water and nutrients for growth in saline environments. Evidence from these controlled environment experiments warrants the commercial-scale testing of the oxygation technology for application to the agricultural and horticultural industries especially to add value to growers’ investments in SDI and to diminish potential negative impacts of over-use of irrigation water.