Oxygation to Unlock Yield Potential of Crops: A Review

Plant productivity and crop yields under any environment depend on a balanced combination of genotype, aerial environment, production inputs such as fertilizer and water and the equally important but often overlooked rhizosphere environment (Wolf 1999). Plant roots require an adequate supply of oxygen in rhizosphere to enable normal respiration and sound metabolic function of the root (Everard, 1985 and Grable, 1966). Insufficient available oxygen triggers a range of adaptive responses, often at the expense of yields and water use efficiencies (Drew, 1992, Barrett Lennard, 2003, Visser et al., 2000). Reduced root respiration caused by low oxygen availability severely impedes plant growth by reducing transpiration and increasing indiscriminate flow of ions and increasing salt ingress into the plant which can reach toxic levels (Kuzyakov and Cheng 2001). Firstly, hypoxia affects the growth, namely of the roots, followed by shoot growth (Kafkafi et al., 1996); Secondly, it impairs the process of solute movement across membranes and thirdly, the effects of hypoxia are expressed in terms of reduced stomatal conductance and/or leaf water potential, inducing symptoms resembling water stress (Rhoades and Loveday 1990). Physical, chemical and biological soil characteristics that influence crop growth and yield depend on the relative proportions of the liquid and gas phases within the root zone (Goorahoo et al., 2000). Hence, ideal composition of air and water is required as shown in Fig 1.
 

       
The conventional methods of irrigation will have transient to long term depressive effects on soil oxygen content. The depressive effect of irrigation on soil oxygen is higher for a given soil water potential on heavy clay soils (e.g., for vertisols) than on lighter soils (McKenzie et al., 1999). Hence plants suffered from suboptimal oxygen supply in the root zone that causes hypoxia and anoxia. In case of drip and subsurface drip irrigated systems, crop receives frequent irrigation and development sustained wetting fronts in the rhizosphere. This condition led to poor diffusion of oxygen causing temporal and spatial hypoxia.
       
In general the hypoxia stress on crops suffocated the transportation of oxidative phosphorylation electrons, Decreases water uptake, reduces the nutrient transportation to above ground portion, increases ABA synthesis in leaves and led to reduction in net photosynthetic rate, decline in organic matter decomposition, reduced root growth and alteration of the balance and supply of plant growth regulators. To overcome these effects, micro irrigation with air injection (oxygation) is adopted; mainly drip and sub-surface drip method of irrigation. The Idea of the soil aeration was explored by “M. Enveart”.
       
‘Oxygation’ is the term used to describe the aeration of irrigation water (Dhungel et al., 2012). It is relatively new innovation but simply involves the injection of atmospheric air into the irrigation water at an appropriate location such as a pump or valve.
 
Approaches to improve the soil aeration
 
Soil based options
 
This option involves the use of soil amendments that improve soil physical parameters and in turn increase air porosity. Amendments are aimed at ameliorating the effects of compaction, salinity and sodicity through an increase in porosity and therefore, in soil aeration.
 
Plant-based options
 
Plant anatomical properties that support oxygen diffusion from shoots to roots and radial oxygen loss from root to the rhizosphere are well described as adaptive mechanisms of plants when flooded. In some species such as rice, oxygen diffusion from shoot to the root, facilitated by greater aerenchymatous tissues and radial oxygen diffusion from the roots, substantially contributes toward satisfying oxygen demand of the roots in saturated soils.
 
Chemical-based options
 
A number of chemicals can change the structure of the soil following application, improving aeration and drainage. The chemical formulations available in the market, including products such as groundbreakers, clay breakers and Claygon are recommended for hard soils to improve the porosity and, therefore, air diffusion in the crop rhizosphere.
 
Management/irrigation-based options
 
Flooding and oxygen depletion can cause root injury if soil is not drained within one or two days, particularly for susceptible species such as cotton and tomato and the injury is severe if anoxia occurs during critical growth stages. Complete anoxia results in a much quicker suppression of root growth (Hodgson and Chan, 1982), for tap roots of cotton and soybean roots died following 30 minutes of exposure to anoxia.
 
Oxygation-based options
 
Oxygation is the process of aerating irrigation water and employing SDI to deliver it to the root zone. Hyper aerating irrigation water to increase the oxygen concentration is accomplished by either mixing air or by mixing peroxides such as hydrogen peroxide (H₂O₂) with irrigation water before it is distributed through the irrigation lines. Oxygation offers plant roots and soil biota extra oxygen with water during, or prior to, finishing, each irrigation cycle when soil air has been replaced by irrigation water. With the current oxygation technology, the additional oxygen is provided directly into the rhizosphere during irrigation with air injection into the irrigation stream or close to the end of each irrigation cycle as with hydrogen peroxide injection.
 
Methods of air injection
 
Air injection irrigation is adding air to the irrigation or aeration following irrigation. Usually, we use air pump, super micro bubble generating system or venturi injector to deal with irrigation water. These methods are also named oxygation, subsurface oxygation or aerated irrigation.
 
Method of ventilation after irrigation
 
In the soil around the roots, drip irrigation belts are buried and the hose with holes connected to the water and air compressor above ground portion of belts. Compressor was used to inject pure oxygen or compressed air into the root zone of soil. This method is named as ventilation after irrigation (Fig 2).
 

       
The advantage of this approach is that will have little bad effects on the original soil. Secondly, this method can increase the anti-clogging ability of dripper; therefore, it extends the service life of the drip irrigation belts (Zhai et al., 1999).
       
The major disadvantages of this method were it was similar to traditional SDI, the emitters were usually submerged at certain depth and it results into insufficient soil water for topsoil. Hence, hinders seed germination and seedling emergence, due to the chimney effect, injection of air alone is expensive and the injected air moves away from the root zone (Bhattarai et al., 2005 and Bhattarai et al., 2006) and this method cannot control accurately the amount of gas.
 
Method of water irrigation and air simultaneously
 
Before irrigation, it is disposing the water by super micro bubble generating system, so that the water enriched with bubble whose diameter is less than 3 μm. This is the method of irrigation water and air simultaneous. Only a few studies used for it to irrigation and some studied in irrigated rice (Zhu et al., 2012).
 
Method of mixing gases with irrigation
 
At the entrance of underground irrigation system, venturi injectors are used to inject a certain amount of air, which is transported to the soil in crop root zone by water. This method is called mixing gases with irrigation (Fig 3).
 

       
It works on the Bernoulli’s law, when water flows through the venturi in a certain pressure, the flow velocity will increase, due to the smaller radius of throat. Water pressure energy converts into kinetic energy and it also reduces on side wall. If the side wall pressure is lower than the atmospheric pressure, a negative pressure will be formed and then air will be sucked into the pipes.
 
Method of using chemical materials to increase oxygen
 
Use urea and calcium peroxide as fertilizer and add low concentration hydrogen peroxide to the water in the irrigation. These substances will release oxygen to add oxygen to the root zone soils slowly. This method is named using chemical materials to increase oxygen. This method is named using chemical materials to increase oxygen (Bhattarai et al., 2004, Zhao et al., 2010 and Zhao et al., 2011). In the 1940s, Melsted used hydrogen peroxide to improve oxygen concentration in the root zone soil (Melsted et al., 1949).
       
However, limitations of this method is greater, such as hydrogen peroxides easiness can be decomposed, the same as the inconvenience of transportation and storage, strong oxidizing and potential hazards for crop, soil structure and soil organisms (Goorahoo et al., 2002).
 
Other methods
 
Dig a groove before planting and lay an arched iron net at the bottom of the groove that is covering gauze on the iron net. Finally, covering the soil and beneath the iron net, what formed underground air reservoir. Using the underground air layer, that will penetrate the soil and replenish soil air. This method is named aerated by groove or substrate-aeration (Fig 4) (Zhao et al., 2010).
 

 
Research Studies
 
Effect of oxygation on growth and yield attributes
 
Dhungel et al., 2012 reported that total industrial fruit yield of pineapple was highest in the oxygation (Air injection at 12 per cent by volume of water) treatment (73.3 t/ha), as followed by control (69.2 t/ha) and no-irrigation (65.9 t/ha). The marketable fruit yield in the industry harvest was greater by 11% due to oxygation as compared to the no-irrigation and 6 per cent compared to the control treatment. Similarly, Goorahoo et al., (2000) reported that air injection in drip irrigation increased the fruit yield and size of melons is larger as compared to control. There was 46 per cent increased in large size melons over water application alone (control). Higher number of pepper (100 for 10 plants) and weight of pepper (13 kg for 10 plants^-1) in air injection as compared to water injection (Goorahoo et al., 2000). Bhattarai et al., 2004 reported that, air injection (at the rate of 12 per cent by volume of the irrigation water employing Mazzei air injector model 384-X (42 mg l^-1) and hydrogen peroxide treatment (H₂O₂ (50 % v/v) was mixed in the irrigation water at the rate of 1 ml litre^-1 of irrigation water.) resulted in increase in lint yield of cotton by 28 and 14 per cent respectively, compared to the non-aerated control. The lint yield was significantly higher in air injection followed by hydrogen peroxide and the control. The increase in yield was due to greater rates  of photosynthesis and more and heavier bolls in cotton.  Aeration increased root weight and length whenever measured and promoted greater total soil respiration. Vivek et al., (2015) reported that air injection irrigation system increased the yield of radish by 9.87 per cent over the control drip irrigated system. Radish yield obtained in air injection irrigation plot was 2.47 t ha^-1 against control drip irrigated plot was 2.24 t ha^-1. The improvement in yield was upto 50 per cent in root mass of vegetables when air is injected. Bhattarai et al., 2010 revealed that total marketable yield (8.86 and 89 t ha^-1 in watermelon and pumpkin respectively) with oxygation as compared to control. Similar, trend followed in case of physiological parameters and seasonal long water use efficiency in both crops. In both crops, increase in yield and water use efficiency was associated with greater light interception by canopy, leaf chlorophyll content, specific leaf area and leaf gas exchange parameters except for WUE and PN/gs due to better root respiration. Niu et al., 2013 reported that post-irrigation aeration enhanced greenhouse cucumber plant height and stem diameter. These parameters generally increased with increasing aeration levels under both Furrow irrigation (FI) and sub-surface drip irrigation (SSD). It might be due to the ratio of the air volume applied to the area of the wetted zone might be a factor controlling the observed better effect under SDI than FI on cucumber plant growth and yield with increasing post-irrigation aeration levels. Torabi (2010) observed the response of four vegetable species to oxygation. The SPAD value of onion (57), bean (37) and beet root (36) was higher in aerated treatment as compared to non-aerated. This was due to higher photosynthetic rate and easy availability of oxygen in rhizosphere.
 
Effect of oxygation on quality parameters
 
Niu et al., (2013) reported that increasing post-irrigation aeration levels produced a corresponding increase in desirable characteristics (high contents of vitamin C, soluble protein, soluble sugar and soluble solids) in the first cucumber fruits that were harvested under both Furrow irrigation (FI) and sub-surface drip irrigation (SSD). It is not clear how post-irrigation aeration enhanced cucumber fruit quality. Shahien et al., (2014) reported that air injection with subsurface trickle irrigation treatment (I₃) recorded most of quality parameters {the specific gravity (1.12), cholorophyll SPAD value (56.61), total carbohydrates (65.83), soluble sugar (6.667) and insoluble sugar (59)} as compared to surface trickle irrigation (I₁) and subsurface trickle irrigation (I₂) in potato. Bhattarai, 2010 revealed that oxygation increased the TSS content by 19 and 4 per cent in watermelon and pumpkin, respectively compared to control treatments.
 
Effect of oxygation on water use efficiency
 
Dhungel et al., 2012 noticed higher WUE of harvested (52.98 t Ml^-1) and industrial yield (29.02 t Ml^-1) in pineapple for oxygation treatment compared to other methods of irrigation. Bhattarai et al., (2010) reported that higher WUE of 70 per cent for air injection treatment (at the rate of 12% by volume of the irrigation water employing Mazzei air injector model 384-X (42 mg l^-1) and 54 per cent for hydrogen peroxide treatment [H₂O₂ (50 % v/v)] was mixed in the irrigation water at the rate of 1 ml litre^-1 of irrigation water.) as compared to control treatment in soybean. Bhattarai et al., (2004) reported that, air injection achieved the highest WUE (3.65 g l^-1) which was on par with hydrogen peroxide (3.32 g l^-1) and the least with control (2.15 g l^-1) in cotton. Shahien et al., 2014 observed that, air injection with subsurface trickle irrigation treatment recorded significantly lower irrigation water amount (5178 m^-3 ha^-1), plant water consumption (5493.53 m^-3 ha^-1) and higher yield (37.53 t ha^-1), WUE (6.83 kg m^-3) in potato as compared to surface trickle irrigation (I₁) and subsurface trickle irrigation (I₂).
 
Effect of oxygation on root attributes
 
Bhattarai et al., 2004 observed higher root weight (87.3 g m^-2), root diameter (0.149 mm) and root length (10.32 km m^-2) in cotton for Air injection treatment followed by hydrogen peroxide compared to control treatment. They also studied on vegetable soybean crop and observed that significantly higher root weight (139.7 g m^-2) and root length (19.18 km m^-2) was observed in hydrogen peroxide treatment followed by air injection and control treatment. Pendergast et al., 2013 reported that, significantly greater total root mass per plant (17 %) was recorded with oxygation treatment compared to the control treatment (i.e. without oxygation supply). Oxygation also resulted in higher RLD (53%), greater fibrous root mass (2%) and significantly larger tap roots per plant (26%) than the control. Increased root activities in the oxygated treatment are implied, with greater access to oxygen driving a higher rate of root respiration in the oxygated treatment. Mei et al., (2013) reported that compared to the control, the rice root dry matter accumulation of Xiushui 09 under oxygenation was significantly increased by 44.64 per cent whereas that of Guodao 6 increased only by 18.40 per cent. There were no significant differences between the two varieties. Rice radicular absorption region is related to root oxygen content. After aeration, the root radicular absorption area of Guodao 6 was 1.99 fold higher than that of the control, while only 1.13 fold for Xiushui 09. Aeration increased rice root length and root volume, but reduced root number significantly. There were significant differences in the root activity of Guodao 6 but not in that of Xiushui 09. In comparison with the control, the root oxidation intensity of Guodao 6 increased by 28.67 per cent under aeration treatment, while that of Xiushui 09 increased only by 0.41 per cent. There were significant differences in the effects of oxygenation on radicular absorption region, root length, root number and root volume between the two varieties.

Oxygation assures optimal root function, microbial activity and mineral transformations which leads to enhanced yield and water use efficiency under hypoxic (aerobic) conditions. It also improves plant performance and yield under irrigated conditions previously considered to be satisfactory for crop growth and offers scope to offset some of the negative impacts of compaction and salinity related to poor soil aeration on crop growth. The aeration mechanism improves oxygen environment of rhizosphere and sufficient oxygen guarantees the normal activities of soil microorganisms, root activity, that enhances the uptake of water, minerals absorption, canopy light interception rate and photosynthetic efficiency. Thus, enhances the crop yield and quality.