Can bioirrigation help farmers in rain-fed areas?
The period of Green Revolution is considered as the most productive period in terms of crop yield, particularly for cereals i.e. shallow-rooted plants. However, few areas remained untouched by the impact of the green revolution. Dr. Norman E. Borlaug, The Father of Green Revolution, while delivering his acceptance speech for Nobel Peace Prize made a very critical statement concerning the rain-fed agriculture.
“Cereal production in the rain-fed areas still remains relatively unaffected by the impact of the green revolution, but significant change and progress are now becoming evident in several countries.” - Nobel lecture, December 11, 1970
The above statement represents challenges associated with the cultivation of shallow-rooted crops (mainly cereals) in rain-fed areas where farmers totally depend on rainfall for cultivation. Since cereals are not able to actively take up the deep soil moisture due to its shallow-root, they face early onset of drought when there is low rainfall or intermittent drought period that leads to drought-induced yield loss in cereal. To avoid such drought-induced yield loss of shallow-rooted crops, farmers in rain-fed areas mainly select deep-rooted plants for cultivation. The selective cultivation of deep-rooted plants leads to scarcity of choices for cereals and thus reduces the agricultural diversity in the area.
The concept of "bioirrigation" has been suggested as potential solution to mitigate drought-induced yield loss of shallow-rooted crops. Bioirrigation has been defined as the transfer of hydraulically lifted water by a deep-rooted plant to a neighbouring shallow-rooted plant which cannot access deep soil moisture. In bioirrigation based system, a shallow-rooted plant has the possibility to receive some fraction of water released by deep-rooted plant, during hydraulic redistribution, to maintain its water-relations during drought period.
Bioirrigation provides a ray-of-hope to crop physiologists to develop a system with pair of deep- and shallow-rooted plant that can coexist together, and deep-rooted could support the shallow-rooted plant during the period of intermittent drought when topsoil layer is under severe drought. Although bioirrigation provides a very positive picture for mitigation of drought-induced yield loss for cereals (shallow-rooted plants) in rainfed area, it deserves a careful evaluation of various factors affecting the balance between facilitative and competitive interactions between two species when the rhizosphere of two species are in close proximity.
Research studies (mostly without mycorrhizal network), in this regard, have reported mixed results where facilitative effects have mostly resulted into improved water-relations of shallow-rooted crops during drought period, but total yield in intercropping system reduced due to strong competitive interactions for soil resources (such as nutrients) between two species. In order to reduce competitive interactions, the distance between rhizosphere of two species must be at optimal level where facilitative effect overwhelms the effect of competitive interactions and overall yield of the system is increased.
Facilitative interactions can be enhanced while maintaining an optimal distance between two species by connecting the rhizosphere through mycorrhizal network. Since, rhizosphere can tightly hold up a large fraction of water released by deep-rooted plants, it is essential that a proper connecting channel is established to create a source-sink relation between a deep- and shallow-rooted plant. There are several studies which show beneficial effect of plant-mycorrhizal symbiosis, and mycorrhizal network has been shown to act as a pathway for nutrient and water transfer for plants. Yet there is still not enough evidence if mycorrhizal symbiosis should be considered in farming and researchers are debating if farmers should manage mycorrhizal symbiosis in farming.
In our recent study, we established a proof-of-concept for mycorrhiza driven bioirrigation based intercropping model where not only water-relations but also longevity of shallow-rooted crop was enhanced under extreme drought condition in the topsoil layer. While bioirrigation promoted survival of shallow-rooted crops, it did not translate into biomass increase due to strong competitive interactions in our microcosm set up. Further studies with careful consideration of optimal distance between rhizosphere, under field condition, are required to enhance facilitative effective to increase total yield by managing mycorrhizal symbiosis. There is much scope for fine-tuning of the intercropping partner and ideal facilitator (species-specific mycorrhizal symbiosis) of bioirrigation.