By: Ibrahima Diedhiou, Univ. of Thies, Senegal Nathan Bogie, Univ. of California, Merced, USA Teamrat Ghezzehei, Univ. of California, Merced, USA Amanda Davey, Ohio State Univ., USA Richard Dick, Ohio State Univ., USA
Published: 2019-01-29

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More than 4 million people in the West African Sahel are facing hardship following dry spells and increased food prices. In June, the FAO sent nearly $10 million in aid for drought-stricken farmers in the region, who depend upon rainfed agriculture. Recurring drought, increased populations, and degraded landscapes are ongoing threats for the semi-arid region. Throughout the Sahel, total production has dramatically decreased over the last 50 years, leading to a loss in resiliency for farming communities. The land consists of nutrient-poor soils with low fertilizer inputs. Farmers desperately need agricultural systems that can improve yields and conserve soil using locally available resources. 

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Figure 11. Millet intercropped with Guiera senegalensis. Source: Matthew Bright

Fortunately, a shrub-intercropping system has been scientifically validated that meets the ecological and food security challenges of the Sahel. The system is based on two shrubs that dominate and co-exist with crops throughout the Sahel (though their densities are low): Guiera senegalensis and Piliostigma reticulatum. Although farmers recognize the value of shrubs, they typically coppice them in the spring to clear fields, then burn the residue, depriving soils of needed organic inputs. Our research, funded by the National Science Foundation, has shown multiple benefits of an Optimized Shrub System (OSS; Figure 11) in which shrub density is increased from current levels (<200-350 shrub/ha) to about 1500 shrubs/ha, and in which coppiced shrub biomass is annually incorporated into the soil. With the OSS, we have seen that soil quality improves, more carbon (C) is sequestered, microbial diversity and activity increase, more nutrients are available, water availability improves, and yields increase up to three-fold (28 peer-reviewed articles [See the further reading section for publications with more information on how shrubs were integrated with grain crops.]). A truly remarkable finding, and the focus of this article, is that the shrubs “bio-irrigate” crops via hydraulic lift at night; combined with improved soil quality, this significantly reduces crop water stress during in-season drought. 

The process called hydraulic lift (also known as hydraulic redistribution) occurs when shrubs move water from the wet deep subsoil up through their root systems. The roots release water into the dry upper soil layers at night when leaf stomata close and photosynthesis stops. Hydraulic lift has been found in environments that undergo periodic droughts. It has been shown to increase the ability of shallow roots to take up nutrients and to maintain higher levels of transpiration and photosynthesis. 

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Figure 12. Millet grown for a hydraulic lift tracer study under irrigation midseason 2014. Note the contrast between millet grown without (left) and with (right) shrubs. Source: Nathaniel Bogie

We hypothesized that each day, nearby pearl millet (Pennisetum glaucum) crops take advantage of some of the water that is drawn to the surface by the shrubs. To test our hypothesis, we set up an experiment in Senegal during the dry season. This allowed us simulate drought conditions and, in the absence of rainfall, to precisely control the timing and amounts of water delivered via low-volume irrigation (Figure 12). We monitored the soil moisture with sensors. Once we observed the daily drying and nightly re-wetting of the soil that is characteristic of hydraulic lift, we began our study. First, we attached bottles---with deuterium-labeled water---to the deep roots of the shrub as shown in Figure 13. The deuterium (“heavy hydrogen”) in the water could then be measured in plant tissue to trace the movement of the labeled water. Then, over a five-day period, we collected above ground leaf samples from the shrub and from adjacent millet plants. The leaves were analyzed for the tracer (deuterium). We found evidence of the tracer in a shrub on the first day after injecting the labeled water, and just one day later we found the tracer in the nearby millet. This finding confirms that water lifted hydraulically by shrubs can be transferred to the adjacent millet crop. We do not yet know the exact route of the water from the shrub roots to the crop, but we are confident that a pathway exists. Although hydraulic lift has been documented in many ecosystems, this study was the first to show that interplanted woody species in an agricultural field could “irrigate” adjacent crops with water obtained through hydraulic lift.

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Figure 13. Diagram showing the hydraulic lift, and transfer to millet plants, of deuterium-labeled water from a vial attached to shrub roots about 1 m below the soil surface. Source: Bogie et al. 2018

We have conducted long-term studies on Guiera senegalensis and Piliostigma reticulatum interplanted with a groundnut-pearl millet rotation. Crops interplanted with the shrubs have shown dramatic yield increases, with or without added fertilizers. The system has also helped remediate degraded soils, as shown by a doubling of the total amount of carbon in the topsoil with 10 years of shrub intercropping. The sequestration of carbon in the soil is an important mechanism for capturing atmospheric carbon dioxide to offset global climate change.    

We now understand many fundamental mechanisms of the OSS rhizosphere hydrology and soil microbiology. With that knowledge base, we are ready to pilot and begin to scale the system. To that end, we created the  Agro-Shrub Alliance, a non-profit organization that provides on-farm research and technical training on shrub-based farming to smallholder farming families across the West African Sahel. Our Alliance of researchers, extension specialists, and farmers is currently working on designing OSS Farmer Field Schools for first-ever, on-farm trials. In conjunction with these initial trainings, we will conduct the necessary socio-economic analyses and incorporate indigenous knowledge for locally adapted OSS. Our ultimate objective is to work collaboratively with farmers to develop outreach platforms that enable widespread adaptation and adoption of the OSS from Senegal to Chad, with the potential to reach 5 million smallholder farm families and regenerate the natural resource base for future generations. Please visit our website and the resources below to learn more about the OSS.

Further reading:

Bogie, N.A, R. Bayala, I.Diedhiou, M.H. Conklin, M.L. Fogel, R.P. Dick, and T.A. Ghezzehei. 2018. Hydraulic Redistribution by Native Sahelian Shrubs: Bioirrigation to Resist In-Season Drought. Front. Environ. Sci. 6:98. Bright, M., I. Diedhiou, R. Bayala, K.

Assigbetse, L. Chapuis-Lardy, Y. Ndour, and R.P. Dick. 2017. Long-term Piliostigma reticulatum intercropping in the Sahel: Crop productivity, carbon sequestration, nutrient cycling, and soil quality. Agriculture, Ecosystems and Environment 242:9–22. Dossa, E.L., I.

Diedhiou, M. Khouma, M. Sene, A. Lufafa, F. Kizito, S.A.N. Samba, A.N. Badiane, S. Diedhiou, and R.P. Dick. 2012. Crop Productivity and Nutrient Dynamics in a Shrub (Guiera senegalensis)–Based Farming System of the Sahel. Agron. J 104:1255–1264.