Variability is the only guarantee when it comes to the rainfall of the Sahel, the transitional zone between the parched Sahara Desert and the wetter savanna in the south. The rains often arrive late, and sometimes they barely come at all.
This can lead to devastating crop failures and famine in a region that relies heavily on the rain to grow most of its food. Over the centuries, farmers across the Sahel have adapted to the fickle rainfall by growing crops such as millet, sorghum, peanut, and cowpea, which are well suited to produce grain even during periods of drought stress.
Sometimes, however, the crops’ adaptation is not enough to protect them from extended droughts, and grain yields plummet due to lack of water. To confound the already dire problem, the population of the Sahel is growing and crop yields are not increasing in step.
In these nutrient-poor soils with low fertilizer input, the land is in desperate need of agriculture systems that can provide adequate yields and soil conservation with minimal inputs.
This research on soil hydrology in the Sahel is part of a larger project funded by the National Science Foundation investigating how traditional techniques practiced by some farmers can be adapted to further increase crop yields, even during times of drought stress. We found that planting food crops together with the native woody shrub Guiera senegalensis can improve crop growth with minimal costs. It’s a simple method that can be used not only in the Sahel but may be applicable in many other areas that are periodically inhospitable to agriculture.
Drinking from the same straw
In some other agroforestry systems where crops are grown in association with trees, the trees can outcompete the crops for water and reduce the growth and yield of the crops nearby.
In our work, we investigated a process called hydraulic lift, also known as hydraulic redistribution, whereby the G. senegalensis shrubs pump water from deep in the soil up through their root systems during the night and deposit it in the dry upper soil layers when they are not actively photosynthesizing. Hydraulic lift has been seen in environments that undergo periodic drought spells and has been shown to increase the ability of shallow roots to take up nutrients and maintain higher levels of transpiration and photosynthesis. The process of hydraulic lift in G. Senegalensis was first observed by Dr. Fred Kizito and his colleagues as part of a similar National Science Foundation project in 2003-2004.
Our hypothesis was that each day, nearby pearl millet crops take advantage of some of this water that is drawn up by the shrubs. To test the hypothesis of water transfer, we set up a field study in Senegal under the harsh Sahelian conditions. This work is part of an ongoing research study looking at the growth of crops in association with the shrubs during the rainy cropping season. In order to create the necessary dry conditions we used irrigation to manipulate the amount and timing of water delivered for this experiment.
We monitored the soil moisture by using soil moisture sensors. Once we observed the daily drying and nightly re-wetting of the soil that is characteristic of hydraulic lift, we began our study.
Hydraulic lift occurs when roots remove water from the upper soil layers during the day, causing the soil to dry out. Then with the wet soil deep down and dry soil near the surface, the water moves up through the plant along a gradient, just like sucking water through a straw. Once it gets to the upper soil layers, it is pulled into the dry soil surrounding the roots in the same way a dry sponge sucks up a spill on a countertop. Our hypothesis is that some of this water that is released from the roots is used by nearby millet plants.
To carry out the experiment we attached bottles of water that included a chemical tracer to the deep roots of the shrub. Then we collected aboveground samples of the shrub and crop stems growing nearby over a period of five days to see where the tracer water went.
We found evidence of the tracer in a shrub on the first day after injecting the water, and shortly thereafter we found it in a crop growing nearby. This finding strongly supports the idea that water moved up through the roots of the shrub and into the crop. The exact route from the shrub roots to the crop is a topic of further investigation, but we are confident that a pathway exists. This combination of hydraulic lift and water transfer between species has long been hypothesized but rarely seen. It has never been observed as a component of an agroforestry system with such profound effects on crop production.
Our team consisted of American, French and Senegalese professors and graduate students, researchers from multiple African countries, and local farmers hired as field technicians. We rented land from a farmer named Saliou Diouf who works closely with the researchers at the Senegalese Agricultural Research Institute. Saliou and his sons gave us invaluable perspective on cultivating millet and managing the shrubs. With the proceeds from research activities on his land, Saliou’s family has been able to build a drip irrigation system and buy water from the communal well in the village to grow and maintain a thriving vegetable business.
Power of native plants
The amount of water transferred between plants appears to be quite small. However, the quantity of water deposited may be much less important than the location where it is released.
The small cylinder of soil that surrounds plant roots, called the rhizosphere, plays a crucial role in a plant’s growth. It is the gateway through which most of the necessary water and nutrients must pass to nourish the plant, and it harbors a high concentration of microbes that perform a wide variety of tasks. Therefore, maintaining the viability of the rhizosphere to perform its functions under water stress is extremely important, and this zone is precisely where hydraulic lift deposits water.
In our tests, we observed the roots of the crops engulfing the shallow shrub roots and we speculate that may be related related to the uptake of water.
G. senegalensis grows extensively throughout the Sahel. The management technique under investigation involves planting the shrubs at a density higher than their natural distribution on the landscape so that they can help promote crop growth in a number of ways.
The ways in which hydraulic lift can affect the structure and function of the soil microbial community are also being investigated by our interdisciplinary team with the NSF Partnership for International Research and Education (PIRE) program. In other species hydraulic redistribution has been observed to significantly alter the function of the microbial communities as well. We are also looking at how the presence of the shrubs, along with their potentially higher rates of micro and mesofauna – small organisms that live in soil, can affect the structure and characteristics of the surrounding soil.
The technique may be applicable across wide swaths of the Sahel with minimal additional inputs of labor and money. As the world population is increasingly forced to cultivate arid and marginal lands, we must not forget to look at how native plants adapt to their surroundings and use this knowledge to optimize our own systems, especially in places with limited resources.