‘Bio-irrigation’ - How it works
The shrubs have deep tap roots that suck up water from the wet deep sub-soil and deposit this small amount of water in the dry upper soil layers at night when photosynthesis stops. Surrounding crops take advantage of some of this water that is drawn up. This is called hydraulic lift or hydraulic redistribution. We call it bio-irrigation.
To test they hypothesis of water transfer, we set up a simulated drought experiment in Senegal during the dry season using irrigation to manipulate the amount and timing of water delivery.
Once we observed the daily drying and nightly re-wetting of the soil that is characteristic of hydraulic lift, we began our study.
We attached bottles with labelled water (deuterium) tracer to the deep roots of the shrub. Then, we collected aboveground leaf samples of the shrub, and adjacent millet plants growing over a period of five days which were analyzed for the tracer.
We found evidence of the tracer in a shrub on the first day after injecting the labelled water, and then a day later we found it in the millet growing nearby.
This finding confirms for the first time that hydraulically lifted water by shrubs can be transferred to the adjacent millet crop.
Shrubs are Islands of Fertility and Sequester Carbon
Two 11 yr field studies (initiated in 2004) at elevated densities (Piliostigma reticulatum (PS) at 1000 and Guiera Senegalensis (GS) at 1500 ha-1) and annual coppiced residue soil amendments (with 0 to 1.5X recommended N-P-K rates) showed that shrubs:
increased total soil C by >11% (PR) to 127 % (GS) from 2012 to 2015. Notably, total C decreased (23 - 39%) in the minus shrub plots.
The most dramatic effects on soil C was with GS on the sandy soil of northern Senegal. Here POM-C, the easily mineralized C, rose by 39% in plus shrub plots but decreased by 36% in minus shrub plots from 2007 to 2015.
Four of the six macronutrients (N, K, Ca, Mg) and 1 of 4 analyzed micronutrients (Mn) were significantly greater during at least 2 of the sampling periods (planting, midseason or harvest) in both 2014 and 2015 for both shrub species.
Soil inorganic N and plant available PO4 were consistently greater in the presence of both shrub species during the growing season.
In conclusion, optimized shrub intercropping significantly promoted C sequestration and soil quality based on plant nutrient measures.
Shrubs Increase Yields! —11 Years of Evidence
Two long-term experiments (11 years) were conducted in Senegal that showed the intriguing ability of G. senegalensis and P. reticulatum (optimized at ~1000 to 1500 shrubs ha-1 with residue returns to soil) to dramatically increase yields of millet and groundnut, with or without fertilizer additions and maintain yields in drought years.
At the P. reticulatum site it took 4 years before it consistently and significantly increased yields over non-shrub plots.
For G. senegalensis there was an immediate and consistent yield response over 11 yrs which averaged 126% across all fertilizer treatments (0.5X-1.5X).
Importantly, shrubs reduce time to harvest by about 15 days - a valuable asset for the semi-arid Sahel with its erratic rainfall patterns.
Yields for G. senegalensis were highly correlated with particulate organic C.
An important mechanism for the yield response was increased water use efficiency (WUE) (kg grain ha-1 mm-1 precipitation), with both shrub species that was maintained in very dry years compared to non-shrub plots where this yield ratio tracked rainfall.
These results are also attributed to hydraulic lift (described above) and improved nutrient availability, and soil quality.
Shrub-intercropping is advantageous for subsistence farmers, because it is a local resource they are familiar with that does not require external inputs or new infrastructure.