Title : Shrub-intercropping hydraulic lift: Bio irrigating crops and driving soil microbial functions in semi-arid West Africa
Abstract:
Recurring in-season drought, exacerbated by climate change, threatens food security for the majority rural small farm holder households of the Sahel, West Africa. Furthermore, the region is plagued with degraded soils due to population growth and concurrent intensification of crop and livestock production. Our team has discovered an innovative and appropriate system to address this challenge for the Sahel – the Optimized Shrub-intercropping System (OSS) that utilizes Guiera senegalensis or Piliostigma reticulatum as companion plants to crops that are found in farmers’ fields and native to the Sahel. Our previous research found that OSS significantly reduced drought stress from which we hypothesized that this response could be due to hydraulic lift (HL) (movement of water via deep roots from wet sub- to dry surface-soil at night). Water balance studies supported HL of OSS but to confirm that HL was beneficial for crops, an in-season simulated drought experiment where pearl millet (Pennisetum glaucum) was grown (in the presence or absence of G. senegalensis) with sufficient irrigation until early flowering and then water was withheld for 41 days until harvest. At the same time the fate of enriched deuterium (2H) water applied to G. senegalensis roots (1 m depth) was tracked into shrub and millet leaves. Twelve days after irrigation was stopped, water potential in both plus and minus shrub plots was <-3.0 Mpa. Sole millet produced no panicles whereas millet plus shrub produced panicles and had 900% greater biomass than sole millet. More profoundly, G. senegalensis transferred isotopically labeled HLed water to adjacent millet plants - meaning shrubs are “bioirrigating” crops. Our other research has shown that the microbial community is maintained and active throughout the dry season in soil influenced by shrubs, which is attributed to HL, and litter and root turnover of C inputs. This is a very practical outcome because decomposition proceeds throughout the long (> 9 mo) dry season, mineralizing and accumulating available nutrients accumulates for the subsequent crop, changing the paradigm that organic matter decomposition can only occur during the rainy season of a semi-arid environment. These findings are very significant as they open up a whole new avenue of “bioirrigation” for semi-arid crops. This is of particular importance for subsistence farmers because G. senegalensis is locally available throughout the Northern Sahel and that it can co-exist in cropped fields because farmers use animal traction and manual labor.
