Hydraulic redistribution by deeply rooted grasses and its ecohydrologic implications in the southern Great Plains of North America

Perennial bioenergy crops with deep (>1 m) rooting systems, such as switchgrass (Panicum virgatum L.), are hypothesized to increase carbon storage in deep soil. Deeply rooted plants may also affect soil hydrology by accessing deep soil water for transpiration, which can affect soil water content and infiltration in deep soil layers, thereby affecting groundwater recharge. Using stable H and O isotope (delta H-2 and delta O-18) and 3 H values, we studied the soil water conditions at 20-30 cm intervals to depths of 2.4-3.6 m in paired fields of switch-grass and shallow rooted crops at three sites in the southern Great Plains of North America. We found that soil under switch-grass had consistently higher soil water content than nearby soil under shallow-rooted annual crops by a margin of 15%-100%. Soil water content and isotopic depth profiles indicated that hydraulic redistribution of deep soil water by switch-grass roots explained these observed soil water differences. To our knowledge, these are the first observations of hydraulic redistribution in deeply rooted grasses, and complement earlier observations of dynamic soil water fluxes under shallow-rooted grasses. Hydraulic redistribution by switch-grass may be a strategy for drought avoidance, wherein the plant may actively prevent water limitation. This raises the possibility that deeply rooted grasses may be used to passively subsidize soil water to more shallow-rooted species in inter-cropping arrangements.

Automated summary

The study found that soil under the deeply rooted switchgrass had consistently higher water content compared to nearby soil under shallow-rooted crops by 15%-100%, likely due to hydraulic redistribution. This redistribution may be a drought avoidance strategy for switchgrass and could potentially be used to supplement soil water for shallow-rooted species in intercropping arrangements.