Root system architecture remodeling under oxygen-mediated subsurface drip irrigation: a mechanistic hypothesis framework linking multiphase mass transfer to root foraging decisions.

As climate variability and water scarcity intensify, subsurface drip irrigation (SDI) has emerged as a key water-saving technology across arid and semiarid regions. However, in practice, the wetted soil volume around each emitter remains close to saturation, disrupting the continuity of the soil gas phase and restricting oxygen diffusion into the root zone. Subsequently, the water and oxygen supplies may compete for the same pore space, which is an intrinsic constraint of SDI. Aerated SDI, particularly systems that use micro- and nanobubbles (MNBs), has shown potential to improve crop yield and water-saving performance, although the effects vary with soil, crop, and management conditions. A unified, cross-scale account of the influence of these engineered gas-liquid inputs on root system architecture (RSA) remodeling remains lacking. Therefore, based on the optimal foraging theory (OFT), we propose an oxygen-mediated root foraging cost framework as a working mechanism hypothesis. This framework treats oxygen not as a growth-promoting factor per se, but as a candidate regulatory variable that may modulate the metabolic cost of root resource acquisition; the sensing-signaling-response chain linking local oxygen gradients to architectural decisions remains to be causally validated. By altering foraging costs, oxygen availability may alter how roots perceive, trade off, and invest in heterogeneous underground resources. Guided by this framework, this review first traces how SDI has evolved technologically, from supplying water through the liquid phase to coupling gas and liquid through MNBs. It then examines how local oxygen gradients may be sensed through upstream oxygen-sensing pathways (including the N-end rule pathway), reactive oxygen species (ROS), nitric oxide (NO), and downstream ethylene-auxin interactions, and how these signals may shape patterns of RSA investment under localized SDI conditions, and proposes the root foraging-accessible domain (FAD) as a candidate concept for evaluating the spatial compatibility between root foraging and the coupled water-oxygen environment. The present evidence supports this conceptual framework, but field-scale causal validation, separation of oxygen effects from confounding factors (nutrients, mechanical impedance, microbial activity), and techno-economic evaluation of aerated SDI remain necessary before the framework can guide irrigation practice.