A critical question for the critical zone: how do plants use rock water?
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COMMENTARY
A critical question for the critical zone: how do plants use rock water? Susanne Schwinning
Received: 22 April 2020 / Accepted: 22 July 2020 # Springer Nature Switzerland AG 2020
Abstract Background The paper by Korboulewsky and coauthors in this issue of Plant and Soil address some of the central questions of critical zone ecohydrology: how do plants interact with rocks that exclude roots but hold plant-available water? Scope I compare plant water uptake from stony soils and fractured bedrock in the critical zone, suggesting that the two cases may represent endpoints of a continuum along which the proportion of available space for root growth changes. Conclusions Rhizosphere models could be improved and generalized by structuring the layers of the critical zone into volume fractions that can be rooted and fractions from which roots are excluded. I hypothesize that plant-available water capacity of the rooted fraction governs productivity, while plant-available water in the unrooted fraction governs drought resilience. Keywords Fractured bedrock . Model . Mycorrhizae . Plant-water uptake . Rhizosphere . Rock water . Stony soil
Plant roots and rocks have a complex relationship. Roots enhance the chemical weathering of rock by Responsible Editor: Rafael S. Oliveira S. Schwinning (*) Biology Department, Texas State University, 601 University Drive San Marcos TX 78666 USA e-mail: [email protected]
transporting assimilated carbon underground, where plants and microbes release CO2 and organic acids. Acidic water dissolves minerals in rocks, opening pore space in the rock matrix (Hasenmueller et al. 2017; Koele et al. 2014). In time, as pores become increasingly connected, the tortuosity of the flow path for water declines and decreases the resistance to flow in and out of the rock. At this stage, weathered rock, while still having the rigidity of rock, can readily take up, store and release water (Graham et al. 2010; Jones and Graham 1993). Meanwhile, the cell pressures of living roots expand flow paths through weathered materials and physical forces drive the weathering front further into fresh bedrock (Phillips et al. 2019). In this way, through the relentless activities of innumerable roots, their associations with other biota and interaction with physical processes, Earth’s critical zone is created; the thin reactive transition zone between the land surface and bedrock, within which water circulates and fresh rock is continually exposed to chemical weathering (Grant and Dietrich 2017). Processes that happen deep in the critical zone are difficult to observe, but some of the interactions between roots, rocks and water are also observable in shallow, stony soils. There are similar knowledge gaps in the ecohydrology of stony soils and of the deep critical zone (Dawson et al. 2020; Zhang et al. 2016), both grappling with the fact that there are two pore domains that contain water for plants. In one domain, pores are sufficiently wide, connected and/or pliable enough for growing root tips to navigate and
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