Hydraulic redistribution by plants and nutrient stoichiometry: Shifts under global change

Abstract

Hydraulic lift, water movement from deep to upper soil layers by roots, is a widespread process in temperate and semi-arid environments. It can contribute 17-81% of total water transpired and favour the uptake of nutrients available mainly from soil organic matter decomposition (e.g. N). Downward siphoning, water movement from upper to deep soil layers, can represent 10-60% of total transpired water, favouring the uptake of nutrients supplied mainly from the leaching of bedrock minerals (e.g. P and K). These vertical water movements also can affect the N : P ratio of runoff waters when, in the case of hydraulic lift, they open the possibility for a given pulse of water to circulate multiple times across the N-rich upper soil layers. Plants, thus, affect the stoichiometry of nutrients in soils and groundwater not only through the physical protection of the soil and through the water uptake but also through water redistribution. Soil water redistribution can also play an outstanding role in the ecosystem responses to global change drivers. The increase in soil patchiness in current and future arid lands modifies runoff fluxes, hydraulic lift and downward siphoning, allowing plants to dispose of higher water and nutrient availabilities. The higher use of hydraulic lift and/or downward siphoning by alien species is a possible cause of alien plant success. Further mechanistic and quantitative research is thus warranted to discern the plant role in water and nutrient cycling and in the responses to global change.

Seasonality of hydraulic redistribution by trees to grasses and changes in their water‐source use that change tree–grass interactions

“…Our results show that for African savanna trees hydraulic redistribution may be more common than previously thought because the three tree species we studied, all of which exhibited hydraulic redistribution, are abundant and widespread in southern African savannas, suggesting that the community and ecosystem impacts of hydraulic redistribution in savannas could be potentially large. Dry season water facilitation by trees to grasses may have significant ecological benefits as suggested by several hypotheses on inter-plant interactions (Ryel et al, 2003;Neumann and Cardon, 2012;Prieto et al, 2012;Sardans and Peñuelas, 2014). Nevertheless, our data demonstrates, as reported in a few studies that shifts between competition and facilitation among cooccurring plants can occur (Callaway et al, 1991;Holmgren et al, 1997;Kikvidze et al, 2006).…”

“…Walter's two-layer model for savannas suggests that for trees and grasses to co-occur, trees are expected to use deep sources of water while grasses use shallow sources (Ward et al, 2013). Neumann and Cardon, 2012;Prieto et al, 2012;Sardans and Peñuelas, 2014). The suggestion of tree-tograss facilitation is based on assessing understory biomass and nutrient content, with higher plant biomass or nutrient content under the tree-canopy implying facilitation by trees.…”

“…For instance, the complex source-sink system for water flow exists not only between plant-soil interface but also within the plant that in turn is influenced by a complex set of factors (Prieto et al, 2012;Sardans and Peñuelas, 2014 and references therein).…”

Soil water potential determines the presence of hydraulic lift of Populus euphratica Olivier across growing seasons in an arid desert region

“…Therefore, P. euphratica might spend more energy to lift much amount of water from deep into shallow soil layers. August was the middle stage of growing season in the arid desert region, the requirements in minerals in this period was lower than that in an early stage, whereas the water requirement was opposite (Sardans, Peñuelas 2014). Thus, P. euphratica might spend little energy to lift water in minerals absorption, whereas in turn shift fine roots distribution from shallow to deep layers to obtain water.…”

Deep soil: Quantification, modeling, and significance of subsurface nitrogen

“…C horizons are strongly affected by soil genesis and biological activity (Richter and Markewitz, 1995); sampling only surface horizons (<0.5 m) ignores all deeper N that is biologically available, and can substantially bias N pool estimates (Whitney and Zabowski, 2004). While deep roots represent a relatively small proportion of total root biomass, they have been consistently shown to be important for maintaining productivity during dry seasons and droughts through the passive movement of water from depth to the surface (called hydraulic redistribution) (Brooks et al, 2002;Brooks et al, 2006;Sardans and Penuelas, 2014). These same roots facilitate N cycling in deep soil not only by direct uptake, but also by increasing rates of decomposition of organic matter and mineralization of N (Manzoni and Porporato, 2009;Armas et al, 2012).…”