River water quality shaped by land–river connectivity in a changing climate

This paper aims to provide a state-of-the-art review on the effects of vegetation roots on the soil structure and soil hydraulic properties. After a thorough review of current studies, the effects of vegetation roots are summarized into four: root exudation, root penetration, root water uptake and root decay. Root exudates alter the size and stability of aggregates, the contact angle of soil, and the viscosity and surface tension of pore fluid; root exudates of crops always increase the soil water retention capacity and decrease the soil saturated hydraulic conductivity. Root penetration creates new pores or clogs existing pores during root growth, and root parameters (e.g., root biomass density, root diameter and root length density) are well correlated to soil hydraulic properties. Root water uptake can apparently increase the soil water retention capacity by providing an additional negative pressure and induce micro-fissures and macropores in the rhizosphere soil. Root decay modifies the pore structure and water repellency of soil, resulting in the increase of soil macro-porosity, soil water retention, and the saturated hydraulic conductivity or steady infiltration rate. Some of the above four effects may be difficult to be distinguished, and most importantly each is highly time-dependent and influenced by a multitude of plant-related and soil-related factors. Therefore, it remains a significant challenge to comprehend and quantify the effects of vegetation roots on the soil structure and soil hydraulic properties. Unsolved questions and disputes that require further investigations in the future are summarized in this review. 

Effects of vegetation roots on the structure and hydraulic properties of soils: A perspective review.

Rooting depth is an ecosystem trait that determines the extent of soil development and carbon (C) and water cycling. Recent hypotheses propose that human-induced changes to Earth's biogeochemical cycles propagate deeply into Earth's subsurface due to rooting depth changes from agricultural and climate-induced land cover changes. Yet, the lack of a global-scale quantification of rooting depth responses to human activity limits knowledge of hydrosphere-atmosphere-lithosphere feedbacks in the Anthropocene. Here we use land cover data sets to demonstrate that root depth distributions are changing globally as a consequence of agricultural expansion truncating depths above which 99% of root biomass occurs (D99) by ∼60 cm, and woody encroachment linked to anthropogenic climate change extending D99 in other regions by ∼38 cm. The net result of these two opposing drivers is a global reduction of D99 by 5%, or ∼8 cm, representing a loss of ∼11,600 km3 of rooted volume. Projected land cover scenarios in 2100 suggest additional future D99 shallowing of up to 30 cm, generating further losses of rooted volume of ∼43,500 km3, values exceeding root losses experienced to date and suggesting that the pace of root shallowing will quicken in the coming century. Losses of Earth's deepest roots—soil-forming agents—suggest unanticipated changes in fluxes of water, solutes, and C. Two important messages emerge from our analyses: dynamic, human-modified root distributions should be incorporated into earth systems models, and a significant gap in deep root research inhibits accurate projections of future root distributions and their biogeochemical consequences. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2022EF002897

Global‐Scale Shifts in Rooting Depths Due To Anthropocene Land Cover Changes Pose Unexamined Consequences for Critical Zone Functioning

Soil biota generates carbon that exports vertically to the atmosphere (CO2) and transports laterally to streams and rivers (dissolved organic and inorganic carbon, DOC and DIC). These processes, together with chemical weathering, vary with flow paths across hydrological regimes; yet an integrated understanding of these interactive processes is still lacking. Here we ask: How and to what extent do subsurface carbon transformation, chemical weathering, and solute export differ across hydrological and subsurface structure regimes? We address this question using a hillslope reactive transport model calibrated using soil CO2 and water chemistry data from Fitch, a temperate forest at the ecotone boundary of the Eastern temperate forest and mid-continent grasslands in Kansas, USA. Model results show that droughts (discharge at 0.08 mm/day) promoted deeper flow paths, longer water transit time, carbonate precipitation, and mineralization of organic carbon (OC) into inorganic carbon (IC) (∼98% of OC). Of the IC produced, ∼86% was emitted upward as CO2 gas and ∼14% was exported laterally as DIC into the stream. Storms (8.0 mm/day) led to carbonate dissolution but reduced OC mineralization (∼88% of OC) and promoted DOC production (∼12% of OC) and lateral fluxes of IC (∼53% of produced IC). Differences in shallow-versus-deep permeability contrasts led to smaller difference (<10%) than discharge-induced differences and were most pronounced under wet conditions. High permeability contrasts (low vertical connectivity) enhanced lateral fluxes. Model results generally delineate hillslopes as active CO2 producers and vertical carbon transporters under dry conditions, and as active DOC producers and lateral carbon transporter under wet conditions. 

From Soils to Streams: Connecting Terrestrial Carbon Transformation, Chemical Weathering, and Solute Export Across Hydrological Regimes

The idea that soil or the pedosphere is connected to the other terrestrial spheres dates back at least to 1880. Some 26 years later, the interdependency of the terrestrial spheres was established, but a fully integrative approach to addressing the interdependence of the biosphere, pedosphere, atmosphere, hydrosphere, geosphere, and toposphere did not emerge until the advent of Earth Systems Science during the 1980s. A significant development within the Earth Systems approach was a re-evaluation of the pedosphere’s role in the global system, the outcome of which is at least twofold: first, an appraisal of the pedosphere as a two-way interactor with the other terrestrial spheres, the study of which has given rise to some “new” pedologies—biopedology, geopedology, topopedology, hydropedology, and anthropopedology; and second, the pedosphere as a key component of what has become known as the Earth’s Critical Zone. The background, current status, and prospects of these hybrid pedologies and the extent to which they truly deal with interdependencies within the environment are the focus of this article.

https://journals.sagepub.com/doi/pdf/10.1177/03091333221147655

Soil as part of the Earth system

Embracing the dynamic nature of soil structure: A paradigm illuminating the role of life in critical zones of the Anthropocene

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Embracing the dynamic nature of soil structure: A paradigm illuminating the role of life in critical zones of the Anthropocene