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

Abstract

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.

Expanding the Spatial Reach and Human Impacts of Critical Zone Science

“…Simultaneously with erosion, the imposition of modern crops across landscapes is responsible for the removal of what otherwise would be perennial vegetation. Averaged globally, roots have shallowed since the imposition of agriculture on our planet; indeed, of the ∼48% of Earth's land surface that has experienced root shallowing, 2.3 × 10 7 km 2 have experienced root shallowing due to agricultural expansion (Hauser et al., 2022). Because roots and the microbes that thrive in the rhizosphere serve as biotic weathering agents, root shallowing could conceivably reduce deep soil‐weathering rates.…”

Phenotypic evolution of agricultural crops

“…Provided that Earth´s soils have lost ca. 14.000 km 3 of rooted volume due to the agricultural conversion of native habitats (Hauser et al, 2022), it is a priority to breed crop root traits that recover part of this carbon sink. (4) Other dimensions of crop phenotypes that should be targets of sustainability‐oriented breeding include improved floral reward for pollinators in pollinator‐dependent crops (Garibaldi et al, 2013), local adaptation of traits through backcrossing elite varieties with landraces (Meseka et al, 2015) and breeding for nutritional quality instead of plain calorie supply (Graham et al, 2007).…”

Considerable gaps in our global knowledge of potential groundwater accessibility

“…Accessible to ecosystems implies that groundwater might also be a convenient human water source. Globally, 96.9% of plants root no deeper than 10 m 35 and are projected to get shallower in agricultural areas 16 . We lack global data on groundwater connectivity to aquatic ecosystems, but two-thirds of US streams that potentially gain water from their surrounding aquifers lie in regions with water table depths no deeper than 10 m 7 .…”

“…As vegetation may rely on groundwater directly or through capillary rise 4 , knowledge about the depth of the groundwater table is a central building block in developing global carbon policy 14 . Recent studies showed that tropical forests may even change from carbon sinks to carbon sources due to water stress 15 and that global land cover changes affect rooting depth and thus carbon and water cycling 16 .…”