Afforestation triggers water mining and a single pulse of water for carbon trade-off in deep soil.

Clarifying the water-root-carbon nexus in the entire root zone is crucial for unlocking the potential of afforestation in mitigating climate change. But the nexus in deep soil (depth > 1 m) remains poorly understood. Here we report contrasts in deep soil water and root distributions across 72 paired sites of adjacent farmlands, representing typical pre-afforestation conditions, and tree plantations, representing modern afforestation across the Loess Plateau of China. Ranging from 6 to 25 m of depth, these profiles included plantations of 13 tree species ranging from 1 to 25 years of age. The observations revealed sustained water mining in deep soil following afforestation with mean soil water decline of 75.2 ± 9.8 mm yr−1 that were accompanied by root deepening rates of 1.00 ± 0.06 m yr−1 with an associated biomass input of 0.18 ± 0.04 Mg C ha−1 yr−1. A water for carbon trade-off in deep soil become evident, likely involving a single pulse of C gains and water losses as no signs of soil rewetting under tree plantations where observed total soil water exhaustion that accumulated the equivalent of up to 2.8 years of mean annual precipitation inputs. The reported water-root-carbon nexus reveals overlooked hydrological costs and, more importantly, over-optimistic expectations of sustained C sequestration under afforestation that may rather represent a single-pulsed C gain supported by deep soil water exhaustion. • Afforestation on the Loess Plateau led to continuous water loss in deep soil. • Water loss led trees to root deeper, causing over 50% of fine roots growing in deep soil. • Root biomass increased as soil water declined, implying a water for carbon trade-off. • Limited soil water replenishment led to a single pulse of water for carbon trade-off. [ABSTRACT FROM AUTHOR]