Water mining from the deep critical zone by apple trees growing on loess

Abstract

There have been significant recent advances in understanding the ecohydrology of deep soil. However, the links between root development and water usage in the deep critical zone remains poorly understood. To clarify the interaction between water use and root development in deep soil, we investigated soil water and root profiles beyond maximum rooting depth in five apple orchards planted on farmland with stand ages of 8, 11, 15, 18, and 22 years in a subhumid region on the Chinese Loess Plateau. Apple trees rooted progressively deeper for water with increasing stand age and reached 23.2 ± 0.8 m for the 22‐year‐old trees. Soil water deficit in deep soil increased with tree age and was 1,530 ± 43 mm for a stand age of 22 years. Measured root deepening rate was far great than the reported pore water velocity, which demonstrated that trees are mining resident old water. The deficits are not replenished during the life‐span of the orchard, showing a one‐way mining of the critical zone water. The one‐way root water mining may have changed the fine root profile from an exponential pattern in the 8‐year‐old orchard to a relative uniform distribution in older orchards. Our findings enhance our understanding of water‐root interaction in deep soil and reveal the unintended consequences of critical zone dewatering during the lifespan of apple trees.

Deficit and Recovery of Deep Soil Water Following a Full Cycle of Afforestation and Deforestation of Apple Trees on the Loess Plateau, China

“…Our group has already performed many observations to clarify how apple trees use water in deep soil, and therefore how this affects groundwater recharge. These studies found a continuous decrease in soil water content in deep soil following afforestation [19,44], and the decreased soil water content in deep soil prevent groundwater recharge [34,45]. In this study, we first synthesize our previous published and newly collected soil water profiles to enrich our understanding of apple trees' water use in deep soil, and then investigate the water recovery process by measuring soil water profiles in deforested apple orchards with different ages.…”

“…Twelve paired-plot sites were sampled in May 2014, with each site thus containing a standing apple orchard and an adjacent cropland. The apple orchard ages were 5,8,11,12,17,18,19,19,20,22,24 and 26 years at the 12 different sites. Here, the annual cropland serves as the control site to study how soil water is depleted due to growing apple trees relative to the annual cropland.…”

“…The standing apple orchard in the pair was directly established on the long-term crop field and, therefore, was the first woody species grown on the land since the 1950s. The soil water profiles in the 5-, 8-, 12-, 17-and 24-year-old orchards were newly added data in this study, and the remaining water profiles were from our previous publications [19,34]. We combined these newly collected profiles with the published water profiles to improve our understanding of an apple tree's water-use processes in deep soil.…”

“…Deep-rooted plants preferentially absorb water from shallow soil, but can soak up deep soil water that would otherwise recharge groundwater [17,18]. Li et al [19] conceptualized deep soil water depletion as a process of one-way mining: Tree roots grow deeper to mine deeper soil water and thus deplete deep soil water, which, in turn, stimulates further root growth [20]. However, when the deep-rooted trees are felled and shallow-rooted crops are planted, soil water content begins to recover, and groundwater recharge could increase by an order of magnitude after several decades of land-use change [21].…”

“…However, when the deep-rooted trees are felled and shallow-rooted crops are planted, soil water content begins to recover, and groundwater recharge could increase by an order of magnitude after several decades of land-use change [21]. There is abundant information on the evolution for soil water content during the development of apple orchards [19,[22][23][24][25][26][27][28][29]; however, there is limited information on the evolution of soil water after the trees are felled.…”