Identifying the contrasting water uptake patterns of dominant afforestation species along an aridity gradient on the Chinese Loess Plateau using stable water isotopes

Abstract

Inappropriate afforestation has caused severe soil desiccation and ecosystem degradation on the Chinese Loess Plateau (CLP) due to poor knowledge of plants’ water uptake patterns along the aridity gradient. In this study, the spatiotemporal difference of water uptake patterns by dominant afforestation species of tree (Robinia pseudoacacia), shrubs (e.g., Hippophae rhamnoides, Caragana microphylla, and Artemisia Ordosica) and natural grasses are explored by stable isotopes (δ¹³C, δ²H and δ¹⁸O). Results from the MixSIAR model showed that water uptake fractions from various soil layers changed from low to middle to high precipitation sites in the drier (2015) and wetter year (2016). The afforested shrubs derived the highest proportions of deep soil water at the low precipitation (LP) site, however, their root uptake depth remarkably shifted towards the shallow soil layers at the high precipitation (HP) site in 2015. In contrast, shallow soil water gradually became a dominant water source for tree and shrubs because of its higher shallow water availability during 2016. This changed root water uptake depth, indicating that the tested afforested tree and shrubs had a plastic ability to adapt their water uptake patterns in variable environments. In addition, results also showed natural grasses predominantly depended on the shallow soil water on the CLP despite the drier and wetter year. We observed significant differences in the relationship between δ¹³C values in the leaves of trees, shrubs, and grasses and annual precipitation. Specifically, the leaf carbon isotope (δ¹³C) values indicated that the tested trees and shrubs had a higher intrinsic water use efficiency (WUE_i) for leaf-level water efficiency than grasses, implying that trees and shrubs presented a competitive advantage on water use over grasses on the CLP. This study also improved our understanding of spatial water usage strategies on CLP plantation species, which is essential to guide future selection of further plantation species selections and management in similar regions.