Stable isotope analysis of soil and plant water in a pair of natural grassland and understory of planted forestland on the Chinese Loess Plateau

A series of revegetation programs have been carried out since 1950s, and a large area of cropland has been converted to forestland and grassland through tree plantation and natural vegetation restoration. However, it is still inadequately addressed how plant revegetation changes the characteristics of soil-plant waters. In this study, we compared δ18O and δ2H values of different waters from precipitation, soil water, root water, and leaf water between different herbaceous plants (understory of the planted forestland and natural recovered grassland) in two neighboring catchments. There was a temporal consistence in δ18O and δ2H among different waters, showing that root water and leaf water could record well the signals of soil water and precipitation, followed by temperature change over month. A series of water line equations between δ18O and δ2H indicate that herbaceous plants have higher soil evaporation for grassland than understory of the forestland, instead of leaf transpiration. Moreover, our study found significant differences in δ18O and δ2H between root water and soil water, probably associated with isotopic fractionation during root water uptake, leaf surface water pools and ecohydrological separation. This study will be helpful to better understand the effect of plant revegetation programmes (planted forestland catchment versus natural grassland catchment) on ecohydrological processes on the Chinese Loess Plateau.