Impacts of re-vegetation on soil water dynamics in a semiarid region of Northwest China

Abstract

Understanding how vegetation (shrub) cover in drylands affects local-to-regional soil water dynamics and associated water balances is of immense importance because of the abundance of afforestation projects worldwide. Vegetation's role in the control of soil water presents a particular challenge to soil water storage (SWS) management in the drylands of China. To address this knowledge gap, we conducted a two-year study in the Mu Us Desert of northwest China. The study involved the acquisition of in-situ soil water measurements within the first 180 cm of soil at three sand dune sites characterized by their differences in % shrub cover. The sand dunes varied from a vegetation-free, bare-ground sand dune site (BF) and two partly vegetated sites, one with medium-level (40 %) and another with high shrub cover (80 %; MF and HF, respectively). Results revealed that the site with the high shrub cover (HF) suffered a net reduction in soil water content (SWC) by up to 32.7 and 39.8 % in the shallow and deep subsoil (0-100 and 100-180 cm), respectively, when compared to corresponding changes at the BF site. Soil water content was shown to be largely influenced by site properties, namely shrub biomass and litter density (p < 0.05). Due to aboveground vegetation and rainfall interception by the litter, 32.2 mm of effective rainfall was reduced to the soil for every 10 %-increase in shrub cover. Bands of soil water depletion during the dry year did not fully recover during the following wet year, resulting in the development of a dried soil layer with an average SWC of 4.6-7.8 %. Increased evapotranspiration (ET_total) led to a decrease in SWS and relative extractable soil water (REW), which caused ET_total at HF to be lower than the rate observed at MF. These findings highlight the need for improvements in current restoration strategies, meant at striking a balance between vegetation restoration and SWC by developing optimal plant-community cover and mosaicked vegetation systems.

Keywords: Dried soil layer; Drylands; Shrub cover; Soil water deficit; Soil water dynamics; Water balance.