Soil water and temperature dynamics in shrub-encroached grasslands and climatic implications: Results from Inner Mongolia steppe ecosystem of north China

Abstract

Shrub encroachment occurs in arid grasslands worldwide, however, little experimental works on shrub encroachment has been conducted in China. Moreover, interactions and feedbacks between soil moisture/temperature and woody plants remain poorly investigated in arid and semiarid regions. We continuously measured soil water content and soil temperature at 10-min intervals beneath shrub canopy and inter-space grass at different topographic positions of a hillslope in Caragana microphylla encroached grassland of Inner Mongolia in north China from June 2009 to April 2011. The findings of this study revealed that, contrary to most previous results, soil moisture was not always greater in shrub patches than that in the inter-space grass patches, but was strongly affected by precipitation size and calcic layers. Significant higher soil water content and quicker response to summer rainstorm in the shallow soil layer (0–20cm) for grass patches than the adjacent shrub patches, however, spatial distribution of the deeper soil water (40–100cm) in the profile was controlled by the position of calcic horizon functioned as a ‘dam’ to store water in wet period and release water to plant in drought period, highlighting positive feedbacks between shrub establishment and CaCO3 horizons. Diurnal soil temperature and daily soil temperature was significantly higher for shrub than grass in winter, but the trend was reversed in summer. The summer soil cooling and winter soil warming effects would favor shrub to resist drought and freeze damage ensuring continuing shrub growth and survival under arid and clod conditions. These evidences suggest that feedbacks between C. microphylla and soil moisture/temperatur were overall positive.