Spatial and temporal variation in soil bulk density and saturated hydraulic conductivity and its influencing factors along a 500 km transect

Soil bulk density (BD) and saturated hydraulic conductivity (Ks) are two critical soil physical parameters that affect soil water holding capacity, infiltration, carbon stocks, and many land and surface processes. Although variations in spatial and temporal BD and Ks are widely recognized and known to differ at different scales, little large-scale data is available. This study explores changes in spatial and temporal BD and Ks along a 500 km transect on the Chinese Loess Plateau (CLP). On seven different occasions, we took BD and Ks measurements in undisturbed soil samples within the surface (0–10 cm) and subsurface (10–20 cm) soil layers from August 2015 to October 2018 along this long transect. This transect covers two climate zones (subhumid and semiarid) and mainly includes five land-use types. BD and log10-transformed Ks (LogKs) exhibited weak and moderate spatial variations during different sampling periods along the long transect, respectively. The highest LogKs values of the two soil layers were observed in summer. Only BD (LogKs) in the surface soil layer of semiarid zone was significantly higher (lower). Grassland had slightly lower BD and higher Ks compared to the other land-use types. Climate had no significant effect on BD and Ks variability. Land-use type and season separately had a significant impact on BD and Ks variability, while sampling time significantly influenced both BD and Ks variability. Ks was negatively correlated with soil water content in spring, summer and autumn. In this long transect, spatial and temporal variations in BD and Ks are determined by land-use types, seasonal changes, sampling time and soil water condition. Understanding temporal BD and Ks variations and their dominant factors will help in the selection of soil physical parameters to optimize hydrological models more accurately at a regional scale.