Background The Qinghai-Tibetan Plateau (QTP) is sensitive to climate change and human disturbance. There is large amount of gravel in the soil of the QTP due to the weak chemical and biological processes. However, few attentions have been paid to the thermal and hydraulic properties of soil with gravel. Gravel in soil affects soil composition, water content, soil bulk density, soil thermal conductivity and other soil properties. The study on hydrothermal properties of gravel was few. The widely used parameterization schemes of soil water and thermal properties only considers the sandy, silt and clay soil, the understanding of the importance of gravel is not enough.

Methods According to the different content of gravel in soil, three types of grassland were selected as the research object. In each type, three plots were selected, totally 9 plots. A 1.6 m deep soil profile was dug in each plot. Each profile was divided into four layers, namely:0-20, 20-40, 60-100, and 100-160 cm. Soil samples were taken from each soil layer and brought to the laboratory. The saturated hydraulic conductivity of soil samples was measured by constant-head permeameter and variable-head permeameter in the laboratory. The thermal conductivity of soil samples in the study area was analyzed by thermal characteristic analyzer (KD2Pro, DECAGON, USA). The relationship between saturated hydraulic conductivity, thermal conductivity and gravel content was analyzed by origin software. and the significance was tested by SPSS.

Results The saturated hydraulic conductivity increases sharply when the content of gravel is more than 60%. The saturated hydraulic conductivity reaches 1.5 cm/s when the content of gravel reaches 80%. Saturated hydraulic conductivity of tillage and supplementary sowing land increases with the increase of gravel content. For all soil layers, the thermal conductivity is basically the same under two conditions when the soil moisture is zero. The thermal conductivity of frozen soil is lower than that of unfrozen soil. With the increase of soil moisture, the thermal conductivity of frozen soil is higher than that of unfrozen soil at about 10%-20%. In addition, when the moisture content of soil samples is approximately the same, the thermal conductivity of frozen and unfrozen soil samples with larger proportion of gravel is higher than that of soil samples with smaller proportion of gravel. The saturated hydraulic conductivity of soil, the unfrozen and frozen thermal conductivity of soil at saturation are positively correlated with soil density, the content of gravel with particle size larger than 2 mm, and negatively correlated with porosity and the content of non-gravel with particle size smaller than 2 mm.

Conclusions The content of gravel has a significant effect on the saturated hydraulic conductivity and thermal conductivity, and future studies need to consider the effect of gravel on the soil water and heat properties.