Influence of different coverage and meteorological factors on soil thermal conductivity and heat flux during freezing and thawing period

In order to study the characteristics of soil thermal conductivity and heat flux under different mulching conditions in freezing and thawing period, we measured the soil temperature with 5 different treatments, i.e. bare land (BL), snow cover (SC), 6 000 kg/hm2 straw mulching + snow cover (SM1), 12 000 kg/hm2 straw mulching + snow cover (SM2), and 18000 kg/hm2 straw mulching + snow cover (SM3), and then calculated the soil thermal conductivity and heat flux. Results showed that the soil thermal conductivity increased when soil was freezing, maintained constant when soil was frozen completely, and decreased when soil was thawing. In the freezing period, straw mulching and snow cover could delay the increase of soil thermal conductivity. In the thawing period, straw mulching and snow cover could delay the decrease of soil thermal conductivity. The rising time of soil thermal conductivity of BL was 7 d earlier than the other 4 treatments in 20 cm soil layer. During the freezing period, the average soil thermal conductivity of BL was the largest, while during the thawing period the average soil thermal conductivity of BL was the lowest. The soil thermal conductivity of BL was 1.55 W/(m·K) in 20 cm soil layer during the freezing period, which was 12%, 26%, 41% and 49% higher than those of SC, SM1, SM2 and SM3, respectively. The thermal conductivity of BL in 20 cm soil layer was 0.78 W/(m·K) in the thawing period, which was 29%, 25%, 22% and 15% smaller than those of SC, SM1, SM2 and SM3, respectively. Straw mulching and snow cover could reduce the range of soil thermal conductivity and the active layer depth of soil thermal conductivity. Soil thermal conductivity had the best correlation with aqueous vapour pressure in BL, and the correlation coefficient was-0.84 (P<0.01). Soil thermal conductivity had the worst correlation with wind speed in BL, and the correlation coefficient was-0.43 (P<0.05). Soil thermal conductivity had the best correlation with ambient temperature under straw mulching and snow cover, and had the worst correlation with wind speed. Straw mulching and snow cover could reduce the influence of atmospheric environment on soil thermal conductivity. When the environment temperature was higher than-3℃, the soil thermal conductivity maintained a lower value and remained unchanged. When the environment temperature was lower than-3℃, the soil thermal conductivity maintained a higher value and remained unchanged. During the freezing period, the soil heat transfer upward and the soil heat flux increased first and then decreased; during the thawing period, the soil heat transfer downward and the soil heat flux increased gradually. Straw mulching and snow cover reduced the soil heat flux and its variation. In the whole freezing and thawing period, the average value of soil heat flux of BL was the largest and the soil heat flux of SM3 was the least in the same soil layer. Straw mulching and snow cover could reduce the active layer depth of soil heat flux. The heat flux of surface soil was significantly higher than that of deep soil. Soil heat flux had the best correlation with solar radiation, and all the correlation coefficients were above 0.85 in the 5 treatments. Soil heat flux had the worst correlation with wind speed, and all the correlation coefficients were below 0.55. Straw mulching and snow cover could reduce the influence of atmospheric environment on soil heat flux. The correlation coefficient between solar radiation and soil heat flux of BL was-0.91 (P<0.01), while the correlation coefficient of SM3 was only-0.88 (P<0.01). The results can provide scientific references for regulation of soil temperature in winter and prediction of soil temperature in spring. © 2017, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.