Effects of Biochar Amendment on Soil Thermal Properties in the North China Plain

Few studies have examined the effects of biochar amendment on soil thermal properties. Soil thermal properties dominate the storage and conduction of heat in soil, affect soil temperature and water and heat movement, and consequently influence plant growth and soil biochemical processes. A 7-yr field experiment was performed to investigate the effects on soil thermal properties of biochar amendment at three application rates: 0 (control), 4.5 t ha(-1) yr(-1) (B4.5), and 9.0 t ha(-1) yr(-1) (B9.0). Soil heat capacity (C), thermal conductivity (lambda), and thermal diffusivity (alpha) of the 0- to 5-cm-depth topsoil were determined throughout a corn (Zea mays L.)-wheat (Triticum aestivum L.) growing period using a heat pulse method. The results showed that soil bulk density was decreased significantly by biochar (P < 0.05). There was no significant difference in soil water content between the control and biochar treatments. There was no significant difference in the soil volumetric heat capacity between the control and biochar treatments. Soil thermal conductivity and thermal diffusivity were decreased significantly by biochar (P < 0.05). There was a strong positive relationship between soil thermal conductivity and bulk density, and between soil thermal conductivity and water content (P < 0.001, n = 99). In contrast, there was a strong negative relationship between soil thermal conductivity and biochar amount (P < 0.001, n = 99). The reduction in soil thermal diffusivity may be attributed largely to the low thermal diffusivity of biochar and the decrease in soil bulk density caused by biochar.