Effect of snow-straw collocation on the complexity of soil water and heat variation in the Songnen Plain, China

The goal of this study was to explore the effect of the collocation coverage of snow and straw on the complexity variation of soil water and heat and its appropriate evaluation method during freeze-thaw periods in the Songnen Plain of China. Based on field trials, the characteristics of the spatial variability in soil water and heat and the correlation between the water/heat in snow and in soil were analyzed under conditions of bare land (BL), natural snow (FB0), natural snow plus 5-cm-thick straw (FB5) and natural snow plus 10-cm-thick straw (FB10). Additionally, a spectral entropy algorithm based on Fourier transform was used to calculate the spectral entropy of the soil water and heat sequences, and the spatial variation law of soil water and heat complexity in vertical section was explored; furthermore, the positive effect of straw on the soil water and heat environment was revealed. (1) During the freezing period (i.e., the rapid freezing period and stable freezing period), the collocation coverage of straw and snow weakened the differences between the surface soil and the deeper soil; furthermore, the presence of straw hindered the energy exchange between snow and soil, and as the amount of straw coverage increased, the difference between snow water/heat and that in soil increased. During the melting period, the infiltration of snowmelt water affected the variation of soil water and heat, whereas the straw, due to its water storage characteristics, absorbed large amounts of snowmelt water and provided a stable water supply to the soil. With the increase of straw coverage, the correlation between snow water/heat and that in soil was increased under the FB5 and FB10 treatments. (2) The spectral entropy algorithm evaluation values were in good agreement with the classical statistical analysis, demonstrating that the algorithm can be used to efficiently and accurately evaluate the complexity of soil water and heat time series. The spectral entropy of soil water and heat in the 10-cm soil layer under the treatments of FB0, FB5 and FB10 was less than the corresponding entropy under the BL treatment during rapid freezing and stable freezing periods. As the soil depth increased, the spectral entropy decreased under the different treatments. Affected by the maximum freezing depth, the soil water and heat spectral entropy values peaked in the vertical section during the stable freezing period; as the amount of straw coverage increased, the position of the peak value gradually increased. However, during the melting period, the complexity of soil water and heat under the three snow cover conditions increased significantly relative to that under the BL treatment. As the straw coverage increased, the complexity of soil water and heat decreased. The complexity evaluation method of the spectral entropy algorithm is reliable, and the measurement accuracy meets the evaluation requirements. The algorithm has application value for the evaluation of soil water and heat variation in cold regions.