As an critical part of the global biogeochemical cycle, the winter soil carbon and nitrogen cycles are extremely sensitive to climate warming. Furthermore, the black soil in northeast China is fertile and rich in organic matter and is a vital production base of commodity grains in China. For as long as half a year, the black soil is in a freezing-thawing state. Climate warming will change the snow cover thickness and soil freezing degree on the surface of the black soil in the winter and affect the freezing-thawing cycle frequency and timing of the soil, thus exerting a profound influence on the fixation, transformation, and release of soil carbon and nitrogen during the freezing period and throughout the year. To better understand the effects of climate warming on the black soil carbon and nitrogen dynamics during the freezing period, an experiment was conducted with two warming levels (W1 and W2) using an infrared radiometer to simulate soil warming. The warming increased the surface soil temperature (0 cm soil temperature) by 1.54℃ (W1) and 4.10℃ (W2), respectively, and significantly increased the soil moisture content compared with the control (C) during the freezing period, most likely because of the melting snow. The snow cover thickness, soil freezing depth, soil organic carbon (SOC), and labile organic carbon (LC) content were reduced by both warming treatments. However, the effect of the temperature increase during the freezing period on the key processes and indicators of the nitrogen cycle in black soil was relatively more complicated. With the increase in temperature, the content of nitrate nitrogen (NO3--N) decreased significantly, and the content of total nitrogen (TN) and net nitrogen nitrification rate increased significantly, while the ammonium nitrogen (NH4+-N), total inorganic nitrogen (TIN) content, and the net nitrogen mineralization rate exhibited a significant increase first and then decreased. In summary, climate warming will bring a warmer and more humid environment to the black soil during the freezing period, and the resulting changes in the soil carbon and nitrogen content and transformation processes will have a profound impact on the structure, productivity of the plants and microbial communities, and carbon and nitrogen cycles in the subsequent growing season. The results provide a scientific basis for studying the carbon and nitrogen cycle mechanisms of the northeast black soil during the freezing period.
Keywords: black soil; carbon cycle; climate change; freezing period; nitrogen cycle.