Effects of freezing intensity on soil solution nitrogen and microbial biomass nitrogen in an alpine grassland ecosystem on the Tibetan Plateau, China

The change of freeze-thaw pattern of the Tibetan Plateau under climate warming is bound to have a profound impact on the soil process of alpine grassland ecosystem; however, the research on the impact of the freeze-thaw action on nitrogen processes of the alpine grassland ecosystem on the Tibetan Plateau has not yet attracted much attention. In this study, the impact of the freezing strength on the soil nitrogen components of alpine grassland on the Tibetan Plateau was studied through laboratory freeze-thaw simulation experiments. The 0–10 cm topsoil was collected from the alpine marsh meadow and alpine meadow in the permafrost region of Beilu River. In the experiment, the soil samples were cultivated at–10°C,–7°C,–5°C,–3°C and–1°C, respectively for three days and then thawed at 2°C for one day. The results showed that after the freeze-thaw process, the soil microbial biomass nitrogen significantly decreased while the dissolved organic nitrogen and inorganic nitrogen significantly increased. When the freezing temperature was below–7°C, there was no significant difference between the content of nitrogen components, which implied a change of each nitrogen component might have a response threshold toward the freezing temperature. As the freeze-thaw process can lead to the risk of nitrogen loss in the alpine grassland ecosystem, more attention should be paid to the response of the soil nitrogen cycle of alpine grasslands on the Tibetan Plateau to the freeze-thaw process.