祁连山岛状冻土活动层土壤氮库对模拟冻融响应

气候变化改变了青藏高原降水和温度，并影响非生长季土壤冻融格局。本研究采用冻融模拟试验，探究了祁连山岛状冻土区高寒草甸活动层(0～20 cm)土壤在长期昼夜冻融交替(149次)过程中土壤氮库对融化温度(5℃,10℃,20℃)和含水量(15%,30%,45%)的响应。结果表明：昼夜冻融过程中土壤有效氮大量释放期长达50天，以溶解性有机氮(Dissolved organic nitrogen, DON)、铵态氮(NH⁺₄)累积为主。微生物量氮(Microbial biomass nitrogen, MBN)通过氮固持增加，发挥土壤有效氮“缓存库”作用；土壤含水量增加有利于冻融过程中DON和MBN积累，但含水量对NH⁺₄累积存在阈值效应；融化温度对水分饱和土壤氮组分含量影响差异不显著，而干燥和湿度适中土壤DON,NH⁺₄及硝态氮净累积量对低温融化响应较高温条件更敏感。气候变化驱动下冻融过程中温度和水分耦合效应对冻土区氮转化影响机制值得深入探讨。

Response of Soil Nitrogen Pools to A Simulated Freeze-Thaw Test in the Active Layer of Segregated Permafrost on the Qilian Mountains

Global climate change is profoundly changing the characteristics of precipitation and temperature in the Qinghai-Tibet Plateau,and further affecting the soil freeze-thaw (F-T) pattern during the non-growing season. We used simulation experiments to test how changes in soil available nitrogen relate to different thawing temperatures (5℃,10℃,20℃) and soil waters (15%,30%,45%) under 149 F-T cycles in topsoil (0~20 cm) of an alpine meadow with discontinuous permafrost in the Qilian Mountains. We found that the soil available nitrogen flush occurred in early 50 diurnal F-T cycles and mainly showed the accumulation of dissolved organic nitrogen (DON) and ammonium nitrogen (NH⁺₄). In this process,microorganisms would increase nitrogen fixation and play the role of soil-available nitrogen cache. During F-T process,increased soil water could regulate soil nitrogen availability by promoting the accumulation of DON and MBN. However,the NH⁺₄ had a threshold in response to moisture. Thawing temperature did not affect the nitrogen pool components of water-saturated soil,while the net accumulations of DON,NH⁺₄, and NO^-₃ in dry and moderate moisture soils had more sensitive to low thawing temperature. Shortly,under the background of global climate change,the mechanisms of coupling effect of temperature and water in F-T for the alpine ecosystem are worth of further study.