青藏高原多年冻土区两种高寒草地生态系统土壤氮季节变化及其与环境因子的关系

在气候变化背景下，青藏高原多年冻土区生态环境发生着一系列变化并进一步影响土壤氮循环过程，但目前冻融循环及植被生长周期中土壤氮的动态变化还不清楚。以青藏高原腹地的风火山和特大桥地区的两种典型草地生态系统为研究对象，分析了土壤可利用氮（NH₄⁺-N、NO₃^--N、DON）及微生物量氮（MBN）的季节变化。结果表明：土壤铵态氮（NH₄⁺-N）及可溶性有机氮（DON）含量在非生长季高于生长季，土壤硝态氮（NO₃^--N）在生长季高于非生长季；风火山地区高寒草甸生态系统中土壤NH₄⁺-N在融化期含量较高；土壤MBN在植被生长旺盛期降低，在植被生长后期升高；风火山地区高寒草甸生态系统中土壤MBN含量、特大桥地区高寒草原生态系统中土壤可利用氮总量与土壤全氮（TN）含量显著正相关。这表明，土壤全氮含量、植被吸收以及冻融作用均可引起土壤可利用氮及MBN的季节变化。

Seasonal variation of soil nitrogen and its relationship to environmental factors under two alpine grassland ecosystems in permafrost regions on the Qinghai-Tibet Plateau

The largest area of permafrost in China is located on the Qinghai-Tibet Plateau， where the ecosystem is very sensitive to climate change. Changes in air temperature and precipitation have been reported to affect soil thermodynamics and hydrodynamics， which are thought to be critical for thawing and freezing processes， and this process further affects the seasonality of soil nitrogen cycling. However， the seasonal dynamics of soil nitrogen on the Qinghai-Tibet Plateau and its relationship with temperature and moisture remain unclear. The study sites are located in permafrost regions along the Qinghai-Tibet Highway， which has a typical cold and dry continental alpine climate. We selected two typically different alpine ecosystems， alpine meadows and alpine grasslands， located in the Fenghuo Mountain and Tedaqiao areas， respectively. Soil samples were collected at 10 cm depth during each month from April 2016 to March 2017 （the entire thaw-freeze cycle of the active layer in the permafrost regions， including a growing season and a non-growing season）， except for December 2016 and February 2017. Soil temperature was also measured at 10 cm depth using a thermocouple temperature sensor. Soil samples were transported to the laboratory and analyzed for total nitrogen （TN）， ammonium nitrogen （NH₄⁺-N）， nitrate nitrogen （NO₃^--N）， microbial biomass nitrogen （MBN）， dissolved organic nitrogen （DON）， soil volume water content （VWC）， and pH. The results showed that soil available N and MBN in the two alpine grassland ecosystems varied significantly between months and had similar trends， while the fluctuation range of alpine meadows in the Fenghuo Mountain area was larger than that of alpine grasslands in the Tedaqiao area. This pattern suggests that the trends of soil available N and MBN are influenced by similar climatic conditions and plant growth periods in both grassland ecosystems， rather than by different vegetation types. The large fluctuation range of alpine meadows in the Fenghuo Mountain area was attributed to the high content of TN in the soil. Soil NH₄⁺-N and DON content were higher in the non-growing season than in the growing season， while soil NO₃^--N content was higher in the growing season than in the non-growing season. Because soil moisture content was higher in the growing season than in the non-growing season， higher soil moisture resulted in higher nitrification， which led to higher soil NO₃^--N content. The lower DON content in soils during the growing season was attributed to plant uptake and leaching during the growing season. Alpine meadows in the Fenghuo Mountains region had high soil NH₄⁺-N content during the thawing phase. This was because the strong nitrification during the growing season led to low NH₄⁺-N content in the soil. In alpine meadows of the Fenghuo Mountains region， freeze-thaw cycles could kill microorganisms in the soil and release inorganic N ions and organic N of small molecular mass， while NH₄⁺-N could be absorbed and accumulated by the soil. Soil MBN decreased at the beginning of the growing season and increased at the end of the growing season， and this pattern may be attributed to the relatively low NH₄⁺-N content in the soil during the growing season， while soil microorganisms prefer to assimilate NH₄⁺-N. Further， soil temperature and moisture showed a weak correlation with soil available N and MBN. This pattern could be explained by the fact that soil available N had been assimilated by plants during the growing season， while soil temperature and moisture were higher during the growing season. Soil MBN content in alpine meadows in the Fenghuo Mountain area and available N in alpine grasslands in the Tedaqiao area were positively correlated with TN. This pattern suggests that soil TN can effectively influence soil available N and MBN in alpine meadows in the Fenghuo Mountain area and alpine grasslands in the Tedaqiao area. Our results indicate that soil TN content， vegetation uptake， and freeze-thaw effects may cause seasonal changes in soil available N and MBN under different grassland types in permafrost regions on the Qinghai-Tibet Plateau.