基于大气沉降与径流的乌鲁木齐河源区氮素收支模拟

选取天山乌鲁木齐河源区作为自然状态下内陆河源区的代表,将区域氮循环简化为大气沉降输入与径流输出,模拟了该区域内的氮素收支状况,得出结论:(1)乌鲁木齐河源区(总控制水文点以上区域)的年均氮素干湿沉降量为17.0 t/a,且以有机氮为主,其次为铵态氮与硝态氮;年均氮素沉降通量为5.92 kg·hm-2·a-1,明显小于下游受人类活动影响更为强烈的地区。(2)河源区氮素的径流输出主要包括大气沉降直接随径流流失、冰川融水流失与泥沙输移流失等,在总控制水文点以上区域以泥沙输移为主,其次为大气沉降直接随径流流失,冰川融水携带氮量相对较少。(3)河源区氮素的输入量普遍小于输出量,该区域表现为一个氮源;在仅计算大气沉降与径流的情况下,总控制水文点以上区域的年均氮素净通量为-2.64 kg·hm-2·a-1,若将生物固氮与反硝化作用纳入考虑则氮源效应更加显著。

Nitrogen budget modelling at the headwaters of Urumqi River Based on the atmospheric deposition and runoff

In order to investigate the nitrogen cycle at the headwaters of endorheic rivers in a natural situation, the Urumqi River originated from the northern slope of Chinese Tianshan Mountains was chosen as a typical region in the Northwest China. The regional nitrogen system was simplified into the atmospheric deposition (input) and runoff (output), and the nitrogen budget was modeled according to the chemical data of precipitation, runoff, soil and glacier. The headwaters of Urumqi River were defined as the upward area of the Main Control Hydrological Station (3408 m above sea level). The Glacier No.1 Hydrological Station (3659 m above sea level) and the Empty Cirque Hydrological Station (3805 m above sea level) at the headwaters were applied for observing the differences between glacier area and non-glacier area, respectively. The results indicated: (1) The annual atmospheric deposition (including wet and dry deposition) of total nitrogen (inorganic and organic nitrogen) at the headwaters of Urumqi River was 17.0 t/a, which was dominated by organic nitrogen (10.1 t/a), along with ammonium nitrogen (NH₄⁺-N, 4.88 t/a) and nitrate nitrogen (NO₃^--N, 2.03 t/a). The annual atmospheric deposition flux at the Main Control Hydrological Station, the Glacier No.1 Hydrological Station and the Empty Cirque Hydrological Station was 5.92 kg·hm^-2·a^-1, 4.60 kg·hm^-2·a^-1 and 4.80 kg·hm^-2·a^-1, respectively. The atmospheric deposition of nitrogen at the headwaters was much less than that at the downstream, which was influenced by the anthropogenic activities. (2) The regional nitrogen output in runoff was composed of three main sections as below: runoff loss directly from atmospheric deposition, glacier melt loss and sand transportation loss. At the headwaters of the Urumqi River, the nitrogen output was dominated by sand transportation loss (16.2 t/a, accounting for 66% of the total), followed with the runoff loss directly from atmospheric deposition (5.94 t/a, accounting for 24%) and glacier melt loss (2.52 t/a, accounting for 10%). At the Glacier No.1 Hydrological Station, the glacier melt loss of nitrogen (38% of the total) took a principal role of nitrogen loss, due to its marked glacier cover percentage. At the Empty Cirque Hydrological Station, 55% of nitrogen loss was attributed to the sand transportation, instead of the direct lost from atmospheric deposition (45%). (3) The nitrogen budget showed that the nitrogen input was less than the nitrogen output at the headwaters of Urumqi River. The study area was a regional nitrogen source. Modeled based on the atmospheric deposition and runoff, the annual net flux of nitrogen at the Main Control Hydrological Station, the Glacier No.1 Hydrological Station and the Empty Cirque Hydrological Station were -2.64 kg·hm^-2·a^-1, -1.39 kg·hm^-2·a^-1 and -3.67 kg·hm^-2·a^-1, respectively. The nitrogen loss flux was less significant in glacier area (observed at the Glacier No.1 Hydrological Station) than that in non-glacier area (observed at the Empty Cirque Hydrological Station), which was generally caused by the low content of nitrogen in the embryonic soil near modern glacier. With a consideration of the biological nitrogen fixation and denitrification additionally in the regional nitrogen cycle, the nitrogen output was more significant than the modelling result.