青海湖大气磷沉降通量季节变化及其对湖泊营养状况潜在影响

Over the last century, human activities have led to an increased flux of nitrogen and phosphorus through the atmosphere and thus have altered the global cycles of these elements. Unlike nitrogen, phosphorus has no significant gaseous phase under environmental temperature and pressure and is therefore almost entirely particle-associated in the atmosphere. Therefore, atmospheric deposition of P to ecosystems has traditionally been thought to be negligible in comparison with nitrogen and not typically been included in P cycling studies. However, recent studies have highlighted the role of combustion, primary biogenic and biomass burning aerosols as significant sources of P. These increases may thus strongly impact the terrestrial and oceanic biogeochemistry where sources of P are otherwise low (e.g. alpine oligotrophic lakes). Therefore, recently, atmospheric P deposition has received attention, particularly in alpine oligotrophic lakes where P generally limits phytoplankton growth. Although there are many alpine lakes in China, the study on atmospheric P deposition to them is extremely limited. The objectives of this project are to measure the flux of atmospheric P deposition to Qinghai lake and understand its ecological importance, using field monitoring, lab experiments, field experiments, and model simulation. The major contents of this study are: (1) Flux of atmospheric P deposition to Qinghai lake and its seasonal change; (2) Stoichiometric N/P ratio of atmospheric fallout to Qinghai lake and its seasonal change; (3) Effect of atmospheric P and N deposition to the trophic of Qinghai lake; and (4) P dissolution and phytoavailability of atmospheric fallout in Qinghai lake.

人类活动增加了氮磷大气传输通量，改变了全球氮磷循环。与氮完全不同，大气中磷几乎全部存在于大气颗粒物。因此，与氮相比，生态系统大气磷沉降传统上被忽略了，并被排出在磷循环之外。然而，最近研究发现生物质燃烧产生的气溶胶极大地提高了大气磷含量，使之成为磷循环的重要部分。大气磷排放的增加可能强烈地影响磷短缺的陆地和海洋生态系统生物地球化学。因此，近些年来大气磷沉降受到国外学者广泛关注，尤其是高原贫营养湖泊的大气磷沉降。虽然我国分布有众多高原湖泊，但是关于高原湖泊大气磷沉降的研究十分有限。本项目以青海湖为研究对象，采用观测与室内外模拟等手段，研究青海湖大气磷沉降及其生态效应。内容包括：大气P沉降通量、N/P沉降比例及其季节变化，大气N/P沉降对青海湖营养水平的影响，大气沉降颗粒物中P的形态及植物有效性。本研究可丰富高原咸水湖泊磷生物地球化学过程与循环理论，助于理解大气沉降对青海湖营养水平与演化的影响。

人类活动增加了磷大气传输通量，改变了全球磷循环。另一方面，高原湖泊，尤其是高原贫营养湖泊生态系统对磷地球化学循环的变化十分敏感。因此，研究人类活动影响背景下高原湖泊磷地球化学过程及其对湖泊生态系统的潜在影响具有重要的科学意义。本课题研究了青海湖磷地球化学过程与通量。青海湖湖水中溶解态磷与颗粒态磷浓度接近，而河水中磷以颗粒态为主。湖泊沉积物中总磷含量为172.6-605.2 mg kg-1，以钙磷结合态和有机质结合态为主。青海湖大气湿沉降氮年均浓度为3.33 mg L-1；其中NH4+-N、NO3--N、有机氮分别占59.16%、16.51%和23.12%。NH4+-N浓度雨季高于旱季， NO3--N浓度雨季低于旱季。大气湿沉降中无机磷和有机磷年均浓度分别为0.30 mg L-1和0.07 mg L-1，春季到秋季呈现出逐渐上升的趋势。大气干沉降中水溶态氮和非水溶态氮年均浓度分别为1.88 mg g-1和1.82 mg g-1，水溶态磷和非水溶态磷年均浓度分别为0.044 mg g-1和0.756 mg g-1。青海湖大气氮磷湿沉降通量分别为16.82 和1.86 kg ha-1 yr-1。主要受降水量影响， 2018年4月到2018年8月湿沉降通量逐渐升高，到9月有所降低。大气氮干沉降通量为7.98 kg ha-1 yr-1，水溶态氮和非水溶态氮干沉降通量相当。大气磷干沉降通量为1.73 kg ha-1 yr-1，其中非水溶态磷占95%，表明青大气干沉降中磷生物有效性低。大气氮和磷沉降通量分别为24.79和3.58 kg ha-1 yr-1。湿沉降是青海湖氮沉降的主要形式。磷干沉降通量大于其湿沉降通量，但生物可用磷大部分来源于湿沉降。大气沉降水溶态氮磷摩尔比为23:1，高于湖泊生物营养限制因子氮磷摩尔比16:1。因此，目前青海湖大气沉降N: P比值可维持青海湖浮游植物生长为P限制型。表明人类活动导致的青海湖大气磷沉降升高可促进湖泊浮游植物的生长，存在潜在富营养化风险。结合文献资料，估算了青海湖磷通量，大气磷干湿沉降通量分别为732.0和55.4 t yr-1，河流颗粒态和溶解态磷输入通量分别为285.2和6.9 t yr-1，地下水磷输入通量为6.0 t yr-1，青海湖磷沉积物通量为978.8 t yr-1。