丹江口水库典型消落区土壤氮磷赋存形态及释放特征研究

丹江口水库是南水北调中线工程水源地,调水工程使得水库蓄水并带来水库水位上升,新增淹没区不同土地利用类型土壤由于淹没浸泡可能使其中氮磷等营养物释放,对水库水质构成威胁.本文通过采集丹江口水库典型消落区5种土地利用类型(滩地、草地、荒地、耕地、果园)的表层土壤,选用磷形态标准测试方法(SMT)和氮形态分级浸取方法对消落区土壤氮磷赋存形态进行分析,同时进行土壤氮磷释放模拟实验来考察消落区土壤"淹没-落干"过程的氮磷释放特征.结果表明,东库湾消落区土壤磷素主要以钙结合态磷(Ca-P)存在,其占TP比例范围为54.7%~82.0%,平均值为69.1%.滩地、草地、荒地土壤生物可利用性磷(BAP)占TP比例也基本超过30%.土壤有机硫化物结合态氮(OSFN)含量在139.1~195.0 mg·kg-1之间,占TN比例范围为18.8%~27.0%,是土壤可转化态氮(TF-N)中主要的赋存形态.土壤释放模拟实验显示,耕地、果园这两类新增淹没区土壤在受淹后其更易向水体释放磷素,而消落区土壤氮素整体上呈现向水体缓慢释放的规律."淹没-落干"前后滩地、荒地的BAP含量下降超过20%,果园BAP含量也降低了16%,草地和耕地的BAP含量则维持相对稳定."淹水-落干"实验后耕地、果园土壤IEF-N含量分别下降22.6%、34.2%,这两类新增淹没区土壤在调水淹没后可能会向上覆水水体释放氮素.

Nitrogen and phosphorus fractions and releasing characteristics of the soils from the representative water-level-fluctuating zone of Danjiangkou Reservoir

Danjiangkou Reservoir is the water intaking place of the South-North Water Transfer Project, the water level of the Reservoir would therefore rise during the initial impoundment period. The soils of newly submerged area with different land use types may be a potential source to release nitrogen and phosphorus into overlying water, posing a probable threat to the water quality of the Reservoir. The surface soil samples of 5 types of land use (bottomland, grassland, wasteland, farmland and orchard land) from the representative water-level-fluctuating zone (WLFZ) of Danjiangkou Reservoir were collected to analyze the contents of nitrogen and phosphorus fractions by using sequential extraction process of nitrogen and standard measurement and test (SMT) procedure of phosphorus fractions, respectively. Afterwards, a submerging experiment was conducted to explore the dynamic characteristics of releasing nitrogen and phosphorus during the "submerged-drying" process. The results showed that the dominated fraction in the soil of WLFZ was calcium-phosphate (Ca-P), accounting for 69.1% in the average of total phosphorus (TP) with a range of 54.7% to 82.0%. The content of biological available phosphorus (BAP) in the soil of bottomland, grassland and wasteland all exceeded 140 mg · kg^-1, and accounting for more than 30% in TP. The organic matter-sulfide fraction nitrogen (OSF-N) content in the soil varied from 139.1 mg · kg^-1 to 195.0 mg · kg^-1, and the ratio ranged from 18.8% to 27.0% in TN, indicating that OSF-N was the main fraction of transferable nitrogen (TF-N) in the soil of the representative WLFZ in Danjiangkou Reservoir. From the submerging experiment, it was easier for "new submerged soil" (farmland and orchard land) to release phosphorus from soil to overlying water, and all 5 types of soils slowly released nitrogen from soil to overlying water. After the "submerged-drying" process in the submerging experiment, the content of BAP declined more than 20% in bottomland and wasteland and 16% for orchard land, but remained relatively stable in grassland and farmland. The contents of IEF-N in farmland and orchard land declined 22.6% and 34.2%, respectively, indicating these two typical "new submerged soil" may release nitrogen from the soil to overlying water after the Reservoir is impounded. Therefore, the nitrogen and phosphorus in soils of WLFZ would become potential pollution sources of water after the implementation of Middle Route Project of the South to North Water Transfer.