| 其他摘要 | One of the main problems in water management is eutrophication. Eutrophication is defined as the enrichment of surface waters with nutrient. Of these nutrients nitrogen (N) and phosphorus (P) are important controlling factors for primary productivity in aquatic ecosystems. Since many years the most common strategy for restoration of these eutrophic surface waters is the reduction of the P Loading, because phosphate is considered to be the natural growth-limiting factor in the most of lakes. The success of the recovery of lakes after reduction of the phosphorus loading however is frequently limited. A major reason is the release of phosphorus from the sediments. Naturally, The biogeochemical processes of nutrients in sediments, especially at the sediment/water interface has been always one of main themes of iimnology. We investigated the vertical profiles of nutrients and chemical species of sediments in two reservoirs-Hongfeng Hu and Baihua Hu in Guizhou to elucidate the biogeochemical characterization of nutrients (P, N, and C) within the early diagenesis. Because the water samples of the two reservoirs had high N: P ratio of between 46:1 to 126:1, phosphorus (P) is considered as the limiting factor for primary productivity in Hongfeng Hu and Baihua Hu. Our studies have shown that sediments of the reservoirs Hongfeng Hu and Baihua Hu had very heavy phosphorus loading, with total-P concentration between 1300 tol500μg. g~(-1). Using the sequential scheme developed by Ruttenberg(1992), we examined forms of solid phosphorus fractions in sediments of the two reservoirs. The dominant P-bearing component in these sediments is organic P (50.3-60%), followed in order of relative dominance by iron-bound P (28.25-34%), authingenic P, adsorbed P, and detrital P. Organic P and iron-bound P vary significantly with depth in the sediment cores. In the two reservoirs the sediment P has strong potential capability to remove out, because organic P and iron-bound P represent the soluble reactive part of sediment P. The organic matter is the principal carrier of P to sediment. A part of the organic material(OM) is buried directly. The other part decomposes, resulting in a release of dissolved HPO_4~(2-) to the pore water. This HPO_4~(-2) either returns to the overlying water and becomes available for uptake by phytoplankton, or is retained in the sediment in an organic or inorganic form. The studies on the chemistry profile of NO_3~-、SO_4~(2-)、 Fe~(2+)、Mn~(2+) in pore water showed that the most obvious derogation of organic matter occurs near the sediment-water interface( within 0-2em) in Hongfeng Hu and Baihua Hu. The flux of HPO_4~(2-) across the interface between water and sediment was calculated based on the concentration gradients of HPO_4~(2-) in pore water and overly water. The research in Hongfeng Hu and Baihua Hu shows that HPO_4~(2-) flux from sediment to water varies seasonally, and was higher in the winter and spring (relative oxidic conditions) than in the summer and autumn (relative anoxic conditions), which is completely different from the traditional and common view. We believe it was mainly contributed to the intensity oxidation decompose of reactive organic matter in surface sediments. The effect of sorption on the sediment-water exchange of HPO_4~(2-) is investigated for sediments of Hongfeng Hu and Baihua Hu. Results of kinetics experiments for dry sediment samples show that the sorption process can be described by the Langmuir or by the Freundlich adsorption isotherm. The data indicate that sorption plays an important role in controlling sediment-water exchange of HPO_4~(2-) in Hongfeng Hu and Baihua Hu(S_m is 21.05mgP.g~(-1), 15.83mgP.g~(-1), respectively). Sorption of HPO_4~(2-) to Fe-oxides and Fe-hydroxide is the most important process responsible for the retention of P in sediment. For example, we found the sorption capacity of the samples removed Fe-oxides and Fe-hydroxide out was 90% less than non-treated samples. The one-dimensional reaction-diffusion model describing pore water HPO_4~(2-) and solid phase P profile were applied for sediment cores of Hongfeng Hu and Baihua Hu. The result shows that the (anoxic/oxic) degradation in surface sediment of organic mater (OM) provides a majority of dissolves HPO_4~(2-) for pore water, which could diffuse upwardly across the sediment-water interface. Due to the OM composed of a lager number of fresh, reactivity species, these OM can degrade very quickly both in oxic or anoxic sediments and release too much dissolve HPO_4~2 to be remove by Fe-oxides sorption. Therefore, within the upper layers of sediment cores, the OM degradation is the key factor to control the cycling of sediment P. With the increasing of depth, bacteria gradually consumed Labile OM. Consequently, the reduction/oxidation of iron mineral will become more and more important to the fate of sediment P. We believe that processes of CFA formation seldom occur during the sedimentary history(only 40 years) of Hongfeng Hu and Baihua Hu. We have examined the distribution of exchangeable, fixed, organic, and total nitrogen in sediments and the concentration ofNO_3~-,NH_4~+ in pore water of Hongfeng Hu and Baihua Hu. The average concentration of total nitrogen is 0.36～0.40% in all sediment cores, and 0.69～1.06% in surface sediment, there are more than 85% organic nitrogen in total nitrogen. The profiles of N_(Org) versus depth and N_(total) Versus depth in sediment cores resemble each other and that of organic carbon concentration versus depth, in which N_(Org), N_(total) are en-rich in surface sediment and decreases gradually with increasing of depth. The sediments of Hongfeng Hu and Baihua Hu have higher ability to fix ammonium in mineral crystal lattice than common soils. The concentrations of "fixed nitrogen" are 434.05 mg.kg~(-1) and 416.94mg.kg~(-1), respectively in sediments of Hongfeng Hu and Baihua Hu. The profiles character of NO_3~- and NH_4~+ in pore water implicate that there are the most impetuous nitrogen cycling in lake system occurring near sediment-water interface, which is effective and unique mechanism sequestering nitrogen load of polluted water. The nitrogen cycling near sediment-water interface not only can supply nutrients to overly water, but also sequester nitrogen load of Eutrophication lakes. The vertical profiles of C_(Org) in most of sediment cores collected from Hongfeng Hu and Baihua Flu suggest a significant characteristic by two stages of "decomposition" and "accumulation", rather than the "three stages" distribution of C_(Org) vertical profile observed in those sediment cores from Erhai Lake. However, the characteristic of the sediment core (HF980903-1-2) resemble those sediment cores from Erhai Lake in vertical profile of C_(Org). There are two reasons why the distribution of C_(Org) vertical profile of sediment cores is different between in the two reservoirs and in Erihai Lake: (1) All sediment cores were segmented into 1 cm intervals, but these slices were not fine to represent some detail stages of vertical profile for most of sediment cores collected from Hongfeng Hu and Baihua Hu. (2) the result, on the other hand, reflected the fact that the organic materials in sediment, especially surface sediments, in Hongfeng Hu and Baihua Hu are more fresh, labile and reactive than that in Erhai Lake. Bacteria will decompose these organic materials so quickly that the OM cannot be preserved in sediment for longer time. Studies show that about 35 ～ 76% sediment OM will be degraded after 10 years burial. The OM degradation in surface sediment was considered as the main sources and mechanism of nutrients regeneration. The result of analysis for atomic ratios C_(Org): N_(Org): P_(Org) indicated that external source of nutrients disturbed the status of nutrients of Hongfeng Hu and Baihua Hu. |
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