Contribution Of Phytolith To The Silicon Transport Of The Yellow River

Silicon is not only the second most abundant element of the earth crust, but alsoa very important nutrient in the terrestrial, riverine and marine ecosystems. Silicon ofterrestrial system can be migrated to rivers in dissolved and particulate forms throughsurface runoff processes, and then be transported to oceans by rivers. Silicon’sconcentration, flux convection, spatial and temporal distributions are very importantto the growth of aquatic organisms as well as the stability of the ecosystems. Biogenicsilica is a significant part of the silicon, and phytolith is the main content of biogenicsilica in the terrestrial ecosystem which plays a vital role in silicon’s biogeochemicalcycling.This paper was based on the investigation of the Yellow River and its majortributaries as well as the riparian soil on July2012. Spatial and temporal distributionsand variations of the dissolved and biogenic silica were analyzed. Compared with thesilicon’s variation at the water and sediments regulation period in June2011, weestimated the transportation of the silicon by the Yellow River preliminarily, discussedthe contribution of phytolith to the silicon transport of the Yellow Rive. Through aseries of experiments, the study results are summarized below:(1)The morphologies of phytolith in Yellow River are rich: several forms ofphytolith were found in the river water, such as stick-shaped, dumbbell-shaped,hat-shaped, saddle-shaped, fan-shaped, pyramid, bamboo-shaped, thorns shaped andother irregular forms. The major morphology of phytolith is stick-shaped, taking up77.2%of the total number; and the hat-shaped and short saddle-shaped account for12.6%and5.12%respectively. Phytoliths and diatoms take up85.2%and14.8%respectively of the total amount of biogenic silica, indicating that phytolith is the mainsoure of biogenic silica in Yellow River, and it make important contribution to thetransport of bioavailable silicon by Yellow River to the ocean.(2)There are also rich morphologies of phytoliths in the soil of the Yellow Rivershore: stick-shaped, axe-shaped, saddle-shaped, tooth-shaped, multibell-shaped,fan-shaped and spherical. The main part is still stick-shaped, accounting for53.6%ofthe total number; then17.9%of the phytolith is saddle-shaped, fan-shaped and other forms of phytolith are relatively less. This may largely due to the vegetation types ofthe region. In addition, the BSi content in the vegetation-rich areas (Heze andHuangshui where are corn land) is higer than in the vegetation-free areas,demonstrating that plant residues is the main source of BSi in soil.(3)The distribution of SPM, BSi, PN and POC along the Yellow River have asimilarity trends in the main channel of the Yellow River water. From Guide inQinghai province toYongning in Gansu province, the river flows through the LoessPlateau whose soil was seriously eroded, and their concentrations increase overall.Then there is a decrease trend of those items as the river enters the Hetao Plain andthe velocity of flow slows down. The concentrations don’t fluctuate seriously whenthe Yellow River flows through Shaanxi province and Shanxi province to Luoyang inthe Henan province. At last, it shows a significant growth in Shandong province. Thetrends of the concentrations are closely related to the topography of the basin.(4)The transport of DSi from Lijin in the Shandong province to the downstreamestuary of Yellow River in July2012is9.02×108mol/month, and that of BSi is0.41×108mol/month, the percentage of BSi to total amout of BSi and DSi is4.3%. Itis lower than Yangtze River, whose ratio of BSi/[BSi+DSi] is9.0%, this may be dueto the DSi content of main stream of the Yellow River(average122.1μmol/L) is higherthan that of Yangtze River(Datong Reach in Anhui province,91.0μmol/L). On theother hand, compared to the Yangtze River, there is less vegetation and more serioussoil erosion in Yellow River basin, so less phytolith will be transported to river fromsoil, causing the BSi content in Yellow River main stream (5.06μmol/L) is lower thanthat of Yangtze River (13.4μmol/L). Due to the extremely large sediment content ofwater, the Yellow River annually transports large amounts of sediment and BSi intothe sea, so the BSi may plays an essential role in the silicon cycle of the land and sea.(5)The phytolith in the main stream of Yellow River accounts for85.2%of theBSi, and the percent of Dongying station in Shandong province is100%according tothe microscopic examination. So we estimated that the phytolith flux transported intothe sea is0.41×108mol/month through Dongying station, showing the importantcontribution of phytolith to the silicon flux by the Yellow River.(6)The analysis of the water nutrients in both of the two periods shows that thecontents of nitrogen and silicon are much higer than that of the phosphorus, indicatingthat the growth of the aquatic organisms in the Yellow River are limited byphosphorus. The relationships among SPM、BSi and POC and the analysis of POCsource showed that the terrestrial input is a significant source of those items to themain stream of the Yellow River in2012. While terrestrial input to the water is quitelow in the water and sediment regulation period (WSRP),2011.(7)There are many factors affecting the transport of silicon by Yellow River, themajors are weathering of silicate rocks, pH value of water, contents of SPM, organicmatter and dissolved inorganic phosphorus. High degree of the silicate weathering results in the much DSi in the water. Both of BSi and DSi decrease with the increaseof pH value, reflecting that the biogenic silica solubility increases as the pH increase,but the decrease of DSi indicating that there are some other fators which heavilyinfluence the transformation between DSi and BSi. The BSi content increases with theincrease of SPM and organic matter, demonstrating that all of them have a commonsource-the input of terrestrial products of surface soil and rock weathering.(8)All the parameters of water are greatly impacted by the water flow in thewater and sediment regulation period of Yellow River in2011and all have asignificant increase in the early water regulation. The contents of DSi and BSi areboth less than those of2012, reflecting a certain retention of the reservoir. While thereis a slightly difference of the ratio of BSi/(DSi+BSi) between the mail channel of theHuanghe River and WSRP, indicating that the conversion between the DSi and BSi isa relatively stable process. SPM fraction analysis displays that the sediment of YellowRiver is a silty loam, suggesting the mineral particles produced by soil and rockweathering are the main source of SPM. In contrast, the terrestrial input of organicmatter is quite low, which suggest that the POC may come from the aquatic organismsof the reservoir in WSRP.