胶州湾海洋生态系统国家野外研究站知识库

氮（N）、磷（P）富营养化而溶解硅酸盐（DSi）相对缺乏会造成水体中浮游植物群落优势种由硅藻类向甲藻类演替。硅藻是海洋浮游植物的主要组成部分，对海洋初级生产力具有极其重要的影响，是许多海洋生物的优质饵料。硅酸盐的相对缺乏会限制硅藻的增殖进而通过食物链对整个海洋生态系统造成一系列影响，也会影响近岸水产养殖业的发展。已经有研究证明自然存在的硅酸盐泵能够通过增加水体的硅酸盐供给，促进硅藻的增殖，消耗真光层中过量的氮营养盐。有人据此提出人为增加硅酸盐供给也可达到促进硅藻增殖、去除富营养化的效果。但是，目前国内外还没有找到一种经济有效的硅酸盐肥料。受禾本科植物富集硅特性的启发，本研究考虑用稻壳制备海水硅酸盐肥料，探究其最佳制备条件并对其肥效和安全性进行验证。稻壳是稻谷加工的副产品，燃烧后得到的稻壳灰富含无定形态SiO₂。

本文借鉴前人对工业用途稻壳灰制备条件的研究结果，在实验室内测定不同烧制温度下稻壳灰中硅（Si）、氮、磷等元素成分以及在海水中DSi、溶解无机磷（DIP）、溶解无机氮（DIN）的释放能力，并以此确定最佳硅酸盐肥料的制备条件。选择市售稻壳灰做加富培养实验，通过探究稻壳灰添加到自然海水中对浮游植物群落结构的影响对其肥效进行验证，同时在实验室内验证稻壳灰的DSi释放速率和释放周期。我们还将稻壳灰放入天然水体中，通过测定不同浸泡时间后稻壳灰中重金属含量的变化对其应用与环境中时的安全性进行验证。

在400–800℃烧制温度下得到的稻壳灰中Si的含量为29.18–44.02%，N和P的含量低于1%。不同温度烧制的稻壳灰在海水中DSi释放速率无显著变化，但是SiO₃^2-/PO₄^3-释放速率比随烧制温度的增加而增加，对水体中DIN含量没有显著影响。综合考虑制备成本因素，本研究确定500℃为硅酸盐肥料最佳制备温度，该温度下制备的稻壳灰在海水的SiO₃^2-/PO₄^3-释放速率比为52.26，适用于缓解近岸海域硅、磷同时限制现象。

加富培养实验所用的市售稻壳灰在海水中可持续缓慢地释放DSi和少量的DIP，而不影响水体中DIN含量。DSi、DIP的平均释放速率分别为4.76 mmol/g_稻壳灰/d、0.23 mmol/g_稻壳灰/d，SiO₃^2-/PO₄^3-释放速率比为24.50。将稻壳灰添加到天然海水中，浮游植物迅速出响应。高稻壳灰添加量组中，硅藻迅速增殖，其丰度达到峰值时达到初始水平的近20倍，是对照组硅藻丰度峰值的5倍，水体中氮营养盐被迅速耗尽，随后硅藻群落受到氮营养盐限制开始衰败；甲藻的生长受到硅藻的抑制，硅甲藻比峰值达到对照组峰值的16倍，硅藻在实验后期硅藻群落衰败后迅速增殖。低稻壳灰添加量组中，硅藻丰度不仅没有显著增加反而略低于对照组，而甲藻丰度相对于对照组受到明显的抑制，硅甲藻比也显著增加，最大值是对照组近5倍，但DIN的消耗量低于空白组。上述过程中增值的硅藻主要是中肋骨条藻（Skeletonema costatum），也是水体中初始浮游植物群落的优势种，其次是新月柱鞘藻（Cylindrotheca closterium），而初始水体中丰度第二高的旋链角毛藻（Chaetoceros curvisetus）丰度随实验的进行逐渐降低为0；迅速增殖的甲藻主要是双刺原多甲藻（Protoperidinium bipes），其次是日本原多甲藻（Protoperidinium nipponicum）。

市售稻壳灰放于天然海水中21天后，仅Cd元素少量释放到海水中，应用于环境中时投加量控制在正常所需范围内其环境安全性是可以保障的。稻壳灰对Cu、Zn、Pb和Hg不仅没有释放，还具有一定的吸附作用。

Dissolved inorganic nitrogen (DIN) enrichment accompanied by silicate deficiency in coastal waters can stimulate a shift in phytoplankton community from diatom- to dinoflagellate-dominance. Diatom plays an important role in primary productivity, and is usually considered as the prefered food for most secondary producers. It has been tested in natural environments that additional silicate supply, by natural mechanisms, can stimulate growth of benign diatom species and nitrogen consumption. It is thus expected that artificial silicate addition can work as well in other eutrophicated coastal waters without a naturally formed silicate pump. However, such practices in enormous field environments are limited by the lack of an easily available and slow releasing silicate fertilizer. Inspired by silica accumulation capability of graminaceous plants, we here proposed rice husk ash (RHA) as an approporiate silicate fertilizer. Rice husk, on burning, gives ash containing >90% amorphous silica.

Optimal condition of RHA preparation was determined following previous research results. Element contents and macro-nutrients releasing capability were measured for RHAs burned under different calcining temperature. Onboard incubation was carried out to investigate its impacts on phytoplankton community when silicate limitation presented, and silicate release rate and duration of RHA was measured in laboratory. The environmental safety was evaluated by determined the temporal variation of heavy metals in RHA soaked in naturel seawater.

RHA obtained at temperature ranging from 400–800 °C contain 29.18–44.02% of Si and less than 1% of N and P. When soaked in seawater, RHA formed at these temperatures can release phosphate and silicate, but no DIN. SiO₃^2-/PO₄^3- release ratio increased from 37.77 for 400 °C to 112.96 for 800 °C, while calcining temperature had no clear influence on silicate releasing rate. Based on the energy cost for burning, we proposed that 500 °C was the optimum temperature, RHA obtained at this temperature was applicable to alleviate the simultaneous Si- and P-limitation in coastal area, with SiO₃^2-/PO₄^3- release ratio was 52.26.

Commercial RHA products gave similar performance during dark incubation, releasing dissolved silicate and phosphate slowly and sustainably but no DIN into seawater. The releasing rates of silicate and phosphate of RHA were 4.76 mmol/g/d and 0.23 mmol/g/d, and SiO₃^2-/PO₄^3- release ratio was 24.50. During onboard incubation, diatom abundance increased by 20 times in the first 5 days at high-does addition, accompanied by the exhaustion of DIN. Correspondingly, dinoflagellate showed a trend of decreasing at the beginning and then increased. At low-does addition, diatom abundance was significantly lower than control, but diatom/dinoflagellate ratio increased by 9.5 times as dinoflagellate abundance decreased by 60% at first 5 days. Consumption of nitrogen nutrients was also lower than control. A diatom proliferation was induced mainly by Skeletonema costatum, which numerically dominated through the expriment period in all treatments. Numerical increase was also significant in Cylindrotheca Closterium. However, Chaetoceros curvisetus, the second dominant diatom species, decreased in all treatments. The proliferation of dinoflagellate was induced mainly by Protoperidinium bipes, followed by Protoperidinium nipponicum.

Soaked in natural seawater for 21 days, Mg content in RHA increased by 1-2 times, but only slight changes were observed in heavy metal contents. Specifically, increase was recorded on Cu, Pb and Hg but decrease was observed on Cd. It is suggested that, even though heavy metal absorptivity was outcompeted by other metal ions existing in large amount, RHA can be used as a seawater silicate fertilizer and unlikely intensify environmental pollution.