冻融循环对畜禽粪便中重金属/磷的分布特征影响及驱动机制

Plenty of the heavy metals and phosphorus concentrated in animal manure have been recognized as one of the most important pollutants’ contributor to water-soil-plant systems and non-point source pollution, which have been paid more and more attention and widely studied. In the season when do not need fertilizer, the animal manure has to been collected and “internally” stored in animal farms, the manure-borne components (organic matters, heavy metals and nutrients like phosphorus) has to undergo the various physical, chemical and biological changes. Especially during the typical freeze-thaw cycles process in the winter, which may largely influence and determine the potential environmental and ecological risks of pollutants after manure spreading into “external” soil or water systems. However, limited researches have been carried out focusing on such processes. This study will focus on freeze-thaw cycles process to investigate the effects on organic matters decomposition and physic-chemical properties alteration in animal manure-borne trace metals’ (Cu, Zn, As and Cd) and phosphorus forms as well as on their distribution mechanisms. Meanwhile, given varying physic-chemical-biological parameters during freeze-thaw process, we will explore the adsorption and transfer behaviors and mechanisms of trace metals/P in manure particle sizes and colloids, through laboratory observing. Finally, simulative pot experiments and aging test will be conducted to assess the bioavailability and stability of toxic metals/P bounded on the different sizes of manure particles and organic components. The research may reveal the behaviors and mechanisms of manure-borne heavy metals/P in waste storing system, in addition, the study can assist us not only to develop the technologies to effectively reduce the transport of manure-borne metals and P in water-soil-plant system but also to well control their ecological and environmental pollution.

畜禽粪便中残留的大量重金属污染物及磷等，被证实为土壤、植物系统及面源污染的重要来源。畜禽粪便堆放过程中的变化直接决定着其土地利用时的污染风险，北方冬季非用肥季节不可避免需经历冻融循环过程，但目前关于冻融过程中畜禽粪便组分、结构等变化的研究未见报道。本项目针对畜禽粪便中典型的重金属污染物以及养分磷，探讨经历冻融循环过程后，有机质组分、结构的变化以及与重金属/磷的结合特征和释放特征，探讨粪便组成差异、冻融循环频率、冻融的物理作用及微生物群落和酶活性等微生物作用对粪便物理结构破坏、有机质降解所导致的粪便粒径组成变化和可溶性胶体形成的影响和作用机制；并系统研究粪便中重金属/磷等元素与不同粒径、有机组分结合特征及分配机制。最后，通过盆栽及老化模拟实验，考察冻融循环后粪便颗粒及胶体与重金属/磷结合力的大小，并对影响粪便胶体与离子络合稳定性的因素进行研究，以期在系统揭示畜禽粪便污染的环境行为机理。

我国每年产生近30亿吨畜禽粪便，在冻融区，冬季非用肥季节大量在户外堆积的粪便将会经历冻融循环过程。本项目系统探究了冻融循环过程中，不同粪便有机质组分、结构的变化以及与重金属/磷的结合特征和分配机制；论证了物理、化学、生物作用对粪便粒径组成变化和可溶性胶体形成的影响、作用机制以及冻融后粪便组分的稳定性；估算了我国主要冻融区域粪便P的释放增量，并对畜禽粪便替代化肥潜力进行了分析。研究显示，冻融过程改变了粪便的理化生性质从而影响了有机质、P及重金属的赋存特征。低温菌分解有机质或盐分离子的解离使粪便离子强度增加，同时微生物呼吸生成有机酸和硝化细菌将生成的NH4+氧化为NO2-或NO3-，导致pH值降低。有机质和粒径的变化受到物理、生物、化学三种作用综合影响，但是物理作用为主要驱动机制。冻融过程中形成了大量胶体物质，同时增加了DOM、腐殖酸、木质纤维素、蛋白质等有机组分的不饱和度，产生了更多的氨基酸、-COO-和C=O等基团，与高温堆肥过程相反，冻融导致粪便大分子有机物分解为小分子有机物的同时其芳香性亦降低。冻融过程的膨胀与收缩破碎了粪便中大颗粒组分，平均重量直径（MWD）减少，总体表现为>1000μm粒径减少和<38μm粒径增加，粒径变化主要是物理破碎以及有机质分解和聚合反应的综合结果。重金属趋向在<38μm组分上聚集，其中Cu和Zn在<38μm组分上的比例分别增加了162%和554%。P的变化与重金属相似，冻融使稳定态P向不稳定态转化，冻融循环后Ex-P高达T-P的50%，加大冻融循环次数和粪便含水量趋势更明显，冻融后粪便利于植物生长及P的吸收。冻融后粪便P的释放是复杂限速释放过程，雨水中NO－3 / SO24－ 摩尔比增大会促进P的释放。我国冻融区内畜禽粪便P的释放增量每年约3.20万吨，冻融典型区域应更多地进行果菜茶等栽培施用，并可向有机肥缺乏省份输出，在减少粪便充余地区粪便冻融季节污染风险的同时实现化肥替代。