The Potential Capacity For Paddy Field Ecosystem To Decontaminate Biogas Slurry And Its Risks Assessment

With the adjustment of agricultural structure and the development of livestock and poultry breeding, excessively centralized and the massive disorderly emissions of livestock manure and flushing water not only polluted the adjacent water body and the surrounding environment, but also hindered epidemic prevention of poultry cultivation. Under the guidance of "recycle economy" and supported by the government, the biogas fermentation engineering was rapid developed in Chinese rural region as an economic, social and environment-friendly method for the organic wastewater treatment. However, the knotty problem of biogas project is how to dispose massive biogas slurry, because there are still very high contents of DOM, SS, N, P and nutrients in biogas slurry. Just treating biogas slurry with local fertilizing can't use up so much and continuously increasing biogas slurry, especially for the livestock and poultry breeding in large scale. Paddy field ecosystem is a typical constructed wetland ecosystem and it has a very wide distribution in south and east China. Therefore, the subject of this study aims to build the paddy field as an artificial wetland ecosystem with both functions of rice production and organic wastewater treatment under the premise of rice production and soil quality safety assurances. Accordingly, our research activities focus on how to support and improve rice production and soil quality under high concentration of organic wastewater conditions, how to increase the decontamination potentials of the paddy field ecosystem, and how to prevent risks resulted from the high discharge of wastewater in the field. In this study, field and laboratory experiments were conducted with the heavy irrigation of biogas slurry to investigate the purification process of the slurry and its influences on the rice production and the environment of paddy field ecosystem. The results provide theoretical basis and technical guidance for comprehensive understanding the response of paddy field ecosystem to the heavy irrigation of biogas slurry, exploring the potential capacity of paddy field to decontaminate biogas slurry and optimizing discharge mode for the heavy irrigation of biogas slurry. The main results are summarized as follows:1. Self-purification of biogas slurry mainly depends on physical and physicochemical processes, but not on microbial activities because microbial activities were strong limited under the conditions of biogas slurry itself. After discharging biogas slurry into paddy field, paddy ecosystem plays a leading role in decontaminating and purifying biogas slurry. With the participation of micro-organisms, plants and other biological processes, the abilities of decontamination and purification of biogas slurry significantly increased. Up to 12 days after biogas slurry irrigation, the concentration of N, P, CODMn in surface water basically reached to CK level. Under 4N biogas slurry irrigation treatment (2400 t-ha-1), NH4+-N concentration in the ground water is smaller than 1N chemical fertilizer treatment, and biogas slurry irrigation will not cause NO3--N pollution in the ground water in current season. The CODMn concentration of groundwater had not varied significantly.2. After irrigation of biogas slurry, soil N and P content was significantly increased, and were positively related to the quantity of biogas slurry irrigation. Soil organic matter content and its metabolic processes had no significant enhancement in short term. There were no obvious adverse effects on soil catalase and invertase activities; Soil pH and the electric conductivity of the soil were positively correlated with biogas slurry irrigation quantity. It indicated that biogas slurry could modify soil acidity in some sense, but it can't go so far as soil salinization.3. Biogas slurry flooding decreased the content of total PLFAs compared with non-flooding soil. After flooding by biogas slurry, the characteristic PLFA for bacteria, fungi, aerobic bacteria were almost unchanged, however, the contents and characteristic PLFA for actinomyces, methanotrophic bacteria and sulfate deoxidation bacteria were significantly decreased; The diversity of soil microbial PLFAs was significantly improved due to the exogenous micro-organisms in biogas slurry, conversely, the contents of total PLFAs extracted from the different treated soil were declined, while the dominant soil PLFAs maintain long-term stability under the same condition. It indicated that the micro-ecology of paddy soil could rapidly screen out the priorities and discard the inferior population, adjusting the internal microbial community structure, thus reach the purpose of adaptable environment. Discrimiant analysis and principal component analysis were used to analyze the dominant PLFAs, and the results showed that treatments 0.5N-1N treatments didn't significantly weakened soil dominant microbial community and microbiologic population. Meanwhile, treatment 1.5N had not yet lead to a devastating impact on the paddy soil microbes. After the biogas slurry irrigation, soil-microbial community structure changed little, which indicated soil ecosystem has the resilience and resistance to maintain its health status in short period.4. Soil heavy metals did not show the significant increase in the paddy fields with the biogas slurry irrigation (maximum to 2400 t ha-1). The content of heavy metals all accord with the national environmental quality standards (GB/15618-1995).5.2N biogas slurry treatment resulted in the highest rice yield. Although rice lodging appeared in 4N biogas slurry irrigation treatment, the yield had not declined significantly. The concentrations of all heavy metals in rice grain were not significantly different among the treatments of the heavy irrigation of biogas slurry, chemical fertilization and CK. The contents of heavy metals in rice grain all accord with the National Standard(GB/2762-2005). With increase of the slurry irrigation, the gel consistency and contents of lysine of the rice declined, while the changes of amylase and crude protein were not significant.6. To ensure the safety for rice production, to maintain the sustainable functions of paddy field ecosystem and to consume more biogas slurry in unit paddy field are three goals for this study. But they are not fully consistent with each other. Therefore, the coordination and optimization of these three goals for the paddy filed wetland ecosystem for biogas slurry treatment is a multi-objective optimization problem. According to our research results, two optimization schemes were proposed to make a preliminary estimation of the irrigation quantity of biogas slurry in the experiment areas. The first scheme with rice yield grain as preferential goal followed by environmental safety and soil fertility maintenance respectively was suggested that 2N treatment of biogas slurry irrigation with the total consumption of biogas slurry 1200 t·ha-1 for one rice production season. The second suggested scheme was the maximal consumption of biogas slurry followed by rice yield and soil fertility maintenance respectively:4N treatment of biogas slurry irrigation with the total consumption of biogas slurry 2400 t·ha-1 for one rice production season.