Nitrogen Dynamics In Paddy Field After Irrigation Of Biogas Slurry And Its Impact On The Environment

With the large scale development of livestock and poultry breeding, the emissions of livestock manure and flushing water are excessively centralized; the quantities are increasing rapidly, posing a serious threat to the surrounding environment, especially the aquatic environment. The biogas fermentation engineering has been proved as an economic, social and environment-friendly method for the treatment of the livestock waste, as well as producing biomass energy. However, with the development of biogas engineering, the production of biogas slurry has been rapidly increasing, as is estimated that over 200 million tons of biogas slurry are produced every year. The traditional ways of treating biogas slurry, such as fertilization, can not meet the increasing demands of disposal of biogas slurry. Biogas slurry has been proved as a high quality organic fertilizer in rice production; thus, this study proposed that the application of paddy field as wetland ecosystems to digest the high ammonium concentration biogas slurry. In this study, biogas slurry was irrigated three times in a month after transplanting and we studied the nitrogen degradation in the surface water of paddy field during the periods of the biogas slurry irrigation and soil drying in field. And we also studied the influences of biogas slurry irrigation on ammonia volatilization, nitrous oxide emission, and the nitrogen content in topsoil, nitrogen concentration in groundwater and the growth and yield of rice. The conclusions are as follows:1. Under the flooding condition of the paddy field, the concentration of TN,DN,NH4+-N in surface water reached the highest within the first day and significantly decreased within three days after irrigation. Thus, these days are not only the critical period for the digestion of nitrogen in the biogas slurry, but also important to control the potential nitrogen pollution in the runoff. The concentration of NO3--N undergo a process of decrease-increase-decrease and recaeched the peak in the six day; the peak concentration was significantly higher than that in the first day after irrigation, so if the nitrate was not absorbed by the rice plant, it may cause the nitrate pollution in ground waer. With the increasing amount of biogas slurry irrigation, the concentration of TN,DN,NH4+-N in surface water and the time for TN,DN,NH4+-N to reduce to the level of CK increased, accordingly. The process of soil drying in field has no significant effects on the concentration of TN,DN,NH4+-N,NO3--N in surface water.2. The average daily rate of ammonia volatilization reached the highest within the first day after irrigation and decreased rapidly in the following six days; the average rate increased increased with the increasing amount of biogas slurry irrigation. Six days after irrigation are the critical period for the loss of nitrogen through ammonia volatilization; the process of soil drying in filed has no significant effects on ammonia volatilization.3. In this study, under the high background value of soil nitrogen and flooding conditions, the change of the average daily rate of nitrous oxide emission with time was not obvious; also, the average daily rate of nitrous oxide emission was not affected the increasing amount of biogas slurry irrigation.4. Under the soil type, texture and structure in this study and flooding conditions, the concentrationof nitrogen in groundwater was below the national drinking water health standards (GB/5749-2006). Thus, the irrigation of biogas slurry would not lead to the nitrogen pollution of ground water. The concentration ofTN,NH4+-N and NO3--N did not increase with the amount of biogas slurry irrigation.5. Under the high background value of soil nitrogen in this study, the content of soil nitrogen in the topsoil of the CK treatment at the tillering stage decreased and the order was 0-2cm layer>2-10cm layer> 10-20cm layer; for the biogas slurry irrigation treatments, soil nitrogen increased in the 0-2cm and 2-10cm layer and decreased in the10-20cm layer. Compared wih the tillering stage, soil nitrogen of CK and biogas slurry irrigation treatments decreased in all layers and the order was 0-2cm layer>10-20cm layer> 2-10cm layer.6. Under the flooding condition of the paddy field, with the increase amount of biogas slurry irrigation, the number of tillers and the height of the rice plant significantly increased, but the rice seed setting rate and 1000-grain weight significantly decreased. The relationship between the nitrogen content and the dry weight of the rice plant and grain and the amount of biogas slurry irrigation was complicated. The overall trend of the yield of rice was 4N treatment>2N treatment> 1N treatment, which indicated that the excessive application of biogas slurry has no influence on the formation of rice yield. The notrogen uptake of the rice plant and grain of the tillering and havesting stages increased with the increase amount of biogas slurry irrigation, while 55.70-66.19% of the nitrogen uptake took place from the stage of transplanting to tillering. The nitrogen utilization efficiency significantly decreased with the increasing amount of biogas slurry irrigation.