Effects Of Biogas Slurry On Rice Quality And Environmental Quality Of Soil

Biogas slurry, a kind of high quality organics, contains abundant macroelements and microelements necessary for plant growth, bioactive substances such as hydrolases and matters inhibiting diseases and pests. Biogas slurry is applied widely in agricultural production. However, so far that the environmental capacity of biogas slurry application on farmland is still not clear, such as the effects on crop yield, on environmental quality of soil, and the best tradeoff biogas slurry application amount for good yield, high crop quality, food safety and soil environmental quality. Sichuan Province is developed in livestocks breeding and grain production, thus it may be a good alternative to utilize biogas slurry fermented with dejection of livestock farm in grain production.Field experiments were conducted to investigate the scientific application amount of biogas slurry (CODCr 7033 mg/L, Total N 323.2 mg/L, NH4+-N 290.0 mg/L) fermented with manure of pig farm bred with Tongwei feedingstuff on rice production in paddy soil developed from yellow soil in the hill area of Sichuan Province. Biogas slurry fermented with manure of a large pig farm was applied to plant paddy in Qionglai, Sichuan province. Effects of biogas slurry on growth, yield, grain quality and food safety of rice and environmental quality of soil were investigated under different application amount of biogas slurry with water and chemical fertilizer as controls. The best amount of biogas slurry application was discussed. The main results are listed as below:The amount of rice tillers, the rate of productive tillers and chlorophyll content in leaves all increased in biogas slurry treatments. With the increase of biogas slurry application amount, yield of rice increased first and then dropped, and when the application amount exceeded 2000 kg/666.7m2, yield was lower than chemical fertilizer treatment. When the amount of biogas slurry was in range of 750～1250 kg/666.7m2, paddy yield reached highest levels, which increased production from 12.21% to 15.52% and 6.78% to 9.93%, compared with water control and chemical fertilizer treatment, respectively.With the increase of biogas slurry application, protein content of rice ascended first and then declined. Protein contents of rice in all biogas slurry treatments were over 9.0%, and the highest levels were reached when the application amount of biogas slurry was in range of 1000～1500 kg/666.7m2. There were very significantly positive logarithmic relationships between Fe, Mn content of rice and the amount of biogas slurry, respectively, and content of Ca and Mg were both increased firstly and then decrease along with the increase amount of biogas slurry. With biogas slurry application amount of 1000～1500 kg/666.7m2, content of all mineral elements of rice were at the highest levels. Considering growth, yield and nutritional quality of rice,1000～1250 kg/666.7m2 is the optimal biogas slurry application amount.With the increase of application amount of biogas slurry, nitrite content of rice fell first and then rose, all lower than the limited value maximum levels of contaminants in foods (GB 2762-2005). With the application amount of biogas slurry of 1000～1500 kg/666.7m2, the lowest level of nitrite content was reached. There were very significantly positive linear relationships between Pb, Cd, Ni content of rice and the application amount of biogas slurry, respectively, a significantly positive exponential relationship between Cr, Cu content and the application amount was observed, non-significant relationships between As, Hg content and the application amount, respectively, and Zn content of rice was increased firstly and then decrease along with an increase amount of biogas slurry. All heavy metals (except for Cu and Pb) content of rice in all treatments were lower than the maximum levels of contaminants in foods(GB 2762-2005); As to Cu, calculated from the fitted exponential model of rice Cu content and biogas slurry amount, the food safe biogas slurry application amount is no more than 1673 kg/666.7m2, or rice Cu content might exceed the maximum level in rice (10.0 mg/kg); As to Pb, when the application amount of biogas slurry was over 1790 kg/666.7m2, Pb content of rice went beyond the limit. Considering food safety, the food safe application amount of biogas slurry is 1000～1673 kg/666.7m2.Take all of rice yield, quality and food safety into account, the optimal application amount of biogas slurry is 1000～1250 kg/666.7m2, a higher yield, nicer and safer quality of rice would be got and good economic benefits of biogas slurry would be achieved. pH values of soils treated with biogas slurry were heightened and acidification of soil would be alleviated effectively. Content of total nitrogen, available nitrogen, and available potassium were all significantly improved, while organic matter and CEC of soil rose first and then fell with the increment of application amount of biogas slurry. The variations of contents of available Fe, Mn, Cu, Zn and Ca were similar to OM and CEC, while Mg content ascended gradually as application amount of biogas slurry increased. When the amount of biogas slurry was in range of 1000～1750 kg/666.7m2, contents of secondary and trace elements were all at higher levels.After treated with biogas slurry, contents of heavy metals (Pb, Cd, Cr, As, Hg, Ni) of soil were all higher than before the application, but all lower than the standard of environment quality standards for soils (GB15618-1995):Pb and Hg contents of soil in all treatments measured up to GradeⅡstandard; Cr, As and Ni content measured up to GradeⅠstandard; although Cd content (including that of control) exceeded the GradeⅡstandard value of GB 15618-1995, Cd contents of rice were all lower than maximum levels of contaminants in foods (GB 2762-2005). When the amount of biogas slurry was 1250～1500 kg/666.7m2, all heavy metals of soil were at lower levels.In view of soil fertility and environmental quality of soil, the optimal application amount of biogas slurry is 1250～1500 kg/666.7m2, higher soil fertility and nicer environmental quality of soil could be obtained and good environmental benefits of biogas slurry achieved.In conclusion, take rice yield, quality, food safety, soil fertility and environmental quality of soil into integrated account, in paddy soil developed from yellow soil in the hill area of Sichuan Province, the best safe application amount of biogas slurry (CODCr 7033 mg/L, Total N 323.2 mg/L, NH4+-N 290.0 mg/L) fermented with manure of pig farm bred with Tongwei feedingstuff for planting rice in paddy soil in massif areas is about 1250 kg/666.7m2, and at this level, an optimal balance between economic and environmental benefits would be reached.