Effect Of Biogas Slurry Irrigation On Plants, Water And Soil Environment

With the rapid development of biogas plants, reasonable disposition of biogas slurry is particularly important. Currently, most of biogas slurry produced by the large and middle scale biogas plants, was used to irrigate on agricultural lands, as compared with only a few biogas slurry was treated before discharging.Because of a great amount of biogas slurry generated by biogas plants contain high concentrations of COD, total nitrogen (TN), total phosphorus (TP) and ammonia nitrogen, besides, there still are no appropriate technical guidances for biogas slurry irrigation for agricultural land usages, which may bring uncertain effect to the crop safety and soil environment quality. Therefore, the objectives of this paper were to investigate the final results of crop yield and crop security when biogas slurry was irrigated, three species of crop including rice, lettuce and ryegrass in different planting methods respectively, and the corresponding effects of different biogas slurry irrigation levels on soil, surface water and leaching water respectively, will be discussed, meanwhile the environmental effect of biogas slurry irrigation was also evaluated for understanding the potentiality of biogas slurry for farmland ecological system, which provides theory and technology support for further application.Biogas slurry irrigation can significantly increase crop yields. Increasing Rice yield ranged in29.84%～112.69%, and67.39%～88.24%increasing for lettuce, For ryegrass, fresh grass yield increased from53.04%to96.64%, hay increasing production ranged in13.04%～19.03%, However, the crop yields did not continue to grow with the irrigation level increases, under high biogas slurry application rate, the growth can be inhibited, and yields of crop will reduced. Therefore biogas slurry irrigation should be appropriate.There was no obvious effect on nitrate content of crops for biogas slurry irrigation, and the nitrate content of lettuce and ryegrass respectively was far below the national standard limitation. Heavy metal contents of the edible part in plants were lower than the national standard limitation, but high irrigation amount of biogas slurry may have significant risk to cause the heavy metals accumulation in plants, the As content in rice, Pb content in lettuce were closed to national standard limitation. Overall, the heavy metal concentration of ryegrass are far below national standard limitation. In order to ensure crop security, the maximum irrigation amounts of biogas slurry in single season was2970m3/hm2for rice,1584m3/hm2for lettuce,2112m3/hm2for ryegrass. Under different land use types, biogas slurry irrigation had different effects on soil physicochemical properties. The pH value of the soil can increase for the biogas slurry irrigation in flooded water, the pH decline for the biogas slurry irrigation in dry land. In sum, the pH has changed slightly. Biogas slurry irrigation can effectively improve soil nutrient contents and soil fertility. Under the three different land use types, soil organic matter, nitrogen and phosphorus contents all increased which showed a certain increasing trend in pot experiments, and meanwhile the nitrate-nitrogen concentration increased as the irrigation amount of biogas slurry increased.However, the Cu, Zn, Pb, Cd, and As contents all increased in soil resulted from biogas slurry irrigation, but single-season irrigation of biogas slurry did not lead to heavy metal contents accumulation which exceed national standard limitation in soil. The accumulation risk of heavy metals is small in soil, and biogas slurry irrigation volume is insufficient to threat soil security. In potted plant experiments, single season irrigation amount was3960m3/hm2for rice,2112m3/hm2for lettuce,2112m3/hm2for ryegrass respectively, which was acceptable. The suitable doses and irrigation cycle of biogas slurry on farmland need to be studied further.Biogas slurry irrigation has little effect on surface water of rice, the pH of surface water changed slightly in different irrigation amounts, and COD, nitrogen and phosphorus contents changed markedly during early time on the biogas slurry irrigation, while tended to stabilize in the later period of rice growth, and the content was very low. Heavy metals contents of surface water also changed slightly in the whole process of rice growth. Single-season irrigation did not pollute the rice surface runoff.Biogas slurry irrigation had a significantly effects on leaching water. In rice plant experiments, eluviation effects were serious in high irrigation treatment irrigation, which exceed the loads of rice eco-system. After the8th irrigation, respectively, the COD, TN, ammonia nitrogen, nitrate nitrogen, TP and DP concentration all increased significantly in leaching water. Under the dryland farming, both in the lettuce and ryegrass environmental, the nutrient contents of leaching water were also higher evidently than that under control; Pose a threat to the water environment. Some effects of biogas slurry irrigation on heavy metals contents in water also have been detected, Hwoever in the whole growth period of all the tested plants, Cu, Zn, Pb and As contents in leaching water was far below the groundwater quality standards (GB/T-14848-93), while the high levels Cd content may be associated with the high background values in bilgas slurry. In order to ensure the safety of surface water and leaching water, the greatest single season irrigation amount of biogas slurry were recommended as2970m3/hm2for rice, and1320m3/hm2for both lettuce and rye in a single growth season.From the view of biogas slurry irrigation back to lands, the maximum capacity as a primary objective for environmental safety, and maximum capacity for the second goal in plant safety, the single season irrigation amount should be controlled within2970m3/hm2for rice,1320m3/hm2for both lettuce and ryegrass.