Effects Of Biogas Slurry Application With PH Adjustment On Its Fertilizer Efficiency And Environmental Impacts In Paddy Field

In recent years, it’s an important resource recovery method to irrigate paddy field by biogas slurry (BS) instead of nitrogen fertilizer. The complicated composition of BS can provide crops with nutrient substance such as N, P, K and a large quantity of organic matter, however, the broad application of BS can also lead to pollution of soil, water and air. A large number of studies focus on the effect of paddy field ecosystem with the direct irrigation of BS. In this study, paddy soil was irrigated with different treatment of BS three times in a month after transplanting. We studied the impact of irrigation by BS on soil fertility, soil microbial community structure, soil enzyme activities, soil heavy metal concentration and heavy metal bio-availability. We also studied the impact of adjusting the pH of BS on ammonia volatilization amount, nitrogen concentration in surface water. The conclusions are as follows:1. With irrigation of the BS, content of nitrogen, phosphorus and organic matter in soil raised, which are positively correlated with BS irrigation quantity. In our study, the irrigation of biogas slurry can improve the acid and alkali of soil and far from soil salination. After irrigating of BS, the pH of surface water decreased in a short-time and then increased. Activities of soil urease, catalase and invertase were significantly higher than that in control group, Activities of soil urease and catalase are in positive correlation with BS irrigation quantity. But there were no significant difference of three enzyme activity between the application of chemical fertilizer and irrigation of biogas slurry. When adjusting pH of BS, urease activity in2N,4N BS irrigated soil was significantly reduced, but no significant influence was found on the invertase and catalase.2. With the BS irrigation quantity increased, soluble nitrogen, total nitrogen and ammonium nitrogen concentration increases accordingly in surface water, and the process of degradation to control level need more time. Ammonia volatilization reduced because the decrease concentration of surface water after reducing pH level, and nitrate nitrogen was not significantly affected. It’s the key time of ammonia volatilization loss in the paddy field in the first6days of biogas slurry irrigation. Adjusting pH of BS can reduce ammonia nitrogen content and ammonia volatilization in surface water, increase fertilizer efficiency in paddy field ecosystem. BS irrigation to paddy field had little impact on soil phosphorus and potassium content.3. The DGGE fingerprint of soil bacterial community show that BS irrigation maintain soil bacterial diversity. The similarity of different treatments is between74%-90%. Compare with control, more than83%similarity was found in treatment of pH adjusted BS irrigation. The stability of the microbe is good for the health of ecological environment and the normal process of BS digestion. The BS irrigation had no adverse effect on the health and stability of soil microbial ecosystem in normal BS treatment level.4. After BS irrigating, the content of Cr have accumulated in topsoil, the content of Cu and Pb is high in surface soil without obvious down movement; the migration of Cr is obvious, and Cd is less than Cr. Under the existing irrigation level of BS, concentration of soil heavy metal did not improve obviously and had no adverse effect on soil quality according to national soil environmental quality secondary standard (GB/15618-1995). Total amount of soil heavy metal did not change significantly with decreasing of BS pH.5. The ammonia volatilization amount was significant reduced by50.30%,32.87%and53.49%after irrigating4N*of adjusted BS (marked with*in the follow text) compare with unadjusted BS. At the first and the third time of irrigation, the volatilization loss of ammonia was significantly decreased with treatment of1N*and2N*. The experiment proved that pH of surface water is one of key factors in ammonia volatilization. The effect is more significant with more pH adjusted BS irrigation, for example in this experiment, irrigation of4N*BS was beneficial for reducing loss of nitrogen.