Control Of Soil Nitrogen Leaching During Biogas Slurry Irrigation By Slurry C/N

There is a growing concern over the environmental risk of the disposal of biogas slurry, produced by the developing animal husbandry, through field application. A typical characteristic of biogas slurry is that it is high in NH₄⁺-N content but low in C/N ratio. Studies have shown that incubation of the soil that received biogas slurry can enhance nitrification and the subsequent nitrogen leaching. However, it remains to be answered that whether the biogas slurry irrigation promotes soil nitrification and nitrogen leaching, and that can nitrogen leaching be reduced by increasing the C/N（DOC/TN, similarly hereinafter） raito of biogas slurry. In this study, the biogas slurry irrigation was applied to soil columns and plots. The nitrogen leaching and the processes to circulate nitrogen in soil were explored; and the influence of DOC/TN ratio of biogas slurry on nitrogen leaching, mineralization and the fate of soil nitrogen were discussed. This study can be highlighted as follows, the nitrogen leaching was noticed, characterized, reduced and explained, and N₂O emission was explored. Results are detailed as follows.Biogas slurry irrigation was applied to two groups of soil plots at various nitrogen application rates. The irrigation lasted for 12 weeks at rate of 25 mm/week, especially to explore the nitrogen leaching. The nitrogen leaching became enhanced as the nitrogen application rate increased. The rates at least 336 kg N/hm2 inhibited soil heterotrophs but promoted autotrophic nitrifiers. As a result, soil mineralization decelerated, but soil nitrification and nitrogen leaching got promoted. Furthermore, the nutrient loads in leachate derived from the high nitrogen application rates were most noticeable during the 4th⁶th and 10th¹1th weeks.Irrigation using biogas slurries with various DOC/TN ratios was applied to soil columns. Irrigation lasted for 12 weeks at rate of 12 mm/week, especially to explore the nitrogen leaching. The nitrogen leaching became low as the DOC/TN ratio of biogas slurry increased. 31.1, 26.6, 2.26, 2.47 and 3.45 percent of the applied nitrogen leached when biogas slurry DOC/TN ratios were 0.9, 2.3, 7.3, 12.3 and 17.3, respectively. The decrease of nitrogen leaching for the biogas slurries with high DOC/TN ratios was probably due to the enhancement of soil denitrification and nitrogen immobilization. DOC/TN 7.3 being compared with DOC/TN 12.3 and 17.3 accelerated soil denitrification, while the nitrogen immobilization for DOC/TN 12.3 and 17.3 was more significant than that for DOC/TN 7.3. Therefore, nitrogen leaching can be effectively controlled by increasing the DOC/TN ratio of biogas slurry. Meanwhile, soil microbial communities were analyzed with the technologies related to phospholipid fatty acid（PLFA） and 16 S r RNA. In addition, the weighted gene co-expression network analysis（WGCNA） was conducted to identify the bacteria genera which may promote or inhibit the soil nitrification. As the DOC/TN ratio of biogas slurry increased, both abundance and diversity of the soil microbial communities became promoted. All the detected nitrifiers including Nitrosomonadaceae_uncultured, Nitrospira, Nitrosospira and Nitrosomonas were most abundant in the DOC/TN 0.9⁷.3 treatments, but least abundant in the DOC/TN 12.3 and 17.3 treatments. Acidobacteriaceae_Subgroup_1uncultured, Rhizomicrobium, Massilia and Lysobacter possibly contributed to soil nitrification, and Trichococcus, etc. possibly inhibited soil nitrification.Irrigation using biogas slurries with different DOC/TN ratios was applied to the soil columns having high soil water contents（SWC）. Irrigation lasted for 49 d at rate of 1.78².06 mm/d to explore the processes to circulate nitrogen in the soil, and to determine the amounts of nitrogen leached from soil, retained in soil and emitted as N₂O from soil. High DOC/TN ratios enhanced nitrogen immobilization and soil respiration, but reduced nitrification and N₂O emission. Under conditions of high SWC, 3.28⁴.21, 67.7¹04 and 0.77¹.03 percent of the applied nitrogen was leached, emitted into the air and emitted as N₂O, respectively. Nitrogen leaching was unremarkable even when DOC/TN of biogas slurry was low, probably due to the high SWC, however, an increase of the temperature in the environment made it significant again.The main innovation point of this study was as follows. The mathematical methods based on nutrient load index（NLI）, to identify the intervals when the nutrient loads in leachate need be concerned and to select the acceptable nitrogen application rate, were put forward.Unfortunately, the soil, biogas slurry and carbon source added into biogas slurry all had only one type, therefore, whether the results of this study can be generalized to wider conditions remains to be answered. The influence of soil water content on the processes circulating nitrogen in soil needs be studied further; the relations of the bacteria identified by WGCNA with soil nitrification need be studied further as well.