Resource Utilization of Biogas Waste as Fertilizer in China Needs More Inspections Due to the Risk of Heavy Metals

The utilization of livestock waste has attracted increasing attention in recent years. The presence of high levels of heavy metals is a major obstacle to the utilization of biogas as a fertilizer resource. In this study, the heavy metal contents in biogas residue, slurry, and discharged sewage from three representative farms of gooseries, henneries, and dairy farms in the Yangtze River Basin were investigated and assessed. The results demonstrated that heavy metals, including Cd, Mn, As, Cu, Pb, Cr, Zn, etc., could be detected in all biogas residues, with significantly different contents between farm types (p < 0.005). Specifically, biogas residues from the goosery and the dairy farms met “China’s Organic Fertilizer Standards” (COF Standards); however, Cd concentrations in biogas residues from hennery farms exceeded the limits by five times. The concentrations of Cd and Pb in biogas slurries from all of the farms exceeded the limits of the “China Farmland Irrigation Water Quality Standard” (CFIWQ Standard). In particular, the Pb concentrations in biogas slurry from the dairy farms exceeded the limits by 29 times, and the discharged sewage from all three farm types complied with the comprehensive sewage discharge standards in China; however, only that from the goosery farms was suitable for irrigation. Thus, it is recommended to increase the feed selection, biogas engineering, and biological-purification-supporting technology, and to carry out regular sampling inspections of the biogas residue, slurry, and discharged sewage for heavy metals, so that environmental and crop pollution risks can be reduced when they are used as sources of nutrients for eco-friendly agriculture.

Comparative Effectiveness of Biogas Residue Acidification and Nitrification Inhibitors in Mitigating CO2 and N2O Emissions from Biogas Residue-Amended Soils

Owing to their high carbon and nitrogen contents, biogas residues may lead to higher carbon dioxide (CO2) and nitrous oxide (N2O) emissions from soils. Acidification of biogas slurry and application of nitrification inhibitors (NIs) could mitigate the emission of these gases. An incubation experiment was therefore carried out to investigate the effect of NIs, DMPP (3, 4-dimethylpyrazole phosphate), and PIADIN (active ingredients: 3.00–3.25% 1,2,4-triazole and 1.50–1.65% 3-methylpyrazole), on CO2 and N2O emissions from soils fertilized with biogas residues and acidified biogas residues. Biogas residues produced higher ammonium-nitrogen (NH4+-N) and nitrate-nitrogen (NO3−-N) concentrations in soils which resulted in higher emissions of CO2-C and N2O-N than that from acidified biogas residues. Both DMPP and PIADIN significantly decreased the emissions of CO2-C (8.1–55.8%) and N2O-N (87–98%) and maintained lower NH4+-N and NO3−-N concentrations when compared to control (without nitrification inhibitors). However, the DMPP had a higher reduction capability for CO2-C emissions than PIADIN in acidified biogas residue applied soil. In conclusion, the acidification of biogas residues and application of NIs are effect in reducing gaseous emission from biogas residue fertilized soils and thus could improve the fertilizer effectiveness of the residues.

Organic acids induce by metal-tolerant Pantoea sp. WP-5 and biogas residues enhanced phytostabilization of cadmium in soil

This study investigated the phytoremediation potential of maize (Zea mays L.) in Cd contaminated soil through co-inoculation of metal tolerant plant beneficial rhzobacteria (MtPBR: Pantoea sp. strain WP-5) with organic manures (PM: poultry manure and BGR: biogas residues). The objectives of this study were to i) examine comparative efficiency of MtPBR, PM and BGR alone or in combined form to improve maize biomass and physiology, and ii) understand the role of organic acid production in root exudates of maize for Cd accumulation and translocation. Pantoea sp. WP-5 showed tolerance to high Cd concentration (1000 mg L− 1), thereby inoculated to maize seeds sown in soil spiked with 75 mg Cd kg− 1 soil and 500 g each of the organic manures per pot. The co-inoculation of MtPBR + BGR significantly (P < 0.05) increased chlorophyll contents, root/shoot dry weight, photosynthetic rate, stomatal conductance and relative water contents, whereas decreased electrolyte leakage, malondialdehyde contents, ascorbate peroxidase and catalase activity in maize over the control treatment. The co-inoculation of MtPBR + BGR produced significantly (P < 0.05) higher concentrations of acetic and citric acid (52.7 ± 0.5 and 22.8 ± 0.08 µg g− 1 root fwt, respectively) in root exudates of maize, which immobilized Cd within plant roots inferred by the positive relation (root Cd vs. organic acids; R2 = 0.80–0.92) and reduced Cd translocation to shoots inferred by the negative relation (shoot Cd vs. organic acids; R2 = 0.81–0.90). It is concluded that the application of MtPBR + BGR enhanced organic acid induced phyto-stabilization and accumulation of Cd in roots and restricted its translocation to shoots.

Acidified Biogas Residues Improve Nutrient Uptake and Growth of Young Maize

Biogas residues (BGR) contain a variety of plant nutrients and are, therefore, valuable fertilizers. However, ammonia (NH3) emissions occur during slurry and BGR application. These emissions can be reduced by lowering the pH of the BGR. Acidification technology works well for slurry, but little is known about the effects on fertilizer properties of acidified BGR (ABGR). This study aimed to examine the impact of acidification on the chemical composition of BGR and its influence on plant growth of juvenile maize and the soil pH, as well as the soluble soil phosphorous (P) and manganese (Mn), after application of ABGR. The soluble amount of nutrients in BGR was compared with that in ABGR. In an outdoor pot experiment, BGR and ABGR were incorporated in soil, and maize was grown for 8 weeks. Two different BGR P levels were compared with (NH4)2HPO4 and a control treatment without additional P. BGR acidification increased dissolved amounts of P from 15% to 44%, calcium from 6% to 59%, magnesium from 7% to 37%, and Mn from 2% to 15%. The dry matter of ABGR-fertilized maize was 34%, 45% higher than that of BGR-fertilized maize. The soluble Mn content in the soil was 74% higher with the low ABGR dose and 222% higher with the higher ABGR dose than the BGR treatments. The fertilizer efficiency of ABGR was higher than that of BGR, indicating that the absolute amount of applied fertilizer could be reduced in systems using ABGR.