Unraveling the relationship between soil carbon-degrading enzyme activity and carbon fraction under biogas slurry topdressing.

Biogas slurry, a by-product of the anaerobic digestion of biomass waste, predominantly consisting of livestock and poultry manure, is widely acclaimed as a sustainable organic fertilizer owing to its abundant reserves of essential nutrients. Its distinctive liquid composition, when tactfully integrated with a drip irrigation system, unveils immense potential, offering unparalleled convenience in application. In this study, we investigated the impact of biogas slurry topdressing as a replacement for chemical fertilizer (BSTR) on soil total organic carbon (TOC) fractions and carbon (C)-degrading enzyme activities across different soil depths (surface, sub-surface, and deep) during the tasseling (VT) and full maturity stage (R6) of maize. BSTR increased the TOC content within each soil layer during both VT and R6 periods, inducing alterations in the content and proportion of individual C component, particularly in the topsoil. Notably, the pure biogas slurry topdressing treatment (100%BS) compared with the pure chemical fertilizer topdressing treatment (CF), exhibited a 38.9% increase in the labile organic carbon of the topsoil during VT, and a 30.3% increase in the recalcitrant organic carbon during R6, facilitating microbial nutrient utilization and post-harvest C storage during the vigorous growth period of maize. Furthermore, BSTR treatment stimulated the activity of oxidative and hydrolytic C-degrading enzymes, with the 100%BS treatment showcasing the most significant enhancements, with its average geometric enzyme activity surpassing that of CF treatment by 27.9% and 27.4%, respectively. This enhancement facilitated ongoing and efficient degradation and transformation of C. Additionally, we screened for C components and C-degrading enzymes that are relatively sensitive to BSTR. The study highlight the advantages of employing pure biogas slurry topdressing, which enhances C component and C-degrading enzyme activity, thereby reducing the risk of soil degradation. This research lays a solid theoretical foundation for the rational recycling of biogas slurry.

Disentangling the impact of biogas slurry topdressing as a replacement for chemical fertilizers on soil bacterial and fungal community composition, functional characteristics, and co-occurrence networks.

The application of biogas slurry topdressing with drip irrigation systems can compensate for the limitation of traditional solid organic fertilizer, which can only be applied at the bottom. Based on this, we attempted to define the response of soil bacterial and fungal communities of maize during the tasseling and full maturity stages, by using a no-topdressing control and different ratios of biogas slurry nitrogen in place of chemical fertilizer topdressing. The application of biogas slurry resulted in the emergence of new bacterial phyla led by Synergistota. Compared with pure urea chemical topdressing, the pure biogas slurry topdressing treatment significantly enriched Firmicutes and Basidiomycota communities during the tasseling stage, in addition to affecting the separation of bacterial and fungal α-diversity indices between the tasseling and full maturity stages. Based on the prediction of community composition and function, the changes in bacterial and fungal communities caused by biogas slurry treatment stimulated the ability of microorganisms to decompose refractory organic components, which was conducive to turnover in the soil carbon cycle, and improved multi-element (such as sulfur) cycles; however it may also bring potential risks of heavy metal and pathogenic microbial contamination. Notably, the biogas slurry treatment reduced the correlation and aggregation of bacterial and fungal symbiotic networks, and had a dual effect on ecological randomness. These findings contribute to a deeper comprehension of the alterations occurring in soil microbial communities when substituting chemical fertilizers treated with biogas slurry topdressing, and promote the efficient and sustainable utilization of biogas slurry resources.

Synergistic effect on soil health from combined application of biogas slurry and biochar.

Biogas slurry and biochar, as typical by-products and derivatives of organic waste, have been applied in agricultural production to improve the soil carbon (C) pool. However, whether the combined application of biogas slurry and biochar produces synergistic effects on the soil C pool and soil health requires quantitative clarification. In this study, we performed a pot experiment to analyze the changes of soil organic carbon (SOC), potassium permanganate-oxidized carbon (POXC), mineralizable carbon (MC), soil ß-glucosidase (S-ß-GC), and soil protein (SP) in different treatments at the flowering and fruit-setting stages, and full fruit stage of tomato by establishing two base fertilizer modes (base fertilizer N and base biogas slurry N), three topdressing modes (topdressing chemical fertilizer N, topdressing 50% biogas slurry N + 50% chemical fertilizer N, and topdressing biogas slurry N), and two biochar levels (no addition and 3% biochar addition). During the full fruit period, the SOC content of bottom applications of biogas slurry and topdressings of biogas slurry significantly increased by 9.92-15.52% and 13.02-18.26%, respectively (P < 0.05), when compared to chemical fertilizer bottom applications and topdressings of chemical fertilizer. When compared to non-biochar treatment, the SOC content of the biochar considerably increased by 52.56-58.94% (P < 0.05). Moreover, biogas slurry treatment increased the MC, steady-state C, and C pool index, and decreased the S-ß-GC, C pool efficiency, C pool activity, and C pool activity index. Application of biogas slurry initially reduced POXC, SP, the C pool management index, and the soil quality index; nonetheless, these indicators eventually recovered or even exceeded the result of single application chemical fertilizer. Overall, the combined application of biogas slurry and biochar strongly increases the soil C pool, improves soil health, and reduces the short-term negative effects of using only biogas slurry.

Biogas slurry topdressing as replacement of chemical fertilizers reduces leaf senescence of maize by up-regulating tolerance mechanisms.

Worldwide physiological research has aimed to decelerate the aging of crop leaves by optimizing fertilization measures to improve crop or biomass yield. Solid organic fertilizers can be combined with chemical fertilizers to delay the aging of crop leaves. Biogas slurry is a liquid organic fertilizer produced by the anaerobic fermentation of livestock and poultry manure and other resources, and it can partially replace chemical fertilizers in field application via drip irrigation systems. However, the impact of biogas slurry topdressing on leaf aging remains unclear. This study investigated treatments with no topdressing (control, CK) and five topdressing patterns of biogas slurry replacing chemical fertilizer (nitrogen) at 100%, 75%, 50%, 25%, and 0% (100%BS, 75%BS, 50%BS, 25%BS, CF). The effects of different proportions of biogas slurry on leaf senescence rate, photosynthetic pigments, osmotic adjustment substances, antioxidant defense enzymes, and nitrogen metabolism related enzyme activities of maize were analyzed. Subsequently, the mechanisms of biogas slurry topdressing on the leaf senescence rate of maize were explored. The results showed that the mean decreasing rate of relative green leaf area (Vm) treated with biogas slurry decreased by 3.7%-17.1% and the duration of leaf area duration (LAD) increased by 3.7%-17.1% compared with the results for CK. The maximum senescence rate of 100%BS was delayed by 4.4 and 5.6 d compared to the results for CF and CK, respectively. During the senescence of maize leaves, the use of biogas slurry topdressing increased the content of chlorophyll, decreased the water loss and the accumulation rate of malondialdehyde and proline in leaves, and increased the activities of catalase, peroxidase, and superoxide dismutase in the later growth and development periods of maize. In addition, biogas slurry topdressing improved the nitrogen transport efficiency of the leaves and ensured continuous and efficient ammonium assimilation. Furthermore, there was a strong correlation between leaf senescence and the investigated physiological indices. Cluster analysis showed that the 100%BS treatment exhibited the most prominent effect on leaf senescence. Biogas slurry topdressing as a substitute for chemical fertilizer can be potentially used as an anti-aging regulation measure for crops to decrease the damage induced by senescence.

Flushing control strategies to improve the stability of a biogas slurry drip irrigation system: Behavioral characteristics and mechanisms.

Biogas slurry drip irrigation can mitigate environmental pollution and reduce the use of chemical fertilizers to enable sustainable development. However, the stability of the biogas slurry drip irrigation system (BSDIS) is disrupted by emitter clogging; hence, it is essential to explore the flushing control strategy of BSDIS. By means of combining actual measurement and simulation, this study investigates the BSDIS stability based on the three technical parameters of the flushing control strategy. Appropriate flushing control strategies can improve system stability and cause spatial differences on the drip irrigation tape. Under various flushing control strategies, the system stability primarily undergoes delays, sensitivity, and ineffectiveness of flushing with time. Compared with the without flushing and emitter outlet downward-oriented treatment, the optimal flushing combination (the high frequency flushing + emitter outlet upward-oriented treatment) reduces the emitter clogging content by approximately 70.97% and increases system stability by 189.1%. In the internal hydrodynamics, the laying direction of emitters does not change the movement characteristics of water flow, although the clogging particles do not completely follow the water flow, with some particles settling owing to gravity, thereby clotting the emitters. When clogging occurs, the increase in flushing speed is conducive to the increase in turbulent kinetic energy on the inlet surface of the emitter, which facilitate the flushing of clogged substances. This study proposes optimal flushing strategy parameters along with a new management mode for the waste liquid represented by biogas slurry.

Characterization of labyrinth emitter-clogging substances in biogas slurry drip irrigation systems.

Drip irrigation is important for efficiently returning biogas slurry to fields. Elucidating the characteristics and components of clogging substances produced by labyrinth emitters in biogas slurry drip irrigation systems will help to develop various clogging substance-remediation strategies. However, previous studies were unable to characterize the clogging substances in emitters. Thus, we aimed to characterize and quantify the substances clogging emitters in a biogas slurry drip irrigation system and determine the micromorphology and dominance of microbial communities. Here, emitter discharge changes and the micromorphologies, phase compositions, and biological communities of clogging substances were studied via hydraulic performance tests, scanning electron microscopy-energy depressive spectra (SEM-EDS), and high-throughput sequencing. The degree of emitter-clogging increased over time (first quickly, then slowly) and was deeper at the end of the drip irrigation tape than at the head. The clogging substances were viscous agglomerations primarily comprising 0.3-1.5-µm particles. Their formation was affected by settlement with gravity, water pressure adhesion, and mobile biological adhesion. The dominant microbial communities in the clogging substances included Firmicutes (29.7%) and Proteobacteria (19%); the emitter-clogging substances primarily comprised water (85%) and composite dry matter. The water, dry matter, and extracellular polymer substance (EPS) weights in the clogging substances increased over time, but their relative proportions remained stable. In the composite dry matter, typical physical (organic carbon, Al2O3, and SiO2), chemical (CaCO3 and MgCO3), and biological (EPS) clogging substances accounted for >50, 9, and 5.62% of the total dry matter mass, respectively. This study provides a good foundation and reference idea and will be very helpful to propose targeted solutions for solving the clogging of biogas slurry drip irrigation system.