Substituting urea with biogas slurry and hydrothermal carbonization aqueous product could decrease NH3 volatilization and increase soil DOM in wheat growth cycle

Recycling C and N from biogas slurry and wastewater of hydrothermal carbonization to rice-paddy systems: Enhanced soil dissolved C and N retention

This study explores a sustainable agricultural approach by mixing biogas slurry (BS) with hydrothermal carbonization aqueous phase (HCAP) to recycle carbon and nitrogen to rice-paddy system. Over two years, the effects of combining swine-derived BS with vegetable-derived HCAP as an alternative for synthesized nitrogen fertilizer in a rice-paddy system were evaluated. Four nitrogen substitution rates were tested: 0% (control group, CKU), 50% (low nitrogen substitution rate, BSVL), 75% (medium nitrogen substitution rate, BSVM), and 100% (high nitrogen substitution rate, BSVH). Results demonstrated combined application of BS and HCAP significantly improved soil dissolved organic matter (DOM) and total nitrogen content by 28.5‒82.5% and 5.8‒7.2%, respectively, with positive correlations to the increase in nitrogen substitution rate (P < 0.05). The DOM components revealed substantial increases in microbial by-product-like and fulvic acid-like substances in the soil, by 0.6‒2.0 folds and 2.8‒10.3 folds, respectively. Ammonia volatilization was significantly reduced by 15.6‒46.3% and 2.2‒12.6% across two years, correlating with pH and ammonium levels in floodwater (P < 0.05). Additionally, substituting chemical nitrogen fertilizer with BS and HCAP maintained grain nitrogen content without compromising rice nitrogen uptake. The results of structural equation model indicate that substituting nitrogen with BS and HCAP enhanced the recycling of carbon and nitrogen in paddy soil by improving soil DOM and total nitrogen accumulation. Overall, this study presents a viable strategy for recycling carbon and nitrogen from BS and HCAP into paddy soil, thereby substituting chemical fertilizers and enhancing soil fertility.

Aerobic composting with hydrothermal carbonization aqueous phase conditioning: Stabilized active gaseous nitrogen emissions

The losses of reactive gaseous nitrogen (N), including ammonia (NH3) and nitrous oxide (N2O), represent a pressing environmental issue during composting. However, the impact of hydrothermal carbonization aqueous phase (HAP) on compost gaseous N emissions and the underlying mechanisms remain largely unexplored. Herein, Quercus acutissima leaves-derived HAP and its modified HAP (MHAP) were added to the chicken manure compost at 5 % (w/w) and 10 % (w/w) applied rates to observe changes in NH3 and N2O fluxes, compost properties and bacterial communities. Results showed that high application of HAP significantly decreased compost cumulative NH3 volatilization by 23-26 % compared to the control and MHAP. Compost NH3 and N2O emissions were significantly influenced by compost temperature and inorganic N concentrations. Moreover, HAP and MHAP at high rates reduced the relative abundance of Bacteroidota (5-29 %) and Proteobacteria (11-35 %), compared to those at low rates. Compost environmental factors and bacterial diversity were identified as dominant factors affecting gaseous N emissions, with 54 % and 25 % explanatory rates, respectively. Furthermore, high application rates of HAP are expected to reduce annual NH3 emissions from poultry manure compost by 40000 t. These findings provide insights into rational resource utilization of HAP and gaseous N emission reduction from composting.

Co-application of hydrothermal carbonization aqueous phase and biogas slurry reduced ammonia volatilization in paddy

Application of biogas slurry (BS) can promote ammonia (NH3) volatilization. Algae sludge and Quercus acutissima leaves are rich in resources and nutrients, and can be effectively converted into valuable products. Hydrothermal carbonization technology (HTC) is a sustainable method for the treatment of wet biomass. However, the large amount of hydrothermal carbonization aqueous products (HAP) contains harmful substances that require effective management. The combined application of HAP and BS can mitigate NH3 emissions and facilitate resource recovery, presenting an eco-friendly approach to both nutrient recycling and pollution mitigation. This study explored the joint application of HAP and BS in paddy to decrease NH3 volatilization and the factors influencing NH3 volatilization. In this study, the HAP prepared from algae sludge and Quercus acutissima leaves at 180 °C and 220 °C was mixed with BS at a 1:1 total nitrogen content ratio, and the mixture was used instead of 25% or 50% urea. The experimental results indicated that the rice yield with the application of HAP and BS was equivalent to the control treatment only with urea (CK). Compared to the CK, HAP and BS treatments reduced soil NH3 volatilization by 6.9%-55.5% and increased soil dissolved organic matter (DOM) by 2.7%-59.4%. The treatments using algae sludge and Quercus acutissima leaves prepared at 220 °C as substitutes for 50% of urea reduced NH3 volatilization by 43.9% and 55.5%, respectively. Ammonium nitrogen, pH, total organic carbon, urease, and DOM were important factors influencing NH3 volatilization. This study showed that substituting part of urea with HAP and BS for field application reduced NH3 volatilization and increased soil organic matter content.

Biochar and hydrochar application influence soil ammonia volatilization and the dissolved organic matter in salt-affected soils

Biochar, which including pyrochar (PBC) and hydrochar (HBC), has been tested as a soil enhancer to improve saline soils. However, the effects of PBC and HBC application on ammonia (NH3) volatilization and dissolved organic matter (DOM) in saline paddy soils are poorly understood. In this research, marsh moss-derived PBC and HBC biochar types were applied to paddy saline soils at 0.5 % (w/w) and 1.5 % (w/w) rates to assess their impact on soil NH3 volatilization and DOM using a soil column experiment. The results revealed that soil NH3 volatilization significantly increased by 56.1 % in the treatment with 1.5 % (w/w) HBC compared to the control without PBC or HBC. Conversely, PBC and the lower application rate of HBC led to decrease in NH3 volatilization ranging from 2.4 % to 12.1 %. Floodwater EC is a dominant factor in NH3 emission. Furthermore, the fluorescence intensities of the four fractions (all humic substances) were found to be significantly higher in the 1.5 % (w/w) HBC treatment applied compared to the other treatments, as indicated by parallel factor analysis modeling. This study highlights the potential for soil NH3 losses and DOM leaching in saline paddy soils due to the high application rate of HBC. These findings offer valuable insights into the effects of PBC and HBC on rice paddy saline soil ecosystems.

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.

The dynamic features and microbial mechanism of nitrogen transformation for hydrothermal aqueous phase as fertilizer in dryland soil

Hydrothermal aqueous phase (HAP) contains abundant organics and nutrients, which have potential to partially replace chemical fertilizers for enhancing plant growth and soil quality. However, the underlying reasons for low available nitrogen (N) and high N loss in dryland soil remain unclear. A cultivation experiment was conducted using HAP or urea to supply 160 mg N kg-1 in dryland soil. The dynamic changes of soil organic matters (SOMs), pH, N forms, and N cycling genes were investigated. Results showed that SOMs from HAP stimulated urease activity and ureC, which enhanced ammonification in turn. The high-molecular-weight SOMs relatively increased during 5-30 d and then biodegraded during 30-90 d, which SUV254 changed from 0.51 to 1.47 to 0.29 L-1 m-1. This affected ureC that changed from 5.58 to 5.34 to 5.75 lg copies g-1. Relative to urea, addition HAP enhanced ON mineralization by 8.40 times during 30-90 d due to higher ureC. It decreased NO3-N by 65.35%-77.32% but increased AOB and AOA by 0.25 and 0.90 lg copies g-1 at 5 d and 90 d, respectively. It little affected nirK and increased nosZ by 0.41 lg copies g-1 at 90 d. It increased N loss by 4.59 times. The soil pH for HAP was higher than that for urea after 11 d. The comprehensive effects of high SOMs and pH, including ammonification enhancement and nitrification activity inhibition, were the primary causes of high N loss. The core idea for developing high-efficiency HAP fertilizer is to moderately inhibit ammonification and promote nitrification.

Artificial humic acid coated ferrihydrite strengthens the adsorption of phosphate and increases soil phosphate retention

Phosphorus (P) leaching loss from farmland soils is one of the main causes of water eutrophication. Thus, effective methods must be developed to maintain sustainability in agricultural soils. Herein, we design artificial humic acid (A-HA) coated ferrihydrite (Fh) particles for fixing P in soil. The experiments in water and soil are successively conducted to explore the phosphate adsorption mechanism and soil P retention performance of A-HA coated ferrihydrite particles (A-Fh). Compared with unmodified ferrihydrite (Fh), the phosphate adsorption capacity of A-Fh is increased by 15 %, the phosphate adsorption speed and selectivity are also significantly improved. The ligand exchange, electrostatic attraction and hydrogen bonding are the dominant mechanisms of phosphate adsorption by A-Fh. In soil experiments, the addition of 2 % A-Fh increases the soil P retention performance from 0.15 to 0.7 mg/kg, and A-Fh are able to convert more phosphate adsorbed by itself into soil available P to improve soil fertility. Overall, this work highlights the importance of this a highly effective amendment for improving poor soils.

Co-application of concentrated biogas slurry and pyroligneous liquor mitigates ammonia emission and sustainably releases ammonium from paddy soil

Biogas production causes vast amounts of biogas slurry (BS). Application of BS to croplands can substitute chemical fertilizers while result in higher ammonia emissions. Tremendous variation of ammonium concentration in different BSs induces imprecise substitution, while concentrated BS holds higher and more stable ammonium. Pyroligneous liquor, an acidic aqueous liquid from biochar production, can be used with concentrated BS to reduce ammonia emission. However, the effects of combining concentrated BS with pyroligneous liquor on ammonia emission and soil (nitrogen) N transformation have been poorly reported. In this study, a field experiment applying concentrated BS only, or combining with 5 %, 10 %, and 20 % pyroligneous liquor (v/v) for substituting 60 % N of single rice cultivation was conducted by contrast with chemical fertilization. The results showed that substituting chemical N fertilizers with concentrated BS increased 24.6 % ammonia emission. In comparison, applying 5 %, 10 %, and 20 % pyroligneous liquor with concentrated BS reduced 4.9 %, 20.3 %, and 24.4 % ammonia emissions, respectively. Applying concentrated BS with more pyroligneous liquor preserved higher ammonium and dissolved organic carbon in floodwater, and induced higher nitrate concentration after fertilization. Whereas soil ammonium and nitrate contents were decreased along with more pyroligneous liquor application before and after the topdressing and exhibited sustainable release until rice harvest. In comparison, the soil N mineralization and nitrification rates were occasionally elevated, while the activities of soil urease, protease, nitrate reductase, and nitrite reductase had multiple responses. Applying concentrated BS only, or combining with 5 %, 10, and 20 % pyroligneous liquor, have little effect on soil basic properties but inorganic N. In summary, applying concentrated BS with >10 % pyroligneous liquor could preserve more N with sustainable release and potentially lower N loss to the atmosphere, and we proposed that applying 13.5 % pyroligneous liquor in concentrated BS could achieve maximum soil fertility and minimum ammonia emission.

Hydrothermal carbonization of biogas slurry and cattle manure into soil conditioner mitigates ammonia volatilization from paddy soil

Hydrothermal carbonization of biogas slurry and animal manure into hydrochar could enhance waste recycling waste and minimize ammonia (NH3) volatilization from paddy fields. In this study, cattle manure-derived hydrochar prepared in the presence of Milli-Q water (CMWH) and biogas slurry (CMBSH), and biogas slurry-based hydrochar embedded with zeolite (ZHC) were applied to rice-paddy soil. The results demonstrated that CMBSH and ZHC treatments could significantly mitigate the cumulative NH3 volatilization and yield-scale NH3 volatilization by 27.9-45.2% and 28.5-45.4%, respectively, compared to the control group (without hydrochar addition), and significantly correlated with pH and ammonium-nitrogen (NH4+-N) concentration in floodwater. Nitrogen (N) loss via NH3 volatilization in the control group accounted for 24.9% of the applied N fertilizer, whereas CMBSH- and ZHC-amended treatments accounted for 13.6-17.9% of N in applied fertilizer. The reduced N loss improved soil N retention and availability for rice; consequently, grain N content significantly increased by 6.5-14.9% and N-use efficiency increased by 6.4-16.0% (P < 0.05), respectively. Based on linear fitting results, NH3 volatilization mitigation resulted from lower pH and NH4+-N concentration in floodwater that resulted from the acidic property and specific surface area of hydrochar treatments. Moreover, NH3-oxidizing archaea abundance in hydrochar-treated soil decreased by 40.9-46.9% in response to CMBSH and ZHC treatments, potentially suppressing NH4+-N transformation into nitrate and improving soil NH4+-N retention capacity. To date, this study applied biogas slurry-based hydrochar into paddy soil for the first time and demonstrated that ZHC significantly mitigated NH3 and increased N content. Overall, this study proposes an environmental-friendly strategy to recycle the wastes, biogas slurry, to the paddy fields to mitigate NH3 volatilization and increase grain yield of rice.

Hydrothermal carbonization aqueous phase promotes nutrient retention and humic substance formation during aerobic composting of chicken manure

The aqueous phase (AP) of hydrothermal carbonization is rich in humic substances (HSs), which could influence the poultry manure composting process and the product quality. Here, raw AP and its modified product (MAP) with different nitrogen (N) contents were added into chicken manure composting at low (5%) or high (10%) rate. Results showed that all APs addition decreased the temperature and pH but AP-10% increased total N, HSs, and humic acid (HA) of compost by 12%, 18% and 27%, respectively. MAP applications increased the total phosphorus by 8-9% and MAP-10% enhanced the total potussium content by 20%. Additionally, both AP and MAP additions increased the contents of three major components of dissolved organic matter by 20-64%. In conclusion, both AP and MAP can generally improve the chicken manure compost quality, which provides a new idea for the recycling of APs derived from agro-forestry wastes during hydrothermal carbonization.

Tofu by-product soy whey substitutes urea: Reduced ammonia volatilization, enhanced soil fertility and improved fruit quality in cherry tomato production

Soy whey is an abundant, nutrient-rich and safe wastewater produced in tofu processing, so it is necessary to valorize it instead of discarding it as sewage. Whether soy whey can be used as a fertilizer substitute for agricultural production is unclear. In this study, the effects of soy whey serving as a nitrogen source to substitute urea on soil NH₃ volatilization, dissolved organic matter (DOM) components and cherry tomato qualities were investigated by soil column experiment. Results showed that the soil NH₄⁺-N concentrations and pH values of the 50% soy whey fertilizer combined with 50% urea (50%-SW) and 100% soy whey fertilizer (100%-SW) treatments were lower than those of 100% urea treatment (CKU). Compared with CKU, 50%-SW and 100%-SW treatments increased the abundance of ammonia oxidizing bacteria (AOB) by 6.52-100.89%, protease activity by 66.22-83.78%, the contents of total organic carbon (TOC) by 16.97-35.64%, humification index (HIX) of soil DOM by 13.57-17.99%, and average weight per fruit of cherry tomato by 13.46-18.56%, respectively. Moreover, soy whey as liquid organic fertilizer reduced the soil NH₃ volatilization by 18.65-25.27% and the fertilization cost by 25.94-51.87% compared with CKU. This study provides a promising option with economic and environmental benefits for soy whey utilization and cherry tomato production, which contributes to the win-win effectiveness of sustainable production for both the soy products industry and agriculture.

Partial Substitution of Urea with Biochar Induced Improvements in Soil Enzymes Activity, Ammonia-Nitrite Oxidizers, and Nitrogen Uptake in the Double-Cropping Rice System

Biochar is an important soil amendment that can enhance the biological properties of soil, as well as nitrogen (N) uptake and utilization in N-fertilized crops. However, few studies have characterized the effects of urea and biochar application on soil biochemical traits and its effect on paddy rice. Therefore, a field trial was conducted in the early and late seasons of 2020 in a randomized complete block design with two N levels (135 and 180 kg ha^-1) and four levels of biochar (0, 10, 20, and 30 t ha^-1). The treatment combinations were as follows: 135 kg N ha^-1 + 0 t B ha^-1 (T1), 135 kg N ha^-1 + 10 t B ha^-1 (T2), 135 kg N ha^-1 + 20 t B ha^-1 (T3), 135 kg N ha^-1 + 30 t B ha^-1 (T4), 180 kg N ha^-1 + 0 t B ha^-1 (T5), 180 kg N ha^-1 + 10 t B ha^-1 (T6), 180 kg N ha^-1 + 20 t B ha^-1 (T7) and 180 kg N ha^-1 + 30 t B ha^-1 (T8). The results showed that soil amended with biochar had higher soil pH, soil organic carbon content, total nitrogen content, and mineral nitrogen (NH₄⁺-N and NO₃^--N) than soil that had not been amended with biochar. In both seasons, the 20 t ha^-1 and 30 t ha^-1 biochar treatments had the highest an average concentrations of NO₃^--N (10.54 mg kg^-1 and 10.25 mg kg^-1, respectively). In comparison to soil that had not been treated with biochar, the average activity of the enzymes urease, polyphenol oxidase, dehydrogenase, and chitinase was, respectively, 25.28%, 14.13%, 67.76%, and 22.26% greater; however, the activity of the enzyme catalase was 15.06% lower in both seasons. Application of biochar considerably increased the abundance of ammonia-oxidizing bacteria (AOB), which was 48% greater on average in biochar-amended soil than in unamended soil. However, there were no significant variations in the abundances of ammonia-oxidizing archaea (AOA) or nitrite-oxidizing bacteria (NOB) across treatments. In comparison to soil that had not been treated with biochar, the average N content was 24.46%, 20.47%, and 19.08% higher in the stem, leaves, and panicles, respectively. In general, adding biochar at a rate of 20 to 30 t ha^-1 with low-dose urea (135 kg N ha^-1) is a beneficial technique for improving the nutrient balance and biological processes of soil, as well as the N uptake and grain yield of rice plants.