Addition of zeolite and superphosphate to windrow composting of chicken manure improves fertilizer efficiency and reduces greenhouse gas emission

The biological mechanism of a lower carbon/nitrogen ratio increases methane emissions during vegetable waste composting

The initial carbon/nitrogen (C/N) ratio is one of the most important factors impacting composting processes, such as methane (CH4) emissions. However, the effects of the C/N ratio on CH4 emissions and the associated biological mechanisms during vegetable waste composting are largely unknown. In this study, a lab-scale experiment was conducted to investigate the effects of different C/N ratios on CH4 emissions and the mechanisms associated with methane-metabolizing microorganisms (methanogens and methanotrophs) during capsicum straw composting. The initial C/N ratios were set to 18, 30 and 50 to simulate the low (L), medium (M) and high (H) C/N ratios, respectively. The results showed that CH4 emissions were mainly concentrated in the thermophilic phase and that the cumulative CH4 emissions were significantly greater in the L treatment than in the M and H treatments by 10.8 and 15.4 times, respectively. During the methanogenic process, the relative abundance of the dominant genus Methanoculleus (47.59 % ∼ 76.92 %) was higher than in the L treatment than in the M and H treatments at the thermophilic and maturation stages, and the Chao1 index and the mcrA gene abundance followed the order of L > M > H at each composting stage. During the methanotrophic process, the dominant genus unclassified_d_bacteria (51.3 % ∼ 91.87 %), Chao1 index, pmoA gene abundance and CO2 emissions were in the order of L > M > H at each composting stage. This pattern suggests that a lower C/N ratio simultaneously enhanced CH4 production and oxidation. A structural equation model further revealed that the methanogenic community, which was driven directly by the relative contents of hemicellulose and cellulose in the substrates, as indicated by the C/N ratio, made greater contributions to CH4 emissions than did the methanotrophic community. In conclusion, a lower C/N ratio increased CH4 emissions mainly by regulating the population and composition of methanogen community.

Integrating bioprocess and metagenomics studies to enhance humic acid production from rice straw

Rice straw burning annually (millions of tons) leads to greenhouse gas emissions, and an alternative solution is producing humic acid with high added-value. This study aimed to examine the influence of a microbial consortium and other additives (chicken manure, urea, olive mill waste, zeolite, and biochar) on the composting process of rice straw and the subsequent production of humic acid. Results showed that among the fungal species, Thermoascus aurantiacus exhibited the most prominent impact in expediting maturation and improving compost quality, and Bacillus subtilis was the most abundant bacterial species based on metagenomics analysis. The highest temperature, C/N ratio reduction, and amount of humic acid production (Respectively in lab 61 °C, 54.67%, 298 g kg-1 and in pilot level 65 °C, 72.11%, 310 g kg-1) were related to treatments containing these microorganisms and other additives except urea. Consequently, T. aurantiacus and B. subtilis can be employed on an industrial scale as compost additives to further elevate quality. Functional analysis showed that the bacterial enzymes in the treatments had the highest metabolic activities, including carbohydrate and amino acid metabolism compared to the control. The maximum enzymatic activities were in the thermophilic phase in treatments which were significantly higher than that in the control. The research emphasizes the importance of identifying and incorporating enzymatically active strains that are suitable for temperature conditions, alongside the native strains in decomposing materials. This strategy significantly improves the composting process and yields high-quality humic acid during the thermophilic phase.

Greenhouse gas emission characteristics and influencing factors of agricultural waste composting process: A review

China, being a major agricultural nation, employs aerobic composting as an efficient approach to handle agricultural solid waste. Nevertheless, the composting process is often accompanied by greenhouse gas emissions, which are known contributors to global warming. Therefore, it is urgent to control the formation and emission of greenhouse gases from composting. This study provides a comprehensive analysis of the mechanisms underlying the production of nitrous oxide, methane, and carbon dioxide during the composting process of agricultural wastes. Additionally, it proposes an overview of the variables that affect greenhouse gas emissions, including the types of agricultural wastes (straw, livestock manure), the specifications for compost (pile size, aeration). The key factors of greenhouse gas emissions during composting process like physicochemical parameters, additives, and specific composting techniques (reuse of mature compost products, ultra-high-temperature composting, and electric-field-assisted composting) are summarized. Finally, it suggests directions and perspectives for future research. This study establishes a theoretical foundation for achieving carbon neutrality and promoting environmentally-friendly composting practices.

Effects of dicyandiamide, phosphogypsum and superphosphate on greenhouse gas emissions during pig manure composting

This study investigated the effects of dicyandiamide, phosphogypsum and superphosphate on greenhouse gas emissions and compost maturity during pig manure composting. The results indicated that the addition of dicyandiamide and phosphorus additives had no negative effect on organic matter degradation, and could improve the compost maturity. Adding dicyandiamide alone reduced the emissions of ammonia (NH₃), methane (CH₄) and nitrous oxide (N₂O) by 9.37 %, 9.60 % and 31.79 %, respectively, which was attributed that dicyandiamide effectively inhibited nitrification to reduce the formation of N₂O. Dicyandiamide combined with phosphogypsum or superphosphate could enhance mitigation of the total greenhouse gas (29.55 %-37.46 %) and NH₃ emission (18.28 %-21.48 %), which was mainly due to lower pH value and phosphoric acid composition. The combination of dicyandiamide and phosphogypsum exhibited the most pronounced emission reduction effect, simultaneously decreasing the NH₃, CH₄ and N₂O emissions by 18.28 %, 38.58 % and 36.14 %, respectively. The temperature and C/N content of the compost were significantly positively correlated with greenhouse gas emissions.

Carbon and N conservation during composting: A review

Composting, as a conventional solid waste treatment method, plays an essential role in carbon and nitrogen conservation, thereby reducing the loss of nutrients and energy. However, some carbon- and nitrogen-containing gases are inevitably released during the process of composting due to the different operating conditions, resulting in carbon and nitrogen losses. To overcome this obstacle, many researchers have been trying to optimize the adjustment parameters and add some amendments (i.e., pHysical amendments, chemical amendments and microbial amendments) to reduce the losses and enhance carbon and nitrogen conservation. However, investigation regarding mechanisms for the conservation of carbon and nitrogen are limited. Therefore, this review summarizes the studies on physical amendments, chemical amendments and microbial amendments and proposes underlying mechanisms for the enhancement of carbon and nitrogen conservation: adsorption or conversion, and also evaluates their contribution to the mitigation of the greenhouse effect, providing a theoretical basis for subsequent composting-related researchers to better improve carbon and nitrogen conservation measures. This paper also suggests that: assessing the contribution of composting as a process to global greenhouse gas mitigation requires a complete life cycle evaluation of composting. The current lack of compost clinker impact on carbon and nitrogen sequestration capacity of the application site needs to be explored by more research workers.

Zeolite application increases grain yield and mitigates greenhouse gas emissions under alternate wetting and drying rice system

Clinoptilolite zeolite (Z) has been widely used for reducing nutrient loss and improving crop productivity. However, the impacts of zeolite addition on CH₄ and N₂O emissions in rice fields under various irrigation regimes are still unclear. Therefore, a three-year field experiment using a split-plot design evaluated the effects of zeolite addition and irrigation regimes on greenhouse gas (GHG) emissions, grain yield, water productivity and net ecosystem economic profit (NEEP) in a paddy field. The field experiment included two irrigation regimes (CF: continuous flooding irrigation; AWD: alternate wetting and drying irrigation) as the main plots, and three zeolite additions (0, 5 and 10 t ha^-1) as the subplots. The results indicated that AWD regime decreased seasonal cumulative CH₄ emissions by 54%-71% while increasing seasonal cumulative N₂O emissions by 14%-353% across the three years, compared with CF regime. Consequently, the yield-scaled global warming potential under AWD regime decreased by 10%-60% while grain yield, water productivity and NEEP improving by 4.9%-7.9%, 19%-27% and 12%-14%, respectively, related to CF regime. Furthermore, 5 t ha^-1 zeolite addition mitigated seasonal cumulative CH₄ emissions by an average of 36%, but did not significantly affect N₂O emissions compared with non-zeolite treatment. In addition, zeolite addition at 5 and 10 t ha^-1 significantly increased grain yield, water productivity and NEEP by 11%-21%, 13%-20% and 13%-24%, respectively, related to non-zeolite treatment across the three years. Therefore, zeolite addition at 5 t ha^-1 coupled with AWD regime could be an eco-economic strategy to mitigate GHG emissions and water use while producing optimal grain yield with high NEEP in rice fields.

Effects and microbial mechanisms of phosphogypsum and medical stone on organic matter degradation and methane emissions during swine manure composting

The degradation of organic matter (OM) and CH₄ emissions during composting greatly influence the composting efficiency and greenhouse effect. This study evaluated the effects of adding phosphogypsum (PPG) and medical stone (MS) on OM breakdown, CH₄ emissions, and their underlying mechanisms. MS accelerated the breakdown of OM in the early composting stage, whereas PPG increased it in the cooling and maturation periods. At the ending of composting, humification was also significantly promoted by PPG and MS (P < 0.05). Moreover, MS and PPG reduced CH₄ emissions by 27.64% and 23.12%, respectively, and significantly inhibited the activities of methanogens in terms of their abundance (mcrA) and composition (dominant genera such as Methanobrevibacter, Methanocorpusculum, and Methanothermus) (P < 0.05). Interestingly, MS enhanced the activity of enzymes and bacterial metabolism related to OM degradation in the early composting stage, whereas PPG promoted them during the cooling and maturity stages. MS and PPG inhibited the activities of enzymes related to CH₄ release during the cooling and maturity stages. Therefore, PPG and MS may have influenced OM degradation and CH₄ releases during composting via changes in bacterial metabolism and enzyme activity levels. PPG and MS could have altered the activities of methanogens to influence the transformation of carbon and CH₄ emissions according to network analysis and partial least-squares path modeling analysis. These findings provide insights at the molecular level into the effects of adding PPG and MS on OM degradation and CH₄ emissions during composting, thereby facilitating the application of PPG and MS in composting systems.

Decreased Methane Emissions Associated with Methanogenic and Methanotrophic Communities in a Pig Manure Windrow Composting System under Calcium Superphosphate Amendment

With the rapid growth of livestock breeding, manure composting has evolved to be an important source of atmospheric methane (CH4) which accelerates global warming. Calcium superphosphate (CaSSP), as a commonly used fertilizer, was proposed to be effective in reducing CH4 emissions from manure composting, but the intrinsic biological mechanism remains unknown. Methanogens and methanotrophs both play a key role in mediating CH4 fluxes, therefore we hypothesized that the CaSSP-mediated reduction in CH4 emissions was attributed to the shift of methanogens and methanotrophs, which was regulated by physicochemical parameter changes. To test this hypothesis, a 60-day pig manure windrow composting experiment was conducted to investigate the response of CH4 emissions to CaSSP amendment, with a close linkage to methanogenic and methanotrophic communities. Results showed that CaSSP amendment significantly reduced CH4 emissions by 49.5% compared with the control over the whole composting period. The decreased mcrA gene (encodes the α-subunit of methyl-coenzyme M reductase) abundance in response to CaSSP amendment suggested that the CH4 emissions were reduced primarily due to the suppressed microbial CH4 production. Illumina MiSeq sequencing analysis showed that the overall distribution pattern of methanogenic and methanotrophic communities were significantly affected by CaSSP amendment. Particularly, the relative abundance of Methanosarcina that is known to be a dominant group for CH4 production, significantly decreased by up to 25.3% accompanied with CaSSP addition. Only Type I methanotrophs was detected in our study and Methylocaldum was the dominant methanotrophs in this composting system; in detail, CaSSP amendment increased the relative abundance of OTUs belong to Methylocaldum and Methylobacter. Moreover, the increased SO42- concentration and decreased pH acted as two key factors influencing the methanogenic and methanotrophic composition, with the former has a negative effect on methanogenesis growth and can later promote CH4 oxidation at a low level. This study deepens our understanding of the interaction between abiotic factors, function microbiota and greenhouse gas (GHG) emissions, as well as provides implication for practically reducing composting GHG emissions.

Carbon footprint of maize production in tropical/subtropical region: a case study of Southwest China

Maize production is critical in tropical/subtropical regions, especially in developing countries where maize is a staple food. However, its environmental costs remain unclear. Southwest China is a tropical/subtropical region with large-scale maize production in each of its sub-regions. In the present study, we used Southwest China as a case study to evaluate the greenhouse gas (GHG) emissions and carbon footprint (CF) of maize production during 1996-2015 using life cycle assessment to identify the driving factors behind the GHG emissions and CF and to propose potential mitigation strategies. The mean GHG emissions of maize production per year during 1996-2015 was 4132 kg CO2-eq·ha-1, and the CF during this period was 961 kg CO2-eq·Mg-1. The GHG emissions and CF in Southwest China were 2-4 times higher than those of other major maize-producing regions worldwide. The GHG emissions and CF were both significantly correlated with the N surplus. The N surplus was also linearly correlated with annual precipitation, annual temperature and growing degree days, but not significantly related with soil pH. Scenario testing showed that the CF of maize production in Southwest China could be reduced by 41%, i.e. to 437 kg CO2-eq·Mg-1, by farmers adopting a comprehensive strategy including recommended fertiliser application rates, innovative fertilisers, and crop management to decrease GHG emissions and achieve the yield potential in the region. Integrated soil and crop management is essential for sustainable maize production in tropical/subtropical regions with complex and changeable ecological conditions, especially in developing countries where maize is a staple food.

Corn cobs efficiently reduced ammonia volatilization and improved nutrient value of stored dairy effluents

Dairy farms produce considerable quantities of nutrient-rich effluent, which is generally stored before use as a soil amendment. Unfortunately, a portion of the dairy effluent N can be lost through volatilization during open pond storage to the atmosphere. Adding of covering materials to effluent during storage could increase contact with NH₄⁺ and modify effluent pH, thereby reducing NH₃ volatilization and retaining the effluent N as fertilizer for crop application. Here the mitigation effect of cover materials on ammonia (NH₃) volatilization from open stored effluents was measured. A pilot-scale study was conducted using effluent collected at the Youran Dairy Farm Company Limited, Luhe County, Jiangsu, China, from 15 June to 15 August 2019. The study included seven treatments: control without amendment (Control), 30-mm × 25-mm corn cob pieces (CC), light expanded clay aggregate - LECA (CP), lactic acid (LA) and lactic acid plus CC (CCL), CP (CPL) or 20-mm plastic balls (PBL). The NH₃ emission from the Control treatment was 120.1 g N m^-2, which was increased by 38.1% in the CP treatment, possibly due to increased effluent pH. The application of CC reduced NH₃ loss by 69.2%, compared with the Control, possibly due to high physical resistance, adsorption of NH₄⁺ and effluent pH reduction. The lactic acid amendment alone and in combination with other materials also reduced NH₃ volatilization by 27.4% and 31.0-46.7%, respectively. After 62 days of storage, effluent N conserved in the CC and CCL treatments were 21.0% and 22.0% higher than that in the Control (P < 0.05). Our results suggest that application of corn cob pieces, alone or in combination with lactic acid, as effluent cover could effectively mitigate NH₃ volatilization and retain N, thereby enhancing the fertilizer value of the stored dairy effluent and co-applied as a soil amendment after two months open storage.

Effects of additives on nitrogen transformation and greenhouse gases emission of co-composting for deer manure and corn straw

Compost can realize the recycling of organic waste. However, it also emits NH₃ and greenhouse gases (GHGs) to the environment, which leads to nitrogen loss and global warming. Adding additives to compost can alleviate the emission of NH₃ and GHGs. The mechanism of nitrogen transformation and GHGs emission was studied with deer manure and corn straw as compost substrate, and biochar and zeolite as additives. The results showed that the addition of zeolite in compost is good for prolonging high-temperature composting time. The addition of zeolite reduced the transformation of NH₃-N and the N₂O emission. The addition of zeolite is beneficial to reduce nitrogen loss during composting. CH₄ emission is an important factor affecting the global warming potential of composting, and it is necessary to improve ventilation conditions in order to alleviate anaerobic. This study is of great significance to reduce nitrogen loss and improve composting effect.

The influence on carbon, nitrogen recycling, and greenhouse gas emissions under different C/N ratios by black soldier fly

Currently, sustainable utilization, including recycling and valorization, is becoming increasingly popular in waste management. Black soldier fly larvae (BSFL) can convert the carbon (C) and nitrogen (N) from organic waste into biomass and improve properties of the substrate to reduce greenhouse gas and NH₃ emissions. In this study, the recycling of C and N and the emissions of greenhouse gas and NH₃ during BSFL bio-treatment of mixtures of pig manure and corncob were investigated under different C/N ratios. The results indicated that initial C/N ratios of feedstock are a crucial parameter affecting the biomass generation of larvae. The BSFL recycled approximately 4.17-6.61% of C and 17.45-23.73% of N from raw materials under different C/N ratios. Cumulative CO₂, CH₄, NH₃, and N₂O emissions at the different C/N ratios ranging from 15 to 35 were 107.92-151.68, 0.08-0.76, 0.14-1.17, and 0.91-1.18 mg kg^-1, respectively. Compared with conventional composting, BSFL treatment could reduce the total greenhouse gas emissions by over 90%. The study showed that bio-treatment of mixtures of pig manure and corncob with a proper C/N ratio by BSFL could become an avenue to achieve higher nutrient recycling, which is an eco-friendly process.