Prediction models for bioavailability of Cu and Zn during composting: Insights into machine learning.

Bioavailability assessment of heavy metals in compost products is crucial for evaluating associated environmental risks. However, existing experimental methods are time-consuming and inefficient. The machine learning (ML) method has demonstrated excellent performance in predicting heavy metal fractions. In this study, based on the conventional physicochemical properties of 260 compost samples, including compost time, temperature, electrical conductivity (EC), pH, organic matter (OM), total phosphorus (TP), total nitrogen, and total heavy metal contents, back propagation neural network, gradient boosting regression, and random forest (RF) models were used to predict the dynamic changes in bioavailable fractions of Cu and Zn during composting. All three models could be used for effective prediction of the variation trend in bioavailable fractions of Cu and Zn; the RF model showed the best prediction performance, with the prediction level higher than that reported in related studies. Although the key factors affecting changes among fractions were different, OM, EC, and TP were important for the accurate prediction of bioavailable fractions of Cu and Zn. This study provides simple and efficient ML models for predicting bioavailable fractions of Cu and Zn during composting, and offers a rapid evaluation method for the safe application of compost products.

Greenhouse gas emission during swine manure aerobic composting: Insight from the dissolved organic matter associated microbial community succession.

Greenhouse gas emissions during aerobic composting is unavoidable, but good practices can minimize emission. Therefore, to explore the key factors influencing the release of greenhouse gas emissions during composting, the inaction of organic matter conversion, greenhouse gas emissions and bacterial community structure during co-composting with different ratio (pig manure and corn straw) over a 6-week period was studied. The excitation-emission matrix fluorescence spectroscopy with the parallel factor was used to identify that dissolved organic matter associated microbial community succession mainly influenced greenhouse gas emissions. Protein-like fractions of dissolved organic matter were more likely to decompose and promote CH4 and CO2 emissions, while the humic-like fractions of dissolved organic matter positively affected N2O emissions. The largest of greenhouse gas emissions was appeared in MR2 with 12.7 kg CO2-eq, and the MR3 and MR4 reduced greenhouse gas emissions by 26.8 % and 11.4 %, respectively.

Dissolved organic carbon, a critical factor to increase the bioavailability of phosphorus during biochar-amended aerobic composting.

Considerable research efforts have been devoted to increase phosphorus (P) availability during aerobic composting. However, there is little discussion weather the dissolved organic carbon (DOC) controls the transformation among P-fractions. Thus, we investigated the changes in DOC compositions and P-fractions during biochar-amended composting (wet weight basis, 5% and 10%). TP content continuously increased since the 'concentration effect' during aerobic composting. NaHCO3-Pi, NaOH-Pi and HCl-Pi were main P-fractions, and biochar can improve P-bioavailability by transforming NaOH-Pi and HCl-Pi into NaHCO3-Pi. Structure equation models (SEMs) indicated that biochar enhanced the P-bioavailability through regulating the decomposition of DOC. Our results at least hint that the activation mechanism on P under the influence of DOC during biochar-amended composting.

Binding characteristics of humic substances with Cu and Zn in response to inorganic mineral additives during swine manure composting.

Composting is suitable for recycling livestock manure into valuable organic fertilizer, which can improve soil quality while mitigating potential risk of heavy metal pollution. Humic substances (HS) in compost have been demonstrated to play a key role in regulating the redistribution of heavy metal fractions. However, limited direct information have been reported on how different components of HS complexes with heavy metals to affect their bioavailability during composting. In this study, sequential extraction procedures (H2O, KCl, Na4P2O7, NaOH and HNO3) were used to assess the characteristics that HS bound with Cu and Zn during composting of swine manure and straw added either 5% boron waste (BW) or 5% phosphate rock (PR). Organically complexed fraction extracted by Na4P2O7 contained only 33-41% of the Cu but most of the Zn (81-87%). During composting, initially mobile fractions of Cu and Zn (extracted by H2O or KCl) changed into more stable fractions (extracted by NaOH and HNO3), and both organic matter and fulvic acids (FA) were identified as critical factors to explain this redistribution based on redundancy analysis. Over 80% of Cu and Zn were complexed with FA of HS. However, exogenous additives (phosphate rock and boron waste) enhanced Cu conversion by promoting humification (Humic acid/Fulvic acids, HA/FA) whereas they had limited influence on Zn, due to the relatively weak binding relationship between Zn and HA.

Tetracycline hydrochloride-stressed succession in microbial communities during aerobic composting: Insights into bacterial and fungal structures.

Available information that whether antibiotics affect the succession in microbial communities during aerobic composting remains limited. Thus, this work investigated the dynamic changes in bacterial and fungal structures during aerobic composting amended with tetracycline hydrochloride (TCH: 0, 50, 150 and 300 mg kg-1). Composting phases significantly affected bacterial and fungal communities, but only fungi strongly responded to antibiotics, while bacteria did not. Firmicutes, Proteobacteria, Bacteroidota and Actinobacteriota were primary bacterial phylum. Neocallimastigomycota was dominant fungal phylum at temperature-heating phase, then Basidiomycota and Ascomycota became main fungal phylum at thermophilic and temperature-colling phases. Low TCH concentration promoted Chytridiomycota growth, while high TCH concentration inhibited mostly fungal activity in TCH-amended composting. Nitrogen species were critical factors controlling the succession in bacterial and fungal communities during composting process. These results cast a new light on understanding about microbial function during aerobic composting.

Additive grain-size: An innovative perspective to investigate the transformation among heavy metal and phosphorus fractions during aerobic composting.

Considerable researches have been devoted to ascertain the transformation among heavy metal (HM) or phosphorus (P) fractions during aerobic composting. However, available information that additives with different grain-sizes regulate the activation mechanism on P through influencing the passivation effect on HMs remains limited. Thus, this work aimed to investigate the dynamic changes in HM-fractions and P-fractions, and ascertain the interaction pathway between HMs and P during aerobic composting amended with medical stone (Coarse medical stone, 3-5 mm; Fine medical stone, < 0.1 mm). Medical stone, especially for coarse-grained medical stone, significantly enhanced the HM-passivation and P-activation during the composting (P < 0.05). The bioavailability factor of HMs decreased by 48.05% (Cu), 20.65% (Pb), 15.58% (Cd) and 6.10% (Zn), and the content of labile available P (LAP) increased by 6.45%. HMs, with the explanatory capacity of 65.9%-84.9%, was important parameter superior to temperature (0.8%-5.4%), moisture content (MC, 0.1%-1.7%), pH (0.1%-8.7%), electric conductivity (EC, 0.8%-9.8%), carbon-to-nitrogen (C:N, 0.3%-2.3%) ratio and dissolved organic carbon (DOC, 0.4%-3.1%), to evaluate the transformation among P-fractions. Our results cast a new light on P-activation with respect to HM-passivation during aerobic composting.

Critical passivation mechanisms on heavy metals during aerobic composting with different grain-size zeolite.

Available information about the passivation effect on heavy metals (HMs) through adsorption and humification during zeolite-amended composting remains limited. Thus, this study explored the dynamic changes in HM-fractions (Zn, Cu, Cd, Cr and Pb) during aerobic composting added with different grain-size zeolite (Fine zeolite, < 0.1 mm, ZF; Coarse zeolite: 3-5 mm, ZC). Compared to the control (without zeolite, CK) and ZF treatments, ZC treatment got the highest temperature in the thermophilic phase, and significantly reduced the bioavailability factor (BF) of HMs, especially for Cu (45.13%), Cd (16.11%) and Pb (25.49%). Redundancy analysis (RDA) and structural equation models (SEMs) indicated that zeolite accelerated the passivation effect on Cd and Pb through regulating the electrical conductivity (EC) as a result of surface adsorption, and on Cu by influencing total carbon (TC) under the function of humification. These results increase our understanding of the passivation mechanisms of HMs during aerobic composting.