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Fast and accurate SOM estimation and spatial mapping are significant for cultivated land planning and management, crop growth monitoring, and soil carbon pool estimation. It is a key problem to construct a fast and efficient estimation model based on hyperspectral remote sensing image data to realize the inversion mapping of SOM in large areas. In order to solve the problem that the estimation accuracy is not high due to the influence of hyperspectral image quality and soil sample quantity during the estimation model construction, this study explored a method for constructing an estimation model of SOM contents based on a new stacking ensemble learning algorithm and hyperspectral images. Surface soil samples in Huangzhong County of Qinghai Province were collected, and their ZY1-02D hyperspectral remote sensing images were investigated. As input data, a feature band dataset was constructed using the Pearson correlation coefficient and successive projections algorithm. Based on the dataset, a new SOM estimation model under the stacking ensemble learning framework combined with heterogeneous models was developed by optimizing the combination of base and meta-learners. Finally, the spatial distribution map of SOM was plotted based on the result of the model over the study area. The result suggested that the input data quality of the estimation model is improved by constructing a feature band dataset. The multi-class ensemble learning estimation model with the combination strategy of the base and meta-learners has better predictive effects and stability than the single-algorithm and single-level ensemble models with homogeneous learners. The coefficient of determination is 0.829, the residual prediction deviation is 2.85, and the predictive set root mean square error is 1.953. The results can provide new ideas for estimating SOM content using hyperspectral images and ensemble learning algorithms, and serve as a reference for mapping large-scale SOM spatial distribution using space-borne hyperspectral images.

Returning straw and biochar to the fields can change the structure of humic acid in soil and affect the content of soil organic matter. However, returning straw and biochar to the fields in successive years also increases the insect pests of crops and causes nitrogen competition, thus increasing the soil burden. Therefore, to provide a theoretical basis for the study of the time effect of returning fields, three treatments were set in this paper to study the effects of straw and biochar returning every other year on the soil organic matter and humic acid (HA) structure. Elements, Fourier transform infrared spectroscopy (FTIR), thermogravimetric spectroscopy (TG), three-dimensional fluorescence spectroscopy, and solid state 13C-NMR spectroscopy were used to comprehensively investigate the structural characteristics of humic acid in soil. The results showed that after straw and biomass charcoal are returned to the field every other year, compared with 2018, after planting in 2020, the average content of soil organic carbon will increase by 29.49% and 36.14%, respectively. HA showed a phenomenon of increased aliphatic property and decreased aromaticity; aromatic carbon of HA decreased, while alkyl carbon and alkoxy carbon increased; the molecular structure of HA developed toward simplification. In the short term, the effect of biomass charcoal returning was more significant. This study can be used as a reference for the research on the time effect of returning straw and biomass carbon to the fields, and can also provide a basis for the research on the composition of humus after returning organic materials to the fields.

Soil pH is an essential indicator for assessing soil quality and soil health. In this study, based on the Chinese farmland soil survey dataset and meteorological dataset, the spatial distribution characteristics of soil pH in coastal eastern China were analyzed using kriging interpolation. The relationships between hydrothermal conditions and soil pH were explored using regression analysis with mean annual precipitation (MAP), mean annual temperature (MAT), the ratio of precipitation to temperature (P/T), and the product of precipitation and temperature (P*T) as the main explanatory variables. Based on this, a model that can rapidly estimate soil pH was established. The results showed that: (a) The spatial heterogeneity of soil pH in coastal eastern China was obvious, with the values gradually decreasing from north to south, ranging from 4.5 to 8.5; (b) soil pH was significantly correlated with all explanatory variables at the 0.01 level. In general, MAP was the main factor affecting soil pH (r = −0.7244), followed by P/T (r = −0.6007). In the regions with MAP < 800 mm, soil pH was negatively correlated with MAP (r = −0.4631) and P/T (r = −0.7041), respectively, and positively correlated with MAT (r = 0.6093) and P*T (r = 0.3951), respectively. In the regions with MAP > 800 mm, soil pH was negatively correlated with MAP (r = −0.6651), MAT (r = −0.5047), P/T (r = −0.3268), and P*T (r = −0.5808), respectively. (c) The estimation model of soil pH was: y = 23.4572 − 6.3930 × lgMAP + 0.1312 × MAT. It has been verified to have a high accuracy (r = 0.7743, p < 0.01). The mean error, the mean absolute error, and the root mean square error were 0.0450, 0.5300, and 0.7193, respectively. It provides a new path for rapid estimation of the regional soil pH, which is important for improving the management of agricultural production and slowing down soil degradation.

Long-term intensive farming has degraded the structural stability of black soil in Northeast China. This study evaluated the effects of fermentation-derived materials and fungal-derived chitin on water-stable aggregates and microbial functional potential in this soil. Four treatments were established: sterile water control (CK), uninoculated fermentation broth substrate (W), live Trichoderma asperellum fermentation broth (P), and cell-free fermentation filtrate (F). Aggregate stability was monitored during a 60-day incubation, and metagenomic sequencing was performed on the most responsive 0.5–0.25 mm dry-sieved fraction. An exogenous chitin addition experiment was also conducted to evaluate the potential contribution of fungal cell-wall-derived chitin to aggregate stabilisation. The W, P, and F treatments increased the proportion of water-stable aggregates >0.25 mm, mean weight diameter, and geometric mean diameter, while decreasing fractal dimension. Among the treatments, the uninoculated fermentation broth substrate showed the strongest effect, particularly in the 0.5–0.25 mm dry-sieved fraction. Metagenomic analysis showed that the uninoculated fermentation broth substrate altered microbial community composition, changed the relative abundances of taxa such as Sphingomonas sediminicola, Priestia megaterium, and Trichoderma asperellum, and increased the relative abundance of carbohydrate-active enzyme-related genes, including those encoding glycosyltransferases, carbohydrate esterases, and glycoside hydrolases. Chitin addition also improved aggregate stability and altered microbial community structure. These findings suggest that the uninoculated fermentation broth substrate and fungal-derived chitin improved black soil aggregate stability, potentially through shifts in microbial community composition and carbohydrate-related functional potential. This study provides a scientific basis for using fermentation-derived materials to improve the structure of degraded black soil.

The freezing winter temperatures in Northeast China hinder the breakdown speed of straw, making it challenging to implement widespread straw return in rice fields, meaning that crop remnants are not efficiently utilised. This study involved a pot experiment conducted on rice plants that were treated with varying quantities of biochar: CK (no biochar); BC1 (5 t ha−1); BC2 (10 t ha−1); and BC3 (20 t ha−1). An investigation was conducted to examine the impact of biochar on the soil organic carbon (SOC), humus (HS) composition, humic acid (HA) structure, and rice yield of paddy fields. The findings demonstrated that the use of biochar led to a substantial rise in SOC and HA-C concentrations in the soil layer between 20 and 40 cm. Additionally, biochar’s application enhanced soil humification. Notably, the treatment with BC3 (20 t ha−1) had the most pronounced impact. The O/C ratio in the HA and the relative strength of the peaks at 1620 cm−1 on the infrared spectra showed a more pronounced response to the BC3 treatment compared to the other biochar treatments. However, the application of the BC1 treatment at a rate of 5 t ha−1 and the BC2 treatment at a rate of 10 t ha−1 had a minimal impact on the fluorescence intensity of humic acid (HA). The application of the BC3 treatment increased the aromatic nature of the humic acid (HA) in paddy soil, leading to the formation of an intricate and enduring HA structure. Furthermore, the use of the BC3 treatment resulted in a notable enhancement in the quantity of spikes, spike weight, and number of grains per spike. Additionally, it positively impacted the accumulation of dry matter in the spike, leading to a substantial 13.7% increase in the rice yield. Applying biochar at a rate of 20 t ha−1 is a sensible and effective approach to enhance the soil organic carbon (SOC) content, enhance the stability of the humic acid (HA)’s structure, and raise the rice yield in the rice-growing area of Northeast China. This study’s findings will establish a theoretical foundation for utilising soil biochar in the rice fields located in Northeast China.

The majority of the studies on humification culture presently use livestock and poultry manure as the primary raw material. There is significantly less research on the changes and structural characteristics of humic acid-like compounds (HAL) formed by humification using real food waste as the raw material. This paper aims to study the changes in humic components and the structure of HAL in the humification process through the addition of biochar to pre-meal waste and post-meal swill as the primary raw materials. Kitchen waste + corn straw (KC) and kitchen waste + corn straw + biochar carbon (KCBr) were humified for 24 days, respectively, using the indoor static composting method, where the samples were collected at days 0, 12, and 24 of incubation. The HAL were analyzed using elemental analysis, infrared spectroscopy, and differential thermal techniques. The results demonstrated that KCBr first entered the high-temperature phase, which lasted for 12 days above 50 °C. The total organic carbon (TOC) demonstrated a decreasing trend in both treatments, while the total nitrogen (TN) demonstrated an upward trend. The HAL and fulvic acid-like (FAL) contents of the two treatments increased and decreased with an increase in the incubation time, respectively. The relative HAL content and humification index (PQ%) of KCBr and KC at the end of humification were 22.76% and 19.69% and 74.30%, and 73.11%, respectively. In terms of the HAL structure, the KCBr treatment demonstrated lowered condensation, reduced oxidation, decreased aliphatic, enhanced aromatization, and increased thermal stability of HAL compared with the KC treatment.

Lignocellulase production from straw fermentation has been widely investigated but the research has neglected to quantify fermentation-derived residue transformation to the humus-like substance (HULIS). To investigate the conversion efficacy of corn stalk residue to HULIS, the amount of HULIS associated with chemical composition and structural changes of humic acid-like substances (HAL) was investigated in a 30 L solid-state fermentation tank during a short period of eight days. The results show that the highest decomposition rate of corn stalk and the highest activity of cellulase, xylanase, and β-glucosidase appeared at the fourth day. At the end of fermenting process, the amount of humic acid-like substances (HAL) and the percentage of HAL in humus acid (PQ value) increased 17.5% and 8.9%, respectively, indicating Trichoderma reesei facilitates the transformation of corn stalk residue to HAL. Fatty acids decreased while aromatic carbon and carboxyl content significantly increased during the ongoing fermentation, which had a positive impact on the HAL thermal stability. The FTIR spectral and thermal analysis revealed an improvement in HAL degrees of condensation, oxidation, and aromatization. The present study suggests that the residue of corn stalks fermented with T. reesei might be a good fertilizer to improve soil characteristics.

Biochar is considered to have potential use in carbon (C) sequestration and has been widely used in soil amendment. Humic substances (HSs), assigned as the stable organic C, have obvious agronomic benefits. However, the response mechanisms of these carbonaceous substances to biochar are unclear in biochar-amended soils. In a two-year experiment, the δ13C technique was employed to trace the fate of the biochar-derived C in HSs and evaluate the effects of four treatments, including no biochar control (CK) and biochar addition at dosages of 6 t ha−1 (BC6), 12 t ha−1 (BC12), and 24 t ha−1 soil (BC24), on soil organic carbon (SOC) and HSs. Compared to CK, biochar application significantly improved total SOC contents and the C pool index, whereas the C labile index declined. Humic acid (HA) and humin were distinctly enhanced in bulk soil. Moreover, the aliphaticity was intensified in the chemical composition of HA. In particular, native HA contents substantially increased by 16.30–55.95%. Biochar-applied C of 4.08–6.43% was finitely involved in HA formation over the two years, which resulted in a genetic relationship between soil HA and biochar to some extent. The low dosages of biochar at 6 t ha−1, 12 t ha−1 and 24 t ha−1 did not obviously affect native SOC contents. Moreover, BC24 had less of an effect on HA formation compared to BC6 and BC12, but had the highest SOC. These results demonstrate that biochar application can improve SOC stocks, reduce C instability and promote HSs formation, and they suggest that determining and keeping an optimum dosage of biochar application can represent an effective strategy (i.e., not only sequestrate C, but also improve soil quality), which is beneficial to sustainability in the ecological environment and agriculture.

In our previous studies, we filtered out fungus (Trichoderma reesei) to have the best ability to transform corn straw into a humic-acid-like substance through laboratory incubation experiments. In order to further verify our former findings, we set up a 360 d field experiment that included three treatments applied under equal carbon (C) mass: (i) corn straw returned to the field (CS), (ii) fermented corn straw treated with Trichoderma reesei returned to the field (FCS-T), and (iii) blank control treatment (CK). Soil organic carbon (SOC), soil labile organic C components, soil humus composition, and the management levels of SOC pools under the three treatments were analyzed and compared. The results showed that the SOC content of CS and FCS-T treatments increased by 12.71 % and 18.81 %, respectively, compared with CK at 360 d. The humic acid carbon (HA-C) content of the FCS-T treatment was 0.77 g kg−1 higher than in the CS treatment. Applying FCS-T appeared to promote a significant increase in SOC content, carbon pool activity index, and carbon pool management index through the accumulation of HA-C, humin carbon, and easily oxidizable organic carbon. Applying fermented corn straw treated with Trichoderma reesei (FCS-T) is more valuable and conducive to increasing soil easily oxidizable organic C (EOC) and humus C content than direct application of corn straw.

The contribution of microbial residues formed on lignin and cellulose to the formation of humus (HS) was investigated. The microbial residues formed by Aspergillus niger (A. niger) in the cultures of cellulose and lignin in a fluid medium were structurally characterized by elemental analysis, differential thermal analysis (DTA), FTIR spectroscopy and CP/MAS 13C NMR spectroscopy. Compared to cellulose itself, the microbial residue from cellulose contains more aromatic compounds and N-containing compounds and fewer carbohydrates and carboxylic compounds. A. niger improved the thermal stability and aromaticity of the cellulose. However, compared with that on lignin, more N-containing compounds, carbohydrates and carboxylic acid derivatives and less aromatic material were found in the microbial residue from lignin. Regardless of whether the carbon source was cellulose or lignin, A. niger utilized the N in the fluid medium to synthesize its own cells, and eventually, they could transfer the N into the microbial residue; in addition, the O-alkyl species dominated over the alkyl and aromatic compounds in the microbial residue. Although the molecular structures of the components of the microbial residue from lignin tended to be simpler, they were more alkylated, more hydrophobic and less aliphatic than those from cellulose. During culture with A. niger, the cellulose underwent degradation and then a polymerization, which led to an increased degree of condensation but a lower degree of oxidation, providing essential precursor substances for HSs formation. However, lignin underwent oxidative degradation. The microbial residue from lignin had a lower degree of condensation and a higher degree of oxidation.