3.1. The Effect of Hydrochar on Soil Properties and Crop Growth
The imposition of both duck farming biomass wastes showed improvements in the electrical conductivity (EC) of the topsoil compared to CF, with increases ranging from 16% to 44%, as shown in
Table 2. Compared to the direct application of duck manure and litter (FM), the EC values were generally higher when hydrochar was applied, except in cases where hydrochar without duck manure had lower EC values. Among the treatments, the FH2 treatment showed the most significant increase. Although it has been reported that an increase in EC may lead to higher salt content in crops [
10], usually it does not inhibit crop growth in a shorter cultivation period, and the more severe the soil salinisation, the higher the pH, whereas in the present study, no change in soil pH was observed in the treatment groups. Due to the nutrient retention and slow-release properties of biomass charcoal, it can be effectively used to fertilise and manage crops with long growth cycles [
11]. Compared to CF and FM, the application of hydrochar increased the levels of total nitrogen, available phosphorus and exchangeable potassium in the soil. The gradient of total nitrogen, available phosphorus and exchangeable potassium content across the different hydrochar treatments followed the order FH1 < FH3 < FH2, indicating that the use of char-based fertilisers can improve soil nutrient availability compared to conventional fertilisers or the direct application of duck manure and litter.
The different treatments showed a consistent trend in biomass accumulation in both maize leaves and stems, as shown in
Figure 1a, with the order CF < FH1 < FM < FH2 < FH3. Visual differences were also observed in the early growth stages of the maize plants (
Figure 1b). Compared to CF and FM, the application of hydrochar as a charcoal-based fertiliser increased maize biomass, except in cases where hydrochar without duck manure components did not show a significant promoting effect on maize leaf and stem biomass. Research by Khosravi et al. [
12] indicated that the application of sewage sludge-derived hydrochar had minimal effect on soybean growth, while chicken manure-derived hydrochar significantly increased soybean yield by up to 66.2%. Therefore, the type of feedstock used for hydrochar production significantly influences crop productivity when used as a charcoal-based fertiliser, and selecting the appropriate type of hydrochar can effectively serve as a soil amendment to promote crop growth.
Observations of maize seedling roots revealed a positive interactive effect between hydrochar and plant root systems. Significant differences in maize root biomass between treatment groups were observed at the 0.05 significance level (F = 3.536,
p = 0.048). Specific contrasts indicated that FH2 > CF, FH3 > CF, FH2 > FM and FH3 > FM, highlighting the significant promoting effect of duck manure–litter-derived hydrochar on maize root growth, especially when the hydrochar contained duck manure, litter and humic acid components. From a nutrient perspective, although the direct application of duck manure and litter to the soil increased nitrogen and phosphorus nutrient levels, dynamic nutrient uptake should also be considered. Research has shown that straw biochar application can increase both total inorganic and organic phosphorus in the soil, whereas direct straw application only increases total organic phosphorus [
13]. In addition, the direct application of organic matter may not sustainably increase soil nitrogen supply capacity in the long term. This may explain why the enhancing effect of the direct application of duck manure and litter on maize biomass is lower than the effect of using duck manure and litter carbonisation as a charcoal-based fertiliser.
3.2. Effects of Hydrochar on Soil Carbon Dynamics and Carbon Composition
The addition of hydrochar can alter soil carbon quality by changing organic carbon content and aromaticity. The incorporation of hydrochar into the soil has shown a significant effect in increasing soil organic carbon (SOC) content (
Figure 2a). Due to the high organic matter content of biomass waste from duck farming, when mixed with fertiliser in the soil, it promotes plant growth and root exudation, thereby increasing soil carbon inputs [
14]. Compared to fertiliser-only treatments, increases in soil organic carbon ranged from 78% to 253%. The direct application of duck manure and litter has limited potential to increase soil organic carbon compared to its carbonisated application. Over time, the unstable carbon components in duck manure and litter are gradually consumed and absorbed by microorganisms, promoting soil respiration and leading to a decrease in the rate of SOC increase [
15]. Furthermore, the increase in organic carbon with different hydrochar treatments follows the order FH2 < FH1 < FH3, which is consistent with the different organic matter content in the different types of hydrochar used. Hydrochar with a stable internal structure can increase soil SOC content, reduce the SOC mineralisation rate and increase stability against surface oxidation and microbial degradation. This effect is attributed to organic molecules that enhance adsorption and polymerisation to form stable organic matter through surface catalytic activity [
16]. In rice cultivation, Sun et al. [
17] found that hydrochar reduced the proportion of unstable SOC in the soil by 33.6% to 15.6% while increasing the proportion of stable SOC by 10.3% to 27.0%. This is due to the interaction and association of hydrochar with soil organic matter components of different decomposition stabilities, which also affect the effectiveness of carbon sequestration in the soil [
18]. Further understanding of the molecular composition of organic matter following the addition of hydrochar to soil is therefore required.
Dissolved organic matter (DOM) is a major component of soil organic matter and the most active carbon fraction in soil. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) provides the highest resolution and mass accuracy to characterise the complex molecular composition of DOM, which was further used in the present study to investigate organic carbon-related composition and structure after the addition of hydrochar derived from duck farm biomass waste to the soil. As shown in
Figure 2b, the molecular composition of DOM is dominated by CHON-type molecular compounds, mainly distributed in regions containing lignin, carbohydrates and condensed aromatic structures, providing a good source of carbon and nitrogen for the soil. Sulphur-containing organic carbon compounds (CHOS-type) are mainly present in the soil as minor components of aliphatic/protein and unsaturated hydrocarbon compounds. Research by Ling et al. [
19] indicates that Gemmatimonadetes, Bacteroidia, Alphaproteobacteria and Gammaproteobacteria are the dominant species in biomass char-amended soils, capable of utilising recalcitrant condensed aromatic and tannin compounds in DOM, while
Ramlibacter are primarily involved in lignin utilisation. Bacilli and Clostridia mainly utilise easily degradable polysaccharides, amino acids and other DOM components. Therefore, the diverse composition of DOM after the addition of hydrochar derived from duck farm biomass waste can provide different soil microbial communities with the necessary materials to promote soil organic carbon transformation.
3.3. Effect of Hydrochar on the Particle Size Distribution of Soil Functional Groups and Agglomerates
The stimulatory effect of biomass char on soil carbon mineralisation is another important factor influencing soil carbon storage and sequestration functions. The potential inhibitory effect of hydrochar on soil organic carbon may involve mechanisms such as the adsorption of organic matter, organic–mineral interactions and aggregate formation [
7], which are related to changes in functional groups and the distribution of soil aggregates.
The FTIR spectra of the soil under different treatments are shown in
Figure 3. Peaks in the range of 3300–3600 cm
−1, attributed to -OH stretching vibrations, are commonly found in carbohydrates, proteins and amide compounds. It can be observed that the peak in the FM treatment, where duck manure and litter was applied directly to the soil, overlaps with the CF treatment without significant differences. In contrast to the CF and FM treatments, the addition of hydrochar derived from duck manure significantly increases the peak intensity in this region. However, the peak intensity in the soil treated with hydrochar derived only from litter shows a significant decrease, indicating that the organic matter in such hydrochar is easily degradable, leading to a reduction in hydroxyl groups. The same trend among the treatment groups extends to the peak at 2920 cm
−1, attributed to symmetrical stretching vibrations of O-H, alkyl C-H and ammonium N-H stretching vibrations, indicating an increase in hydroxyl and amine groups in the soil with the addition of hydrochar derived from duck manure. Similarly, peaks in the 1600–1700 cm
−1 range show consistent trends in intensity across treatments, mainly due to the symmetric stretching of C=C in aromatic hydrocarbons and alkenes and the asymmetric stretching of C=O in amides, suggesting that duck manure-derived hydrochar enhances the aromatic structure of the soil by increasing carboxyl and aldehyde groups. The characteristic peak representing aromatic groups at 1458 cm
−1 is also present in the soil of all treatment groups. Unlike the previous peaks, the direct application of duck manure and litter also increases the intensity of the peak at this point, possibly due to the promotion of soil humification by the application of duck manure and litter. The transmission peak at 1100 cm
−1 is attributed to polysaccharides with C-O-C oxygen functional groups and the C-peak of aromatic C-H. Compared to the CF control group, both the FM group, in which the duck manure and litter was applied directly to the soil, and the FH1 group, in which hydrochar derived from the litter was added to the soil, show enhanced peak intensities at this point, with the FH3 treatment group showing the most significant enhancement. The peak at 750 cm
−1, attributed to aromatic C-H stretching vibrations, also indicates a higher carbon content in the soil due to the addition of duck manure-derived hydrochar. Overall, the addition of hydrochar derived from litter alone does not increase the abundance of functional groups in the soil, whereas hydrochar incorporating duck manure significantly enhances the aromatic structure and oxygen functional groups in the soil, especially when both humic acid- and duck manure-derived hydrochar are present as soil amendments.
Soil aggregates are fundamental components of soil structure. Due to the absence of sticky duck manure components and the lower nutrient content, both the CF and FH1 treatments exhibit a sandy loam texture with an extreme distribution of aggregate particle sizes, consisting predominantly of microaggregates (
Figure 4). Compared to the FM group, where the duck manure and litter was applied directly to the soil, the addition of hydrochar facilitated the formation of more macroaggregates in the soil, with the proportion of aggregates in the 0.02–2.0 mm particle size range showing the trend FH3 > FH1 > FH2 (
Appendix A), consistent with the trends in soil organic carbon content across the groups. In general, an increase in soil aggregate particle size leads to an increase in soil organic carbon, and the increase in organic carbon content is beneficial for improving the soil aggregate structure [
20], suggesting an interdependent relationship. On the one hand, organic carbon itself acts as a binding agent that can bind microaggregates into macroaggregates. On the other hand, the addition of hydrochar can indirectly promote the formation of soil humus, carbon compounds, aromatic hydrocarbons and other organic macromolecules, thereby increasing soil organic carbon [
21]. Therefore, it can be inferred that the humic acid macromolecules present in the FH3 group may have contributed to some extent to the formation of macroaggregates. Furthermore, the introduction of hydrochar enriches the soil with more functional groups, thereby increasing the stability of soil aggregates. Functional groups on the surface of hydrochar, such as hydroxyl (-OH) and carboxyl (-COOH) groups, can induce cation–bridging interactions, which are primarily responsible for the formation of macroaggregates in soil [
22].
3.4. Effect of Hydrochar on the Distribution and Composition of Soil Microorganisms
The addition of hydrochar provides a degradable organic carbon source for soil microbes, which regulates microbial growth by enhancing substrate effectiveness, thereby altering soil microbial diversity [
7]. In previous studies, the improvement in soil aggregation may be attributed to the addition of hydrochar, which enhances various organic substances produced by soil bacteria, fungi and plant roots, such as organic acids and lipids [
23]. Further microbial sequencing analysis was therefore carried out. The coverage of soil samples with and without a hydrochar addition reached 100%, indicating that the 16S and ITS gene sequences were sufficient to accurately describe the diversity and richness of the bacterial and fungal microbial communities in the soil. The Chao1 and ACE (observed species) indices were primarily used to describe the quantity of microbial communities, with higher values, indicating greater species richness, while the Simpson and Shannon indices reflect the diversity and evenness of microbial community distribution. The specific assessment indices are shown in
Table 3. For bacteria, the Simpson and Shannon indices were slightly lower in soils with added hydrochar than in soils without hydrochar, but the difference was not significant. However, for Chao1 and ACE, the values in soils with hydrochar were significantly higher than those without hydrochar, showing a clear advantage. Bacteria play a crucial role in the formation of soil macroaggregates and microaggregates, where cyanobacteria, as part of the surface soil biological crust community, can promote soil stability by producing EPS that act as aggregate binders [
24]. In contrast, for fungi, all four indices were lower in the FH3 treatment group with added hydrochar than in the control soil. These results suggest that while hydrochar has a positive effect on bacterial abundance, it has a negative effect on bacterial diversity and both fungal abundance and diversity. This is consistent with reports on the effects of biomass char on soil bacterial and fungal diversity, such as the negative effect of hydrochar on AM fungi reported by [
23], and the findings of Hu et al. [
25] that soil bacterial diversity was higher in biochar amended soils, while biochar reduced soil fungal diversity.
A redundancy analysis (RDA) can better show the influence of one set of variables on another set of variables and then find the main factors of soil environment affecting soil microorganisms. As shown in
Figure 5a, axis 1 explains 94.12% of the correlation between the 5 variables and bacterial communities; SOC explained 93.7% of the variation and was followed by the aggregate explaining 5.5% of the variation. According to RDA, SOC was the main factor affecting the relative abundance of soil bacteria. Further, observing that soil aggregates, the microbial community and organic carbon are more closely linked in RDA compared to other factors, it can be inferred that the application of hydrochar affects soil organic carbon by altering soil aggregates and the microbial bacterial community.
Appendix B and
Appendix C show the abundance clustering dendrograms of operational taxonomic units (OTUs) for bacteria and fungi in soils with and without a hydrochar addition. Consistent with the microbial abundance data, the bacterial abundance values in the tested soil samples were much higher than the fungal abundance values, and the bacterial OTU dendrogram branches were denser than those of the fungi. In most branches of the bacterial clustering, both soils with and without added hydrochar shared common branches. Bacteria in hydrochar-amended soil were more concentrated in the terminal branches, whereas the proportion of bacteria in the terminal branches of soil without hydrochar was lower, confirming the role of hydrochar in increasing bacterial abundance in the soil. In contrast to bacteria, the out abundance clustering dendrograms for fungi in soil with and without hydrochar showed significant differences. Both treatments had distinct branches that participated separately, especially in the secondary branches, where soil without hydrochar had unique branches. In the tertiary branches with four forks, the fungal abundance in soils with added hydrochar disappeared in two of the branches. This suggests that the addition of hydrochar to the soil affects both bacteria and fungi, with fungi being more affected by hydrochar. Using Venn diagrams to compare the number of shared or unique OTUs between soil samples with and without hydrochar (
Figure 5b), it was found that there were significant differences in the community abundance of bacteria and fungi between soils with and without hydrochar, with only 13.7% and 13.28% of OTUs shared for bacteria and fungi, respectively.
In addition to changes in the abundance and diversity of bacteria and fungi in the soil, the addition of hydrochar also leads to changes in the composition of the microbial community structure.
Figure 6 shows the relative abundance of bacterial and fungal community composition at the phylum and genus level in soils with and without a hydrochar addition. Looking at the bacterial phylum classification, the soil microbial community composition is mainly dominated by the phyla Proteobacteria, Actinobacteriota, Bacteroidota and Chloroflexi, with relatively high abundance. The addition of hydrochar to the soil had the most significant effect on the relative abundance of the phylum Actinobacteriota, which increased from 23.6% to 36.9%. In addition, the relative abundance of the phylum Bacteroidota increased by 1.6%, while the phylum Chloroflexi was compressed, reducing its relative abundance to 1.1%. This is consistent with the results of Sun et al. [
17], where the phylum Chloroflexi was significantly reduced in soil after a hydrochar addition (
p < 0.05). Overall, the addition of hydrochar to the soil resulted in relatively uniform changes in the abundance distribution of bacterial phyla, although the relative abundances tended to become more uneven.
Figure 6b further illustrates the changes in bacterial composition at the genus level. At the genus level, the addition of hydrochar to the soil promoted a more even distribution of dominant bacterial genera, with most genera showing increased relative abundances, except for genera such as Sphingomonas and Herpetosiphon, which were adversely affected by hydrochar, leading to a decrease in their relative abundances.
In the fungal realm, the relative abundance distribution at the phylum level is quite extreme, as shown in
Figure 6c, with the phyla Ascomycota and Chytridiomycota accounting for 97.4% to 98.3% of the total phylum relative abundance. Hydrochar had a significant effect on the relative abundance distribution of these two phyla, increasing the relative abundance of Ascomycota from 85.4% to 97.2%, while decreasing the relative abundance of Chytridiomycota from 12.9% to 0.2%. Thus, the addition of hydrochar made Ascomycota the dominant phylum among soil fungi. Furthermore, at the genus level of fungal classification, as shown in
Figure 6d, it is evident that the addition of hydrochar to the soil led to a significant reduction in the diversity of fungal genera, confirming the earlier conclusion that hydrochar has a negative effect on fungal abundance and diversity in the soil. It is worth noting that due to the humic acid composition of the added hydrochar, the presence of the genus
Humicola was observed in the hydrochar-amended soil, together with significant increases in the relative abundance of genera such as
Botryotrichum and
Iodophanus.