【期刊】中国农业大学商建英：生物炭施用于钙质潮土可以稳定表层和深层土壤有机碳

Subsoil (> 1 m) has a great potential to sequester SOC due to the slow carbon turnover rate. The stability of subsoil SOC is controlled by soil mineralogy and microbial processes, and the type of fresh carbon supply. The response of the dynamics and properties of subsoil SOC to the input of external carbon sources (e.g., biochar, straw, and other organic compounds) remains largely unclear. This knowledge gap has become one of the key uncertainties in understanding carbon sequestration in subsoil. Therefore, there is a great need to fully understand the capacity of deeper soil horizons to sequester SOC and the long-term effects of management practices on SOC content at these depths. This study, for the first time, provides a detailed understanding of how SOC turnover responds to long-term biochar presence in the calcareous subsoil. It has significant implications for improving agricultural practices and mitigating climate change by offering field evidence for biogeochemical processes mediated by biochar migration.

Biochar amendment significantly increased topsoil SOC content (0–20 cm) from 5.9 ± 0. 5 g kg⁻¹ to 7.9 ± 0.3 g kg⁻¹ (Fig. 1a). Although biochar amendment did not change SOC contents in the subsoil, subsoil DOC content increased by 162 % (from 0.23 g kg⁻¹ to 0.60 g kg⁻¹) in soil with versus without biochar application (Fig. 1b). The dissolved and colloidal fraction of biochar can be transported to deeper soils along soil pores and fractures due to irrigation, rainfall, and other factors.Although the biochar amendment increased DOC contents in both topsoil and subsoil after ten-year biochar addition into topsoil (Fig. 1b), MBC increased only in the topsoil with versus without biochar amendment (156.8 ± 4.2 mg kg⁻¹ vs. 239.1 ± 4.0 mg kg⁻¹) (Fig. 1c). The MBC content in the biochar-treated subsoil exhibited a slight decrease compared to the control (Fig. 1c).

图1 生物炭施用对表层和深层土壤有机碳含量的影响

Fig. 1  Effects of biochar application on (a) soil organic carbon (SOC, g kg⁻¹), (b) dissolved organic carbon (DOC, g kg⁻¹), and (c) microbial biomass carbon (MBC, g kg⁻¹) for control and biochar-amended soil.

The water-extracted dissolved organic matter (DOM) components with different molecular weights in the control and biochar-amended topsoil (0-20 cm) and subsoil (140-160 cm) are shown in Liquid chromatography organic carbon detection (LC-OCD) analysis (Fig. 2). There was an increase in large functionalized macromolecules (referred to as humic-like substances, HS) and a high concentration of building blocks (polyphenols and polyphenolic acids) for the biochar-amended topsoil. The total water-dissolved organic molecules and hydrophilic organic carbon of biochar-amended subsoil were higher than those of the control soil, while the hydrophobic components were not (Fig. 2c and 2d). The large molecular weight biopolymers, building blocks + HS, and the low molecule weight neutrals in biochar-amended subsoil were higher than the control soils due to most of the hydrophilic carbon and porous colloidal biochar was transported to the subsoil with rainfall and irrigation over ten years.

图2 对照和生物炭处理表层和深层土壤水可溶性有机碳液相色谱有机碳检测图

Fig. 2 Water-extracted dissolved organic content (DOC) of control and biochar-amended soils for topsoil (0–20 cm) and subsoil (140–160 cm), analyzed with LC-OCD.

In topsoil, biochar amendment decreased the relative abundance of Actinobacteria (from 27.4 % to 19.6 %) and increased the Proteobacteria (from 27.1 % to 32.1 %) and Gemmatimonadetes (from 4.4 % to 5.1 %) compared to the control (Fig. 3a). Compared to the control soil, biochar amendment substantially affected the bacterial community compositions in subsoil than in topsoil. For example, biochar amendment decreased the relative abundances of Acidobacteria, Actinobacteria, Gemmatimonadetes, Nitrospirae, Latescibacteria, and Planctomycetes, the decrements were between 21.2 % and 75.4 %, but increased the relative abundance of Proteobacteria from 27.3 % (control) to 40.0 % (biochar-amended soil) (Fig. 3a). Phylum-level species abundance clustering heat maps strongly illustrated that the long-term presence of biochar changed the community composition similarity between the control and biochar-treated soil samples at both topsoil and subsoil (Fig.3b). In addition, biochar amendment decreased microbial diversity and richness as the smaller values of PD_whole_tree, Chao1, Observed_species, and Shannon indices were observed in soil with or without biochar amendment.

图3 生物炭添加对表层和深层土壤细菌的影响

(a) 细菌群落相对丰度，(b) 门水平上物种聚类热图

Fig. 3 Effect of biochar amendment on the soil bacteria in topsoil and subsoil

(a) Mean relative abundance of bacteria (n=3) and (b) species abundance clustering heatmap in the control and biochar-amended soil samples (phylum level) for the representative soil depth.

Biochar amendment altered the SOC assigned within different density fractions in both topsoil and subsoil, significantly increasing the mineral-associated soil organic matter (MAOM) content, thus contributing to the carbon sequestration (Fig.4). Biochar amendment increased SOC contents by 33.3 % in topsoil and 5.5 % in subsoil (Fig. 1a). Such increases are mainly because of the MAOM formation. This is confirmed by the finding that the MAOM content in topsoil increased from 1.21 ± 0.03 g kg⁻¹ to 5.83 ± 0.05 g kg⁻¹ and from 1.43 ± 0.17 g kg⁻¹ to 2.08 ± 0.47 g kg⁻¹ in subsoil after biochar addition. The increase in SOC in the topsoil was due to both the high organic carbon content of biochar itself and the physical and chemical protection mechanisms via organo-mineral interactions causing stabilizing and accumulating SOC after biochar addition. A decade after biochar addition, the different components of the SOC are reassigned and transferred from the o-POM fraction to the MAOM fraction, which can maximize carbon sequestration.

图4 生物炭添加对表层和深层土壤有机碳分配的影响

Fig. 4 Distribution of organic carbon in occluded particulate organic matter (o-POM) and mineral-associated soil organic matter (MAOM) for control and biochar-amended soils.

Here, we propose that after ten years of application of biochar to surface soil, the biochar affected the content, component, and stabilization of subsoil organic carbon (Fig. 5). With the biochar migration, the DOC content and the composition of humus and aromatics in the subsoil were increased. Significantly, the addition of biochar facilitated organo-mineral interactions, thereby enhancing the stabilization of subsoil organic carbon. The sequestration of soil carbon in subsoil due to long-term biochar application is associated with the strong mobility of colloidal and dissolved biochar, which allows biochar to translocate from topsoil to subsoil. A decade after biochar application in soil, fresh biochar can be aged and fragmented in the field, improving the mobility of biochar colloids. The biochar interacted with the topsoil and caused more biochar movement downward in the form of DOC, which led to the increase of DOC in the whole soil profile. In addition, colloidal biochar also has stronger mobility in alkaline environments, which further supports our hypothesis that biochar can migrate to subsoil layers in calcareous soil and change the soil carbon stability, and as a result, plays an important role in subsoil carbon sequestration.

图5 生物炭影响表层和深层土壤稳定性的潜在机理

(a) 细菌群落相对丰度，(b) 门水平上物种聚类热图

Fig. 5 Proposed mechanism of the biochar effect on the SOC composition, bioactivities, and stabilization for topsoil and subsoil a decade after biochar incorporated into calcareous topsoil.

Our study shows that biochar amendment to a calcareous soil leads to carbon stabilization, not only in the topsoil, but also in the subsoil. Biochar mobility in alkaline soils, especially aged biochar, also affects the composition of dissolved organic carbon (DOC), increasing the DOC content in the subsoil. These findings provide experimental and conceptual evidence for the long-term impact of biochar application on carbon sequestration in calcareous soil. Our results suggest that the effect of biochar application on carbon sequestration and its role in mitigating global climate change in terrestrial ecosystems may be underestimated, especially if the stability of subsoil SOC is overlooked. Our study emphasizes the need for more field experiments to better understand the impact of long-term biochar application on subsoil carbon sequestration. Biochar application in the field is a key strategy for global carbon sequestration and climate change mitigation. Future research on the long-term effects of biochar application on SOM stocks and structure will contribute to a deep understanding of the SOC turnover process driven by biochar migration.

本文内容来自ELSEVIER旗舰期刊Sci Total Environ第907卷发表的论文：

Wang, Y., Yin, Y.J., Joseph, S., Markus, F., Wang, X., Tahery, S., Li, B.G. and Shang, J.Y., 2024. Stabilization of organic carbon in top- and subsoil by biochar application into calcareous farmland. Sci Total Environ 907, 168046.

DOI: https://doi.org/10.1016/j.scitotenv.2023.168046