The primary ecological challenges in citrus orchards include soil acidification, nutrient depletion, and significant carbon dioxide emissions resulting from conventional cultivation practices. To address these challenges, citrus peel residues and cassava stalks underwent pyrolysis at 500 °C to generate biochars. Different proportions of these biochars (1%, 2%, and 4%) were applied under controlled laboratory conditions to assess their impact on the mineralization of soil organic carbon in citrus orchards. The results indicated that both types of biochar effectively regulated the soil pH to approximately 5.5. Significantly, the addition of 4% cassava stalk biochar significantly increased the levels of available phosphorus and potassium. The phosphorus levels rose by 512.55%, and the potassium levels surged by 1434.01%. Additionally, the soil organic carbon increased to 16.7 g/kg. Conversely, the citrus peel biochar decreased the availability of phosphorus but resulted in the highest increase in available potassium, at 1523.75%, and elevated the soil organic carbon content to 13 g/kg. Both types of biochar enhanced the soil organic carbon mineralization rate to varying extents with increasing application ratios, simultaneously boosting the cumulative amount of organic carbon mineralized. Among the treatments, cassava stalk biochar displayed the lowest C₀/SOC ratio, of 0.169, indicating its superior carbon retention capacity. Furthermore, cassava stalk biochar showed inhibitory effects on soil catalase and urease activities within the citrus orchard. Overall, the application of 4% cassava stalk biochar appears to be more beneficial for nutrient regulation and carbon sequestration in citrus orchard soils, while also contributing to the reduction in soil acidification by adjusting pH levels.

To examine the mechanisms of organic carbon transformation and sequestration by biochar in citrus orchard soil, a 100-day organic carbon mineralization test was conducted using citrus orchard soil from a 5-year-old forest. Calcium-modified citrus peel biochar (OBC-Ca) was applied at rates of 0%, 1%, 2%, and 4%. The results indicated that different percentages of OBC-Ca significantly influenced the mineralization processes in citrus orchards. Specifically, the cumulative mineralization of soil organic carbon was notably reduced by 8.68% and 17.00% with the application of 2% and 4% OBC-Ca, respectively, compared to the control group. Random forest analysis revealed that microbial biomass carbon (MBC), readily oxidizable carbon (ROC), and dissolved organic carbon (DOC) were critical indicators for predicting the cumulative mineralization of soil organic carbon. MBC and ROC were found to inhibit the cumulative mineralization, while DOC promoted it. As the proportion of OBC-Ca applications increased, MBC rose by 2.63% to 10.46%, ROC increased by 16.41% to 108.59%, and DOC increased by 0.48% to 11.67%. Correlation analysis demonstrated a significant negative correlation between the cumulative mineralization rate of soil organic carbon and soil enzyme activity, with soil sucrase content increasing significantly by 216.42% to 393.44% compared to the control. The application of calcium-modified biochar effectively reduces carbon dioxide emissions from citrus orchard soils, with a 4% application yielding the most favorable outcomes for enhancing soil carbon sinks, thereby positively impacting the carbon sequestration potential of citrus orchard soil.

Arbuscular mycorrhizal (AM) fungi can enhance the uptake of soil nutrients and water by citrus, promoting its growth. However, the specific mechanisms underlying the action of AM fungi in promoting the growth of citrus were not fully elucidated. This study aimed to explore the role of AM fungi Funneliformis mosseae in the regulatory mechanisms of P. trifoliata growth. Pot experiments combined with non-targeted metabolomics methods were used to observe the growth process and changes in metabolic products of P. trifoliata under the conditions of F. mosseae inoculation. The results showed that F. mosseae could form an excellent symbiotic relationship with P. trifoliata, thereby enhancing the utilization of soil nutrients and significantly promoting its growth. Compared with the control, the plant height, stem diameter, number of leaves, and aboveground and underground dry weight in the F. mosseae inoculation significantly increased by 2.57, 1.29, 1.57, 4.25, and 2.78 times, respectively. Moreover, the root system results confirmed that F. mosseae could substantially promote the growth of P. trifoliata. Meanwhile, the metabolomics data indicated that 361 differential metabolites and 56 metabolic pathways were identified in the roots of P. trifoliata and were inoculated with F. mosseae. This study revealed that the inoculated F. mosseae could participate in ABC transporters by upregulating their participation, glycerophospholipid metabolism, aminoacyl tRNA biosynthesis, tryptophan metabolism and metabolites from five metabolic pathways of benzoxazinoid biosynthesis [mainly enriched in lipid (39.50%) and amino acid-related metabolic pathways] to promote the growth of P. trifoliata.

Soil organic carbon (SOC) loss in sloping farmland is a critical challenge for agricultural sustainability. This study investigated how citrus peel biochar (CPB), field snail shell powder (SSP), and their composite (CPB + SSP) differentially regulate SOC dynamics across slope positions (upper, middle, lower) in Guangxi’s citrus orchards. Key findings revealed: CPB significantly increased SOC content (up to 5.5 g·kg⁻¹ at lower slopes) via high carbon input but suppressed mineralization amount in lower slope position (reduction of 17.9%) due to its high C/N ratio. SSP neutralized soil acidity (pH 3.95 to 7.5), stimulating microbial activity and raising mineralization rates by 58.95% (lower slope), yet minimally enhanced SOC (only +0.7 g·kg⁻¹). CPB + SSP effectively balanced carbon stability and active release: dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC) increased by 14.4 mg·kg⁻¹ and 0.22 g·kg⁻¹ (middle slope), while SOC rose significantly (e.g., +2.2 g·kg⁻¹ at lower slope). Slope position effects strongly influenced outcomes: the lower slope (highest initial SOC) responded most strongly to CPB for carbon stabilization, while middle slopes benefited from CPB + SSP to reconcile carbon loss with fertility. These results provide slope-specific strategies for SOC management by integrating amendment synergy and machine learning-driven insights in citrus orchards.