The acidification of yellow-red soil readily induces soil fertility degradation. While inorganic mineral amendments often lead to soil compaction, organic fertilizers exhibit slow improvement effects. As an alkaline organic liquid fertilizer, the efficacy of biogas slurry combined with organic fertilizer in ameliorating acidified yellow-red soil remains unclear. This study comparatively investigated the effects of different amendment measures on rape yield and soil improvement, evaluating the performance of biogas slurry-organic fertilizer co-application based on soil amelioration outcomes and rape agronomic traits. The experiment included five treatments: no fertilization (CK), conventional chemical fertilization (NPK), NPK plus organic fertilizer (M), NPK plus organic fertilizer and hydrated lime (M+L), and biogas slurry substituting chemical fertilizers (with equivalent NPK) plus organic fertilizer (M+B). Comprehensive analyses were conducted on soil acidity indicators, organic matter content, nutrient status, organic/inorganic phosphorus fractions, ¹³C-NMR spectra, enzyme activities, as well as rape yield and nutrient content under different treatments. Pearson correlation analysis was performed to determine the optimal amendment strategy. The results demonstrated that: 1) Compared with NPK treatment, all organic fertilizer treatments significantly increased rape yield and nutrient content (P<0.05). Both M+L and M+B treatments substantially elevated soil pH by 2.36 and 1.42, respectively, while reducing exchangeable aluminum, exchangeable hydrogen, and total exchangeable acidity by 99.68%/92.31%/99.41% (M+L) and 92.89%/52.07%/91.37% (M+B) (P<0.05). The M+B treatment significantly enhanced soil organic matter, available nitrogen, and available potassium by 15.47%, 6.10%, and 148.07% (P<0.05), though total nitrogen and potassium contents showed no significant improvement (P>0.05). 2) Organic fertilizer treatments significantly increased total phosphorus (61.15%-136.00%), inorganic phosphorus (304.65%-500.00%), and available phosphorus (7.46-28.52 fold) compared to NPK (P<0.05), with M+B showing the most pronounced effects. The M+B treatment also significantly improved alkaline phosphatase, neutral phosphatase, and acid phosphatase activities by 50.56%, 16.00%, and 43.35%, respectively, whereas M and M+L treatments significantly suppressed all three phosphatase activities (P<0.05). 3) ¹³C-NMR analysis revealed that organic fertilizer treatments decreased the proportions of alkyl-C and carbonyl-C while increasing O-alkyl-C and aromatic-C fractions. Among M, M+L, and M+B treatments, the alkyl-C/O-alkyl-C ratio followed M+B > M+L > M, while the aliphatic-C/aromatic-C ratio exhibited M > M+L > M+B. These findings indicate that the combined application of organic fertilizer and biogas slurry effectively mitigates soil acidification, enhances phosphorus availability, improves organic matter stability, and promotes nutrient uptake by rape, representing a sustainable approach for ecological restoration of acidified yellow-red soils.

To address the nutrient deficiency and severe degradation of yellow-red soils, this study investigates the effects of different biochar types and application rates on the physicochemical properties, extracellular enzyme activities related to the carbon and nitrogen cycles, aggregate composition, and carbon and nitrogen distribution in dryland yellow-red soils. The experiment included eight treatments: no fertilizer (CK), conventional fertilization (NPK), and conventional fertilization combined with the application of corn stover biochar, rice straw biochar, and pig manure biochar at application rates of 13.5 and 40.5 t/hm² (CSB1, CSB2, RSB1, RSB2, PMB1, and PMB2). The results showed that compared with the NPK treatment, the application of biochar significantly increased the soil organic carbon, total nitrogen, alkali-hydrolyzable nitrogen, and total potassium content, with increases ranging from 6.27% to 99.46%, 4.41% to 83.62%, 8.29% to 72.51%, and 2.84% to 15.51%, respectively. The application of rice straw biochar and corn stover biochar reduced the soil total phosphorus and available potassium contents, whereas the PMB2 treatment increased the soil total phosphorus, available phosphorus, available potassium, and cation exchange capacity by 7.65%, 76.42%, 14.23%, and 23.85%, respectively (P＜0.05). The application of biochars also significantly enhanced the activities of β-xylosidase, β-cellobiohydrolase, Leucine aminopeptidase, and N-acetyl-β-D-glucosaminidase, with increases of 29.36% to 78.84%, 51.91% to 265.03%, 4.16% to 149.80%, and 26.78% to 185.46%, respectively. The RSB1 and RSB2 treatments reduced β-glucosidase activity by 21.74% and 27.42%, respectively (P>0.05), while both corn stover biochar and pig manure biochar increased β-glucosidase activity. Except for the PMB1 treatment, which significantly decreased the 0.25-2 mm aggregate fraction by 35.00% (P＜0.05), other biochar treatments had no significant effects on this aggregate size fraction (P>0.05). The application of biochar did not significantly affect the 0.053-0.25 mm aggregate fraction (P>0.05). However, biochar applications increased the organic carbon and total nitrogen content of the 0.25-2 mm and 0.053-0.25 mm aggregates, with the PMB2 treatment showing the largest increases, by 129.54% and 25.22%, and 102.87% and 35.11%, respectively (P＜0.05). Furthermore, the PMB2 treatment also showed the largest increase in the organic carbon contribution rate of the 0.25-2 mm aggregates. In conclusion, the effectiveness of different biochar in improving the soil physicochemical properties and enzyme activities followed the order: Pig manure biochar, corn stover biochar, rice straw biochar, with PMB2 showing the best performance. The application of 40.5 t/hm² pig manure biochar is an effective strategy for improving the physicochemical properties, enzyme activities, and organic carbon and nitrogen content in aggregate in yellow-red soils and is highly suitable for soil improvement and fertility enhancement in dryland areas.