Background: Soil microbes exist throughout the soil profile and those inhabiting topsoil (0–20 cm) are believed to play a key role in nutrients cycling. However, the majority of the soil microbiology studies have exclusively focused on the distribution of soil microbial communities in the topsoil, and it remains poorly understood through the subsurface soil profile (i.e., 20–40 and 40–60 cm). Here, we examined how the bacterial community composition and functional diversity changes under intensive fertilization across vertical soil profiles [(0–20 cm (RS1), 20–40 cm (RS2), and 40–60 cm (RS3)] in the red soil of pomelo orchard, Pinghe County, Fujian, China. Results: Bacterial community composition was determined by 16S rRNA gene sequencing and interlinked with edaphic factors, including soil pH, available phosphorous (AP), available nitrogen (AN), and available potassium (AK) to investigate the key edaphic factors that shape the soil bacterial community along with different soil profiles. The most dominant bacterial taxa were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, Crenarchaeota, and Bacteriodetes. Bacterial richness and diversity was highest in RS1 and declined with increasing soil depth. The distinct distribution patterns of the bacterial community were found across the different soil profiles. Besides, soil pH exhibited a strong influence (pH ˃AP ˃AN) on the bacterial communities under all soil depths. The relative abundance of Proteobacteria, Actinobacteria, Crenarchaeota, and Firmicutes was negatively correlated with soil pH, while Acidobacteria, Chloroflexi, Bacteriodetes, Planctomycetes, and Gemmatimonadetes were positively correlated with soil pH. Co-occurrence network analysis revealed that network topological features were weakened with increasing soil depth, indicating a more stable bacterial community in the RS1. Bacterial functions were estimated using FAPROTAX and the relative abundance of functional bacterial community related to metabolic processes, including C-cycle, N-cycle, and energy production was significantly higher in RS1 compared to RS2 and RS3, and soil pH had a significant effect on these functional microbes. Conclusions: This study provided the valuable findings regarding the structure and functions of bacterial communities in red soil of pomelo orchards, and highlighted the importance of soil depth and pH in shaping the soil bacterial population, their spatial distribution and ecological functioning. These results suggest the alleviation of soil acidification by adopting integrated management practices to preserve the soil microbial communities for better ecological functioning.

Excess phosphorus (P) accumulation in the soil can change the bioavailability of P and increase the leaching risks, but the quantitative evaluation of these responses in acidic red soil is lacking. This study aimed to investigate the composition of soil P fractions under different phosphorus apparent balances (PAB) in acidic red soil and the bioavailability and the leaching change-points of different P fractions. Five phosphorus (P) fertilization rates were applied (0, 16.38, 32.75, 65.50, 131.00 kg P·ha⁻¹) in every sweet corn cultivation from the field experiment, and the treatments were marked as P0, P1, P2, P3, and P4, respectively. The PAB showed negative values in P0 and P1 which were −49.0 and −15.0 kg P·ha^–1 in two years, respectively. In contrast, PAB in P2 as well as in P3 and P4 were positive, the content ranging from 40.2 to 424.3 kg P·ha⁻¹ in two years. Per 100 kg ha⁻¹ P accumulate in the soil, the total P increased by 44.36 and 10.41 mg kg⁻¹ in the surface (0–20 cm) and subsurface (20–40 cm) soil, respectively. The content of inorganic P fractions, including solution phosphate (Sol-P), aluminum phosphate (Al-P), iron phosphate (Fe-P), reduction phosphate (Red-P), and calcium phosphate (Ca-P), significantly increased by 0.25, 16.22, 22.08, 2.04, and 5.08 mg kg⁻¹, respectively, in surface soil per 100 kg ha⁻¹ P accumulated in the soil. Path analysis showed that the most important soil P fractions contributing to Olsen-P were Sol-P and Al-P, which can directly affect Olsen-P, and their coefficients were 0.24 and 0.73, respectively. Furthermore, the incubation experiments were conducted in the laboratory to investigate the leaching risk of different P fractions, and they showed Sol-P was a potential source of leaching, and the leaching change-points of Al-P and Fe-P were 74.70 and 78.34 mg·kg^–1, respectively. Continuous P that accumulated in soil changed the composition of P fractions, and the bioavailability as well as the leaching risks increased. This is important in optimizing soil P fertilization management in agricultural ecosystems based on the bioavailability and critical levels for leaching of P fractions.

Escherichia coli O157:H7 is a well-recognized cause of human illness. Survival of Escherichia coli O157:H7 in five purple soils from Sichuan Province was investigated. The dynamics of E. coli O157:H7 survival in purple soils were described by the Weibull model. Results showed that this model is suitable to fit survival curves of E. coli O157:H7 in purple soils, with the calculated t_d value (survival time needed to reach the detection limit of 100 CFU·g⁻¹) ranging from 2.99 days to 26.36 days. The longest survival time of E. coli O157:H7 was observed in neutral purple soils (24.49 days), followed by alkalescent purple soil (18.62 days) and acid purple soil (3.48 days). The redundancy analysis (RDA) revealed that t_d values were significantly enhanced by soil nutrition (total organic carbon (OC), total nitrogen (TN), available potassium (AK) and the ratio of humic acid to fulvic acid (Ha/Fa)), but were significantly suppressed by iron and aluminum oxide.

Excessive nitrogen application in tea plantations can accelerate soil acidification and the loss of exchangeable base cations. However, how these changes are partitioned among soil aggregate fractions and linked to tea productivity remains unclear. An 8-year field experiment with five N application rates (0, 150, 300, 450 and 600 kg·ha−1) was conducted in a tea plantation with red soil, initial pH 4.2 and cv. Tieguanyin in Anxi County, Fujian Province. Soil aggregate composition and stability were determined by wet sieving to assess the effects of N rates on aggregate stability, exchangeable base cations and nutrient contents, and tea yield and quality. N application significantly affected the stability of soil aggregates. The low N treatment (150 kg·ha−1 N) gave the highest mean weight diameter (MWD), indicating the greatest aggregate stability. The results demonstrated that N application rates have nonlinear effects on soil properties and tea yield. During this 8-year-long experimental study, the low-nitrogen treatment maintained higher net concentrations of exchangeable alkali metal cations, indicating that its enhanced cation retention capacity is sustainable rather than a transient pulsed effect occurring after fertilizer application. Additionally, this treatment significantly increased the proportion of macroaggregates and the MWD content, thereby optimizing the soil physical structure. This structural improvement, together with enhanced nutrient availability and moderated soil acidification, was associated with the peak yield and quality observed under this treatment. In contrast, excessive N application severely depleted exchangeable Ca2+ and Mg2+ through intensified nitrification and H+ competition, disrupted cationic bridging, and disintegrated large aggregates into smaller fractions, consequently compromising soil stability. These findings confirm that low-nitrogen represents the optimal fertilizer application strategy. Mechanistic analysis indicates that the improved tea quality with addition of 150 kg·ha−1 N is associated with increased levels of alkali-hydrolyzable-N, and exchangeable Ca and Mg within aggregates, which are positively correlated with soil pH. Therefore, to improve soil quality while enhancing tea yield and quality, it is recommended that N fertilizer input be maintained at a low rate (150 kg·ha−1) in tea plantations in southeastern Fujian under the conditions of this study.