Rhizosphere soil microbial communities substantially impact plant growth by regulating the nutrient cycle. However, dynamic changes in soil microbiota under different tree ages have received little attention. In this study, changes in soil physicochemical properties, as well as bacterial diversity and community structures (by high-throughput Illumina MiSeq sequencing), were explored in pomelo trees of different ages (i.e., 10, 20, and 30 years) under red and paddy soils cultivated by farmers with high fertilizer input. Moreover, soil factors that shape the bacterial community, such as soil pH, AP (available phosphorous), AK (available potassium), and AN (available nitrogen), were also investigated. Results showed that pH significantly decreased, while AP, AK, and AN increased with increasing tree age under red soil. For paddy soil, pH was not changed, while AP was significantly lower under 10-year-old pomelo trees, and AK and AN contents were minimum under 30-year-old pomelo trees. Both soil types were dominated by Proteobacteria, Acidobacteria, and Actinobacteria and showed contrasting patterns of relative abundance under different tree age groups. Bacterial richness and diversity decreased with increasing tree age in both soil types. Overall, bacterial community composition was different under different tree ages. RDA analysis showed that soil pH, AP, and AN in red soil, and pH and AP in paddy soil showed the most significant effects in changing the bacterial community structure. A random forest model showed Sinomonas and Streptacidiphilus in red soil, while Actinoallomurus and Microbacterium in paddy soil were the most important genera explaining the differences among different age groups. The ternary plot further revealed that genera enrichment for Age_30 was higher than that for Age_10 and Age_20 in red soil, whereas specific genera enrichment decreased with increasing tree age under paddy soil. Co-occurrence network revealed that bacterial species formed a complex network structure with increasing tree age, indicating a more stable microbial association under 20 and 30 years than 10-year-old pomelo trees. Hence, contrasting patterns of changes in soil physicochemical properties and soil microbial communities were recorded under different tree ages, and tree ages significantly affected the bacterial community structure and richness. These findings provide valuable information regarding the importance of microbes for the sustainable management of pomelo orchards by optimizing fertilizer input for different ages of trees.

Aluminum (Al) toxicity is one of the most serious hazards of soil acidification, which limits root formation and hinders crop growth. Soil acidification in orchards has been widely reported. However, there is little information about the effect of Al3+ on citrus roots in acidified red soil. Taking pomelo (Citrus grandis), the main citrus variety in Southeast China, as an example, soil surveys were conducted to analyze the status and characteristics of acidification in pomelo orchards. Subsequently, we explored whether optimized fertilization and liming can effectively reduce Al toxicity in roots and increase yield by three treatments, including farmer fertilization practice (FFP), NPK optimization (OPT), and OPT combined with lime (OPT+L). The results showed that (1) rapid soil acidification occurred in pomelo orchards in the past 40 years (with a decrease of 0.81 units). The severity of soil acidification increases with orchard age. In addition, the soil pH and base saturation (BS) at the drip line (DL; i.e., fertilization site) were the lowest, while the concentration of exchangeable acid (Ex.Acid) and exchangeable Al (Ex.Al), and the ratio of Ex.Al to cation exchange capacity (Ex.Al%) was highest compared to other locations away from DL. Random forest analysis indicated Ex.Al was the most important factor to reflect soil acidification. (2) Under the OPT+L treatment, the concentrations of soil Ex.Acid and Ex.Al decreased, and BS increased, which markedly reduced the Al3+ concentration in the roots and promoted root growth. OPT+L significantly decreased the Al concentrations in soils (by 71.3%) and roots (by 47.8%) but increased root activity, root length, root tips and root surface area by 119.2%, 80.3%, 124.6% and 136.6%, respectively. In addition, structural equation modeling further proved that increasing soil pH and BS while decreasing Ex.Al was beneficial to citrus production in acidic soil, and optimized fertilization and lime application are necessary.

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