Effects of Drip Fertilization on Soil Nitrogen Accumulation and Bacterial Communities in Citrus Orchards of Different Ages in the Red Soil Region of Southern China

Due to the multiple benefits of drip fertilization, such as water conservation, fertilizer reduction and environmental protection, it has been widely adopted in orchard cultivation in China. However, the effects of drip fertilization on soil nitrogen in citrus orchards of different ages and their mechanisms are still unclear. Therefore, this study carried out soil surveys in three citrus (Citrus reticulata Blanco) orchards of different ages under two irrigation and fertilizer treatments (NF: normal fertilization; DF: drip fertilization) in Nanfeng City, Jiangxi Province, renowned as the “Chinese Citrus Township”. Soil nitrogen and properties, bacterial communities, N-cycling functional genes and their correlations were analyzed. In the results, soil TN content reduced under DF in the orchard that was 1–2 years old but increased in the orchard that was >10 years old. Soil SN under DF was generally higher and more responsive to tree age, mainly driven by SON. Soil bacterial diversity and composition were influenced by DF, but related to tree age, soil N-cycling showed an “active retention” mode under DF. Soil nitrogen was more related to soil physicochemical properties in the irrigation stage but to soil bacterial factors in the non-irrigation stage. This study can provide a theoretical basis to promote the differentiated application of water–fertilizer integrated technology in citrus orchards of different ages.