Effects of Tea Plantation Chronosequence on Soil Nitrogen Fractions and Microbial Community Structure

Abstract

Long-term tea cultivation alters soil nitrogen forms and microbial communities, yet the coupling relationships between soil nitrogen fractions and microbial functions remain poorly understood. Because long-term tea cultivation is accompanied by sustained nitrogen enrichment and progressive acidification that continuously reshape the chemical composition of soil organic nitrogen, resolving this gap is essential for predicting soil nitrogen supply and turnover in intensively managed tea plantations of different ages. A space-for-time chronosequence was established within a large state-owned tea plantation in Shouning County, Fujian Province, China, comprising interspersed stands of 3- (T3), 15- (T15), and 40-year-old (T40) tea plants and adjacent uncultivated secondary forestland (CK). We combined the Bremner acid-hydrolysis method with Illumina high-throughput sequencing to investigate variations in soil organic nitrogen (SON) fractions, microbial community structure, and associated potential functions. The results showed that tea cultivation significantly reduced soil pH. Compared with CK, the concentrations of amino acid nitrogen (AAN) and acid-hydrolyzable ammonium nitrogen (AN) in T40 increased by 223.03% and 137.83%, respectively. Meanwhile, the proportions of acid-hydrolyzable unidentified nitrogen (HUN) and non-hydrolyzable nitrogen (NHN) to total nitrogen (TN) decreased from 27.18% and 51.70% under CK to 23.22% and 40.84% under T40, respectively, indicating a remarkable restructuring of the soil nitrogen pool. For bacterial communities, the relative abundance of Proteobacteria increased from 17.76% in CK to 23.96–44.46% in tea plantations, whereas that of Acidobacteriota declined from 37.80% to 34.47–22.31%, accompanied by a significant enrichment of copiotrophic taxa. Redundancy analysis revealed that AAN and AN were the dominant drivers shaping bacterial community structure. FAPROTAX analysis showed that, compared with forest soils, tea plantations exhibited a significantly lower nitrogen fixation abundance (from 14.35% to 6.81%, p < 0.001) and an enhanced relative abundance of nitrate reduction (p < 0.05). FUNGuild analysis revealed that the dominant forest multi-trophic group (Pathotroph–Saprotroph–Symbiotroph) dropped sharply from 44.53% to 17.12–18.62% in tea plantations (p < 0.01), whereas the Pathotroph group increased from 0.61% to 2.22–2.78% (p < 0.05). Collectively, long-term tea cultivation elevates the supply of soil labile organic nitrogen, selectively enriches copiotrophic microbial communities, and reshapes the functional profiles of soil microbial communities.