Optimizing nitrogen input for sustainable tea production: an integrated assessment of soil aggregate stability, base-cation retention and quality improvement in an 8-year field study

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.