Threshold-Driven Soil Degradation and Productivity Decline Under Long-Term Tea Monoculture in Tropical Highlands: Evidence from Field Data and Predictive Modelling

Long-term monoculture is a major driver of soil degradation in tropical agroecosystems, yet quantitative evidence on critical thresholds governing soil functionality remains limited. This study integrates field-based observations with predictive modelling to characterize degradation dynamics and identify threshold responses in long-term tea (Camellia sinensis) systems in the Central Highlands of Vietnam.

Soils from plantations of different ages (5, 10, and 20 years) were compared with native forest systems across key physicochemical and biological indicators. Results show substantial declines in soil organic carbon (SOC, −48%), available phosphorus and potassium (−48%), and plant-available water capacity (−36%), accompanied by increased bulk density and severe biological degradation.

A multiple regression model explained 76.4% of yield variability, highlighting SOC, available P, total K, and PAWC as dominant predictors of productivity. Importantly, piecewise regression revealed clear non-linear responses and identified critical thresholds at SOC ≈ 12 mg g?¹ and available P ≈ 6 µg g?¹, below which yield and economic returns decline abruptly.

These findings demonstrate that soil degradation follows a threshold-driven process rather than a linear trajectory. The study advances current understanding by providing a quantitative framework for identifying tipping points in soil systems and introduces threshold-based indicators as early-warning tools for sustainable management of perennial cropping systems in tropical highlands.