EFFECTS OF TEA (CAMELLIA SINENSIS) CULTIVATION ON SOIL QUALITY IN THE LAM DONG, VIETNAM

The objectives of the study were to assess soil quality and its relationship to the sustainability of tea cultivation in the Lam Dong, Vietnam. Overall soil quality declined with increasing age of the tea plantations as evidenced by decreases in soil OC, total N, K and S, available P and K, mean weight diameter of aggregates. As well, total P, bulk density and mechanical resistance increased with increasing cultivation intensity. Because these soil properties were sensitive to cultivation effects, they were considered to be good indicators of soil quality. Soil properties that were less sensitive to change, and limited as soil quality indicators included texture, clay mineralogy and sesquioxides, and effective cation exchange capacity. Soil quality changes were greatest during the first 10 years of cultivation and were generally greatest in the surface 0- to 40-cm of soil. Soil and crop management factors were considered to be the most important factors affecting soil quality. Decreases in long-term crop yields were found to correspond with decreases in soil quality. In terms of crop productivity, the most important soil quality indicators (based on a multiple regression analysis) were OC, available P, total K and PAWC. Economic analysis of the yield and production cost data indicated that, under current conditions, tea cultivation in the Lam Dong province is sustainable for periods of about 20 years. Thus, measured values of soil quality indicators in the 20-yr tea soils were considered to represent the "critical levels" for economic sustainability of tea cultivation.

本研究旨在评估越南林同省（Lam Dong）的土壤质量，及其与茶叶种植可持续性的关联。随着茶园树龄的增加，土壤整体质量呈下降趋势，具体体现为土壤有机碳（OC）、全氮、全钾、全硫、有效磷、速效钾以及团聚体平均重量直径均有所降低；与此同时，全磷、土壤容重与机械阻力则随种植强度的提升而升高。由于这些土壤性质对种植活动的影响较为敏感，因此被视为评估土壤质量的优良指标。而对变化敏感性较低的土壤性质，包括土壤质地、黏土矿物学特征、铁铝氧化物以及有效阳离子交换量，则较少被用作土壤质量评价指标。土壤质量的变化在种植初期的前10年最为显著，且整体上以0~40 cm表层土壤的变化幅度最大。土壤与作物管理因素被认为是影响土壤质量的最关键因素。研究发现，作物长期产量的下降与土壤质量的降低呈现显著相关性。就作物生产力而言，经多元回归分析筛选出的最重要土壤质量指标为有机碳（OC）、有效磷、全钾以及植物有效持水量（PAWC）。对产量与生产成本数据的经济分析表明，在当前生产条件下，林同省的茶叶种植可持续周期约为20年。因此，20年树龄茶园的土壤质量指标实测值，被视为茶叶种植经济可持续性的"临界阈值"。