Agroecological Practices for a Sustainable Tea Production in Northern Vietnam
Thèse soutenue le 10 octobre 2023 - Deakin University
Tea is a very important cash crop in Vietnam, providing crucial income and employment for farmers in poor rural areas. Unfortunately, the dominance of long-term, conventional tea cultivation, which strongly relies on agrochemical inputs has caused severe soil health degradation, low tea productivity and quality as well as human health concerns and environmental pollution. At the same time, as tea production may provide a better net income compared with other annual crops such as rice and vegetables, farmers have been converting parts of their allocated land to cultivate tea plants. Little is known about the benefit of agroecological management as an alternative to conventional tea management practices, and thus, there is a need to understand how it can improve tea yields, quality, and the livelihoods of the farmers. In addition, since soil acidification has been one of the major concerns of the tea industry in Vietnam as well as in the world, cost-effective strategies such as liming, the application of agricultural wastes and byproducts have been promoted to control soil acidification, restoration and maintain soil fertility and biodiversity. Among these solutions, the role of liming in ameliorating soil acidity and enhancing soil health and crop productivity is widely documented although poorly understood in the case of tea plantations in Northern Vietnam.
In this study, we investigated the sustainability of agroecological tea production along with the main challenges of conventional tea farming in Northern Vietnam, the mechanisms and consequences of tea soil acidification and soil health degradation; the potential uses of agricultural wastes/composts and liming for managing soil acidification and subsequently improving tea soil health- related properties while enhancing crop productivity and quality. A total of 66 different tea households were selected for assessing the economic efficiency, and afterward, 20 tea farms from these households were selected for field experiments and sampling, including both agroecological and conventional plantations, as well as converted and non-converted fields. Soil physical and chemical attributes, soil fauna, and tea root AMF, as well as tea yield and yield components, were analyzed to compare the impacts of agroecological and conventional tea management methods. To assess the effect of the 9-month liming application on tea soil biodiversity, soil bacterial, fungal, and AMF community richness, and composition were also determined using rDNA and ITS gene sequencing analyses, in addition to soil physicochemical and fauna assessments.
Our study showed that apart from potentially bringing about production economic efficiency due to cheap inputs and high productivity in the short term, the continuity of conventional tea production in Vietnam has led to a series of severe issues, including soil degradation, particularly soil acidification, poor tea quality, environmental pollution, and human health concerns. Agroecological tea management practices such as the application of organic fertilizers, biofertilizers and biopesticides, organic mulching, intercropping as well as integrated pest/disease management have been widely reported to retain soil physicochemical and biological attributes. These beneficial impacts are mainly driven by the additions of organic matter and soil essential macro and micronutrients, which enrich soil organism diversity and functional activities, as well as reduce the use of agrochemical inputs and chemical residues in soil and on tea leaves. Converting conventional tea adoption to agroecological management practices significantly increased tea root AMF intensity by up to 24%, soil macro, and mesofauna by 110% and 60%, respectively, and soil pH by 0.5 units on average. Despite the lower tea yields, our study indicated for the first time that agroecological tea adopters earned around USD 8,400 ha/year more than the farmers still practicing conventional management, which was mainly driven by the premium price of agroecological tea products and other credits from supporting agencies.
Though tea soil acidification has numerous consequences on soil chemical and biological properties, as well as tea quality and productivity, in which the reduction and imbalance of nutrient base cations, including Ca2+, Mg2+, Na+, and K+ have been considered one of the most serious disadvantages, this issue can be mitigated by the applications of liming and/or agricultural wastes and byproducts, which could supply alkaline matter and essential elements to neutralize soil acidity, thus improving soil health-related properties and crop performance. Consequently, 9 months after the application, liming significantly enhanced soil pH (by 0.4 units) and soil OM, while strongly reducing soil exchangeable Al3+ and Mn2+, and P availability. Converting paddies and vegetable fields to tea farms also resulted in higher soil pH values, OM, and soil P availability, but also increased soil Al toxicity risk. Macrofauna observed in tea soils was less abundant than in organic mulch, and liming also had a significant and positive effect on macrofauna abundance recorded in these layers. The lime amendment also significantly enhanced tea AMF intensity and frequency, as well as tea yield and yield components, regardless of the land use history difference. In contrast, soil bacterial, fungal, and AMF relative abundance and composition were strongly responsive to land use history, and the interaction of liming and land use history, because of changes in soil physicochemical properties and crop types. The sole lime application did not lead to any significant impacts on soil microbial richness or community composition, indicating that a 0.4-unit shift in soil pH may not be enough to trigger a significant change in soil microbial communities. Additionally, lime incorporation created a better environment for the growth of some fungal taxa, while suppressing other fungal groups which are preferable to acidic soils, thus fungal diversity appeared to be unaffected by liming. Since the single liming application from this study was not enough for inducing a significant and positive effect on diversity and composition of soil microbial communities, further studies might be needed to investigate other liming strategies, such as frequency and application rates, sampling depth and study period. How lime addition affects other organisms (nematodes, soil microfauna, soil microbial functional diversity) also deserves further investigation to provide a better understanding of liming efficiency in enhancing the food web in tea plantation soils.
Our findings contribute towards understanding changes in soil biodiversity in response to liming and land-use conversion and confirm that appropriate liming could be an effective strategy to ameliorate soil acidity, thus enhancing soil biodiversity and crop productivity. Additionally, this study highlights the important roles of soil microbial communities concerning tea quality indicators and other aspects of tea plantation management in ensuring that suitable and sustainable management practices are promoted for restoring soil fertility in the region. The strategies developed in this study might also be useful in understanding and improving the sustainable management of other regional perennial crops, such as coffee and fruit orchards. Where applicable, the promotion of agroecological farming and other soil acidification management strategies could benefit both the local population and the environment through a reduction of expensive agrochemical inputs and an increased source of income.
