Tea Bag Index: Assessing Soil Microbial Activity Under Different Compost Management Practices in Vineyards

Introduction
The Tea Bag Index (TBI) is a standardized, simple, and accessible method for assessing soil microbial activity by estimating the rate of organic matter decomposition. Compost is an organic-rich fertilizer produced through decomposition processes facilitated by microorganisms. When added to soil, compost improves nutrient content, enhances chemical and physical properties, and boosts fertility, also contributing to soil biodiversity by introducing microbial species.

Methods
We used the Tea Bag Index (TBI) method to assess the effect of compost on the soil microbial activity under different vineyard soil management practices: tilled compost, mulched compost, and tillage without compost (control). Two types of tea were used: green tea, which contains a higher fraction of degradable organic compounds, and rooibos tea, which has a higher amount of recalcitrant compounds. A tea bag of each type was weighed and then buried, midway between two vine plants, at a depth of approximately 12 cm in at least three replicates of each different soil management practice. After a 3-month burial period, the bags were retrieved, dried at 65°C for 48 hours, and then reweighed to calculate mass loss. An ANOVa was used to assess statistical differences among the different soil management practices.

Results
The use of the Tea Bag Index (TBI) according to the original method (Keuskamp et al., 2013; https://doi.org/10.1111/2041-210X.12097) was ineffective due to the apparent incompatibility of the calculation formula. Therefore, as a valid technique for determining soil microbial decomposition activity, we measured the percentage of organic matter degradation in the tea bags: [1-(final tea weight/initial tea weight)] X 100). This approach has been used in other studies as well.

Conclusions
In the two years of compost application, the response of the soil biological activity differed. Particularly in the second year, both the tilled and the mulched compost significantly increased the soil biological activity.