A field assessment to validate the assumptions of the Tea Bag Index (TBI) as a measure of soil health

Abstract

This study evaluated key assumptions underlying the Tea Bag Index (TBI) as a soil health assessment tool through a time series conducted under field conditions. The TBI assesses soil health by measuring the decomposition rates of standardized green and red tea over a 90-day incubation period. Our research was conducted on a degraded 33.6-ha agricultural site in North Saanich, British Columbia, where we compared two plots: a reference site and a grazed site, each representing distinct land use conditions.The first assumption evaluated was that the TBI metrics of S (stabilization rate) and k (decomposition rate) follow exponential decay models. While S adhered to this assumption, k did not, suggesting additional as yet undetermined influences on decomposition rates. We introduced new biological metrics, Bg and Br, which followed polynomial models of decay, and may reflect the decomposition of different materials over time by different soil organisms.The second assumption tested was that a 90-day incubation period adequately captures the timeframe required for labile litter consumption. Our findings indicated that decomposition variance plateaued after 105 days, and that an increase in incubation time by 35 days increased the precision of mathematical decomposition models by a mean value of 18.1 %.The final assumption tested was that litter decomposition rate varies with land use. Our findings affirm that land use does affect litter decomposition rate, although the specific sources of that variance remain unclear.TBI accuracy can be improved by extending the litter incubation time to at least 105 days. Incorporating biological metrics (Bg and Br) increases the TBI's precision. Our results reaffirm the TBI's sensitivity to land use differences, highlighting its potential as a cost-effective tool for assessing soil health under varying conditions.