Effects of initial leaching for estimates of mass loss and microbial decomposition—Call for an increased nuance

Abstract

Decomposition is essential to carbon, nutrient, and energy cycling among and within ecosystems. Several methods have been proposed for studying litter decomposition by using a standardized and commercially available substrate. One of these methods is the Tea Bag Index (TBI) which uses tea bags (green and rooibos tea) incubated for ~90 days. The TBI is now applied all over the globe, but despite its usefulness and wide application, the TBI (as well as other methods) does not explicitly account for the differences in potential loss of litter mass due to initial leaching in habitats with large differences in moisture. We, therefore, studied the short‐term mass losses (3–4 h) due to initial leaching under field and laboratory conditions for green and rooibos tea using the TBI and contextualized our findings using existing long‐term mass loss (90 days) in the field for both aquatic and terrestrial environments. For both tea litter types, we found a fast initial leaching rate, which could be mistaken for decomposition through microbial activity. This initial leaching was higher than the hydrolyzable fraction given in the description of the TBI. We also found that leaching increased with increasing temperature and that leaching in terrestrial environments with high soil moisture (>90%) is almost as large as in aquatic environments. When comparing our findings to long‐term studies, we found that up to 30–50% of the mass loss of green tea reported as decomposition could be lost through leaching alone in high moisture environments (>90% soil moisture and submerged). Not accounting for such differences in initial leaching across habitats may lead to a systematic overestimation of the microbial decomposition in wet habitats. Future studies of microbial decomposition should adjust their methods depending on the habitat, and clearly specify the type of decomposition that the study focuses on.

Litter quality outweighs climate as a driver of decomposition across the tundra biome

“…Although tea undergoes leaching processes, losing mass due to the loss of water-soluble compounds during in situ decomposition 63 , so too do tundra plants 38 . We tested leaching rates in our study, finding ~20% greater mass loss for green tea and ~7% greater mass loss for rooibos tea in two-month incubations rather than in 24-hour incubations in liquid water.…”

Reasons to not correct for leaching in TBI; Reply to Lind et al. (2022)

“…This means that factors that determine leaching overlap with, and are hard to separate from those that determine litter quality and mineralization (Shumilova et al, 2019). Lind et al (2022) advocate that correcting litter decomposition rates for leaching would improve the TBI method (and implicitly other litterbag studies). The TBI method intends to obtain a standardized, easy measurement of mass losses and introducing a leaching correction would complicate its practical use as well as introduce uncertainties in its' interpretation.…”

“…Shorter incubation durations likely increase the probability of this happening. Lind et al (2022) propose to correct for primary leaching using a three-hour interval Harmon, 2016) with the red and the green line representing the initial water-soluble fraction of rooibos and green tea respectively and their standard deviation (Keuskamp et al, 2013). The category 'other' includes graminoids, some lichens but no forbs.…”

“…Pre-leaching may often extract more material from the tea than the environmental settings would. For instance, Lind et al (2022) and Thomas et al (2023) measured 7%-12% of green tea mass to be leached in outdoor soils, whereas >35% of the initial tea mass was leached in indoor treatments that resembled pre-leaching treatments (Figure 1). Therefore, a soaking pre-treatment could make the tea less comparable to local litter (Boulton & Boon, 1991).…”

“…However, due to the small mesh of buried tea bags, fragmentation and bleaching may not contribute much to the mass losses observed in TBI as compared to studies using litter bags with a wider mesh size placed on top of the soil. Like Lind et al (2022), Benfield et al (2017) and Boulton and Boon (1991), we argue to be explicit about definitions in order to minimize confusion in the scientific discussion.…”