Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.
The present study examines the impact of the C source (reserves vs current assimilates) on tree C isotope signals and stem growth, using experimental girdling to stop the supply of C from leaves to stem. Two-year-old sessile oaks (Quercus petraea) were girdled at three different phenological periods during the leafy period: during early wood growth (Girdling Period 1), during late wood growth (Girdling Period 2) and just after growth cessation (Girdling Period 3). The measured variables included stem respiration rates, stem radial increment, delta(13)C of respired CO(2) and contents of starch and water-soluble fraction in stems (below the girdle) and leaves. Girdling stopped growth, even early in the growing season, leading to a decrease in stem CO(2) efflux (CO(2R)). Shift in substrate use from recently fixed carbohydrate to reserves (i.e., starch) induced (13)C enrichment of CO(2) respired by stem. However, change in substrate type was insufficient to explain alone all the observed CO(2R) delta(13)C variations, especially at the period corresponding to large growth rate of control trees. The below-girdle mass balance suggested that, during girdling periods, stem C was invested in metabolic pathways other than respiration and stem growth. After Girdling Period 1, the girdle healed and the effects of girdling on stem respiration were reversed. Stem growth restarted and total radial increment was similar to the control one, indicating that growth can be delayed when a stress event occurs early in the growth period. Concerning tree ring, seasonal shift in substrate use from reserves (i.e., starch) to recently fixed carbohydrate is sufficient to explain the observed (13)C depletion of tree ring during the early wood growth. However, the inter-tree intra-ring delta(13)C variability needs to be resolved in order to improve the interpretation of intra-seasonal ring signals in terms of climatic or ecophysiological information. This study highlighted, via carbohydrate availability effects, the importance of the characterization of stem metabolic pathways for a complete understanding of the delta(13)C signals.
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