Early stage litter decomposition across biomes
Graphical abstract
Introduction
Through litter decomposition >50% of net primary production is returned to the soil (Wardle et al., 2004) and 60 Pg C year−1 is emitted to the atmosphere (Houghton, 2007). Depending on the type of ecosystem, the quantity of soil organic carbon (SOC) in the top 1-m depth range from 30 tons/ha in arid climates to 800 tons/ha in organic soils in cold regions, with a predominant range from 50 to 150 tons/ha (Lal, 2004). The amount of SOC is determined by the balance of carbon inputs from primary production and losses through the decomposition of organic matter over time (Olson, 1963). However, there is a large degree of variability in this balance and more research is needed for a better mechanistic understanding of decomposition processes at various scales and for a more accurate estimation of present and future global carbon budgets (Aerts, 2006).
Decomposition of plant litter may be divided into at least two stages (e.g. Berg and McClaugherty, 2008). The early stage of decomposition (ca. 0 to 40% mass loss) is characterized by leaching of soluble compounds and by decomposition of solubles and non-lignified cellulose and hemicellulose (Couteaux et al., 1995; Heim and Frey, 2004). The late stage (ca. 40–100% mass loss) encompasses the degradation of lignified tissue. In general, microbial decomposition of organic substrates is controlled by both biotic factors (substrate quality and microbial community composition) and abiotic factors (temperature and moisture; Gavazov, 2010). Research to understand the impact of global changes such as climate on decomposition processes has typically been conducted at individual sites and/or through cross-site observations and experiments (e.g. Emmett et al., 2004; Heim and Frey, 2004; García Palacios et al., 2013). This has sometimes lead to controversial conclusions since the observed decomposition may be dependent on local litter quality used in the study and the factors controlling decomposition may be influenced by the methodologies and experimental designs applied. Consequently, comparisons across observations and common conclusions may be hampered. For example, early stage decomposition (mainly microbial) has been reported to be primarily controlled by climate and major nutrients in pine needle litter (Berg and McClaugherty, 2008), by microbial and nematode communities in pine needle litter (García Palacios et al., 2016), by litter content of water soluble substances (Heim and Frey, 2004) and by soil temperature and soil pH for a maize straw-soil mixture (Djukic et al., 2012). At regional and global scales, litter decomposition has been reported to be controlled by climate and litter quality (explaining about 60–70% of litter decomposition rates; Parton et al., 2007) and by soil meso-and microfauna communities (explaining about 7%; Wall et al., 2008). However, at the biome scale the metadata-analysis by García Palacios et al. (2013) showed that the variables controlling decomposition vary with decomposition in cold and dry biomes being mostly controlled by climatic conditions while soil fauna seemed to have a more defining role in warm and wet biomes. Moreover, Bradford et al., (2014) showed that climate has a main control on decomposition only when local-scale variation is aggregated into mean values. In order to pinpoint the specific drivers of litter decomposition across various litter types with different decomposition rates and across multiple sites, standardized studies across sites and regions are needed (Wickings et al., 2012; Handa et al., 2014; Parsons et al., 2014).
Decomposition studies across multiple sites using standardized methods already exist within observational networks or experimental studies such as GLIDE (Global Litter Invertebrate Decomposition Experiment – Wall et al., 2008), LIDET (Long-term Intersite Decomposition Experiment Team – Adair et al., 2008), CIDET (Canadian Intersite Decomposition Experiment – Trofymow and CIDET Working Group, 1998), DIRT (Detrital Input and Removal Experiment – Nadelhoffer, 2004), BioCycle (Biodiversity and biogeochemical cycles: a search for mechanisms across ecosystems - Makkonen et al., 2012), DECO (European Decomposition project - Johansson et al., 1995), CANIF (Carbon and Nitrogen Cycling in Forest Ecosystems project – Persson et al., 2000), MICS (Decomposition of organic matter in terrestrial ecosystems: microbial communities in litter and soil – Cotrufo et al., 2000), VULCAN (Vulnerability assessment of shrubland ecosystems in Europe under climatic changes - Emmett et al., 2004), and VAMOS (Variation of soil organic matter reservoir – Cotrufo et al., 2000). Results from these have been used by predictive models such as Yasso07 (Tuomi et al., 2009) and in meta-analyses such as the ART-DECO project (Cornwell et al., 2008). These studies have all provided important information on the decomposition of litter, but have been limited to specific biomes or ecosystem types or have used site specific litter.
Therefore, despite the many efforts, a general understanding of the litter decomposition process and its driving factors is hampered by (1) use of site- or network/project-specific litters and methodologies (e.g. different study lengths, litter bag mesh sizes, incubation depths, litter type and litter mixes; García Palacios et al., 2013), and (2) the low number of global studies that go across all biomes (Bradford et al., 2016). This study presents results from the TeaComposition initiative which uses standard litters (tea bags - Keuskamp et al., 2013) and a common protocol allowing global and long-term application to overcome these limitations by providing standardized litter decomposition measurements across broad spatial scales. This paramount importance of standardized methods has alo been emphasized by Haase et al., 2018 and Mollenhauer et al., 2018 in press. The study presents early stage litter mass loss across nine biomes with the aim to determine and compare globally the main drivers of decomposition at present climatic conditions. The early stage decomposition is generally expected to show greater mass loss rates and a dynamic response of mass loss to controlling factors (e.g. Heim and Frey, 2004; Pérez-Suárez et al., 2012). Therefore the specific objectives of the study were to estimate the variation in early stage mass loss of two litter types worldwide, to explore the linkage of early stage litter mass loss with key drivers (climate, litter type, land-use), and to explore whether the relative importance of the drivers differ between the litter types. Our research questions are (1) does early stage litter mass loss of Green tea and Rooibos tea vary at the global scale due to the different litter qualities (Didion et al., 2016; Keuskamp et al., 2013), (2) are abiotic drivers controlling the initial stage of mass loss (Bradford et al., 2016) with temperature being the main regulating factor in the cold biomes and precipitation in the warmer biomes (Adair et al., 2008), and (3) does early stage litter mass loss vary between land-use types due to changes in the microclimates (Fig. 1).
Section snippets
Background of the TeaComposition initiative
The TeaComposition initiative was started in summer 2016. The main objective is to investigate long-term litter decomposition and its key drivers at present as well as under different future climate scenarios using a common protocol and standard litter (tea) across nine terrestrial biomes. It is one of the first comprehensive global studies on litter decomposition focusing on the litter decomposition in the topsoil and the degradation of the main litter components (lignin, cellulose and
Relative importance of litter quality on mass loss across biomes
Across all biomes, tea mass remaining after three months of field incubation (Fig. 3) was higher for Rooibos tea (78%, SD = 10.31) than for Green tea (38%, SD = 15.86). Overall, similar mass loss patterns were recorded for both tea types across biomes with tendencies or significantly higher mass loss at warm and humid climates compared to the dry and/or cold biomes. However, there was a significant interaction between biome and tea type (F = 84; P < .01) indicating that some differences between
Discussion
The early stage of litter decomposition is a highly dynamic phase and therefore important for the understanding of litter decay and the controlling factors across biomes and ecosystem types. Here we studied the early stage mass loss of two standardized litter types (Green tea and Rooibos tea) across 336 sites globally and found that the litter type (quality) was the main determinant of the mass loss while climate and land use had little effect.
Conclusions
Our study showed that litter type has the strongest influence on mass loss globally in the early stage of decomposition, while the effect of climate was only important under less favorable climatic conditions and when data were aggregated at the biome scale. This finding is particularly relevant for the general understanding of litter and carbon dynamics in relation to biosphere-atmosphere feedback, since the early stage litter decay is responsible for a significant fraction of the carbon loss
Acknowledgements
This work was performed within the TeaComposition initiative, carried out by 190 institutions worldwide. We thank Gabrielle Drozdowski for her help with the packaging and shipping of tea, Zora Wessely and Johannes Spiegel for the creative implementation of the acknowledgement card, Josip Dusper for creative implementation of the graphical abstract, Christine Brendle for the GIS editing, and Marianne Debue for her help with the data cleaning. Further acknowledgements go to Adriana Principe,
Authorship
ID designed and coordinated the study with extensive input from CB. IKS, SKR, KSL accomplished data collection and preparation. SKR conducted statistical analyses. KV and BB provided inputs for manuscript concept. ID wrote the manuscript with contribution from all authors. The TeaComposition team implemented the study and provided site specific and climatic data. The authors declare no conflict of interest.
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