Direct and understorey-mediated indirect effects of human-induced environmental changes on litter decomposition in temperate forest

Human-induced environmental changes in temperature, light availability due to forest canopy management, nitrogen deposition, and land-use legacies can alter ecosystem processes such as litter decomposition. These influences can be both direct and indirect via altering the performance of understorey vegetation. To identify the direct and indirect effects of environmental changes on litter decomposition, we performed an experiment with standardised green and rooibos teas. The experiment was conducted in a temperate mixed deciduous forest, and treatments (temperature, light, and nitrogen) were applied to mesocosms filled with ancient and post-agricultural forest soil. Both green tea and rooibos teas were more rapidly decomposed in oligotrophic soil than in eutrophic soil. The direct effects of the treatments on litter decomposition varied among the two litter types, incubation times, and soil fertility groups. Warming and agricultural legacy had a negative direct effect on the decomposition of the green tea in the high soil fertility treatment during the early decomposition stage. In contrast, agricultural legacy had a positive direct effect on the decomposition of rooibos tea. Soil enriched with nitrogen had a negative direct effect on the decomposition of green tea in mesotrophic soil in the early decomposition stage and on rooibos tea in later stage. The indirect effects of the treatments were consistently negative, as treatments (especially the temperature and light treatments in the early decomposition stage) had a positive effect on plant cover, which negatively affected litter decomposition. Our results indicate that warming, increased nitrogen deposition, and land use legacy can directly stimulate the decomposition of labile litter on more fertile soils. Furthermore, warming and increased light had stronger positive direct effects on understorey herbaceous cover, which leads to slower decomposition rates, especially in more fertile soils. Therefore, the indirect effects of environmental changes related to the understorey layer on litter decomposition can be more important than their direct effects, thus should not be overlooked

Effects of Climate and Atmospheric Nitrogen Deposition on Early to Mid-Term Stage Litter Decomposition Across Biomes

Litter decomposition is a key process for carbon and nutrient cycling in terrestrial ecosystems and is mainly controlled by environmental conditions, substrate quantity and quality as well as microbial community abundance and composition. In particular, the effects of climate and atmospheric nitrogen (N) deposition on litter decomposition and its temporal dynamics are of significant importance, since their effects might change over the course of the decomposition process. Within the TeaComposition initiative, we incubated Green and Rooibos teas at 524 sites across nine biomes. We assessed how macroclimate and atmospheric inorganic N deposition under current and predicted scenarios (RCP 2.6, RCP 8.5) might affect litter mass loss measured after 3 and 12 months. Our study shows that the early to mid-term mass loss at the global scale was affected predominantly by litter quality (explaining 73% and 62% of the total variance after 3 and 12 months, respectively) followed by climate and N deposition. The effects of climate were not litter-specific and became increasingly significant as decomposition progressed, with MAP explaining 2% and MAT 4% of the variation after 12 months of incubation. The effect of N deposition was litter-specific, and significant only for 12-month decomposition of Rooibos tea at the global scale. However, in the temperate biome where atmospheric N deposition rates are relatively high, the 12-month mass loss of Green and Rooibos teas decreased significantly with increasing N deposition, explaining 9.5% and 1.1% of the variance, respectively. The expected changes in macroclimate and N deposition at the global scale by the end of this century are estimated to increase the 12-month mass loss of easily decomposable litter by 1.1-3.5% and of the more stable substrates by 3.8-10.6%, relative to current mass loss. In contrast, expected changes in atmospheric N deposition will decrease the mid-term mass loss of high-quality litter by 1.4-2.2% and that of low-quality litter by 0.9-1.5% in the temperate biome. Our results suggest that projected increases in N deposition may have the capacity to dampen the climate-driven increases in litter decomposition depending on the biome and decomposition stage of substrate.Peer reviewe

Nitrogen budgets in Europe: a methodology to quantify environmentally relevant flows of reactive nitrogen compounds on a national scale

Reactive nitrogen compounds are responsible for multiple negative impacts while they remain in the environment, changing their state and chemical form. Here we develop a methodology to trace these compounds throughout the environment using a stringent concept to describe their fate consistently and comprehensively. Using an individual country as the system scale, the individual flows of reactive nitrogen are characterized between and within eight pools reflecting human society, economic sectors and environmental spheres, also accounting for transboundary flows, to create a national nitrogen budget. The methodology has been devised for implementation by national agencies in conjunction with greenhouse gas or air pollution emission inventories, hence it links closely with the structures and data derived in these contexts. The guiding methodological principle is the mass conservation of reactive nitrogen, implemented as a material flow analysis that systematically describes all flows and stock changes. Embedding results obtained from five European countries demonstrates the feasibility of the approach. The major environmental pathways of reactive nitrogen compounds can be traced from industrial processes and agricultural production, including the agri-food chain, indicating levers for policy interventions. Spatial and temporal benchmarking of the results demonstrates comparisons between countries or over time. While further results of practical implementation are needed to assess overall robustness, the budget approach allows for multiple opportunities of data checks and verification to visualize the uncertainty associated to many input data, such as lacking information on nitrogen contents and specific flows, or the relevance of so-far unaccounted-for stocks of reactive nitrogen. Useful applications have been identified that link nitrogen budgets to impacts on human health as well as on ecosystems and the climate, indicating that developing and using national nitrogen budgets may shape improved and information-led policies

The TeaComposition Initiative: Unleashing the power of international collaboration to understand litter decomposition

Collected harmonized data on global litter decomposition are of great relevance for scientists, policymakers, and for education of the next generation of researchers and environmental managers. Here we describe the TeaComposition initiative, a global and open research collaborative network to study organic matter decomposition in a standardized way allowing comparison of decomposition rate and carbon turnover across global and regional gradients of ecosystems, climate, soils etc. The TeaComposition initiative today involves 570 terrestrial and 300 aquatic ecosystems from nine biomes worldwide. Further, we describe how to get involved in the TeaComposition initiative by (a) implementing the standard protocol within your study site, (b) joining task forces in data analyses, syntheses and modelling efforts, (c) using collected data and samples for further analyses through joint projects, (d) using collected data for graduate seminars, and (e) strengthening synergies between biogeochemical research and a wide range of stakeholders. These collaborative efforts within/emerging from the TeaComposition initiative, thereby, will leverage our understanding on litter decomposition at the global scale and strengthen global collaborations essential for addressing grand scientific challenges in a rapidly changing world.This work was performed within the TeaComposition and TeaComposition H2O initiatives, carried by 290 institutions worldwide. We thank to UNILEVER for sponsoring the Lipton tea bags. The initiative is supported by the following grants: ILTER Initiative Grants, ClimMani Short-Term Scientific Missions Grants, INTERACT Remote Transnational Access and an Alfred Deakin Postdoctoral Research Fellowship. Nico Eisenhauer gratefully acknowledges the support of iDiv funded by the German Research Foundation (DFG– FZT 118, 202548816). ST-T was supported by the ARC DE210101029 and Deakin University’s ADPR Fellowship. Fernando T. Maestre acknowledges support from the European Research Council (ERC Grant agreement 647038 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041)

Reading tea leaves worldwide: Decoupled drivers of initial litter decomposition mass-loss rate and stabilization

The breakdown of plant material fuels soil functioning and biodiversity. Currently, process understanding of global decomposition patterns and the drivers of such patterns are hampered by the lack of coherent large-scale datasets. We buried 36,000 individual litterbags (tea bags) worldwide and found an overall negative correlation between initial mass-loss rates and stabilization factors of plant-derived carbon, using the Tea Bag Index (TBI). The stabilization factor quantifies the degree to which easy-to-degrade components accumulate during early-stage decomposition (e.g. by environmental limitations). However, agriculture and an interaction between moisture and temperature led to a decoupling between initial mass-loss rates and stabilization, notably in colder locations. Using TBI improved mass-loss estimates of natural litter compared to models that ignored stabilization. Ignoring the transformation of dead plant material to more recalcitrant substances during early-stage decomposition, and the environmental control of this transformation, could overestimate carbon losses during early decomposition in carbon cycle models

Reading tea leaves worldwide: decoupled drivers of initial litter decomposition mass‐loss rate and stabilization

The breakdown of plant material fuels soil functioning and biodiversity. Currently, process understanding of global decomposition patterns and the drivers of such patterns are hampered by the lack of coherent large‐scale datasets. We buried 36,000 individual litterbags (tea bags) worldwide and found an overall negative correlation between initial mass‐loss rates and stabilization factors of plant‐derived carbon, using the Tea Bag Index (TBI). The stabilization factor quantifies the degree to which easy‐to‐degrade components accumulate during early‐stage decomposition (e.g. by environmental limitations). However, agriculture and an interaction between moisture and temperature led to a decoupling between initial mass‐loss rates and stabilization, notably in colder locations. Using TBI improved mass‐loss estimates of natural litter compared to models that ignored stabilization. Ignoring the transformation of dead plant material to more recalcitrant substances during early‐stage decomposition, and the environmental control of this transformation, could overestimate carbon losses during early decomposition in carbon cycle models

The handbook for standardized field and laboratory measurements in terrestrial climate change experiments and observational studies (ClimEx)

1. Climate change is a world‐wide threat to biodiversity and ecosystem structure, functioning and services. To understand the underlying drivers and mechanisms, and to predict the consequences for nature and people, we urgently need better understanding of the direction and magnitude of climate change impacts across the soil–plant–atmosphere continuum. An increasing number of climate change studies are creating new opportunities for meaningful and high‐quality generalizations and improved process understanding. However, significant challenges exist related to data availability and/or compatibility across studies, compromising opportunities for data re‐use, synthesis and upscaling. Many of these challenges relate to a lack of an established ‘best practice’ for measuring key impacts and responses. This restrains our current understanding of complex processes and mechanisms in terrestrial ecosystems related to climate change. 2. To overcome these challenges, we collected best‐practice methods emerging from major ecological research networks and experiments, as synthesized by 115 experts from across a wide range of scientific disciplines. Our handbook contains guidance on the selection of response variables for different purposes, protocols for standardized measurements of 66 such response variables and advice on data management. Specifically, we recommend a minimum subset of variables that should be collected in all climate change studies to allow data re‐use and synthesis, and give guidance on additional variables critical for different types of synthesis and upscaling. The goal of this community effort is to facilitate awareness of the importance and broader application of standardized methods to promote data re‐use, availability, compatibility and transparency. We envision improved research practices that will increase returns on investments in individual research projects, facilitate second‐order research outputs and create opportunities for collaboration across scientific communities. Ultimately, this should significantly improve the quality and impact of the science, which is required to fulfil society's needs in a changing world