P. M. van Bodegom

Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient.

Plant litter decomposition is a key process in terrestrial carbon cycling, yet the relative importance of various control factors remains ambiguous at a global scale. A full reciprocal litter transplant study with 16 litter species that varied widely in traits and originated from four forest sites covering a large latitudinal gradient (subarctic to tropics) showed a consistent interspecific ranking of decomposition rates. At a global scale, variation in decomposition was driven by a small subset of litter traits (water saturation capacity and concentrations of magnesium and condensed tannins). These consistent findings, that were largely independent of the varying local decomposer communities, suggest that decomposer communities show little specialisation and high metabolic flexibility in processing plant litter, irrespective of litter origin. Our results provide strong support for using trait-based approaches in modelling the global decomposition component of biosphere-atmosphere carbon fluxes.

Temperature effects on soil methane production : an explanation for observed variability

Abstract Methane production in anaerobically incubated soil depends strongly on temperature. Reported values of Q10, which is the relative increase in activity after an increase in temperature of 10°C, are between 1.3 and 28. We attempted to explain this large variation by considering processes underlying methane production, namely anaerobic carbon mineralisation, reduction of alternative electron acceptors and increased methanogenic activity, each having a well accepted Q10 value of about 2. Q10 is hypothesised to vary due to a temperature-dependent lag time with alternative electron reduction before methane production starts (in incubation experiments). Slurried peat samples were incubated at 4, 10, 13.5, 20 and 30°C, and carbon dioxide, CH4 and fatty acids were intensively monitored for 98 d. The temperature dependence of anaerobic C-mineralisation could be described with Q10 values of 1.1 (4–13.5°C) and 1.5 (13.5–30°C). For other biological processes we assumed a common Q10 of 2. The experiment at 13.5°C was used to calibrate a temperature adapted version of a recently developed simulation model. This model was used to study the dynamic interactions of the temperature dependent subprocesses and simulated CH4 production at different temperatures reasonably well. At low temperatures, electron acceptors and methanogenic biomass limit methane production for a longer time leading to low methane production at low temperatures. This explains why the overall Q10 value calculated from both model (Q10 2–23) and experiment (Q10 2.9–3.6) varied in time and was larger than the Q10 values of the individual subprocesses (Q10 1.1–2). Some of the reported high overall Q10 values could be explained in the same way.

A process-based model for methane emission predictions from flooded rice paddies

Estimation and prediction of methane emission from flooded rice paddies is impaired by the large spatial and temporal variability in methane emissions and by the dynamic nonlinear relations between processes underlying methane emissions. This paper describes a process-based model on methane emission prediction from flooded rice paddies that can be used for extrapolation. The model is divided into two compartments; rhizosphere, which is a function of root length density, and bulk soil. The production of carbon substrates drives methane emission and originates from soil organic matter mineralization, organic fertilizer decomposition, in both compartments, and root exudation and root decay, in the rhizosphere compartment only. It is assumed that the methanogens are completely outcompeted for acetate by nitrate and iron reducers but that competition takes place with sulfate reducers. Produced methane is transported to the root surface in the rhizosphere or the soil-water interface in the bulk soil. Transport time coefficients are different for the two compartments. Part of the methane is oxidized, a constant fraction of produced methane in the bulk soil, whereas the oxidation fraction varies according to root activity dynamics in the rhizosphere. The remaining methane is emitted to the atmosphere. The model was validated with independent field measurements of methane emissions at sites in the Philippines, China, and Indonesia with only few generally available site-specific input parameters. The model properly predicts methane emission dynamics and total seasonal methane emission for the sites in different seasons and under different inorganic and organic fertilizer conditions. A sensitivity analysis on model assumptions showed that the assumptions made in this model are reasonable and that the division into two compartments was necessary to obtain good results with this model. The combination of proper prediction and the necessity of few input parameters allow model application at regional and global scales.

Litter stoichiometric traits of plant species of high‐latitude ecosystems show high responsiveness to global change without causing strong variation in litter decomposition

• High-latitude ecosystems are important carbon accumulators, mainly as a result of low decomposition rates of litter and soil organic matter. We investigated whether global change impacts on litter decomposition rates are constrained by litter stoichiometry. • Thereto, we investigated the interspecific natural variation in litter stoichiometric traits (LSTs) in high-latitude ecosystems, and compared it with climate change-induced LST variation measured in the Meeting of Litters (MOL) experiment. This experiment includes leaf litters originating from 33 circumpolar and high-altitude global change experiments. Two-year decomposition rates of litters from these experiments were measured earlier in two common litter beds in sub-Arctic Sweden. • Response ratios of LSTs in plants of high-latitude ecosystems in the global change treatments showed a three-fold variation, and this was in the same range as the natural variation among species. However, response ratios of decomposition were about an order of magnitude lower than those of litter carbon/nitrogen ratios. • This implies that litter stoichiometry does not constrain the response of plant litter decomposition to global change. We suggest that responsiveness is rather constrained by the less responsive traits of the Plant Economics Spectrum of litter decomposability, such as lignin and dry matter content and specific leaf area.

Trace elements and carbon and nitrogen stable isotopes in organisms from a tropical coastal lagoon

Trace elements (Fe, Mn, Al, Zn, Cr, Cu, Ni, Pb, Cd, Hg, and As) and stable isotope ratios (δ13C and δ15N) were analyzed in sediments, invertebrates, and fishes from a tropical coastal lagoon influenced by iron ore mining and processing activities to assess the differences in trace element accumulation patterns among species and to investigate relations with trophic levels of the organisms involved. Overall significant negative relations between trophic level (given by 15N) and trace element concentrations in gastropods and crustaceans showed differences in internal controls of trace element accumulation among the species of different trophic positions, leading to trace element dilution. Generally, no significant relation between δ15N and trace element concentrations was observed among fish species, probably due to omnivory in a number of species as well as fast growth. Trace element accumulation was observed in the fish tissues, with higher levels of most trace elements found in liver compared with muscle and gill. Levels of Fe, Mn, Al, and Hg in invertebrates, and Fe and Cu in fish livers, were comparable with levels in organisms and tissues from other contaminated areas. Trace element levels in fish muscle were below the international safety baseline standards for human consumption.

Plant responses to rising water tables and nutrient management in calcareous dune slacks

Plant species of oligotrophic wet dune slacks have dramatically decreased as a result of desiccation and eutrophication. The aim of this study was to test in a field experiment the effects of restoration management in oligotrophic, wet dune slacks (groundwater level rise in combination with topsoil removal or mowing) on abiotic variables and on survival and biomass of four plant species. The effect of groundwater level rise on abiotic variables strongly differed between mown sampling locations and those with topsoil removal. At locations with a mowing treatment, a large rise in water tables led to increased N availability and higher reduced iron concentrations than at other locations. Such effects were absent at locations with recent topsoil removal. No effect of groundwater level rise on P-availability was found. Topsoil removal on average lowered N availability by 13%, P availability by 65% and Fe2+ by 56%. All phytometer species survived better in mown dune slacks than in dune slacks that had received topsoil removal. Survival of all species was negatively related to groundwater level rise. On the short term local extinction risks of small populations may be enhanced by rewetting and topsoil removal. On the long-term, however, such measures are crucial to maintain vegetation of oligotrophic wet dune slacks in a degraded dune landscape.

Effects of interpolation and data resolution on methane emission estimates from rice paddies

Rice paddies are an important source of the greenhouse gas methane (CH4). Global methane emission estimates are highly uncertain and do not account for effects of interpolation or data resolution errors. This paper determines such scaling effects for the influence of soil properties on calculated CH4 emissions for the island of Java, Indonesia. The effects of different interpolation techniques, variograms and neighbor optimization were tested for soil properties by cross-validation. Interpolated organic carbon values were not significantly different from the original soil samples, in contrast to interpolated soil iron contents. Interpolation of soil properties coupled to a process-based model on CH4emissions led to a significant change in distribution of calculated CH4 emissions, i.e., the variance decreased. Effects of data resolution were examined by interpolating soil properties to derive data at different data resolutions and then calculating CH4 emissions by applying the process-based model at these resolutions. The soil properties did not differ significantly for different data resolutions, in contrast to calculated CH4 emissions. These scaling effects were caused by the combination of interpolation and a non-linear model. Real scaling effects may even be larger because small-scale variability was not accounted for. Scaling effects, including those caused by small-scale variability, have to be considered to achieve unbiased and less uncertain global CH4emissions estimates from rice paddies.

Plant traits in response to raising groundwater levels in wetland restoration: evidence from three case studies

ABSTRACT Question: Is raising groundwater tables successful as a wetland restoration strategy? Location: Kennemer dunes, The Netherlands; Moksloot dunes, The Netherlands and Bullock Creek fen, New Zealand. Methods: Generalizations were made by analysing soil dynamics and the responsiveness of integrative plant traits on moisture, nutrient regime and seed dispersal in three case studies of rewetted vs. control wetlands with the same actual groundwater levels. Soil conditions included mineral (calcareous and non-calcareous) soils with no initial vegetation, mineral soils with established vegetation and organic soils with vegetation. Results: The responsiveness of traits to raised groundwater tables was related to soil type and vegetation presence and depended on actual groundwater levels. In the moist-wet zone, oligotrophic species, ‘drier’ species with higher seed longevity occupied gaps created by vegetation dieback on rewetting. The other rewetted zones still reflected trait values of the vegetation prevalent prior to rewetting with fewer adaptations to wet conditions, increased nutrient richness and higher seed longevity. Moreover, ‘eutrophic’ and ‘drier’ species increased at rewetted sites, so that these restored sites became dissimilar to control wetlands. Conclusions: The prevalent traits of the restored wetlands do not coincide with traits belonging to generally targeted plant species of wetland restoration. Long-term observations in restored and control wetlands with different groundwater regimes are needed to determine whether target plant species eventually revegetate restored wetlands. Abbreviations: F = Mean moisture indicator values, lnRR = Natural log of response ratio, N = Nutrient indicator value, PCA = Principal Component Analysis, RDA = Redundancy analysis

Diffusive gas transport through flooded rice systems

A fully mechanistic model based on diffusion equations for gas transport in a flooded rice system is presented. The model has transport descriptions for various compartments in the water-saturated soil and within the plant. Plant parameters were estimated from published data and experiments independent of the validation experiment. An independent experiment is described in which the diffusion coefficient of sulfurhexafluoride (SF6) in water-saturated soil was determined. The model was validated by experiments in which transport of SF6 through soil and plant was monitored continuously by photoacoustics. The independent default settings could reasonably predict gas release dynamics in the soil-plant system. Calculated transmissivities and concentration gradients at the default settings show that transport within the soil was the most limiting step in this system, which explains why most gases are released via plant-mediated transport. The root-shoot interface represents the major resistance for gas transport within the plant. A sensitivity analysis of the model showed that gas transport in such a system is highly sensitive to the estimation of the diffusion coefficient of SF6, which helps to understand diel patterns found for greenhouse gas emissions, and to the root distribution with depth. This can be understood from the calculated transmissivities. The model is less sensitive to changes in the resistance at the root-shoot interface and in the root fraction active in gas exchange. The model thus provides an understanding of limiting steps in gas transport, but quantitative predictions of in situ gas transport rates will be difficult given the plasticity of root distribution.

Quantification of uncertainties in global grazing systems assessment

Livestock systems play a key role in global sustainability challenges like food security and climate change, yet, many unknowns and large uncertainties prevail. We present a systematic, spatially explicit assessment of uncertainties related to grazing intensity (GI), a key metric for assessing ecological impacts of grazing, by combining existing datasets on a) grazing feed intake, b) the spatial distribution of livestock, c) the extent of grazing land, and d) its net primary productivity (NPP). An analysis of the resulting 96 maps implies that on average 15% of the grazing land NPP is consumed by livestock. GI is low in most of worlds grazing lands but hotspots of very high GI prevail in 1% of the total grazing area. The agreement between GI maps is good on one fifth of the worlds grazing area, while on the remainder it is low to very low. Largest uncertainties are found in global drylands and where grazing land bears trees (e.g., the Amazon basin or the Taiga belt). In some regions like India or Western Europe massive uncertainties even result in GI > 100% estimates. Our sensitivity analysis indicates that the input-data for NPP, animal distribution and grazing area contribute about equally to the total variability in GI maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available global level datasets is a precondition for improving the understanding of the role of livestock systems in the context of global environmental change or food security.