Underlying Mineral Soil Research Articles

Implications of alder shrub growth for alpine tundra soil properties in Interior Alaska

ABSTRACT The increase in deciduous shrub growth in response to climate change throughout the Arctic tundra has uncertain implications, in part due to a lack of field observations. Here we investigate how increasing alder shrub growth in alpine tundra in Interior Alaska corresponds to active layer thickness and soil physical properties. We documented increased alder growth by combining biomass harvests and dendrochronology with the analysis of remotely sensed Normalized Difference Vegetation Index and fire history. Active layer thickness was measured with a tile probe and carbon and nitrogen pools were assessed via elemental analysis. Shallower organic layers under increasing alder growth indicate that nitrogen-rich, deciduous litter inputs may play a role in accelerating decomposition. Despite the observed reduction in organic carbon stocks, active layer thickness was the same under alder and adjacent graminoid tundra, implying deeper thaw of the underlying mineral soil. This study provides further evidence that the widely observed expansion of deciduous shrubs into graminoid tundra will reduce ecosystem carbon stocks and intensify soil–atmosphere thermal coupling.

Gamma radiation for the estimation of mineral soil water content in a boreal forest

Continuous monitoring of water quantities in different soil horizons is necessary to understand the behavior of infiltrated water in the soil. Under certain conditions, using measurements of natural ground gamma radiation can help us estimate soil water content measurements over a 100 m2 surface within a 15 cm depth. A CS725 sensor can provide up to four daily estimates of soil water content by detecting the natural emission of gamma radiation. However, in boreal forest environments, gamma radiation mitigated by the water in the thick humus layers (litter, fermented, and humic horizon) can bias the underlying mineral soil water content measurements. The objective of this research was to evaluate the accuracy of methods that incorporate variables describing the surface humus layer into calculations of the underlying mineral soil water content, by measuring the soil’s natural gamma emission with the CS725. Using raw gamma radiation values obtained by CS725 sensors deployed over various boreal soils, we tested two functions. The first one included variables describing the humus layer and the other excluded these variables (manufacturer’s method). The function that included the descriptive humus layer variables showed superior results compared to the function without. The results of this study suggest that the CS725 sensor can adequately estimate mineral soil water content within ±10% absolute of the reference water content when examined with the following humus variables: humus layer thickness, fractioned composition, bulk density, and linear gamma radiation attenuation.

Assessing soil carbon dynamics following mechanical site preparation in boreal lichen woodlands of Québec, Canada

Soil scarification is a common soil mechanical preparation (MSP) method in forestry aimed to enhance the success of reforestation. However, by turning over and displacing the humus (forming a mound; M), and producing furrows of bare mineral soil (F), it is believed that this method has a substantial effect on soil C stocks. Here, we assessed the influence of this MSP on soil C stocks in two boreal lichen woodlands (LWs) in Québec, Canada, scarified ten and two years before sampling (2012 and 2020 sites, respectively).Carbon stocks averaged 11.8–15.4 kg C m−2 in the undisturbed section of the LWs (IF) with 9–15 % located in the vegetation, 36–42 % in the humus and 42–55 % in the mineral soil (0–30 cm). MSP caused a C concentration decline of ∼105 g C kg−1 in the displaced humus. Unexpectedly, this decrease was similar at both sites, suggesting that most C losses from this horizon occurred within two years. Higher C stock in the mineral horizons of the M than of the IF compartment at the 2012 site, suggests that a fraction of the C lost from the displaced humus may not have been released to the atmosphere but rather translocated to the underlying mineral soil. However, no such phenomenon was detected at the 2020 site, where a net C loss from the mineral soil was observed. Overall, the use of mass balance equations did not show evidence of net C losses at the scale of the whole profile in the disturbed sections of both LWs following MSP. This was partly due to site heterogeneity and a sampling bias, but also to mechanisms such as C translocation from the organic to the mineral soil horizons, and the replenishment of the soil C pool owing to natural regeneration occurring at the surface of the furrows. Overall, this study suggests that MSP may not have as strong an impact on soil C stocks as previously believed. Additionally, it highlights the challenges associated with estimating C changes in the soil following MSP and discusses ways to improve the assessment of C dynamics in such heterogeneous habitats.

Catchment characteristics control boreal mire nutrient regime and vegetation patterns over ~5000 years of landscape development

Vegetation holds the key to many properties that make natural mires unique, such as surface microtopography, high biodiversity values, effective carbon sequestration and regulation of water and nutrient fluxes across the landscape. Despite this, landscape controls behind mire vegetation patterns have previously been poorly described at large spatial scales, which limits the understanding of basic drivers underpinning mire ecosystem services. We studied catchment controls on mire nutrient regimes and vegetation patterns using a geographically constrained natural mire chronosequence along the isostatically rising coastline in Northern Sweden. By comparing mires of different ages, we can partition vegetation patterns caused by long-term mire succession (<5000 years) and present-day vegetation responses to catchment eco-hydrological settings. We used the remote sensing based normalized difference vegetation index (NDVI) to describe mire vegetation and combined peat physicochemical measures with catchment properties to identify the most important factors that determine mire NDVI. We found strong evidence that mire NDVI depends on nutrient inputs from the catchment area or underlying mineral soil, especially concerning phosphorus and potassium concentrations. Steep mire and catchment slopes, dry conditions and large catchment areas relative to mire areas were associated with higher NDVI. We also found long-term successional patterns, with lower NDVI in older mires. Importantly, the NDVI should be used to describe mire vegetation patterns in open mires if the focus is on surface vegetation, since the canopy cover in tree-covered mires completely dominated the NDVI signal. With our study approach, we can quantitatively describe the connection between landscape properties and mire nutrient regime. Our results confirm that mire vegetation responds to the upslope catchment area, but importantly, also suggest that mire and catchment aging can override the role of catchment influence. This effect was clear across mires of all ages, but was strongest in younger mires.

Medium-term effects of wildfire severity on soil physical, chemical and biological properties in Pinus halepensis Mill. woodland (Southern Italy): an opportunity for invasive Acacia saligna colonization?

Wildfire frequency and severity have greatly increased in Mediterranean areas in recent decades affecting ecosystems functioning due to alteration in the above- and below-ground process. This study aimed to investigate how wildfire severity, in the medium-term (2–5 years), impacts soil properties within a Pinus halepensis woodland located in the Special Area of Conservation (SAC) of the Natura 2000 network (IT9130006 - Pinewoods of the Ionian Arch). In 2021, four years after a large wildfire in 2017, the woodland still exhibited fire effects with evidence of low, medium or high burn severity in different sites (named LBS, MBS, HBS, respectively). In addition, we observed an area burnt at medium severity that was invaded by Acacia saligna (MBSA site), a fast-growing, highly invasive, drought-tolerant nitrogen-fixing plant, thus we also examined the combined effect of wildfire and A. saligna on the soil. We compared soil properties across burnt sites with a nearby unburnt site (control). Thickness, weight and organic carbon of litter (L) and fermentation (F) layers were measured, alongside physical, chemical and biological properties in the underlying mineral soil (0–10 cm). Our results show that wildfire destroyed the organic layers and these had not recovered four years after the wildfire (except for L-layer within LBS) with a consequent loss (∼2 t C ha−1) of this carbon pool. In mineral soil we identified fourfold increases in N mineralization and nitrification rates across all burnt sites, regardless of the burn severity and A. saligna presence, suggesting an alteration of N-cycle processes. On the contrary, total microbial biomass and soil respiration as well as most of the physical and chemical properties of the soil were comparable between the burnt and control soils. Principal Component Analysis (PCA) highlighted that burn severity affected soil variables with lower changes in LBS than in other burnt soils. Also, the HBS soil did not show greater negative impacts compared to MBS sites. This is probably due to the increased post-fire colonization by herbaceous plants in HBS, favoured by the complete destruction of trees. In this case, waiting for natural vegetation recovery can be a valid management option, but periodic monitoring of fire-soil-vegetation interaction mainly to avoid invasive species widespread is advocated.

Enzymes, Manganese, or Iron? Drivers of Oxidative Organic Matter Decomposition in Soils.

Oxidative decomposition of soil organic matter determines the proportion of carbon that is either stored or emitted to the atmosphere as CO2. Full conversion of organic matter to CO2 requires oxidative mechanisms that depolymerize complex molecules into smaller, soluble monomers that can be respired by microbes. Current models attribute oxidative depolymerization largely to the activity of extracellular enzymes. Here we show that reactive manganese (Mn) and iron (Fe) intermediates, rather than other measured soil characteristics, best predict oxidative activity in temperate forest soils. Combining bioassays, spectroscopy, and wet-chemical analysis, we found that oxidative activity in surface litters was most significantly correlated to the abundance of reactive Mn(III) species. In contrast, oxidative activity in underlying mineral soils was most significantly correlated to the abundance of reactive Fe(II/III) species. Positive controls showed that both Mn(III) and Fe(II/III) species are equally potent in generating oxidative activity, but imply conventional bioassays have a systematic bias toward Fe. Combined, our results highlight the coupled biotic-abiotic nature of oxidative mechanisms, with Mn-mediated oxidation dominating within Mn-rich organic soils and Fe-mediated oxidation dominating Fe-rich mineral soils. These findings suggest microbes rely on different oxidative strategies depending on the relative availability of Fe and Mn in a given soil environment.

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (Fl/t), the migration center (Xc) and the relaxation depth (α) using 99 soil inventory data sets. Fl/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, Fl/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only Xc increased significantly with time according to generalized mixed model analysis (p < 0.01). However, Xc and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high 137Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of 137Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of 137Cs from organic to surface mineral soil layer after the second year. These results indicate that 137Cs retention capacity of the organic layer can affect the apparent vertical migration of 137Cs in the underlying mineral soil layer.

Convergence, divergence or chaos? Consequences of tree trunk decay for pedogenesis and the soil microbiome in a temperate natural forest

The biochemical effects of trees may significantly influence local pedogenesis as well as pedocomplexity, biodiversity and forest dynamics on both stand and landscape scales. One such effect is the decay of tree trunks, which is driven by organisms, and especially by the microbiome. Decomposition modifies soil formation, which due to the existence of many feedbacks affects the composition of the decomposer community. We aimed to uncover general trends in the evolution of Entic Podzols as well as individual trajectories of soil properties below decaying beech trunks in an old-growth mountain forest. In particular, we used mathematical models to distinguish soil convergence, divergence and chaotic behaviour to enhance a general theory of pedogenesis. We further aimed to calculate the depth and time of convergence if this scenario is prevailing in the study plot. Pedogenetic pathways were assessed regarding the changing composition of fungal and bacterial communities in soils to obtain a complex picture of the decaying trunk-soil microbiome system.We sampled the decaying wood layer under 24 lying beech trunks and corresponding organic horizons on adjacent control microsites occupied by decaying beech leaves. At the same time we sampled underlying mineral soil horizons at both microsites (wood vs. leaves), all on Entic Podzols and granite (in total 192 soil samples). Individual trunk fall events were dated using precise dendrochronology, with the resulting chronosequence covering trunks lying for 8–52 years. We analysed decomposition processes (with a wide spectra of organic acids and ions analysed), soil chemistry (28 additional soil properties assessed), and the microbiome composition in both decaying organic matter and soils (relative abundances of the 200 most common bacterial and fungal OTUs analysed).During the first stage of trunk decay, underlying Entic Podzols responded with a significant increase of nutrients, pH, and CEC, and the maximal divergence compared to control sites was reached between 12 and 60 years after the trunk fall. Subsequently, a majority of soil properties slowly converged over a few decades to match the soil properties of control sites. The modelled convergence point occurred at ages between 39 (SO42−) and 229 (Alw) years, with a median convergence time of 53 years. The majority of soil properties converged within 20 cm below the trunk, but mathematical models predicted footprints of some soil properties down to depths of ca 60 cm. In addition, 11 soil properties did not converge at any depth, and for some properties the models even diverged. Differences in bacterial and fungal communities between below-trunk and control positions were relatively minor. Pedochemical drivers of fungal and bacterial communities (nutrients content, Ntot, Cox, Al, Fe, Mn forms) changed significantly in the mineral soil below trunks, and the microbiome partly reflected these depth-related changes. However, we propose that there is a threshold between organic and mineral soil horizons limiting the impacts of trunk decay and pedogenesis in changes to the microbiome.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified 137Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated 137Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the 137Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of 137Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1–2 years after the accident) for Japanese forest ecosystems.

Seasonal evolution of active layer thaw depth and hillslope‐stream connectivity in a permafrost watershed

AbstractTo advance our understanding of permafrost hillslope drainage dynamics and its influence on streamflow hydrochemistry, we instrumented a hillslope‐stream sequence located in the headwaters of the Niaqunguk River watershed, Nunavut, Canada (63°N, 68°W). We combined high spatial resolution field measurements of water and frost tables across the hillslope, with semiweekly measurements of groundwater and streamflow chemistry to track the evolution of streamflow chemistry during active layer thaw. Interestingly, localized differential thaw patterns emerged under near saturation conditions across the instrumented hillslope, the result of a low‐frequency high‐magnitude summer rainfall event. Hillslope structure and uneven active layer thaw created two distinct fill‐and‐spill domains. A subsurface domain defined by frost table microtopography and a surface domain defined by surface topography. We observed a seasonal shift in streamflow chemistry with an increased influence of water flowing through the underlying mineral soils as the active layer thawed. As thaw progressed streamflow chemistry began to match that of the riparian groundwater, which was a mixture of hillslope surface and subsurface water. Hillslope‐stream surface connections were sporadic and occurred when rainfall and saturation conditions across the lower portion of the hillslope were sufficient for water to spill out of midslope surface depressions and across a saturated riparian zone and into the stream. This research shows how hillslope structure and thaw processes influence hillslope‐stream connectivity in permafrost terrain.

Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with 137Cs. It is important to characterize the behavior of 137Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, 137Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and 137Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of 137Cs could be suppressed rapidly in Japanese forests. At all sites, 137Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous 137Cs supply from the organic layer. The diffusion coefficients of 137Cs in the mineral soil were estimated to be 0.042–0.55 cm2 y−1, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited 137Cs will migrate to the mineral soil but only less than 10% of the total 137Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the 137Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

Can alkaline residuals from the pulp and paper industry neutralize acidity in forest soils without increasing greenhouse gas emissions?

Alkaline residuals, such as wood ash and lime mud generated from pulp and paper mills, could be recycled as liming agents in sugar maple (Acer saccharum Marsh.) forests affected by soil acidification. The objectives of this study were (1) to evaluate soil chemistry, in particular soil acidity, after the application of three alkaline residuals from the pulp and paper industry, and (2) to determine if these alkaline residuals altered soil greenhouse gas (GHG) emissions as a result of the change in soil pH or due to their chemical composition. Soil properties and GHG fluxes were monitored for two years after alkaline residuals were applied to six forest sites dominated by sugar maple in southeastern Quebec, Canada. Each site received six treatments: wood ash applied at 5, 10 and 20 t ha−1, lime mud (7.5 t ha−1), a mixture of slaker grits and green liquor sludge (7 t ha−1) and an unamended control. These treatments had acid-neutralizing power from 0 to 9 t ha−1. All alkaline residuals buffered soil acidity as a function of their neutralizing power, and more neutralization occurred in the forest floor layer than in the underlying mineral soil. In the forest floor, the alkaline residual treatments significantly increased pH by more than one unit, nearly doubled the base saturation, and reduced exchangeable acidity, Al and Fe concentrations compared to control plots. The CO2 and N2O fluxes were lower after application of alkaline residuals, and this was related to the soil pH increase and the type of alkaline residual applied. Lime mud was more effective at reducing GHG fluxes than other alkaline residuals. We conclude that these alkaline residuals can effectively counteract soil acidity in sugar maple forests without increasing soil GHG emissions, at least in the short term.

Changing in Water-physical Properties of Drained Peat Soils during Extraction and Exploration of Minerals in the Conditions of the Northern Urals

Results of long-term (1976-2016) researches of water-physical properties of long seasonally-permafrost peat soils of Northern Trans-Urals are stated. It is shown that the shrinkage process has a significant impact on the density of peat soil, especially in the first years after drainage. In the Northern Urals, the annual decrease in the size of peat soil is 1.5 cm in the first five years after drainage, 1-1.2 cm - for 15 years and 0.6-0.7 cm - in subsequent years. In the first five years after drainage, the density of the 0.2 m layer increases by 7.4%, decreasing to 2.1-3.0% in subsequent years. At a depth of 0.6-1.0 m, 5 years after drainage, there are no significant changes in the addition density. A significant increase in the addition density of peaty - gley soil established by plowing the underlying mineral soil. A relatively stable indicator is the density of the solid phase of the soil. For 35 years, the density of the solid phase of the average peat soil in the 0.3 m layer increased by 5.4%; low - power for 20 years - increased by 1.2%. The lowest moisture content of the average peat soil for the 35-year period in the root (0.3 m) layer decreased by 11.5%; at a depth of 0.6 – 1.0 m – by 3.7%. In low-power peat soil reduction of moisture capacity in the 0.3 m layer for 20 years was 6.4%; in peat (0.2 m) layer of peaty - gley soil for the same period – by 10.2%.

Modelling Dissolved Organic Carbon Production in Coniferous Forest Soils

Core Ideas Mechanisms of DOC production affect the quantities and qualities of SOC and DOC. Three models with different physical and microbial processes were analyzed. Comparisons with experimental data provides no strong discrimination between models. The production of dissolved carbon (DOC) in forest soils is still not well understood as a result of difficulties in isolating co‐occurring processes. We have used an extension of the continuous‐quality theory to analyze the consequences of three modes for DOC production in coniferous forest soils: Model A, the production of DOC occurs through physical leaching of soil organic carbon (SOC) and the composition of DOC is proportional to the composition of SOC with respect to carbon (C) qualities; Model B, the production of DOC occurs when the decomposers use SOC and the composition of DOC will be proportional to the composition of SOC that decomposers extract; and Model C, the production of DOC is associated with mortality of decomposers and the composition of DOC reflects the composition of decomposer biomass. The results from Models B and C resemble each other but differ from Model A. Models B and C lead to a larger fraction of C being lost through leaching, a higher quality of the C leached, and a lower 14C age of the leached C than Model A. Models B and C differ with respect to properties of DOC leached from a horizon. All three models were found capable of describing stores of C in and leaching of DOC from an organic horizon and an underlying mineral soil in three temperate/boreal Norway spruce [Picea abies (L.) Karst.] forests. Microbial activity should always be an important agent for DOC production. Model B and, in particular, Model C suggest a lager contribution of microbially processed C in DOC than Model A.

Nitrogen-rich compounds constitute an increasing proportion of organic matter with depth in Oi-Oe-Oa-A horizons of temperate forests

Soil organic matter (SOM) in O horizons of temperate forests forms a natural gradient of increasing decomposition, as operationally separated into the Oi (leaf litter), Oe (partially decomposed), and Oa (highly decomposed) sub-horizons. This study determines the contribution of extractable nitrogen (N)-rich biomolecules and characterizes changes in SOM composition within O horizon decomposition gradients and in underlying A horizons of three temperate forest stands. Four biomolecules were chosen for this investigation: chlorophyll, protein, DNA, and chitin; combined, they represent the majority of identifiable N-rich organic compound classes present in soils. Selective chemical extractions and purification methods provide estimates of concentrations of each distinct biomolecule class, while short-term respiration measurements provide insight into possible relationships with microbial activity. Changes in the molecular character of SOM are probed using Fourier Transform Infrared (FTIR) and Carbon K-edge X-ray Absorption Near Edge Structure (C-XANES) spectroscopies. Differences in biomolecule concentrations and respiration by horizon reveal two contrasting trends with increasing decomposition and depth: chlorophyll and respiration decrease, whereas protein, DNA and chitin increase. Increasing contribution of these three extractable N-rich biomolecules with progressive decomposition and depth at all sites correspond to decreasing concentrations of C and N and to increases in C:N ratio. These trends are consistent with FTIR spectral results, which indicate a relative increase of NH bending and aromatic CN vibrations, and with C-XANES spectral results, which show a relative increase of aromatic CN transitions. These multiple lines of evidence demonstrate that extractable N-rich biomolecules constitute a growing proportion of the organic C and N content within an organic matter decomposition gradient and that this trend continues into the underlying mineral soil. We posit that the persistence of these intact N-containing biomolecules represents a growing contribution from the microbial biomass pool, but also from non-specific interactions among biomolecules in the Oe and Oa horizons and from biomolecule-mineral interactions in mineral soils. The results of this investigation (increases in the proportion of extractable N-rich biomolecules) also provide valuable insight into the potential bioavailability and biochemical structure of SOM in relation to degree of decomposition.

Recovery of carbon pools a decade after wildfire in black spruce forests of interior Alaska: effects of soil texture and landscape position

We measured organic-layer (OL) recovery and carbon stocks in dead woody debris a decade after wildfire in black spruce (Picea mariana (Mill.) B.S.P.) forests of interior Alaska. Previous study at these research plots has shown the strong role that landscape position plays in governing the proportion of OL consumed during fire and revegetation after fire. Here, we show that landscape position likely influences fire dynamics in these stands through changes in mineral soil texture. The content of fine-textured materials in underlying mineral soils was positively related to OL depths measured 1 and 10 years after fire, and there was an interaction between soil texture and elevation in governing OL consumption and OL recovery a decade following fire. OL depths 10 years after fire were 2 cm greater than 1 year after fire, with a range of 19 cm of accumulation to 9 cm of subsidence. Subsidence was inversely related to the percentage of fine textures within the parent material. The most influential factor determining the accumulation of OL carbon stocks a decade following wildfire was the interaction between landscape position and the presence of fine-textured soil. As such, parent material texture interacted with biological processes to govern the recovery of soil organic layers.

Stoichiometric plasticity of microbial communities is similar between litter and soil in a tropical rainforest

Heterotrophic microorganisms are commonly thought to be stoichiometrically homeostatic but their stoichiometric plasticity has rarely been examined, particularly in terrestrial ecosystems. Using a fertilization experiment in a tropical rainforest, we evaluated how variable substrate stoichiometry may influence the stoichiometry of microbial communities in the leaf litter layer and in the underlying soil. C:N:P ratios of the microbial biomass were higher in the organic litter layer than in the underlying mineral soil. Regardless of higher ratios for litter microbial communities, C, N, and P fertilization effects on microbial stoichiometry were strong in both litter and soil, without any fundamental difference in plasticity between these two communities. Overall, N:P ratios were more constrained than C:nutrient ratios for both litter and soil microbial communities, suggesting that stoichiometric plasticity arises because of a decoupling between C and nutrients. Contrary to the simplifying premise of strict homeostasis in microbial decomposers, we conclude that both litter and soil communities can adapt their C:N:P stoichiometry in response to the stoichiometric imbalance of available resources.

Water-physical properties of drained peat soils of Northern Trans-Ural forest-steppe zone

The results of long-term studies (1976-2016) of water-physical properties of long-seasonally-frozen peat soils of Northern Trans-Ural are set out. It is shown that the changes in water-physical properties are functional in nature and primarily due to economic use of soil and the condition of its surface. A significant influence on the composition density of peat soils, especially in the first years after drainage, has the process of shrinking. In the conditions of Northern Trans-Ural annual reduction in the value of peat soils is 1.5 cm in the first five years after drainage, 1-1.2 cm for 15 years and 0.6-0.7 cm in subsequent years. In the first five years after drainage, the density of the composition in the layer 0.2 m increased by 7.4%, declining to 2.1-3.0% in subsequent years. At a depth of 0.6-1.0 m in 5 years after drainage there is no significant change of addition density. A significant increase in the density of the addition of peat-gley soil established by plowing underlying mineral soil. A relatively stable indicator is the density of the solid phase of the soil. For 35 years, the density of the solid phase medium peat soil layer of 0.3 m increased by 5.4%; low power over a 20 year period by 1.2%. The least moisture capacity medium peat soil over a 35-year period in the rooting zone (0.3 m) layer was reduced by 11.5%; at a depth of 0.6-1.0 m - by 3.7%. In thin peat soil reduction of water capacity in the layer of 0.3 m in 20 years was 6.4%; in peat (0.2 m) layer of peaty gley soil during the same period - by 10.2%.

Tropical Montane Cloud Forests in the Orinoco river basin: The role of soil organic layers in water storage and release

The Tropical Montane Cloud Forest (TMCF) is a hydrologically unique and highly vulnerable ecosystem to changes in land-use and climate. Assessing the impacts of these changes needs to consider soil-water dynamics. In particular, the organic layer and its functioning requires attention because its difference in water retention characteristics and root density compared to the underlying mineral soils. A higher root density and a higher water storage capacity of the organic layer compared to the mineral soils suggests that most nutrient and water uptake occurs in this layer. However, hydraulic properties of mineral soils and their impact on hydrology of TMCFs have been poorly studied. Here we provide organic layer water retention curves for TMCF soils that were measured in the laboratory. With these data we assessed the potential land-use and climate change impacts on TMCF soil moisture dynamics. From the land-use change perspective, we estimated the water storage capacity loss by slash-and-burn deforestation practices. These estimates show that the storage loss ranges from 35 to 59mm when TMCFs are converted to pastures. This is several times higher than the estimated TMCF canopy storage capacity (2 to 5mm). Therefore, the higher peak flow observations in deforested catchments might not only be explained by a decreasing canopy water storage but also likely due to a decreasing soil water storage. From the climate change perspective, we evaluated the effect of contrasting dry season conditions on soil moisture and transpiration using a 1-D water-flow model, and assessed the sensitivity of these hydrological variables to uncertainties in saturated hydraulic conductivity. Although transpiration was not limited by soil moisture during the mild dry season, it was affected during the severe dry season and continued to decline under expected climate change with the prolongation of dry spells. Our results show that the organic layer is a key element in TMCF's hydrological functioning and call for an increased focus on the role of organic soils when evaluating effects of land-use change.

Pore water velocity and ionic strength effects on DOC release from peat-sand mixtures: Results from laboratory and field experiments

Organic soils are the most important source of dissolved organic carbon (DOC) in surface water. To date, most studies have focused on natural and re-wetted peatlands, but in Central Europe a large proportion of organic soils are drained and under agricultural use. Furthermore, measures such as deep ploughing or sand addition have been conducted to improve trafficability and have resulted in topsoil horizons consisting of a peat-sand mixture. Very little is known about DOC losses from such soils. Moreover, peat soils frequently feature both mobile zones, characterised by active water and solute transport, and immobile zones, which exchange solutes with the mobile zone by diffusion. Surprisingly, however, the effects of this dual porosity on DOC transport have not yet been explored. This study investigated the physicochemical controls on DOC concentrations in a peat-sand mixture by means of a saturated column experiment with undisturbed columns. The soil came from a former bog in northern Germany where peat layers remaining after peat extraction were mixed with the underlying mineral soil by ploughing. Three pumping rates and two levels of electrical conductivity (EC) were applied. The transport properties of the soil were obtained by analysing breakthrough curves of potassium bromide using the transport model STANMOD, which is based on the two-region non-equilibrium concept. The results of the column study were compared to DOC concentrations measured bi-weekly for two years at the field site from where the columns were taken. Despite a similar texture and soil organic carbon (SOC) content, the fraction of the mobile zone in the columns varied between 51% and 100% of total porosity. Thus even heavily degraded organic soils mixed with sand still showed a dual porosity comparable to degraded peat soils. Percolating the columns with the high EC solution caused low pH values, probably due to ion exchange and cation bridging. The combination of high EC and low pH greatly decreased DOC concentrations at the outlet of the columns. DOC concentrations decreased and fluxes increased as the pumping rates increased. Taking pore water velocity in the mobile zone into account could help to explain the differences between the columns. Overall, transport of DOC did not seem to be limited by production of DOC, but by rate-limited exchange processes. In contrast to the column experiment, field concentrations of DOC were much higher and were not related to pH, but increased with higher electrical conductivity. These higher concentrations could be explained by low pore water velocities and the slightly higher SOC content in the field. This first experiment on DOC transport in peat-sand mixtures taking the dual-porosity nature of organic soils into account clearly demonstrated the importance of pore water velocity and thus the residence time for DOC concentrations. While hydrochemical conditions are frequently addressed in laboratory studies, there is a need for improved understanding of their interaction with hydrology and soil-physical properties, especially when attempting to interpret DOC data on different spatial and temporal scales.