Connecting molecular biomarkers, mineralogical composition, and microbial diversity from Mars analog lava tubes.

Lanzarote (Canary Islands, Spain) is one of the best terrestrial analogs to Martian volcanology. Particularly, Lanzarote lava tubes may offer access to recognizably preserved chemical and morphological biosignatures valuable for astrobiology. By combining microbiological, mineralogical, and organic geochemistry tools, an in-depth characterization of speleothems and associated microbial communities in lava tubes of Lanzarote is provided. The aim is to untangle the underlying factors influencing microbial colonization in Earth's subsurface to gain insight into the possibility of similar subsurface microbial habitats on Mars and to identify biosignatures preserved in lava tubes unequivocally. The microbial communities with relevant representativeness comprise chemoorganotrophic, halophiles, and/or halotolerant bacteria that have evolved as a result of the surrounding oceanic environmental conditions. Many of these bacteria have a fundamental role in reshaping cave deposits due to their carbonatogenic ability, leaving behind an organic record that can provide evidence of past or present life. Based on functional profiling, we infer that Crossiella is involved in fluorapatite precipitation via urea hydrolysis and propose its Ca-rich precipitates as compelling biosignatures valuable for astrobiology. In this sense, analytical pyrolysis, stable isotope analysis, and chemometrics were conducted to characterize the complex organic fraction preserved in the speleothems and find relationships among organic families, microbial taxa, and precipitated minerals. We relate organic compounds with subsurface microbial taxa, showing that organic families drive the microbiota of Lanzarote lava tubes. Our data indicate that bacterial communities are important contributors to biomarker records in volcanic-hosted speleothems. Within them, the lipid fraction primarily consists of low molecular weight n-alkanes, α-alkenes, and branched-alkenes, providing further evidence that microorganisms serve as the origin of organic matter in these formations. The ongoing research in Lanzarote's lava tubes will help develop protocols, routines, and predictive models that could provide guidance on choosing locations and methodologies for searching potential biosignatures on Mars.

Global dataset of soil organic carbon in tidal marshes.

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha-1 in the top 30 cm and 231 ± 134 Mg SOC ha-1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Relating mineral-organic matter stabilization mechanisms to carbon quality and age distributions using ramped thermal analysis.

Organic carbon (OC) association with soil minerals stabilizes OC on timescales reflecting the strength of mineral-C interactions. We applied ramped thermal oxidation to subsoil B horizons with different mineral-C associations to separate OC according to increasing temperature of oxidation, i.e. thermal activation energy. Generally, OC released at lower temperatures was richer in bioavailable forms like polysaccharides, while OC released at higher temperatures was more aromatic. Organic carbon associated with pedogenic oxides was released at lower temperatures and had a narrow range of 14C content. By contrast, N-rich compounds were released at higher temperatures from samples with 2 : 1 clays and short-range ordered (SRO) amorphous minerals. Temperatures of release overlapped for SRO minerals and crystalline oxides, although the mean age of OC released was older for the SRO. In soils with more mixed mineralogy, the added presence of older OC released at temperatures greater than 450°C from clays resulted in a broader distribution of OC ages within the sample, especially for soils rich in 2 : 1 layer expandable clays such as smectite. While pedogenic setting affects mineral stability and absolute OC age, mineralogy controls the structure of OC age distribution within a sample, which may provide insight into model structures and OC dynamics under changing conditions. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Effects of Climate Change on Soil Organic Matter C and H Isotope Composition in a Mediterranean Savannah (Dehesa): An Assessment Using Py-CSIA.

Dehesas are Mediterranean agro-sylvo-pastoral systems sensitive to climate change. Extreme climate conditions forecasted for Mediterranean areas may change soil C turnover, which is of relevance for soil biogeochemistry modeling. The effect of climate change on soil organic matter (SOM) is investigated in a field experiment mimicking environmental conditions of global change scenarios (soil temperature increase, +2-3 °C, W; rainfall exclusion, 30%, D; a combination of both, W+D). Pyrolysis-compound-specific isotope analysis (Py-CSIA) is used for C and H isotope characterization of SOM compounds and to forecast trends exerted by the induced climate shift. After 2.5 years, significant δ13C and δ2H isotopic enrichments were detected. Observed short- and mid-chain n-alkane δ13C shifts point to an increased microbial SOM reworking in the W treatment; a 2H enrichment of up to 40‰ of lignin methoxyphenols was found when combining W+D treatments under the tree canopy, probably related to H fractionation due to increased soil water evapotranspiration. Our findings indicate that the effect of the tree canopy drives SOM dynamics in dehesas and that, in the short term, foreseen climate change scenarios will exert changes in the SOM dynamics comprising the biogeochemical C and H cycles.

Soil Organic Matter Molecular Composition Shifts Driven by Forest Regrowth or Pasture after Slash-and-Burn of Amazon Forest.

Slash-and-burn of Amazon Forest (AF) for pasture establishment has increased the occurrence of AF wildfires. Recent studies emphasize soil organic matter (SOM) molecular composition as a principal driver of post-fire forest regrowth and restoration of AF anti-wildfire ambience. Nevertheless, SOM chemical shifts caused by AF fires and post-fire vegetation are rarely investigated at a molecular level. We employed pyrolysis-gas chromatography-mass spectrometry to reveal molecular changes in SOM (0-10, 40-50 cm depth) of a slash-burn-and-20-month-regrowth AF (BAF) and a 23-year Brachiaria pasture post-AF fire (BRA) site compared to native AF (NAF). In BAF (0-10 cm), increased abundance of unspecific aromatic compounds (UACs), polycyclic aromatic hydrocarbons (PAHs) and lipids (Lip) coupled with a depletion of polysaccharides (Pol) revealed strong lingering effects of fire on SOM. This occurs despite fresh litter deposition on soil, suggesting SOM minimal recovery and toxicity to microorganisms. Accumulation of recalcitrant compounds and slow decomposition of fresh forest material may explain the higher carbon content in BAF (0-5 cm). In BRA, SOM was dominated by Brachiaria contributions. At 40-50 cm, alkyl and hydroaromatic compounds accumulated in BRA, whereas UACs accumulated in BAF. UACs and PAH compounds were abundant in NAF, possibly air-transported from BAF.

Changes in soil organic matter molecular structure after five-years mimicking climate change scenarios in a Mediterranean savannah.

Mediterranean savannahs (dehesas) are agro-sylvo-pastoral systems with a marked seasonality, with severe summer drought and favourable rainy spring and autumn. These conditions are forecasted to become more extreme due to the ongoing global climate change. Under such conditions, it is key to understand soil organic matter (SOM) dynamics at a molecular level. Here, analytical pyrolysis (Py-GC/MS) combined with chemometric statistical approaches was used for the molecular characterization of SOM in a five-years field manipulative experiment of single and combined rainfall exclusion (drought) and increased temperature (warming). The results indicate that SOM molecular composition in dehesas is mainly determined by the effect of the tree canopy. After only five years of the climatic experiment, the differences caused by the warming, drought and the combination of warming+drought forced climate scenarios became statistically significant with respect to the untreated controls, notably in the open pasture habitat. The climatic treatments mimicking foreseen climate changes affected mainly the lignocellulose dynamics, but also other SOM compounds (alkanes, fatty acids, isoprenoids and nitrogen compounds) pointing to accelerated humification processes and SOM degradation when soils are under warmer and dryer conditions. Therefore, it is expected that, in the short term, the foreseen climate change scenarios will exert changes in the Mediterranean savannah SOM molecular structure and in its dynamic.

Soil pH and Soluble Organic Matter Shifts Exerted by Heating Affect Microbial Response.

Fire-induced alterations to soil pH and organic matter play an important role in the post-fire microbial response. However, the magnitude of which each parameter affects this response is still unclear. The main objective of this work was to determine the magnitude in which soil pH and organic matter fire-induced alterations condition the response of viable and cultivable micro-organisms using laboratory heating, mimicking a range of fire intensities. Four heating treatments were applied to unaltered forest soil: unheated, 300, 450, and 500 °C. In order to isolate the effect of nutrient or pH heating-induced changes, different culture media were prepared using soil:water extracts from the different heated soils, nutrient, and pH amendments. Each medium was inoculated with different dilutions of a microbial suspension from the same original, unaltered soil, and microbial abundance was estimated. Concurrently, freeze-dry aliquots from each soil:water extract were analyzed by pyrolysis-gas chromatography/mass spectrometry. The microbial abundance in media prepared with heated soil was lower than that in media prepared with unheated soil. Nutrient addition and pH compensation appear to promote microbial proliferation in unaltered and low-intensity heated treatments, but not in those heated at the highest temperatures. Soil organic matter characterization showed a reduction in the number of organic compounds in soil-heated treatments and a marked increase in aromatic compounds, which could be related to the observed low microbial proliferation.

Fingerprinting macrophyte Blue Carbon by pyrolysis-GC-compound specific isotope analysis (Py-CSIA).

There is a need for tools to determine the origin of organic matter (OM) in Blue Carbon Ecosystems (BCE) and marine sediments to (1) facilitate the implementation of Blue Carbon strategies into carbon accounting and crediting schemes and (2) decipher changes in ecosystem condition over decadal to millennial time scales and thus to understand and predict the stability of BCE in a changing world. Pyrolysis-GC-compound specific isotope analysis (Py-CSIA) is applied for the first time in marine environments and BCE research. We studied Australian mangrove, tidal marsh and seagrass sediments, in addition to potential sources of OM (Avicennia, Posidonia, Zostera, Sarcocornia, Ecklonia and Ulva species and seagrass epiphytes), to identify precursors of different biomacromolecule constituents (lignin, polysaccharides and aliphatic structures). Firstly, the link between bulk δ13C and δ13C reconstructed from compound-specific δ13C showed that the pyrolysis approach allows for the isotopic screening of a representative portion of the OM. Secondly, for all samples, the C isotope fingerprint of the carbohydrate products (plant polysaccharides) was the heaviest (13C enriched), followed by lignin and aliphatic products. The differences in δ13C among macromolecules and the overlap in δ13C among putative sources reflect the limitations of bulk δ13C analyses for deciphering OM provenance. Thirdly, phanerogams specimen had the heaviest carbohydrate and lignin, confirming that seagrass-derived lignocellulose can be traced based on δ13C. Consistent differences for individual compounds were identified between seagrasses and between Avicennia and Sarcocornia using Py-CSIA. Fourth, ecosystem shifts (colonization of seagrass habitats by mangrove) on millenary time scales, hypothesized in previous studies on the basis of bulk δ13C and Py-GC-MS, were confirmed by Py-CSIA. We conclude that Py-CSIA is useful in Blue Carbon research to decipher OM sources in marine sediments, identify ecosystem transitions in palaeoenvironmental records, and to understand the role of different OM compounds in Blue Carbon storage.

Impact of Biochar Amendment on Soil Properties and Organic Matter Composition in Trace Element-Contaminated Soil.

The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha-1 of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375-650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200-375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

Hydrophobicity of soils affected by fires: An assessment using molecular markers from ultra-high resolution mass spectrometry.

Soil water repellency (SWR) is a physical property due to a complex interaction of factors (e.g., fire, soil organic matter, soil texture) that reduces the soil water infiltration capacity. Traditionally, SWR is attributed to the accumulation and redistribution of hydrophobic compounds within soil profile. To obtain further insight into chemical compounds, which could be associated with SWR, a study was done on coarse (1-2 mm) and fine (< 0.05 mm) granulometric fractions of burned and unburned sandy soils under two Mediterranean vegetation biomes from Doñana National Park (Spain). The water drop penetration time (WDPT) test was used to assess the SWR. The molecular composition of extracted humic substances from the soil organic matter (SOM) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS). Partial least squares (PLS) regressions showed that the SWR can be predicted (P = 0.006) solely based on the abundances of approximately 1200 common compounds determined by FT-ICR/MS. This model confirmed the significant correlation between a specific SOM molecular composition and the SWR. The comparative analysis revealed that the SWR in the burned samples was significantly (P < 0.05) related to the abundance of aromatic and condensed compounds, while in the unburned samples there was a significant influence of aromatic hydrocarbons and lignin compounds. In the fine fraction, lipid compounds were significantly associated with the SWR. Contrastingly, the coarse fraction did not show any correlation. Alternatively, soils with a high SWR were significantly related to the presence of lipids and lignin. This analysis showed that combining FT-ICR/MS molecular characterizations with statistical treatments is a powerful approach for exploratory analysis suggesting that the structural features associated with SWR in the studied soils are different depending on the types of vegetation or the soil physical fractions with different particle size.

Effects of technosols based on organic amendments addition for the recovery of the functionality of degraded quarry soils under semiarid Mediterranean climate: A field study.

This study aims to evaluate the effects of technosols made with different organic amendments to restore degraded soils in a semiarid limestone quarry. The effects on soil quality, functionality and organic matter dynamics of the technosols amended with waste of gardening, greenhouse horticultural, stabilized sewage sludge and two mixtures of sludge with both vegetable composts were assessed. Several physical and chemical properties, humus fractions, soil respiration and molecular composition was performed after 6 and 18 months. Un-amended soils, and nearby natural undegraded soils served as reference. Amended technosols increased water retention capacity, electrical conductivity, total organic carbon and nitrogen, respect to not amended and natural soils. Humus fraction composition was not altered over time. Un-amended soils, very poor in organic matter, did not show any pyrolyzable compounds or labile soil organic matter by thermogravimetry. In contrast, the pyrochromatograms of natural soils showed lignocellulosic materials, polypeptides and a noticeable presence of alkylic compounds. In technosols with both types vegetable compost, the organic matter structure was more complex, showing compounds from lignin-derived and long-chain alkyl, polysaccharides, chlorophyll isoprenoids and nitrogen. In sludge technosol, a set of sterols was outstanding. The mixtures showed a molecular fingerprint of materials derived from the decomposition of the organic amendments that formed them. These signs of the contribution of different organic matter forms derived from the amendments were also reported by the series exothermic peaks found in the calorimetry. This short-term study indicates a clear effect of the amendments on the recovery of soil organic matter and presumably of its functionality. After the amendments application, microbial activity and soil respiration rates increased rapidly but ceased 18 months later. The molecular composition of the organic matter of the soils amended with plant compost was very similar to that of natural, non-degraded soils in nearby areas.

Fire effects on C and H isotopic composition in plant biomass and soil: Bulk and particle size fractions.

This work studies carbon (C) and hydrogen (H) isotope composition of plant biomass and soil organic matter (SOM) in an attempt to assess both, changes exerted by fire and possible inputs of charred materials to the soil after a wildfire. Isotope composition of bulk soil, soil particle size fractions and biomass of the dominant standing vegetation in the area (Quercus suber) from Doñana National Park (SW-Spain) were studied by isotope ratio mass spectrometry (IRMS). SOM C isotope composition indicates the occurrence of two SOM pools with different degree of alteration. Coarse soil fractions (>0.5 mm) were found 13C depleted with δ13C values close to those in leaf biomass, pointing to a predominance of poorly transformed SOM. Conversely, fine fractions (<0.1 mm) were found enriched in 13C as corresponds to a more humified SOM. The fire produced no changes in this trend, although a consistent 13C enrichment (c. 1‰) was observed in all soil fractions with decreasing size. Concerning H isotopes, the coarse fractions (>0.5 mm) displayed significant lower δ2H values than the intermediate and fine ones (<0.5 mm), again similar to those in leaf biomass (c. -80‰), whereas the fine fractions were found deuterium (2H)-enriched with significant higher δ2H values (c. 50‰), suggesting physical speciation of H depending on soil particle size. The fire produced a significant 2H depletion (Δ2H c. -10‰) in the finer fractions (<0.1 mm). The study of stable isotope analysis added new information and complements the results obtained by other proxies to better understand the effect of fire on SOM.

Effect of a wildfire and of post-fire restoration actions in the organic matter structure in soil fractions.

The impact of wildfires and of restoration actions on soil organic matter (SOM) content and structure was studied in a soil under pine (Pinus pinea) from Doñana National Park (SW Spain). Samples were collected from burnt areas before (B) and after post-fire restoration (BR) and compared with an unburnt (UB) site. Analytical pyrolysis (Py-GC/MS) was used to investigate SOM molecular composition in whole soil samples and in coarse (CF) and fine (FF) fractions. The results were interpreted using a van Krevelen graphical-statistical method. Highest total organic carbon (TOC) was found in UB soil and no differences were found between B and BR soils. The CF had the highest TOC values and FF presented differences among the three scenarios. Respect to SOM structure, the B soil was depleted in lignin and enriched in unspecific aromatics and polycyclic aromatic hydrocarbons, and in all scenarios, CF SOM consisted mainly of lignocellulose derived compounds and fatty acids. In general, FF SOM was found more altered than CF. High contribution of unspecific aromatic compounds and polycyclic aromatic hydrocarbons was observed in B-FF whereas BR-FF samples comprised considerable proportions of compounds from labile biomass, possibly due to soil mixing during rehabilitation actions. The fire caused a defunctionalisation of lignin-derived phenolics and the formation of pyrogenic compounds. The van Krevelen diagram was found useful to-at first sight-differentiate between chemical processes caused by fire and of the rehabilitation actions. Fire exerted SOM demethoxylation, dealkylation and dehydration. Our results indicate that soil management actions after the fire lead to an increase in aromaticity corresponding to the accumulation of lignin and polycyclic aromatic compounds. This suggests additional inputs from charred lignocellulosic biomass, including black carbon, that was incorporated into the soil during rehabilitation practices.

Discerning natural and anthropogenic organic matter inputs to salt marsh sediments of Ria Formosa lagoon (South Portugal).

Sedimentary organic matter (OM) origin and molecular composition provide useful information to understand carbon cycling in coastal wetlands. Core sediments from threors' Contributionse transects along Ria Formosa lagoon intertidal zone were analysed using analytical pyrolysis (Py-GC/MS) to determine composition, distribution and origin of sedimentary OM. The distribution of alkyl compounds (alkanes, alkanoic acids and alkan-2-ones), polycyclic aromatic hydrocarbons (PAHs), lignin-derived methoxyphenols, linear alkylbenzenes (LABs), steranes and hopanes indicated OM inputs to the intertidal environment from natural-autochthonous and allochthonous-as well as anthropogenic. Several n-alkane geochemical indices used to assess the distribution of main OM sources (terrestrial and marine) in the sediments indicate that algal and aquatic macrophyte derived OM inputs dominated over terrigenous plant sources. The lignin-derived methoxyphenol assemblage, dominated by vinylguaiacol and vinylsyringol derivatives in all sediments, points to large OM contribution from higher plants. The spatial distributions of PAHs (polyaromatic hydrocarbons) showed that most pollution sources were mixed sources including both pyrogenic and petrogenic. Low carbon preference indexes (CPI > 1) for n-alkanes, the presence of UCM (unresolved complex mixture) and the distribution of hopanes (C29-C36) and steranes (C27-C29) suggested localized petroleum-derived hydrocarbon inputs to the core sediments. Series of LABs were found in most sediment samples also pointing to domestic sewage anthropogenic contributions to the sediment OM.

Detection of cylindrospermopsin and its decomposition products in raw and cooked fish (Oreochromis niloticus) by analytical pyrolysis (Py-GC/MS).

The presence of the toxin cylindrospermopsin is increasingly frequent in samples from different ecosystems and it is a serious problem both at environmental level and for animal and human health. To be able to prevent CYN exposure risk, it is important to have suitable analytical methods, but also quick and economical ones. Analytical pyrolysis coupled to GC/MS (Py-GC/MS) represents an important alternative for the rapid detection, characterization or "fingerprinting" of different materials. However, it has been less studied with cyanotoxins up to date. The present work aims to investigate: 1) the suitability of Py-GC/MS for detection of CYN and its decomposition products in raw and cooked fish samples before consumption and 2) the influence of the different cooking methods on the presence of different CYN degradation products detected by Py-GC/MS. For first time, these results present that Py-GC/MS could be a rapid and economical alternative for the detection and monitoring of CYN and its degradation products (DP. m/z 290.1, 169.1 and 336.2) in raw or cooked fish. Moreover, the changes induced in CYN and DP by cooking could be amenable and detected by Py-GC/MS.

Impact of wildfires on subsurface volcanic environments: New insights into speleothem chemistry.

Siliceous speleothems frequently reported in volcanic caves have been traditionally interpreted as resulting from basalt weathering combined with the activity of microbial communities. A characteristic feature in lava tubes from Hawaii, Azores and Canary Islands is the occurrence of black jelly-like speleothems. Here we describe the formation process of siliceous black speleothems found in a lava tube from La Palma, Canary Islands, Spain, based on mineralogy, microscopy, light stable isotopes, analytical pyrolysis, NMR spectroscopy and chemometric analyses. The data indicate that the black speleothems are composed of a hydrated gel matrix of amorphous aluminum silicate materials containing charred vegetation and thermally degraded resins from pines or triterpenoids from Erica arborea, characteristic of the overlying laurel forest. This is the first observation of a connection between fire and speleothem chemistry from volcanic caves. We conclude that wildfires and organic matter from the surface area overlying caves may play an important role in the formation of speleothems found in La Palma and demonstrate that siliceous speleothems are potential archives for past fires.

Quick Analysis of Organic Amendments via Portable X-ray Fluorescence Spectrometry.

The determination of heavy metals in soils and organic amendments, such as compost, manure, biofertilizer, and sludge, generally involves the digestion of samples with aqua regia, and the determination of those in the solution using various techniques. Portable X-ray fluorescence (PXRF) has many advantages in relation to traditional analytical techniques. However, PXRF determines the total elemental content and, until now, its use for the analysis of organic amendments has been limited. The objective of this work is the calibration of a PXRF instrument to determine the aqua regia-soluble elemental contents directly in solid samples of organic amendments. Our proposal will avoid the digestion step and the use of other laboratory techniques. Using a training set of samples, calibration functions were obtained that allow the determination of the aqua regia-soluble contents from the PXRF readings of total contents. The calibration functions (obtained by multiple linear regression) allowed the quantitative determination of the aqua regia-soluble contents of Fe, K, P, S, Zn, Cu, Pb, Sr, Cr, and Mn, as well as the organic matter content and a semi-quantitative assessment of Al, Ca, V, Ba, Ni, and As contents. The readings of Si, Fe, Al, Ca, K, or S were used as correction factors, indicating that the calibrations functions found are truly based on the chemical composition of the sample matrix. This study will allow a fast, cheap, and reliable field analysis of organic amendments and of other biomass-based materials.

Changes in soil organic matter composition after Scots pine afforestation in a native European beech forest revealed by analytical pyrolysis (Py-GC/MS).

The introduction of coniferous species in former deciduous forests may exert changes in soil organic matter, particularly in its molecular composition. In this work, pyrolysis-gas chromatography-mass spectrometry was used to study changes in SOM quality related to the centennial afforestation of Scots pine in an area formerly covered by European beech forest in the NE-flank of the Moncayo Natural Park (NE-Spain). For each soil profile three organic layers (fresh litter, fragmented litter and humified litter) and mineral soil horizons (Ah, E, Bhs and C) were studied. A total of 128 compounds were identified in the pyrograms, and composition differences were detected among the organic and mineral soil layers as well as between soils under beech and pine, for the main compound classes: nitrogen compounds, aromatics, lignin methoxyphenols, polycyclic aromatic hydrocarbons, lipids and polysaccharide-derived moieties. Such chemical differences were found to be derived from the biomass composition of the predominant vegetation type that was incorporated into the soil and from its progression into the soil profile. The analysis of the distribution of alkanes indicated higher SOM stabilization in the native beech forest soil. The signal of beech biomarkers (long chain n-alkanes C31-C33) found in the pine E horizon indicates the permanence of SOM derived from the natural forest ca. 100 years after the afforestation.

Mulching-induced preservation of soil organic matter quality in a burnt eucalypt plantation in central Portugal.

Mulching has amply proven its effectiveness to mitigate post-fire soil erosion but its impacts on soil organic matter (SOM) quality and quantity continue poorly studied. The present study addressed this knowledge gap for a eucalypt plantation in central Portugal that had been burnt and, immediately after the wildfire, mulched with 13.6 Mg ha-1 of eucalypt logging residues some five years before. This was done by performing a range of analytical techniques (elemental and isotope analyses, analytical pyrolysis and 13C NMR spectroscopy) not only on the bulk soil samples but also on their humic acids (HAs) and free organic matter (FOM) fractions. While mulching reduced soil and SOM losses with 91 and 93%, respectively, it also improved SOM quality of the topsoil, in particular in terms of HAs and FOM. At 0-4 cm depth, both HAs and FOM contents were roughly twice as high in the mulched plots as in the control plots. The effects of mulching on the molecular composition of HAs and FOM fractions, however, varied markedly. Analytical pyrolysis (Py-GC/MS) revealed that mulching had led to a noticeable accumulation of labile, aliphatic SOM constituents such as carbohydrate-derived and alkyl compounds (fatty acids and n-alkanes) but that it hardly affected the composition of HAs. Even so, solid-state 13C NMR spectroscopy showed that mulching had resulted in a relative increase in aryl C in the FOM fraction, suggesting an enhanced preservation of the pyrogenic OM. Overall, the combined use of a range of analytical techniques allowed to conclude that, five years after their application, the forest logging residues had led to a greater preservation of the fire-derived pyrogenic OM (mainly aromatic compounds) in the topsoil as well as to higher contents of SOM's most labile molecular constituents (mainly carbohydrates and n-alkyl compounds). The former reflected the reduced erosion rates, while the latter was probably due to a combination of reduced erosion rates with the additional input of fresh organic matter.