Occluded Organic Matter Research Articles

Natural <sup>13</sup>C Abundance of Organic Matter in Water-Stable Aggregates of Haplic Chernozem under Conditions of Contrasting Land Uses

Natural 13C abundance of different organic matter (OM) pools in water-stable macro- and free microaggregates of Haplic Chernozem in contrasting land use variants (steppe and long-term bare fallow) are described. Fractionation of 13C at certain stages of the formation of OM pools is relatively constant, regardless of the level of structural organization. This is demonstrated by the presented conceptual scheme, which allows one to quantify the fluxes of carbon (C) in the system of aggregate/OM pool. It was revealed that the main C fluxes in the OM pools go from the free OM (LFfr) to the Residue fraction through microaggregates within water-stable aggregates (mWSA), the components of which are the occluded OM (LFocc) and the Clay. With a high degree of probability, C flows from macroaggregates (WSAma) to free microaggregates (WSAmi). At the same time, the higher probability of the C flux into the Residue from the mWSA as compared to the direct C fluxes from the LFocc and the Clay, testifies in favor of the hypothesis that the Residue is represented, to a large extent, by parts/fragments of disintegrated mWSAs of 50–1 μm in size. Regardless of the size, the WSA contains a labile OM (as the components of mWSA, along with LFfr – only in macroaggregates) and a stable OM (Residue). Labile OM (LFocc and Clay) within WSAmi is characterized by a lower degree of microbial processing (“lighter” isotopic signature) compared to that within WSAma, which is due to its greater “physical” protection against microbial attacks. However, the most stable OM pool, concentrated in the Residue within WSAmi is enriched in 13C compared to that within WSAma. Considering that the Residue determines the total isotopic composition of C in WSA, the organic matter of free microaggregates is characterized by a higher degree of microbial processing in comparison with that of macroaggregates. Free microaggregates are parts of disintegrated macroaggregates.

Variations in the chemical structure and Carbon-13 natural abundance in water-stable macro- and microaggregates in Haplic Chernozem under the contrasting land use variants

Water-stable macro- (WSAma) and free microaggregates (WSAmi) were isolated from the 2-1mm air-dry macroaggregates from the surface horizons of Haplic Chernozem in contrasting variants of land use: the steppe and the bare fallow. The 13C NMR data and the 13C natural abundance of the Occluded organic matter (OM) (LFoc) and Clay within WSAs in the steppe obviously indicate a lower degree of microbiological processing of OM within WSAmi as compared with WSAma. This is reflected in lower degrees of decomposition (DI) and aromaticity (ARI) of OM and the C/N ratio, as well as lower 13C enrichment. This implies that the "labile" part of OM within WSAmi (LFoc and Clay, which are components of microaggregates within water-stable aggregates (mWSAs)) is more physically protected compared to that within WSAma. However, the heavier total δ13C signature of OM within WSAmi indicates its greater degree of microbiological processing compared to that within WSAma. This seems contrary to the concept of greater physical protection of OM within microaggregates as compared to macroaggregates. It was revealed that the heavier total δ13C signature of OM within WSAmi (greater degree of microbiological processing) is determined by the "oldest" OM located in the inter-aggregate space of WSAs, which is concentrated in the Residue fraction (Res). Due to its quantitative dominance, the Residue fraction is crucial for the total δ13C signature of WSAs. Negative changes in the quality of OM under the long-term bare fallow (52-yr) were reflected in a sharp increase in the integral indices of the chemical structure (DI, ARI), as well as the hydrophobicity index (HI) in all studied OM pools. It was accompanied by their 13C enrichment in the bare fallow compared to the steppe. Free microaggregates (WSAmi) are fragments of disintegrated macroaggregates (WSAma). We found no evidence of their formation within macroaggregates.

Atmospheric pCO2 Reconstruction of Early Cretaceous Terrestrial Deposits in Texas and Oklahoma Using Pedogenic Carbonate and Occluded Organic Matter

Pedogenic carbonate samples collected from three Lower Cretaceous (Aptian–Albian) fossil localities in Texas and Oklahoma were analyzed to develop paleoatmospheric pCO2 estimates by measuring the stable carbon isotopes of pedogenic calcite and their co-existing occluded organic matter. Calcite δ13C values ranged from −10.9‰ to −4.4‰ while occluded organic matter δ13C values ranged from −27.3‰ to −21.1‰. These stable carbon isotope measurements combined with temperature (30 °C) and soil-respired CO2 concentration (839–6047 ppmV) values provided atmospheric pCO2 estimates ranging from 67 ppmV to over 1100 ppmV. These estimates show a significant increase in atmospheric pCO2 during the late Aptian followed by a decrease in atmospheric pCO2 during the late Aptian to early Albian transition period, roughly correlating with the OEA1b event. Given the lack of chronostratigraphic constraints of the Lower Cretaceous geologic units in the study area, these data provide further evidence for the approximate age of the units as well as pertinent paleoclimate insights into greenhouse climate conditions.

Regional Patterns in Miocene‐Pliocene Aridity Across the Chinese Loess Plateau Revealed by High Resolution Records of Paleosol Carbonate and Occluded Organic Matter

AbstractPaleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (δ13Ccarb) is complicated because multiple factors affect δ13Ccarb values and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (δ13Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ13Ccarb values increase by nearly 5‰, while δ13Corg values increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ13C data for C4 vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with δ13Ccarb values becoming higher and more variable to the N‐NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in δ13Ccarb values at Jiaxian, while southern δ13Ccarb records remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with δ13Ccarb values being more sensitive to moisture availability under consistently more arid conditions to the NW.

Physical protection of soil organic carbon through aggregates in different land use systems in the semi-arid region of Brazil

The objectives of this work were to quantify the soil organic carbon (SOC) stock and to evaluate the distribution of soil aggregates and their respective SOC and labile carbon (LC) present inside the aggregates in different perennial land use systems in the semi-arid region of Brazil. Soil samples were collected in four land use systems: two forage palm cultivation systems of different ages, a mango orchard, and natural forest. A higher amount of SOC stored in the 0–60 cm layer was observed in the mango orchard, increasing the soil carbon stock by 16.4% in relation to the natural forest. Palm cultivations presented larger amounts of macroaggregates from the 10–20 cm layer until 60 cm depth, while the mango orchard and natural forest presented a similar pattern in all fraction-size classes. SOC averages of non-occluded particulate organic matter (POM) and aggregate occluded organic matter (AOM) in the macroaggregates of the 0–10 cm layer were higher in the mango orchard and natural forest compared to palm cultivations; however, the LC of AOM was higher in palm crops. Soil microaggregates were little influenced by land use systems, however this class represented an average of 24% of SOC stock.

Features of the chemical structure of different organic matter pools in Haplic Chernozem of the Streletskaya steppe: 13C MAS NMR study

The aim of the current research was to study structural features of four organic matter (OM) pools isolated by the modified method of granulo-densimetric fractionation from two Сhernozems. We purposed to relate these features to the OM allocation and the transformation processes. The pools included: 1) free light fraction located in an inter-aggregate space, 2) light fraction occluded inside the microaggregates, 3) OM bound with clay particles, and 4) OM bound with a residual heavy fraction left after light fractions and clay separation. Soils of contrasting land uses: steppe and long-term permanent bare fallow were selected to assess changes that occur in soil OM during the degradation. We used 13C CP/MAS NMR spectroscopy controlling the quantitativeness of spectra with the aid of 13C DP/MAS NMR. The obtained spectra of the studied fractions clearly differed in the proportion of main functional groups. The occluded OM is more aromatic compared to the free OM. The structural changes observed at transition from the free light fraction to the occluded one indicate an active decomposition of lignin and carbohydrates in the latter fraction. This provides in the occluded OM appropriate conditions for the formation of “young HA”: high local concentration of substrates and spatial proximity of enzymes. At C-deficiency (bare fallow) the chemical structure of the occluded OM is very close to that of humic acids. The chemical structure of the clay bound OM reflects high content of products of microbial origin. OM of the residual heavy fraction differs from that of the clay: the proportions of main functional groups in it are more close to that of the free light fraction, but with higher carboxyl content. Heavy fractions also differ under acid treatment: the residue losses less carbon. The above-mentioned differences show that the division of heavy fractions into two components is reasonable. Various acid hydrolyzability indicates a predominance of strong chemical bonds in the occluded OM and the significant contribution of weak bonds in the clay OM, i.e., the occluded OM is highly condensed, in contrast, components of the clay OM are largely interconnected by hydrogen-, coordinate, hydrophobic and other relatively weak bonds. Soil degradation under extreme land use leads not only to OM scarcity, but also to its greater hydrolyzability, strong enrichment with aromatic fragments and depletion of carbohydrate and aliphatic fragments in all studied fractions. Degradation changes in the occluded OM are most pronounced. Our results demonstrate that the applied fractionation scheme coupled with quantitative 13C CP/MAS NMR spectroscopy is a very promising approach for evaluating processes of soil OM transformation and degradation.

Paleoenvironment of the Central Himalaya during late MIS 3 using stable isotopic compositions of lacustrine organic matter occluded in diatoms and sediments

Marine Isotope Stage (MIS) 3 was an interstadial stage in the climate history of the Earth where the last phase (40–30 ka) showed relatively warmer climate over the northwest China, Tibetan Plateau, and northwestern India compared to other regions of the world. However, not many studies are reported for climate variability during this period from the Himalayan region. Here, an attempt has been made to understand the paleoenvironment of the Central Himalaya during 45–29 ka using stable isotopic compositions of carbon (δ13CTOC) and nitrogen (δ15NTN) along with their elemental ratios (TOC/TN) in bulk organic matter and occluded organic matter within diatom frustules (δ13CDiatom) of a paleolake sequence. The variabilities in δ13CDiatom, δ13CTOC, and TOC/TN ratios indicated intermittent changes in the lake carbon dynamics from bicarbonate dominated system to carbon dioxide dominated regime. These changes in lacustrine carbon cycling provided evidence for existence of sporadic dry events in the Central Himalaya during 45–29 ka. In agreement with the records from the northwest China, Tibetan Plateau, and northwestern India, our results also confirm the existence of a relatively wet and humid period during 40–32 ka in the Central Himalaya. Therefore, it appeared that compared to other parts of the world, relatively wet and humid climate existed throughout this region. However, the mechanism behind this wet and humid phase remains to be explored and warrants a high resolution paleoclimate study in the Himalaya.

Age matters: Fate of soil organic matter during ageing of earthworm casts produced by the anecic earthworm Amynthas khami

The transformation of organic matter (OM) occluded in earthworm casts during the casts' lifetime may affect its composition and accumulation in stabilised OM pools. We collected casts produced by the anecic earthworm Amynthas khami in a tropical woodland of Northern Vietnam presenting different degradation stages. The aim of this study was to assess (1) the biogeochemical properties of OM in earthworm casts compared to control aggregates from the surrounding soil, and (2) the fate of occluded OM during cast ageing. We analysed intact casts and their density fractions at five ageing stages for organic carbon (OC) and nitrogen (N) content, OM chemical composition by analytical pyrolysis, and OM stability by Rock-Eval thermal analysis. Compared to control aggregates, fresh casts had higher OC values (5.3 vs. 2.6%) and increased OC in all density fractions. Cast OM had more lignin (1.1 vs.<0.1%), similar polysaccharide and N-compound contribution and was more thermally labile than those of control aggregates. Changes in OM composition and content during cast ageing appeared at the end of the casts’ lifetime, when they were broken into smaller aggregates (10–13 mm). At these latest stages, OC content decreased (1.5-fold); along with OM in macro-aggregates (2.2-fold) and microaggregates (6.8-fold). Reduced mineral-associated OC was also recorded (1.3-fold). During ageing, the percentage of lignin decreased (5.5-fold) while the thermal stability of OM increased. The distinct OM properties of casts were maintained throughout the cast ageing process, and most properties of the oldest casts were significantly different from the control aggregates. Since OM is retained in cast fragments after they disintegrate, the earthworm influence on soil organic matter stabilisation processes appears to persist beyond the lifespan of casts.

Spectroscopic Discrimination of Sorghum Silica Phytoliths.

Grasses accumulate silicon in the form of silicic acid, which is precipitated as amorphous silica in microscopic particles termed phytoliths. These particles comprise a variety of morphologies according to the cell type in which the silica was deposited. Despite the evident morphological differences, phytolith chemistry has mostly been analysed in bulk samples, neglecting differences between the varied types formed in the same species. In this work, we extracted leaf phytoliths from mature plants of Sorghum bicolor (L.) Moench. Using solid state NMR and thermogravimetric analysis, we show that the extraction methods alter greatly the silica molecular structure, its condensation degree and the trapped organic matter. Measurements of individual phytoliths by Raman and synchrotron FTIR microspectroscopies in combination with multivariate analysis separated bilobate silica cells from prickles and long cells, based on the silica molecular structures and the fraction and composition of occluded organic matter. The variations in structure and composition of sorghum phytoliths suggest that the biological pathways leading to silica deposition vary between these cell types.

Terrestrial microbialites provide constraints on the mesoproterozoic atmosphere

AbstractPalaeoclimate data indicate that Earth surface temperatures have remained largely temperate for the past 3.5 Byr despite significantly lower solar luminosity over this time relative to the present day. There is evidence for episodic early and late Proterozoic glaciation, but little evidence of glaciation in the intervening billion years. A prolonged equable Mesoproterozoic Earth requires elevated greenhouse gas concentrations. Two endmember scenarios have been proposed for maintaining global warmth. These include extremely high pCO2 or more modest pCO2 with higher methane concentrations. This paper reports on the δ13C of organic matter in 1.1 Ga stromatolites from the Copper Harbor Conglomerate (CHC) of the Mesoproterozoic Midcontinent Rift (North America) and δ18O and Δ47 temperatures of inorganic stromatolite carbonate to constrain formation and burial conditions and the magnitude of ancient carbon isotope discrimination. CHC sediments have never been heated above ~125–155°C, providing a novel geochemical archive of the ancient environment. Stromatolite Δ47 data record moderate alteration, and therefore, the occluded organic matter was unlikely to have experienced significant thermal alteration after deposition. The δ13C values of ancient mat organic matter and inorganic carbonate show isotope discrimination (εp) values ~15.5–18.5‰, similar to modern microbial mats formed in equilibrium with low concentrations of dissolved inorganic carbon. In combination, these data are consistent with a temperate climate Mesoproterozoic biosphere supported by relatively modest pCO2. This result agrees with Atmosphere‐Ocean Global Circulation Model reconstructions for Mesoproterozoic climate using 5–10 times present atmospheric levels pCO2 and pCH4 of &gt;28 ppmv. However, given marine modelling constraints of CH4 production that suggest pCH4 was below 10 ppm, this creates a methane paradox. Either an additional source of CH4 (e.g. from terrestrial ecosystems) or another greenhouse gas, such as N2O, would have been necessary to maintain equable conditions in the Mesoproterozoic.

Physicochemical surface properties of different biogenic silicon structures: Results from spectroscopic and microscopic analyses of protistic and phytogenic silica

The significance of biogenic silicon (BSi) for Si cycling in terrestrial biogeosystems has been acknowledged since decades. Its importance originates from the fact that BSi generally is more soluble than silicate minerals and thus i) controls Si fluxes from terrestrial to aquatic ecosystems and ii) plays an important role as source of readily- or plant-available Si (i.e., H4SiO4) in soils. In this context, physicochemical surface properties of BSi structures determine their dissolution kinetics. We applied transmission Fourier transform infrared (FTIR) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) microscopy, and confocal laser scanning microscopy (CLSM) to investigate physicochemical surface properties of different biogenic silica structures. Using these techniques we were able to detect differences on a molecular level (FTIR, DRIFT) and in surface roughness parameters (CLSM) between BSi synthesized by protists (testate amoebae, diatoms) and BSi synthesized by plants (phytoliths) as well as between fresh (extracted from plants) and aged (extracted from soils) phytoliths. While fresh phytoliths showed organic impurities that can be assigned to occluded organic matter, aged phytoliths showed additional impurities of mineral origin. This is due to the fact that the used non-destructive gravimetric extraction of phytoliths is unsuitable for a distinct differentiation between biogenic silica (phytoliths) and non-biogenic (minerogenic or microcrystalline) Si forms in general. We recommend DRIFT microscopy for analyses of phytoliths extracted from soils because this technique allows measurements of selected, single siliceous phytoliths (as well as other BSi structures). Surface roughness parameters of aged phytoliths decreased compared to the ones of fresh phytoliths indicating a decrease of specific surface areas available for dissolution processes. Physicochemical surface properties will help us to better understand the BSi status (BSi quality and quantity) of soils with implications for Si availability in soils and thus Si cycling in terrestrial biogeosystems.

13C-NMR, PAHs, WSOC and water repellence of fire-affected soils (Albic Podzols) in lichen pine forests, Russia

Soil organic matter (SOM) in boreal forest ecosystems can be strongly transformed by fires which, in turn, can affect soil physical and biological properties such as water repellency and nutrient cycling. The article contains the results of a study on SOM content in fire-affected lichen pine forests of the middle taiga subzone of the European part of Russia (Komi Republic). Soils which survived surface fires 2, 10, 16 years ago and unburned area were studied. We investigated changes in SOM composition using 13C NMR spectroscopy, and we measured water-soluble organic carbon (WSOC) content, polycyclic aromatic hydrocarbons (PAHs) concentrations, and contact angles of water droplets on soils. Forest fires increased pyrogenic organic carbon content and also largely transformed SOM composition. 13C NMR spectroscopy showed that the SOM composition of densimetric fractions in pyrogenic and unburned soil greatly differs. Fires increased the aromatic content in SOM composition of light fractions. The highest amount of aromatic carbon probably pyrogenic origin accumulated in fractions of free and occluded organic matter. According to 13C NMR spectroscopy, the aromatic component of organic matter increased in the pyrogenic horizons. The strongest PAHs concentrations occurred in the fractions of free particulate and occluded organic matter, in reference and fire-affected soils. The presence of PAHs in light densimetric fractions may serve as markers of past fire and fire intensity. In the initial years after fire, there was a decrease in the content of WSOC compounds; however, the WSOC content recovered with the recovery of ground vegetation. Our results showed that the greatest changes of contact angles were in upper pyrogenic soil horizons, whereas fires did not modify the contact angle value of lower mineral horizons.

Effects of Different Organic-Matter Sources On Estimates of Atmospheric and SoilpCO2Using Pedogenic Carbonate

Abstract Carbon isotope analysis of paleosol carbonates is one of the most widely used methods for producing quantitative estimates of CO2 levels in the ancient atmosphere, and is increasingly used to estimate soil pCO2 as a proxy for primary productivity. Recent efforts to improve the carbonate CO2 paleobarometer by refining input parameters (e.g., soil temperature, soil CO2 production function) have yielded more accurate estimates of ancient atmospheric pCO2. The carbonate CO2 paleobarometer is especially sensitive to input values for the carbon-isotope composition of soil organic matter (OM), which should ideally reflect the δ13C value of OM present when pedogenic carbonates were forming. Published soil pCO2 estimates derived from pedogenic carbonate in Upper Jurassic paleosols are re-evaluated here using OM occluded within pedogenic carbonates rather than average values of fossilized plant material collected throughout the sampled stratigraphic sections. The new soil pCO2 estimates calculated using occluded OM range from 4,600 to 20,000 ppmV and are much lower than the previously published estimates, which were often in excess of 60,000 ppmV. In order to determine which OM source provides more accurate results, estimates of atmospheric pCO2 obtained using plant material and occluded OM from a carbonate-bearing modern soil are compared with measured, pre-industrial atmospheric CO2 levels. In the modern soil profile, plant OM δ13C is highly variable and slightly more negative than the δ13C of occluded OM. The observed ∼ 1‰ offset between the average δ13C values of plant material and occluded OM is much less than the overall range of ∼ 6‰ in plant OM. Estimates of atmospheric pCO2 from the modern soil that are calculated using occluded OM differ by less than 100 ppmV, on average, from estimates based on plant OM. These results suggest that occluded OM produces reasonably accurate pCO2 estimates when used with the carbonate CO2 paleobarometer. Applying a −1‰ correction to the δ13C of occluded OM also produces accurate pCO2 estimates, but the extreme variability in δ13C of plant matter leads to inaccurate pCO2 estimates, even when samples are averaged.

Composition and hydrophobic properties of organic matter in the densimetric fractions of soils from the Subpolar Urals

Organic matter features in the upper mineral horizons have been studied for four soils in the Subpolar Urals: humus-illuvial podbur (Entic Podzol), gleyic humus-illuvial podbur (Stagnic Entic Podzol), iron-illuvial podzol (Albic Podzol), and eluviated burozem (Leptic Cambisol). Organic matter pools have been separated by densimetric fractionation. The concentrations of carbon and nitrogen and the relative contributions of separate densimetric fractions to the total content of elements in the upper soil horizons reflect the genetic features of formation of the studied soils. The saturation of organic matter with nitrogen increases with increasing density of the fractions. The proportion of heavy fraction HF1 characterized by a high content of fine silt particles increases in the upper horizons of podburs (Entic Podzol, Stagnic Entic Podzol) and burozem (Leptic Cambisol). The contact wetting angles, which characterize the hydrophobic properties of soils, have been determined for the densimetric fractions and horizons of gleyic humus-illuvial podbur (Stagnic Entic Podzol) and iron-illuvial podzol (Albic Podzol). These values vary from 60 to 88° among the soil horizons and from 22 to 137° among the densimetric fractions. It has been found that the hydrophobic properties of the studied soil horizons are largely determined by the contents of free and occluded organic matter fractions.

Abundance and composition of free and aggregate-occluded carbohydrates and lignin in two forest soils as affected by wildfires of different severity

Organic matter is the soil component most affected by wildfires, both in terms of abundance and composition. Fire-induced alteration of soil organic matter (SOM) depends on heating intensity and duration, oxygen availability and other factors related to topography, climate, soil and vegetation features. Particularly affected by fire is the litter layer, but SOM from the uppermost mineral soil can also experience some major changes. In this study, we investigated the direct impact of fire on molecular SOM parameters in density fractions isolated from the top 2.5cm of mineral soil in two forests that recently experienced wildfires of different severity. One, located in Tuscany, Central Italy, is a mixed forest of Downy oak and Maritime pine, developed on Acrisols formed on sandy lacustrine deposits, affected by a moderately severe fire. The other, located in Victoria, South-East Australia, is a mixed-species eucalypt forest, developed on a Cambisol formed on sandy Devonian sediments, affected by an extremely severe fire (the infamous ‘Black Saturday’ fire). The purpose of this study was the assessment of fire-induced changes on amount and composition of the bulk SOM and SOM associated to soil fractions having different densities. We used 1.8Mgm−3 as density cut-off and distinguished between free and aggregate-occluded SOM. In particular, the analyses focused on abundance and composition of two major SOM components, proposed as molecular indicators of fire severity: the non-cellulosic neutral sugars, digested by trifluoroacetic acid (TFA), and the lignin-derived phenolic monomers, released by cupric oxide (CuO) oxidation. The chemical structure of both bulk SOM and SOM fractions were analysed by solid-state 13C nuclear magnetic resonance spectroscopy.In contrast to the moderately severe fire affecting the Italian site, the extremely severe fire at the Australian site caused substantial loss of SOM from the top mineral soil. Both fires had major effects on SOM composition. In spite of the evident impact they experienced, neither hydrolysable sugars nor lignin phenols resulted to be reliable indicators of fire severity. Moreover, both fires apparently broke up soil aggregates, hence promoting the release of some occluded organic matter. The fire-induced changes of SOM observed have implications for the C cycle, so highlighting the critical role of wildfire occurrence and severity in climate change.

Release of potassium accompanying the dissolution of rice straw phytolith

In rice, Si assimilated from the soil solution is deposited in inter- and intracellular spaces throughout the leaf and stems to form silicified structures (so-called phytoliths). Because K is also present in significant concentrations in rice stems and leaves, the question arises if K is immobilized in the mineralized silica during the precipitation of Si. This work determined whether desilification of the phytolith is a factor regulating K release by implementing batch experiments. Solubility of Si and K of the rice straw heated at different temperatures were examined to identify effect of pretreatment.Analyses of phytoliths using SEM–EDX and X-ray tomographic microscopy in conjunction with the results from batch experiments revealed that K might co-exist with occluded organic matter inside the phytolith structure. In the kinetic experiments, corresponding increases of K and Si concentrations in the supernatants were observed which suggested that desilification of the phytolith is a main factor regulating K release. The extent of heat pretreatment of the rice straw is of significant importance with respect to dissolution of the phytolith by affecting organic removal and surface modification. At temperatures lower than 600°C, corresponding increases of the soluble Si and K with heating temperature have been obviously observed. In contrast, the solubility of Si and K gradually decreased at temperatures above 600°C. This work provides insights into factors that control release of K and Si from phytolith and a practical recommendation for practices of burning rice straw that may maximize subsequent release of Si and K for crops.

Factors affecting the molecular structure and mean residence time of occluded organics in a lithosequence of soils under ponderosa pine

Occluded, or intra-aggregate, soil organic matter (SOM) comprises a significant portion of the total C pool in forest soils and often has very long mean residence times (MRTs). However, occluded C characteristics vary widely among soils and the genesis and composition of the occluded organic matter pool are not well understood. This work sought to define the major controls on the composition and MRT of occluded SOM in western U.S. conifer forest soils with specific focus on the influence of soil mineral assemblage and aggregate stability. We sampled soils from a lithosequence of four parent materials (rhyolite, granite, basalt, and dolostone) under Pinus ponderosa. Three pedons were excavated to the depth of refusal at each site and sampled by genetic horizon. After density separation at 1.8 g cm−3 into free/light, occluded and mineral fractions, the chemical nature and mean residence time of organics in each fraction were compared. SOM chemistry was explored through the use of stable isotope analyses, 13C NMR, and pyrolysis GC/MS. Soil charcoal content estimates were based on 13C NMR analyses. Estimates of SOM MRT were based on steady-state modeling of SOM radiocarbon abundance measurements. Across all soils, the occluded fraction was 0.5–5 times enriched in charcoal in comparison to the bulk soil and had a substantially longer MRT than either the mineral fraction or the free/light fraction. These results suggest that charcoal from periodic burning is the primary source of occluded organics in these soils, and that the structural properties of charcoal promote its aggregation and long-term preservation. Surprisingly, aggregate stability, as measured through ultrasonic dispersion, was not correlated with occluded SOM abundance or MRT, perhaps raising questions of how well laboratory measurements of aggregate stability capture the dynamics of aggregate turnover under field conditions. Examination of the molecular characteristics of the occluded fraction was more conclusive. Occluded fraction composition did not change substantially with soil mineral assemblage, but was increasingly enriched in charcoal with depth relative to bulk SOM. Enrichment levels of 13C and 15N suggested a similar degree of microbial processing for the free/light and occluded fractions, and molecular structure of occluded and free/light fractions were also similar aside from charcoal enrichment in the occluded fraction. Results highlight the importance of both fire and aggregate formation to the long-term preservation of organics in western U.S. conifer forests which experience periodic burning, and suggest that the composition of occluded SOM in these soils is dependent on fire and the selective occlusion of charcoal.

Changes in the organic matter of taiga soils during the natural reafforestation after cutting in the middle taiga of the Komi Republic

It has been shown that the distribution features of the hydrophilic and hydrophobic fractions of humic substances (HSs) in the upper genetic horizons of undisturbed podzolic soils and lithobarrier podzols are similar. The concentrations of the separate HS fractions in the mineral horizons are largely determined by the soil mineralogy. In forest litters of secondary phytocenoses developed after the cutting of spruce forests, an increase in the content of hydrophilic compounds has been observed on loamy deposits at a decrease in the portion of ligninlike organic compounds. An in crease in the portion of AlFe-humus compounds has been revealed in the mineral soil horizons of secondary phytocenoses. It has been proposed to use the degree of hydrophilicity ( DH) for characterizing the features of the organic matter in taiga soils. An increase in the content of the fractions of occluded organic matter and that bound to minerals (density of 1.6-2.2 g/cm 3 ) has been observed in the eluvial horizons of a young cutover area.

Limitations on the climatic and ecological signals provided by the δ13C values of phytoliths from a C4 North American prairie grass

Abstract Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ13C values of phytoliths from a C4 grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r2

Effect of landuse on organic matter stabilized in organomineral complexes: A study combining density fractionation, mineralogy and δ13C

Landuse changes for the purposes of cultivation often destabilise a substantial part of the initially stabilized organic matter (OM) in surface soils. However, the mechanisms of OM destabilisation are poorly documented, particularly with respect to organomineral complexes. The aim of this study was to characterize and quantify the effects of landuse on various OM pools, while focusing particularly on mineral-bound OM in a ferralsol. Four different parameters were assessed: the proportion of mineral-bound OM, the nature of the minerals (poorly crystalline aluminosilicates, gibbsite, halloysite and iron oxides) contributing to organomineral complexes, landuse, and soil depth (surface horizons versus non-tilled deeper horizons). The study site had a field with C4 vegetation that had been cultivated for 186 years and compared to an uncultivated reference plot with C3 vegetation. Organomineral complexes were separated by densimetric fractionation and characterized by mineralogical and carbon isotopic methods. Depending on the considered horizon, 58 to 80% of the OM was stabilized through organomineral complexation. Chemosorption of organic compounds at the surface of mineral phases was thus found to be a major stabilisation process in the ferralsol. Although cultivation significantly affected OM pools that were not bound to minerals (particulate non-occluded and particulate occluded OM), these pools represented a low proportion (~ 5%) of the carbon budget variations in the profile. Most variations in carbon stocks within the profile were attributed to organic matter located in organomineral complexes. Several potential processes were highlighted on the organomineral complex scale: (i) cultivation may have modified the physicochemical stability of mineral phases (involving dissolution and/or formation); (ii) cultivation could have led to vertical migration of organomineral complexes and their accumulation deeper in the profile; (iii) cultivation systematically modified the amount of organic compounds linked to minerals, with a decrease in topsoil and an increase in deep soil. In further studies to determine the impact of cultivation on soil carbon stocks, it would thus be interesting to continue assessing mechanisms that control the dynamics of mineral-bound OM while also investigating soil layers to a depth of at least 1 m.