Mineralization of dissolved organic carbon in mineral soil solution of two forest soils

Dissolved organic carbon (DOC) constitutes an important carbon input flux to forested mineral soils. Seepage from mineral subsoils contains only small amounts of DOC because of mineralization, sorption or the formation of particulate organic matter (POM). However, the relation between these processes is largely unknown. Therefore, the objective of this study was to quantify the mineralization of DOC from different depths of forest soils, and to determine degradation rate constants for rapidly and slowly degradable DOC pools. Mineralization of DOC and formation of POM in mineral soil solution from two forested sites in northern Bavaria (Germany) were quantified in a 97 days laboratory incubation experiment. Furthermore, spectroscopic properties such as specific UV absorption and a humification index derived from fluorescence emission spectrometry were measured before and after incubation. DOC in all samples turned out to belong mainly to the stable DOC pool (> 95 %) with half‐lives ranging from years to decades. Spectroscopic properties were not suitable to predict the mineralization of DOC from mineral soils. However, together with data on DOC from the forest floor and long‐term data on DOC concentrations in the field they helped to identify the processes involved in C sequestration in mineral subsoils. Mineralization, formation of POM, and probably sorption seem all to be responsible for maintaining low concentrations of DOC in the upper mineral soil. DOC below the upper mineral soil is highly resistant to mineralization, and thus the further decrease of DOC concentrations in the subsoil as observed under field conditions cannot be attributed to mineralization. Our results suggest that sorption and to some minor extent the formation of POM may be responsible for C sequestration in the subsoil.     

Soil organic matter transformation in Argentinian Hapludolls

Distribution and transformation of SOM in an Argentinian Hapludoll under arable land use and afforested with Pinus radiata was investigated by a combined approach using particle-size fractionation, wet-chemical analysis and ¹³C NMR spectroscopy. The soils showed thick mollic A horizons and had high organic carbon (OC) contents even in the subsoil, clay-sized separates having the highest OC concentrations. Under pine, a thick forest floor was built up. CuO oxidation data indicated low transformation of lignin in the forest floor, but advanced oxidative decomposition in the mineral soil horizon. In contrast, non-cellulosic carbohydrates, appeared to be stabilized in the mineral soil horizon against mineralization. Humic acids extracted from the mineral soil horizons showed an extremely high aromaticity. We assume that this was due to the production of pyrogenic aromatic moieties (black carbon) as a result of frequent fires in this ecosystem. No clear profile differentiation with respect to SOM quality was obtained. Composition of SOM in the mineral soil appeared not yet influenced from land use.     

Vertical Distribution of Soil Pyrogenic Matter: A Review

Alteration of terrestrial biomass during fire produces pyrogenic matter, a large quantity of which is deposited on and may enter soils. The amount of annual burning around the globe implies greater production and storage of carbon in soil pyrogenic organic matter (pyrOM) than is regularly found in investigations of soil pyrogenic matter. A portion of this unaccounted for pyrogenic material may be translocated to subsurface soil depths, which is frequently excluded from soil investigations, and the presence of subsoil charcoal helps to partially fill this pyrOM gap. In this paper, I discuss the mechanisms for vertical redistribution of pyrogenic matter in soils and develop conceptual models to describe the depth, particle size, and age distribution of pyrogenic matter resulting from different translocation mechanisms. Based on the relationships between pyrogenic matter turnover and translocation, the suitability of pyrogenic matter in soils as an indicator for soil processes is discussed. Deciphering the relationships between pyrogenic matter depth, particle size, and age distribution yields valuable insights into the dynamics of soil forming processes as well as soil pyrogenic matter turnover, which should therefore, be a focus of future studies.     

Alkyl C and hydrophobicity in B and C horizons of an acid forest soil

Aliphatic C most probably derived from ester‐bound moieties was found to be present in sandy subsoil horizons. The hydrophobic nature of such compounds may increase their stabilization potential. Therefore, the aim of this study was to investigate the potential of aliphatic compounds in mineral soil horizons along a Dystric Cambisol profile under beech forest to increase hydrophobicity. The conceptual approach included the analyses of soil samples before and after solvent extraction and base hydrolysis for elemental and isotopic composition. Additionally, the advancing contact angle was measured to quantify hydrophobicity. Curie‐point pyrolysis GC/MS was carried out to characterize the nature of alkyl C present in subsoil samples. A close correlation between the ¹⁴C activity and the stable‐C‐isotope ratio (δ¹³C) indicates isotopic fractionation upon C stabilization in subsoils. Free lipids contributed less than 10% to the organic C found in subsoil horizons. Base hydrolysis revealed very high amounts of hydroxyalkanoic acids in the B horizons of the acid forest soil. Hydrophobicity of SOM was not found to be correlated to esterified‐ or free‐lipid content. The contact angle was in a similar range for all bulk soil horizons, suggesting greater hydrophobicity of organic matter in subsoil horizons considering their very low concentrations of organic C compared to the A horizon. The quantity and nature of pyrolysis products change with increasing depth in the soil profile. Aliphatic products cannot be detected in B and C horizons by Curie‐point pyrolysis GC/MS.     

Charcoal quality does not change over a century in a tropical agro-ecosystem

Charcoal stocks were determined in a chronosequence of soils which have been converted to agricultural land use by slash-and-burn up to 100 years ago. With time, opposite to our assumptions, the charcoal chemical quality, as measured by molecular markers for pyrogenic carbon, did not change and charcoal stocks did not show a clear decrease. Our results indicate that charcoal may resist chemical degradation even when exposed to intense weathering in a tropical climate.     

The effect of pre‐industrial charcoal kilns on chemical properties of forest soil of Wallonia,Belgium

In Wallonia, Belgium, intensive in situ charcoal production that was linked closely to pre‐industrial smelting and steel‐making affected a large part of the forested area in the late eighteenth century. Charcoal kiln relics can be detected under forest as domes of about 10 m in diameter, with the topsoil greatly enriched with charcoal residues. We sampled 19 charcoal kiln sites and the adjacent reference soil by soil horizon on four different soil types (Arenosols, Luvisols, Cambisols and Podzols). Data were analysed with linear mixed models to assess the effect of the charcoal kiln site on soil properties in relation to depth and soil conditions. We also addressed the evolution of soil properties over time by a comparison of the soil characteristics at a currently active kiln site. The charcoal‐rich topsoil has a larger C:N ratio and cation exchange capacity (CEC) per unit of organic carbon than the reference soil. The largest CECs per unit of carbon were observed on soil with coarser textures. On acidic soil, the increase in base saturation in the subsoil reflects the past liming effect of ash produced by wood charring, whereas the topsoil is re‐acidified. The acidity of carbonate‐rich Cambisols, however, is not reduced. Regardless of soil type, the kiln topsoil is greatly depleted in exchangeable K⁺ and available P, which may be attributed to the small affinity of the exchange complex of charcoal for K⁺ and a decrease in P availability with time. Therefore, we recommend further research on the long‐term effects of biochar on the dynamics of plant nutrients.     

Effects of pyrolysis temperature and feedstock type on biochar characteristics pertinent to soil carbon and soil health: A meta-analysis

Biochar amendment to soil is utilized globally as an approach to enhance carbon storage and to improve soil functioning. However, biochar characteristics and related improvements of soil functioning depend on biochar production conditions. Systematic evaluation of corresponding biochar characteristics is needed for more targeted and efficient biochar application strategies. Herein, we systematically review the effects of biochar pyrolysis temperature (175–950°C) and feedstock (corn stover, switchgrass and wood) on selected biochar characteristics (carbon content, H/C ratio, nitrogen content, pH, specific surface area, ash content and pore volume). These specific characteristics were selected as being pertinent to soil organic carbon sequestration and soil health improvement. Despite numerous studies on these topics, few have numerically quantified the effects of pyrolysis temperature. Our results show that high pyrolysis temperature (>500°C) increased carbon content and pore volume for wood biochar compared with low pyrolysis temperature (≤500°C). The high pyrolysis temperature decreased the H/C ratio and nitrogen content but increased pH, specific surface area and ash content regardless of feedstock. Compared with corn stover biochar and switchgrass biochar, wood biochar had higher carbon content and larger specific surface area but lower nitrogen and ash contents regardless of pyrolysis temperature. The higher biochar carbon content might be derived from higher lignin and cellulose contents of wood feedstock. Wood feedstock had 76%–109% more lignin and 27%–47% more cellulose than corn stover and switchgrass. Corn stover biochar had higher pH, and switchgrass biochar had larger pore volume than wood biochar. Our study indicates that the targeted production of biochar with specific characteristics can be facilitated by the selection of pyrolysis temperature and feedstock type. For amending soil with biochar, more operationally defined biochar production conditions and feedstock selection might be a way forward to wider acceptance and better predictability of biochar performance under field conditions.     

Vertical gradients of potential enzyme activities in soil profiles of European beech,Norway spruce and Scots pine dominated forest sites

Management of forest sites has the potential to modulate soil organic matter decomposition by changing the catalytic properties of soil microorganisms within a soil profile. In this study we examined the impact of forest management intensity and soil physico-chemical properties on the variation of enzyme activities (β-glucosidase, β-xylosidase, α-glucosidase, phenol oxidase, N-acetyl-glucosaminidase, l-leucine aminopeptidase, phosphatase) in the topsoil and two subsoil horizons in three German regions (Schorfheide-Chorin, Hainich-Dün, Schwäbische Alb). The sandy soils in the Schorfheide-Chorin (SCH) showed lower ratios of the activity of carbon (C) acquiring enzymes (β-glucosidase) relative to nitrogen (N) acquiring enzymes (N-acetyl-glucosaminidase + l-leucine aminopeptidase), and activity of C acquiring enzymes relative to phosphorous (P) acquiring enzymes (phosphatase) than the finer textured soils in the Hainich-Dün (HAI) and Schwäbische Alb (ALB), indicating a shift in investment to N and P acquisition in the SCH. All enzyme activities, except phenol oxidase activity, decreased in deeper soil horizons as concentrations of organic C and total N did, while the decrease was much stronger from the topsoil to the first subsoil horizon than from the first subsoil to the second subsoil horizon. In contrast, phenol oxidase activity showed no significant decrease towards deeper soil horizons. Additionally, enzyme activities responsible for the degradation of more recalcitrant C relative to labile C compounds increased in the two subsoil horizons. Subsoil horizons in all regions also indicate a shift to higher N acquisition, while the strength of the shift depended on the soil type. Further, our results clearly showed that soil properties explained most of the total variance of enzyme activities in all soil horizons followed by study region, while forest management intensity had no significant impact on enzyme activities. Among all included soil properties, the clay content was the variable that explained the highest proportion of variance in enzyme activities with higher enzyme activities in clay rich soils. Our results highlight the need for large scale studies including different regions and their environmental conditions in order to derive general conclusions on which factors (anthropogenic or environmental) are most influential on enzyme activities in the whole soil profile in the long term at the regional scale.     

Modeling soil respiration based on carbon, nitrogen, and root mass across diverse Great Lake forests

The variability in the net ecosystem exchange of carbon (NEE) is a major source of uncertainty in quantifying global carbon budget and atmospheric CO₂. Soil respiration, which is a large component of NEE, could be strongly influential to NEE variability. Vegetation type, landscape position, and site history can influence soil properties and therefore drive the microbial and root production of soil CO₂. This study measured soil respiration and soil chemical, biological and physical properties on various types of temperate forest stands in Northern Wisconsin (USA), which included ash elm, aspen, northern hardwood, red pine forest types, clear-cuts, and wetland edges. Soil respiration at each of the 19 locations was measured six times during 1 year from early June to mid-November. These data were combined with two additional data sets from the same landscape that represent two smaller spatial scales. Large spatial variation of soil respiration occurred within and among each forest type, which appeared to be from differences in soil moisture, root mass and the ratio of soil carbon to soil nitrogen (C:N). A soil climate driven model was developed that contained quadratic functions for root mass and the ratio of soil carbon to soil nitrogen. The data from the large range of forest types and site conditions indicated that the range of root mass and C:N on the landscape was also large, and that trends between C:N, root mass, and soil respiration were not linear as previously reported, but rather curvilinear. It should be noted this function appeared to level off and decline at C:N larger than 25, approximately the value where microbial nitrogen immobilization limits free soil nitrogen. Weak but significant relationships between soil water and soil C:N, and between soil C:N and root mass were observed indicating an interrelatedness of (1) topographically induced hydrologic patterns and soil chemistry, and (2) soil chemistry and root production. Future models of soil respiration should address multiple spatial and temporal factors as well as their co-dependence.     

不同森林植被下土壤有机碳的分解特征及碳库研究

分析了不同森林植被和同一植被不同林龄的人工杉木林下土壤有机碳的分解特征及土壤有机碳中的活性碳库、缓效性碳库和惰效性碳库的大小和周转时间。结果表明:不同森林植被下土壤有机碳的分解速率不同,总的趋势都是:培养前期分解速度快,后期分解速度慢,土壤剖面A层>剖面B层。在剖面A层中:不同森林植被下分解速率的大小顺序为常绿阔叶林>人工杉木林,不同林龄的人工杉木林为成熟林>中龄林>幼龄林;在剖面B层中:分解速率差异不大。不同森林植被下不同土壤剖面上的土壤活性碳库、缓效性碳库和惰效性碳库的库容和分解速率不同,土壤活性碳库碳含量一般占总有机碳的0 99%～2 89%,田间平均驻留时间为10～23天;土壤缓效性碳一般占总有机碳的17 17%～55 46%,田间平均驻留时间为1 6～24 2年;土壤惰效性碳一般占总有机碳的42 05%～80 66%,田间平均驻留时间为假定的1000年。     

Effect of Land-Use History on Soil Carbon and Nitrogen in a Mediterranean Catchment

The impact of land use on soil organic matter was investigated. Five land cover types (pine forest, olive groves, wheat, wheat/maize cultivation systems, and a shrub pasture) belonging to three land-use categories from the same catchment in the island of Lesvos, Greece, were used. The soils developed under similar pedogenetic processes and accepted similar agricultural practices for at least 30 years. The results showed that the land-use and cover types ranged according to their total soil carbon (C) content as follows: forest > double cultivation > wheat > olive > pasture. Crop plantations contained 31 to 40% less C at their upper 0- to 45-cm layer than forest. Pasture had shallow soils with a small C accumulation but high C concentration, whereas olive groves had the lowest concentrations of both soil C and nitrogen (N). Olive grove soils were the most prone to degradation but possessed the greatest potential for C sequestration.     

Controls on carbon accumulation and storage in the mineral subsoil beneath peat in Lakkasuo mire, central Finland

What processes control the accumulation and storage of carbon (C) in the mineral subsoil beneath peat? To find out we investigated four podzolic mineral subsoil profiles from forest and beneath peat in Lakkasuo mire in central boreal Finland. The amount of C in the mineral subsoil ranged from 3.9 to 8.1 kg m^?2 over a thickness of 70 cm and that in the organic horizons ranged from 1.8 to 144 kg m^?2. Rates of increase of subsoil C were initially large (14 g m^?2 year^?1) as the upland forest soil was paludified, but decreased to < 2 g m^?2 year^?1 from 150 to 3000 years. The subsoils retained extractable aluminium (Al) but lost iron (Fe) as the surrounding forest podzols were paludified beneath the peat. A stepwise, ordinary least‐squares regression indicated a strong relation (R² = 0.91) between organic C concentration of 26 podzolic subsoil samples and dithionite–citrate–bicarbonate‐extractable Fe (negative), ammonium oxalate‐extractable Al (positive) and null‐point concentration of dissolved organic C (DOC_np) (positive). We examined the ability of the subsoil samples to sorb dissolved organic C from a solution derived from peat. Null‐point concentration of dissolved C (DOC_np) ranged from 35 to 83 mg l^?1, and generally decreased from the upper to the lower parts of the profiles (average E, B and C horizon DOC_np concentrations of 64, 47 and 42 mg l^?1). The DOC_np was positively correlated with percentage of soil C and silt and clay content. The concentration of dissolved organic C in pore water in the peat ranged from 12 to 60 mg l^?1 (average 33 mg l^?1), suggesting that the sorptive capacity of the subsoil horizons for C had been exhausted. We suggest that the increase of C contents in the subsoil beneath mires is related to adsorption of dissolved organic C and slow mineralization under anaerobic conditions.     

Charcoal production at kiln sites affects C and N dynamics and associated soil microorganisms in Quercus spp. temperate forests of central Mexico

Temperate forests dominated by Quercus spp. cover large parts of Central Mexico and rural communities depend on these forests for wood and charcoal. The impacts of charcoal production on selected chemical properties including C and N dynamics, and populations of ammonifiers, nitrifiers and denitrifiers were investigated on surface soils (0–15 cm) collected during the dry and rainy season of these forests. Organic C was halved in soil at the kiln sites compared to undisturbed forest soil. Concentrations of exchangeable Ca²⁺, K⁺ and Mg²⁺ increased >1.6 times at kiln sites and pH increased from 4.5 in undisturbed soil to 7.0 at kiln sites. The kiln sites had 1.3 times and 2.4 times lower microbial biomass C and N, respectively, than undisturbed forest sites during the rainy season. Although the effect of charcoal production on NH₄⁺, NO₂^? and NO₃^? concentrations was small, the ammonifying, nitrifying and denitrifiers were 16 times lower at the kiln sites than in the undisturbed forest soil. This research found that the charcoal production had a negative effect on the cultivable microorganisms involved in N cycling and the soil microbial biomass C and N compared to undisturbed forest soil. Differences in inorganic N dynamics were more affected by seasonality, i.e. precipitation, than by charcoal production.     

复配改良剂表施对高降雨量地区茶园底层强酸性土壤酸度的调控

Strongly acidic soils (pH < 5.0) are detrimental to tea (Camellia sinensis) production and quality. Little information exists on the ability of surface amendments to ameliorate subsoil acidity in the tea garden soils. A 120-d glasshouse column leaching experiment was conducted using commonly available soil ameliorants. Alkaline slag (AS) and organic residues, pig manure (PM) and rapeseed cake (RC) differing in ash alkalinity and C/N ratio were incorporated alone and in combination into the surface (0--15 cm) of soil columns (10 cm internal diameter × 50 cm long) packed with soil from the acidic soil layer (15--30 cm) of an Ultisol (initial pH = 4.4). During the 120-d experiment, the soil columns were watered (about 127 mm over 9 applications) according to the long-term mean annual rainfall (1 143 mm) and the leachates were collected and analyzed. At the end of the experiment, soil columns were partitioned into various depths and the chemical properties of soil were measured. The PM with a higher C/N ratio increased subsoil pH, whereas the RC with a lower C/N ratio decreased subsoil pH. However, combined amendments had a greater ability to reduce subsoil acidity than either of the amendments alone. The increases in pH of the subsoil were mainly ascribed to decreased base cation concentrations and the decomposition of organic anions present in dissolved organic carbon (DOC) and immobilization of nitrate that had been leached down from the amended layer. A significant (P < 0.05) correlation between alkalinity production (reduced exchangeable acidity -- N-cycle alkalinity) and alkalinity balance (net alkalinity production -- N-cycle alkalinity) was observed at the end of the experiment. Additionally, combined amendments significantly increased (P < 0.05) subsoil cation concentrations and decreased subsoil Al saturation (P < 0.05). Combined applications of AS with organic amendments to surface soils are effective in reducing subsoil acidity in high-rainfall areas. Further investigations under field conditions and over longer timeframes are needed to fully understand their practical effectiveness in ameliorating acidity of deeper soil layers under naturally occurring leaching regimes.     

华南花岗岩侵蚀区不同植被类型坡面土壤有机碳分布和团聚体稳定性

植物是影响土壤有机碳含量和土壤团聚体稳定性的重要因素。选取华南典型花岗岩侵蚀区荒草地、桉树林、湿地松林和木荷林4种植被类型径流小区的土壤为研究对象,分析测定不同坡位、不同土层深度的土壤有机碳特性和团聚体稳定性等指标,评价不同植被类型对土壤养分的分布特性以及团聚体稳定性差异,明确花岗岩侵蚀退化区较为理想的生态恢复措施,旨在为合理利用土壤、重建坡面植被和改善土壤结构提供科学依据。结果表明:土壤总有机碳(TOC)、全氮(TN)和溶解性有机碳(DOC)含量随土层加深逐渐降低,而林地小区土壤碳氮比(C/N)则相反,荒草地碳氮元素的坡面变异系数(CV)显著高于其他3种林地,其中桉树林地TOC、TN、DOC和C/N的坡面分布的变异系数较荒草地分别降低40%,56.18%,68.5%和25.81%;湿地松林地TOC、TN、DOC和C/N的坡面分布的变异系数较荒草地分别降低62.73%,33.71%,46.46%,58.06%;木荷林地TOC、TN、DOC和C/N的坡面分布的变异系数较荒草地分别降低41.82%,38.2%,51.18%,48.39%,表明林地较荒草地更有利于土壤碳氮在坡面的均质化和有机质的积累。荒草地和木荷林地0.25 mm粒径以上的团聚体在上、中坡位的质量分数显著高于其他植被类型,而林下植被生物量较高的木荷林地的平均质量直径(MWD)和几何平均直径(GMD)显著高于其他植被类型。其中木荷小区水稳性团聚体平均质量直径(MWD)较荒草地、桉树和湿地松分别高20.10%,19.58%,23.20%;几何平均直径(GMD)较荒草地、桉树和湿地松分别高20.00%,19.54%,22.23%,表明在花岗岩侵蚀区林地空间结构较好的林草模式有利于土壤有机碳的积累和土壤结构的稳定。     

Charcoal effects on soil solution chemistry and growth of <Emphasis Type="BoldItalic">Koeleria macrantha</Emphasis> in the ponderosa pine/Douglas-fir ecosystem

We conducted laboratory and greenhouse experiments to determine whether charcoal derived from the ponderosa pine/Douglas-fir ecosystem may influence soil solution chemistry and growth of Koeleria macrantha, a perennial grass that thrives after fire. In our first experiment, we incubated forest soils with a factorial combination of Douglas-fir wood charcoal generated at 350°C and extracts of Arctostaphylos uva-ursi with and without the addition of glycine as a labile N source. These results showed that charcoal increased N mineralization and nitrification when glycine was added, but reduced N mineralization and nitrification without the addition of glycine. Charcoal significantly reduced the solution concentration of soluble phenols from litter extracts, but may have contributed bioavailable C to the soil that resulted in N immobilization in the no-glycine trial. In our second experiment, we grew K. macrantha in soil amended with charcoal made at 350°C from ponderosa pine and Douglas-fir bark. Growth of K. macrantha was significantly diminished by both of these charcoal types relative to the control. In our third experiment, we grew K. macrantha in soil amended with six concentrations (0, 0.5, 1, 2, 5, and 10%) of charcoal collected from a wildfire. The data showed increasing growth of K. macrantha with charcoal addition, suggesting some fundamental differences between laboratory-generated charcoal and wildfire-produced charcoal. Furthermore, they suggest a need for a better understanding of how temperature and substrate influence the chemical properties of charcoal.     

Carbon mineralization, fungal and bacterial growth, and enzyme activities as affected by contact between crop residues and soil

The degree of contact between crop residues and the soil matrix, as determined by the method of residue incorporation, affects decomposition dynamics both under natural and experimental conditions. In microcosm experiments we tested the hypothesis that poor residue-soil contact reduces the decomposition of structural plant constituents through delayed colonization by microorganisms degrading cellulose and hemicellulose. Barley straw or red clover foliage was either confined in buried mesh bags or homogeneously mixed into a loamy topsoil or a silty subsoil to create poor or intimate residue-soil contact in microbiologically rich and less rich environments, respectively. Soil type had no effect on decomposition of the easily degradable clover residues, but cumulative mineralization of barley straw C after 52 days at 15°C was less in the subsoil than in the topsoil by 12% of initial C. For clover material, poor soil contact increased cumulative C mineralization by 5% of initial C in the loamy topsoil but had no effect in the silty subsoil. For the more slowly degradable, cellulose- and hemicellulose-rich straw, on the other hand, poor soil contact reduced C mineralization by 6% of initial C. The results from the loamy topsoil were confirmed in a second experiment in a sandy topsoil. The reduced decomposition of straw with poor soil contact could not be explained by less favourable abiotic conditions, N deficiency nor exclusion of larger animals by mesh bags. Reduced straw-soil contact delayed measured increases in fungal ergosterol concentration, ratio of fungal to bacterial substrate-induced respiration, number of cellulase-producing, colony-forming bacterial units and activity of cellulases and hemicellulase on the residues. Thus, the results supported our hypothesis and underscore the importance of ensuring representative conditions for the soil microflora when decomposition dynamics are studied in microcosms experiments designed to mimic field conditions.     

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.     

Properties and bioavailability of particulate and mineral‐associated organic matter in Arctic permafrost soils,Lower Kolyma Region,Russia

Permafrost degradation may cause strong feedbacks of arctic ecosystems to global warming, but this will depend on if, and to what extent, organic matter (OM) is protected against biodegradation by mechanisms other than freezing and anoxia. Here, we report on the amount, chemical composition and bioavailability of particulate (POM) and mineral‐associated OM (MOM) in permafrost soils of the East Siberian Arctic. The average total organic carbon (OC) stock across all soils was 24.0 ± 6.7 kg m^?2 within 100 cm soil depth. Density fractionation (density cut‐off 1.6 g cm^?3) revealed that 54 ± 16% of the total soil OC and 64 ± 18% of OC in subsoil horizons was bound to minerals. As well as sorption of OM to clay‐sized minerals (R² = 0.80; P < 0.01), co‐precipitation of OM with hydrolyzable metals may also transfer carbon into the mineral‐bound fraction. Carbon:nitrogen ratios, stable carbon and nitrogen isotopes, ¹³C‐NMR and X‐ray photoelectron spectroscopy showed that OM is transformed in permafrost soils, which is a prerequisite for the formation of mineral‐organic associations. Mineral‐associated OM in deeper soil was enriched in ¹³C and ¹⁵N, and had narrow C:N and large alkyl C:(O‐/N‐alkyl C) ratios, indicating an advanced stage of decomposition. Despite being up to several thousands of years old, when incubated under favourable conditions (60% water‐holding capacity, 15°C, adequate nutrients, 90 days), only 1.5–5% of the mineral‐associated OC was released as CO₂. In the topsoils, POM had the largest mineralization but was even less bioavailable than the MOM in subsoil horizons. Our results suggest that the formation of mineral‐organic associations acts as an important additional factor in the stabilization of OM in permafrost soils. Although the majority of MOM was not prone to decomposition under favourable conditions, mineral‐organic associations host a readily accessible carbon fraction, which may actively participate in ecosystem carbon exchange.     

长白山森林土壤有机碳库大小及周转研究

主要分析不同森林植被下有机碳的分解动态和土壤碳库各组分大小、周转时间。结果表明：土壤样品培养90天，CO2累计释放量表层大致为1723～5065mg／kg、下层大致为178～642mg／kg。分解速率总的趋势是前期快，后期慢，表层明显大于下层。大小顺序为：冷杉林〉针阔混交林和阔叶林〉针叶林。在不同植被下的表层和下层土壤中，活性碳占总有机碳的0．54％～1．67％，0．45％～5．48％．平均驻留时间为11～56天、60～88天；缓效性碳占总有机碳的23．0％～63．3％，33．2％～72．2％，平均驻留时间为4～70年、24～161年；惰效性碳占总有机碳的35．5％～75．5％．26．0％～65．%。表层土壤的总有机碳、活性碳、缓效性碳和惰效性碳含量都明显大于下层。凋落物的化学组成主要决定活性碳库、缓效性碳库含量，土壤的粘粒含量等性质主要决定惰效性碳库含量。