Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Organic matter composition and degree of humification in lignite-rich mine soils under a chronosequence of pine

In the Lusatian mining district, in the eastern part of the Federal Republic of Germany, organic matter of reclaimed mine soils consists of a mixture of lignite and recently formed soil organic matter (recent carbon). The aim of the study was to investigate the recent carbon accumulation and the degree of humification of a chronosequence of young mine soils under forest. The lignite content of the forest floor, Ai (0–5 cm) and Cv horizons (1 m depth) was determined by ¹⁴CU activity measurements and the structural composition of the organic matter was characterised by ¹³C CPMAS NMR spectroscopy. To obtain a characterisation of the degree of humification, the soil samples were analysed for the content of polysaccharides, proteins, lignin and lipids by wet chemical methods. ¹⁴C activity measurements indicate that at the oldest site, comparable amounts of carbon accumulated in the first few centimetres of the soil profile than in natural forest soils. ¹³C CPMAS NMR spectra of the organic matter in the Ai horizons of the three soil profiles were dominated by aromatic and alkyl carbon species characteristic for lignite, but indicated as well an increasing contribution of carbon species from decomposing plant litter with soil age. When the results from wet chemical analyses were normalised to the total carbon content no changes with age could be noticed. After normalisation of the amount of litter compounds to the recent carbon content, the carbon identified by plant litter compound analysis decreased with increasing depth and increasing age of the soils. After 32 years the values are comparable to those of natural forest soils. These observations were confirmed by increasing degree of lignin alteration with stand age and soil depth. The data of wet chemical analyses complement data obtained by ¹⁴C activity measurements and ¹³C CPMAS NMR spectroscopy and lead to the conclusion that 32 years after reforestation the degree of humification of the soil organic matter is in the same range as those of natural sites. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Anthropogenic N deposition increases soil organic matter accumulation without altering its biochemical composition

Accumulating evidence indicates that future rates of atmospheric N deposition have the potential to increase soil C storage by reducing the decay of plant litter and soil organic matter (SOM). Although the microbial mechanism underlying this response is not well understood, a decline in decay could alter the amount, as well as biochemical composition of SOM. Here, we used size‐density fractionation and solid‐state ¹³C‐NMR spectroscopy to explore the extent to which declines in microbial decay in a long‐term (ca. 20 yrs.) N deposition experiment have altered the biochemical composition of forest floor, bulk mineral soil, as well as free and occluded particulate organic matter. Significant amounts of organic matter have accumulated in occluded particulate organic matter (~20%; oPOM); however, experimental N deposition had not altered the abundance of carboxyl, aryl, alkyl, or O/N‐alkyl C in forest floor, bulk mineral soil, or any soil fraction. These observations suggest that biochemically equivalent organic matter has accumulated in oPOM at a greater rate under experimental N deposition, relative to the ambient treatment. Although we do not understand the process by which experimental N deposition has fostered the occlusion of organic matter by mineral soil particles, our results highlight the importance of interactions among the products of microbial decay and the chemical and physical properties of silt and clay particles that occlude organic matter from microbial attack. Because oPOM can reside in soils for decades to centuries, organic matter accumulating under future rates of anthropogenic N deposition could remain in soil for long periods of time. If temperate forest soils in the Northern Hemisphere respond like those in our experiment, then unabated deposition of anthropogenic N from the atmosphere has the potential to foster greater soil C storage, especially in fine‐texture forest soils.     

Aspects of the chemical structure of soil organic materials as revealed by solid-state13C NMR spectroscopy

Solid-state cross-polarisation/magic-angle-spinning³C nuclear magnetic resonance (CP/MAS¹³C NMR) spectroscopy was used to characterise semi-quantitatively the organic materials contained in particle size and density fractions isolated from five different mineral soils: two Mollisols, two Oxisols and an Andosol. The acquired spectra were analysed to determine the relative proportion of carboxyl, aromatic, O-alkyl and alkyl carbon contained in each fraction. Although similar types of carbon were present in all of the fractions analysed, an influence of both soil type and particle size was evident.The chemical structure of the organic materials contained in the particle size fractions isolated from the Andosol was similar; however, for the Mollisols and Oxisols, the content of O-alkyl, aromatic and alkyl carbon was greatest in the coarse, intermediate and fine fractions, respectively. The compositional differences noted in progressing from the coarser to finer particle size fractions in the Mollisols and Oxisols were consistent with the changes noted in other studies where CP/MAS¹³C NMR was used to monitor the decomposition of natural organic materials. Changes in the C:N ratio of the particle size fractions supported the proposal that the extent of decomposition of the organic materials contained in the fine fractions was greater than that contained in the coarse fractions. The increased content of aromatic and alkyl carbon in the intermediate size fractions could be explained completely by a selective preservation mechanism; however, the further accumulation of alkyl carbon in the clay fractions appeared to result from both a selective preservation and anin situ synthesis.The largest compositional differences noted for the entire organic fraction of the five soils were observed between soil orders. The differences within orders were smaller. The Mollisols and the Andosol were both dominated by O-alkyl carbon but the Andosol had a lower alkyl carbon content. The Oxisols were dominated by both O-alkyl and alkyl carbon.A model describing the oxidative decomposition of plant materials in mineral soils is proposed and used to explain the influence of soil order and particle size on the chemical composition of soil organic matter in terms of its extent of decomposition and bioavailability.     

Linking microbial activity and soil organic matter transformations in forest soils under elevated CO2

Soil organic matter (SOM) dynamics ultimately govern the ability of soil to provide long‐term C sequestration and the nutrients required for ecosystem productivity. Predicting belowground responses to elevated CO₂ requires an integrated understanding of SOM transformations and the microbial activity that governs them. It remains unclear how the microorganisms upon which these transformations depend will function in an elevated CO₂ world. This study examines SOM transformations and microbial metabolism in soils from the Duke Free Air Carbon Enrichment site in North Carolina, USA. We assessed microbial respiration and net nitrogen (N) mineralization in soils with and without elevated CO₂ exposure during a 100‐day incubation. We also traced the depleted C isotopic signature of the supplemental CO₂ into SOM and the soils' phospholipid fatty acids (PLFA), which serve as biomarkers for living cells. Cumulative net N mineralization in elevated CO₂ soils was 50% that in control soils after a 100‐day incubation. Respiration was not altered with elevated CO₂. C : N ratios of bulk SOM did not change with elevated CO₂, but incubation data suggest that the C : N ratios of mineralized organic matter increased with elevated CO₂. Values of SOM δ¹³C were depleted with elevated CO₂ (?26.7±0.2 vs. ?30.2±0.3‰), reflecting the depleted signature of the supplemental CO₂. We compared δ¹³C of individual PLFA with the δ¹³C of SOM to discern incorporation of the depleted C isotopic signature into soil microbial groups in elevated CO₂ plots. PLFA i15:0, a15:0, and 10Met18:0 reflected significant incorporation of recently produced photosynthate, suggesting that the bacterial groups defined by these biomarkers are active metabolizers in elevated CO₂ soils. At least one of these groups (actinomycetes, 10Met18:0) specializes in metabolizing less labile substrates. Because control plots did not receive an equivalent ¹³C tracer, we cannot determine from these data whether this group of organisms was stimulated by elevated CO₂ compared with these organisms in control soils. Stimulation of this group, if it occurred in the elevated CO₂ plot, would be consistent with a decline in the availability of mineralizable organic matter with elevated CO₂, which incubation data suggest may be the case in these soils.     

Adsorption Characteristics and the Effect of Dissolved Organic Matter on Mercury(II) Adsorption of Various Soils in China

To evaluate the adsorption characteristics and effects of dissolved organic matter (DOM) from wheat straw (DOM_W) and swine manure (DOM_S) on mercury [Hg(II)] adsorption by soils, a series of experiments was conducted on 26 soils with different soil properties.Results showed that the values of K_d (a solid–liquid partition coefficient) of soils varied within a range of 0.768–14.386 L g^?1. K_d values were mainly controlled by soil organic matter (SOM), cation exchange capacity (CEC), soil nitrogen (N), and soil sulfur (S). When DOM_W and DOM_S were added to the soil samples, the amount of Hg(II) adsorbed by the soils decreased significantly. Furthermore, based on the decreased percentage of adsorption of Hg(II) by soil upon adding DOM_W (P_DOMW) or DOM_S (P_DOMS), about 73.08% soils, the values of P_DOMS were smaller than those of P_DOMW. The P_DOMW values were related to SOM, pH, free Fe oxide content, and CEC, whereas P_DOMS values were related to soil free Fe/Al oxide contents. Therefore, we should pay more attention about mercury risk caused by the addition of exogenous organic matter in soils, especially for the soils with low or high pH, less SOM, low CEC, and less free Fe/Al oxide.     

Cultivation effects on the nature of organic matter in soils and water extracts using CP/MAS 13C NMR spectroscopy

The objective of this study was to examine the chemical structure of the organic matter (SOM) of Oxisols soils in slash and burn agriculture, in relation to its biological properties and soil fertility. The CP/MAS ¹³C technique was used to identify the main structural groups in litter and fine roots as SOM precursors; to identify the changes on the nature of the SOM upon cultivation and the proportion of labile and stable components; and to identify the nature of the organics present in water extracts (DOC). Carbohydrates were the main structural components in litter whereas components such as carbonyl C, carboxyl C,O-alkyl C and alkyl C were more common in SOM. Phenolic C and the degree of aromaticity were similar in litter and SOM. Cultivation resulted in a small decrease in the relative proportion of carbohydrates in SOM, little change in the levels of O-alkyl C and carbonyl C, but an increase in carboxyl C, phenolic C and aromaticity of the SOM. The level of alkyl C in soil was higher than the level of O-alkyl C, indicating the importance of long-chain aliphatics along with lignins in the stabilization of the SOM in Oxisols. The SOM of Mollisols from the Canadian Prairies differed from the Oxisol, with a generally stronger expression of aromatic structures, particularly in a cultivated soil in relation to a native equivalent. Carbohydrate components were the predominant structures in the DOC, indicating their importance in nutrient cycling and vertical translocations in the Oxisol.     

Soil organic carbon stock and chemical composition in four plantations of indigenous tree species in subtropical China

Subtropical China has more than 60% of the total plantation area in China and over 70% of these subtropical plantations are composed of pure coniferous species. In view of low ecosystem services and ecological instability of pure coniferous plantations, indigenous broadleaf plantations are being advocated as a prospective silvicultural management for substituting in place of large coniferous plantations in subtropical China. However, little information is known about the effects of tree species conversion on stock and stability of soil organic carbon (SOC). The four adjacent monospecific plantations were selected to examine the effects of tree species on the stock and chemical composition of SOC using elemental analysis and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. One coniferous plantation was composed of Pinus massoniana (PM), and the three broadleaf plantations were Castanopsis hystrix (CH), Michelia macclurei (MM), and Mytilaria laosensis (ML). We found that SOC stock differed significantly among the four plantations in the upper (0–10 cm) layer, but not in the underneath (10–30 cm) layer. SOC stocks in the upper (0–10 cm) layer were 11, 19, and 18% higher in the CH, MM, and ML plantations than in the PM plantation. The differences in SOC stock among the four plantations were largely attributed to fine root rather than aboveground litterfall input. However, the soils in the broadleaf plantations contained more decomposable C proportion, indicated by lower percentage of alkyl C, higher percentage of O-alkyl C and lower alkyl C/O-alkyl C ratio compared to those in the PM plantation. Our findings highlight that future strategy of tree species selection for substituting in place of large coniferous plantations in subtropical China needs to consider the potential effects of tree species on the chemical composition in addition to the quantity of SOC stock.     

Incorporation of Plant Residues into Soil Organic Matter Fractions With Grassland Management Practices in the North American Midwest

Disturbed grassland soils are often cited as having the potential to store large amounts of carbon (C). Fertilization of grasslands can promote soil C storage, but little is known about the generation of recalcitrant pools of soil organic matter (SOM) with management treatments, which is critical for long-term soil C storage. We used a combination of soil incubations, size fractionation and acid hydrolysis of SOM, [C], [N], and stable isotopic analyses, and biomass quality indices to examine how fertilization and haying can impact SOM dynamics in Kansan grassland soils. Fertilized soils possessed 113% of the C possessed by soils subjected to other treatments, an increase predominantly harbored in the largest size fraction (212–2,000 μm). This fraction is frequently associated with more labile material. Haying and fertilization/haying, treatments that more accurately mimic true management techniques, did not induce any increase in soil C. The difference in ¹⁵N-enrichment between size fractions was consistent with a decoupling of SOM processing between pools with fertilization, congruent with gains of SOM in the largest size fraction promoted by fertilization not moving readily into smaller fractions that frequently harbor more recalcitrant material. Litterfall and root biomass C inputs increased 104% with fertilization over control plots, and this material possessed lower C:N ratios. Models of incubation mineralization kinetics indicate that fertilized soils have larger pools of labile organic C. Model estimates of turnover rates of the labile and recalcitrant C pools did not differ between treatments (65.5 ± 7.2 and 2.9 ± 0.3 μg C d⁻¹, respectively). Although fertilization may promote greater organic inputs into these soils, much of that material is transformed into relatively labile forms of soil C; these data highlight the challenges of managing grasslands for long-term soil C sequestration.     

Carbon isotopes reveal soil organic matter dynamics following arid land shrub expansion

 Over the past century, overgrazing and drought in New Mexico’s Jornada Basin has promoted the replacement of native black grama (Bouteloua eriopoda Torr.) grass communities by shrubs, primarily mesquite (Prosopis glandulosa Torr.). We investigated the effects of shrub expansion on the distribution, origin, turnover, and quality of light (LFC) and heavy (HFC) soil organic matter (SOM) fractions using δ¹³C natural abundance to partition SOM into C₄ (grass) and C₃ (shrub) sources. Soil organic matter beneath grasses and mesquite was isotopically distinct from associated plant litter, providing evidence of both recent shrub expansion and Holocene plant community changes. Our δ¹³C analyses indicated that SOM derived from mesquite was greatest beneath shrub canopies, but extended at least 3 m beyond canopy margins, similar to the distribution of fine roots. Specific ¹⁴C activities of LFC indicated that root litter is an important source of SOM at depth. Comparison of turnover rates for surface LFC pools in grass (7 or 40 years) and mesquite (11 or 28 years) soils and for HFC pools by soil depth (∼150–280 years), suggest that mesquite may enhance soil C storage relative to grasses. We conclude that the replacement of semiarid grasslands by woody shrubs will effect changes in root biomass, litter production, and SOM cycling that influence nutrient availability and long-term soil C sequestration at the ecosystem level. Received: 17 May 1996 / Accepted: 12 November 1996     

Vegetation Effects on Soil Organic Matter Chemistry of Aggregate Fractions in a Hawaiian Forest

We examined chemical changes from leaf tissue to soil organic matter (SOM) to determine the persistence of plant chemistry into soil aggregate fractions. We characterized a slow (Dicranopteris linearis) and fast-decomposing species (Cheirodendron trigynum) and surface (O), and subsurface (A-horizon) SOM beneath each species using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS), with and without derivatization. The live tissues of Dicranopteris had greater lignin content whereas Cheirodendron had a greater lipid, N-bearing, and polysaccharide component. Despite this difference in leaf chemistry, SOM chemistry was similar between soil aggregate fractions, but different between horizons. The O-horizon contained primarily lignin and polysaccharide biomarkers whereas the A-horizon contained polysaccharide, aromatic, and N-derived compounds, indicating considerable microbial processing of plant litter. The soils beneath Cheirodendron inherited a greater lipid signal composed of cutin and suberin biomarkers whereas the soils beneath Dicranopteris contained greater aromatic biomarker content, possibly derived from plant lignins. The soils beneath both species were more similar to root polysaccharides, lipids, and lignins than aboveground tissue. This study indicates that although plant-derived OM is processed vigorously, species-specific biomarkers and compound class differences persist into these soils and that differences in plant chemical properties may influence soil development even after considerable reworking of plant litter by microorganisms.     

Soil organic matter quality influences mineralization and GHG emissions in cryosols: a field‐based study of sub‐ to high Arctic

Arctic soils store large amounts of labile soil organic matter (SOM) and several studies have suggested that SOM characteristics may explain variations in SOM cycling rates across Arctic landscapes and Arctic ecosystems. The objective of this study was to investigate the influence of routinely measured soil properties and SOM characteristics on soil gross N mineralization and soil GHG emissions at the landscape scale. This study was carried out in three Canadian Arctic ecosystems: Sub‐Arctic (Churchill, MB), Low‐Arctic (Daring Lake, NWT), and High‐Arctic (Truelove Lowlands, NU). The landscapes were divided into five landform units: (1) upper slope, (2) back slope, (3) lower slope, (4) hummock, and (5) interhummock, which represented a great diversity of Static and Turbic Cryosolic soils including Brunisolic, Gleysolic, and Organic subgroups. Soil gross N mineralization was measured using the ¹⁵N dilution technique, whereas soil GHG emissions (N₂O, CH₄, and CO₂) were measured using a multicomponent Fourier transform infrared gas analyzer. Soil organic matter characteristics were determined by (1) water‐extractable organic matter, (2) density fractionation of SOM, and (3) solid‐state CPMAS ¹³C nuclear magnetic resonance (NMR) spectroscopy. Results showed that gross N mineralization, N₂O, and CO₂ emissions were affected by SOM quantity and SOM characteristics. Soil moisture, soil organic carbon (SOC), light fraction (LF) of SOM, and O‐Alkyl‐C to Aromatic‐C ratio positively influenced gross N mineralization, N₂O and CO₂ emissions, whereas the relative proportion of Aromatic‐C negatively influenced those N and C cycling processes. Relationships between SOM characteristics and CH₄ emissions were not significant throughout all Arctic ecosystems. Furthermore, results showed that lower slope and interhummock areas store relatively more labile C than upper and back slope locations. These results are particularly important because they can be used to produce better models that evaluate SOM stocks and dynamics under several climate scenarios and across Arctic landscapes and ecosystems.     

Changes in soil bacterial community triggered by drought‐induced gap succession preceded changes in soil C stocks and quality

The aim of this study was to understand how drought‐induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought‐induced die‐off, is being replaced by Holm‐oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and ¹³C solid‐state Nuclear Magnetic Resonance (CP‐MAS ¹³C NMR) to soils within areas of influence (defined as an surface with 2‐m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close‐chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below‐ground before above‐ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r‐strategic bacteria) further gives indications of how drought‐induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.     

Interactions between Carbon and Nitrogen Mineralization and Soil Organic Matter Chemistry in Arctic Tundra Soils

We used long-term laboratory incubations and chemical fractionation to characterize the mineralization dynamics of organic soils from tussock, shrub, and wet meadow tundra communities, to determine the relationship between soil organic matter (SOM) decomposition and chemistry, and to quantify the relative proportions of carbon (C) and nitrogen (N) in tundra SOM that are biologically available for decomposition. In all soils but shrub, we found little decline in respiration rates over 1 year, although soils respired approximately a tenth to a third of total soil C. The lack of decline in respiration rates despite large C losses indicates that the quantity of organic matter available was not controlling respiration and thus suggests that something else was limiting microbial activity. To determine the nature of the respired C, we analyzed soil chemistry before and after the incubation using a peat fractionation scheme. Despite the large losses of soil C, SOM chemistry was relatively unchanged after the incubation. The decomposition dynamics we observed suggest that tundra SOM, which is largely plant detritus, fits within existing concepts of the litter decay continuum. The lack of changes in organic matter chemistry indicates that this material had already decomposed to the point where the breakdown of labile constituents was tied to lignin decomposition. N mineralization was correlated with C mineralization in our study, but shrub soil mineralized more and tussock soil less N than would have been predicted by this correlation. Our results suggest that a large proportion of tundra SOM is potentially mineralizable, despite the fact that decomposition was dependent on lignin breakdown, and that the historical accumulation of organic matter in tundra soils is the result of field conditions unfavorable to decomposition and not the result of fundamental chemical limitations to decomposition. Our study also suggests that the anticipated increases in shrub dominance may substantially alter the dynamics of SOM decomposition in the tundra. Received 31 January 2002; accepted 16 July 2002.     

Phenanthrene toxicity and dissipation in rhizosphere of grassland plants (Lolium perenne L. and Trifolium pratense L.) in three spiked soils

Rhizodegradation is a technique involving plants that offers interesting potential to enhance biodegradation of persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Nevertheless, the behaviour of PAHs in plant rhizosphere, including micro-organisms and the physico-chemical soil properties, still needs to be clarified. The present work proposes to study the toxicity and the dissipation of phenanthrene in three artificially contaminated soils (1 g kg^-1 DW). Experiments were carried out after 2 months of soil aging. They consisted in using different systems with two plant species (Ryegrass—Lolium perenne L. var. Prana and red clover—Trifolium pratense L. var. fourragère Caillard), three kinds of soils (a silty-clay-loam soil “La Bouzule”, a coarse sandy-loam soil “Chenevières” and a fine sandy-loam soil “Maconcourt”). Phenanthrene was quantified by HPLC in the beginning (T ₀) and the end of the experiments (30 days). Plant biomass, microbial communities including mycorrhizal fungi, Rhizobium and PAH degraders were also recorded. Generally phenanthrene contamination did not affect plant biomass. Only the red clover biomass was enhanced in Chenevières and La Bouzule polluted soils. A stimulation of Rhizobium red clover colonisation was quantified in spiked soils whereas a drastic negative phenanthrene effect on the mycorrhization of ryegrass and red clover was recorded. The number of PAH degraders was stimulated by the presence of phenanthrene in all tested soils. Both in ryegrass and red clover planted soils, the highest phenanthrene dissipation due to the rhizosphere was measured in La Bouzule soils. On the contrary, in non-planted soils, La Bouzule soils had also the lowest pollutant dissipation. Thus, in rhizospheric and non-rhizospheric soils the phenanthrene dissipation was found to depend on soil clay content.     

Atmospheric N Deposition Increases Organic N Loss from Temperate Forests

Atmospheric deposition of nitrogen (N) resulting from fossil fuel combustion has increased N inputs to temperate forests worldwide with large consequences for forest productivity and water quality. Recent work has illustrated that dissolved organic N (DON) often dominates N loss from unpolluted forests and that the relative magnitude of dissolved inorganic N (DIN) loss increases with atmospheric loading. In contrast to DIN, DON loss is thought to be controlled by soil dynamics that operate independently of N supply and demand and thus should track dissolved organic carbon (DOC) following strict stoichiometric constraints. Conversely, DON loss may shift with N supply if soil (SOM) or dissolved organic matter (DOM) is stoichiometrically altered. Here, we assess these two explanations of DON loss, which we refer to as the Passive Carbon Vehicle and the Stoichiometric Enrichment hypotheses, by analyzing patterns in soil and stream C and N in forest watersheds spanning a broad gradient in atmospheric N loading (5–45 kg N ha⁻¹ y⁻¹). We show that soil N and DON losses are not static but rather increase asymptotically with N loading whereas soil C and DOC do not, resulting in enrichment of organic N expressed as decreased soil C:N and stream DOC:DON ratios. DON losses from unpolluted sites are consistent with conservative dissolution and transport of refractory SOM. As N supply increases, however, N enrichment of organic losses is greater than expected from simple dissolution of enriched soils, suggesting activation of novel pathways of DON production or direct N enrichment of DOM. We suggest that our two hypotheses represent domains of control over forest DON loss as N supply increases but also that stoichiometric enrichment of bulk soils alone cannot fully account for large DON losses in the most N-polluted forests.