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1.
The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm−3 (free POM<1.6), occluded particulate organic matter with densities <1.6 g cm−3 (occluded POM<1.6) and 1.6-2.0 g cm−3 (occluded POM1.6-2.0) and mineral-associated SOM (>2.0 g cm−3) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the 13C/12C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM<1.6>occluded POM<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.  相似文献   

2.
 The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg–1) and lowest under natural fallow (6.3 mg kg–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg–1) and lowest in continuously cropped plots (107 mg N kg–1). After addition of all nutrients except N, a highly significant linear relationship (R 2=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R 2 increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability. Received: 9 April 1999  相似文献   

3.
Humus forms may be the first tool to assess qualitatively organic matter turnover in soils; as such they should be related to the stocks of organic C a soil can store, to the characteristics of organic matter that affect its stability and, more generally, to the factors of soil formation. In this work, we tested these hypotheses in 27 forest soils of northwestern Italy. Site variables representing the pedogenic factors allowed classifying the plots into three clusters, which were significantly different for soil and humus types. The average stocks of organic C in the humic episolum (organic and top mineral horizons) ranged from 2.7 kg m−2 in Eumulls to 9.5 kg m−2 in Amphimulls. A clear trend in C stocks was visible and related both to the increasing presence of organic layers where the environmental conditions do not favour a rapid turnover of organic matter and to the good mixing of organics and minerals in “bio-macrostructured” A horizons. The characteristics of organic matter were also linked to humus forms: The proportion of humified complex substances was the highest in the most active forms, and conversely, non-humified extracted substances formed a considerable part of organic matter only where the environmental conditions limit organic matter degradation. Humus forms seem therefore to reflect several mechanisms of organic matter stabilisation and are clearly related to the capacity of the soil to store C.  相似文献   

4.
In the grassland/forest ecotone of North America, many areas are experiencing afforestation and subsequent shifts in ecosystem carbon (C) stocks. Ecosystem scientists commonly employ a suite of techniques to examine how such land use changes can impact soil organic matter (SOM) forms and dynamics. This study employs four such techniques to compare SOM in grassland (Bromus inermis) and recently forested (∼35 year, Ulmus spp. and Quercus spp.) sites with similar soil types and long-term histories in Kansas, USA. The work examines C and nitrogen (N) parameters in labile and recalcitrant SOM fractions isolated via size and density fractionation, acid hydrolysis, and long-term incubations. Size fractionation highlighted differences between grassland and forested areas. N concentration of forested soils’ 63-212 μm fraction was higher than corresponding grassland soils’ values (3.0±0.3 vs. 2.3±0.3 mg gfraction−1, P<0.05), and N concentration of grassland soils’ 212-2000 μm fraction was higher than forested soils (3.0±0.4 vs. 2.3±0.2 mg gfraction−1, P<0.05). Similar trends were observed for these same fractions for C concentration; forested soils exhibited 1.3 times the C concentration in the 63-212 μm fraction compared to this fraction in grassland soils. Fractions separated via density separation and acid hydrolysis exhibited no differences in [C], [N], δ15N, or δ13C when compared across land use types. Plant litterfall from forested sites possessed significantly greater N concentrations than that from grassland sites (12.41±0.10 vs. 11.62±0.19 mg glitter−1). Long-term incubations revealed no differences in C or N dynamics between grassland and forested soils. δ13C and δ15N values of the smallest size and the heavier density fractions, likely representing older and more recalcitrant SOM, were enriched compared to younger and more labile SOM fractions; δ15N of forested soils’ 212-2000 μm fraction were higher than corresponding grassland soils (1.7±0.3‰ vs. 0.5±0.4‰). δ13C values of acid hydrolysis fractions likely reflect preferential losses of 13C-depleted compounds during hydrolysis. Though C and N data from size fractions were most effective at exhibiting differences between grassland and forested soils, no technique conclusively indicates consistent changes in SOM dynamics with forest growth on these soils. The study also highlights some of the challenges associated with describing SOM parameters, particularly δ13C, in SOM fractions isolated by acid hydrolysis.  相似文献   

5.
The 14C age of soil organic matter is known to increase with soil depth. Therefore, the aim of this study was to examine the stabilization of carbon compounds in the entire soil profile using particle size fractionation to distinguish SOM pools with different turnover rates. Samples were taken from a Dystric Cambisol and a Haplic Podzol under forest, which are representative soil types under humid climate conditions. The conceptual approach included the analyses of particle size fractions of all mineral soil horizons for elemental composition and chemical structure of the organic matter by 13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The contribution of phenols and hydroxyalkanoic acids, which represent recalcitrant plant litter compounds, was analyzed after CuO oxidation.In the Dystric Cambisol, the highest carbon concentration as well as the highest percentage of total organic carbon are found in the <6.3 μm fractions of the B and C horizons. In the Haplic Podzol, carbon distribution among the particle size fractions of the Bh and Bvs horizons is influenced by the adsorption of dissolved organic matter. A relationship between the carbon enrichment in fractions <6.3 μm and the 14C activity of the bulk soil indicates that stabilization of SOM occurs in fine particle size fractions of both soils. 13C CPMAS NMR spectroscopy shows that a high concentration of alkyl carbon is present in the fine particle size fractions of the B horizons of the Dystric Cambisol. Decreasing contribution of O-alkyl and aromatic carbon with particle size as well as soil depth indicates that these compounds are not stabilized in the Dystric Cambisol. These results are in accordance with data obtained by wet chemical analyses showing that cutin/suberin-derived hydroxyalkanoic acids are preserved in the fine particle size fractions of the B horizons. The organic matter composition in particle size fractions of the top- and subsoil horizons of the Haplic Podzol shows that this soil is acting like a chromatographic system preserving insoluble alkyl carbon in the fine particle size fractions of the A horizon. Small molecules, most probably organic acids, dominate in the fine particle size fractions of the C horizons, where they are stabilized in clay-sized fractions most likely due to the interaction with the mineral phase. The characterization of lignin-derived phenols indicated, in accordance with the NMR measurements, that these compounds are not stabilized in the mineral soil horizons.  相似文献   

6.
The long-term storage of soil organic matter (SOM) in forest soils is still poorly understood. In this study, particle size fractionation in combination with accelerator mass spectroscopy (AMS) and solid state 13C nuclear magnetic resonance (NMR) spectroscopy was applied to investigate organic carbon (OC) stabilisation in Cambisol and Luvisol profiles under spruce (Picea abies) and beech (Fagus sylvatica L.) forests. In most samples, OC was preferentially associated with <2 μm fractions. Throughout soil profiles the contribution of OC in the clay fraction to the total OC increased from 27%-53% in A horizons to 44-86% in E, B and EB horizons. The 200-2000 μm fractions from all sites and all depths showed a percentage of modern C (pmC)>100. They were enriched in 14C owing to high inputs of recent material from leaves and roots. Clearly less active material was associated with <2 and 2-20 μm fractions. This demonstrated that the particle size fractionation procedure applied to our study was capable to isolate a young OC fraction in all samples. The pmC values were strongly decreasing with depth but the decrease was much more pronounced in the fine fractions. The <2 and 2-20 μm fractions of B, E and EB horizons revealed radiocarbon ages between 512 and 4745 years before present which indicated that the SOM in those horizons was little affected by the recent vegetation. The major components of labile and stable SOM pools in topsoils and subsoils were always O/N-alkyl C (28-53%) and alkyl C (14-48%) compounds. NMR spectra of bulk soils and particle size fractions indicated that high alkyl C and O/N-alkyl C proportions throughout the soil profile are typical of Cambisols and Luvisols which were not subjected to regular burning. A relation between radiocarbon age and chemical composition throughout soil profiles was not observed. This suggests that the long-term stabilisation of SOM is mainly controlled by the existence of various mechanisms of protection offered by the soil matrix and soil minerals but not by the chemical structure of SOM itself.  相似文献   

7.
In order to evaluate the sustainability and efficiency of soil carbon sequestration measures and the impact of different management and environmental factors, information on soil organic matter (SOM) stability and mean residence time (MRT) is required. However, this information on SOM stability and MRT is expensive to determine via radiocarbon dating, precluding a wide spread use of stability measurements in soil science. In this paper, we test an alternative method, first developed by Conen et al. (2008) for undisturbed Alpine grassland systems, using C and N stable isotope ratios in more frequently disturbed agricultural soils. Since only information on carbon and nitrogen concentrations and their stable isotope ratios is required, it is possible to estimate the SOM stability at greatly reduced costs compared to radiocarbon dating. Using four different experimental sites located in various climates and soil types, this research proved the effectiveness of using the C/N ratio and δ15N signature to determine the stability of mOM (mineral associated organic matter) relative to POM (particulate organic matter) in an intensively managed agro-ecological setting. Combining this approach with δ13C measurements allowed discriminating between different management (grassland vs cropland) and land use (till vs no till) systems. With increasing depth the stability of mOM relative to POM increases, but less so under tillage compared to no-till practises. Applying this approach to investigate SOM stability in different soil aggregate fractions, it corroborates the aggregate hierarchy theory as proposed by Six et al. (2004) and Segoli et al. (2013). The organic matter in the occluded micro-aggregate and silt & clay fractions is less degraded than the SOM in the free micro-aggregate and silt & clay fractions. The stable isotope approach can be particularly useful for soils with a history of burning and thus containing old charcoal particles, preventing the use of 14C to determine the SOM stability.  相似文献   

8.
Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm−3) and cropland (1.14 g cm−3), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

9.
《Geoderma》2005,124(1-2):143-155
With respect to carbon sequestration in soil, attempts have been made to identify soil organic matter (SOM) fractions that respond more rapidly to changes in land-use than bulk SOM, which could thus serve as early indicators for the overall stock change. We used a combination of physical fractionation (size and density separation) and chemical characterisation (C-to-N ratios, CuO lignin signature, 13C NMR spectroscopy) to identify sensitive SOM fractions in an agricultural system with sandy dystric cambisols in Bavaria, Germany, 7 years after a land-use change. Land-use types included long-term arable land and grassland, and conversion from one system to the other. Soil carbon and nitrogen contents in 0–3 cm increased from 14 to 39 mg organic carbon g−1 soil, and from 1.7 to 3.9 mg nitrogen g−1 soil in the following order: permanent arable, conversion grassland to arable, conversion arable to grassland, and permanent grassland. Wet sieving and ultrasonic dispersion with 22 J ml−1 released <5% and 60% to 80%, respectively, of the amount of particles >20 μm relative to complete dispersion. The most sensitive fraction, with respect to land-use, was SOM in the fraction >20 μm not released after sequential wet sieving and ultrasonic dispersion. In contrast, the proportion of free light (wet sieving, density <1.8 g cm−3) and occluded light (ultrasonic dispersion with 22 J ml−1, <1.8 g cm−3) particulate organic matter (POM) showed no clear response to land-use. The structural composition of POM indicated its vegetation origin with a selective enrichment of lignin and a loss of O-alkyl C relative to its plant precursors. Decomposition of the occluded light POM was only slightly advanced relative to the free light POM. In mineral fractions <20 μm, SOM was significantly more transformed than in the coarse fractions, as shown by NMR spectroscopy; however, it revealed no specific land-use pattern. An exception to this was the proportion of O-alkyl C in the clay fraction, which increased with SOC content. Ratios of alkyl to O-alkyl C in mineral fractions <20 μm differentiated samples gave a better differentiation of samples than the C-to-N ratios. We conclude that neither free nor occluded light POM are appropriate early indicators for changes in land-use at the investigated sites; however, total SOM, its distribution with depth, and SOM allocated in stable aggregates >20 μm were more sensitive.  相似文献   

10.
We examined whether grass species and soil nitrogen (N) availability could enhance Carbon (C) and N turnover during root litter decay in grassland. Three species with increasing competitiveness (Festuca ovina, Dactylis glomerata and Lolium perenne) were grown at two N fertiliser levels in an undisturbed grassland soil, in which soil organic fractions derived for the last 9 years from Lolium root litter which was 13C-depleted. During the subsequent experimental year, the C turnover was calculated using the respective δ13C values of the old and new C in the root phytomass, in two Particulate Organic Matter (POM) fractions above 200 μm and in the lightest part of the aggregated soil fraction between 50 and 200 μm. Soil N availability was monitored during the regrowth periods with ion exchange resins (IER). The C decay rates of each particle size fraction were calculated with a simple mechanistic model of C dynamics. The N mineralisation immobilisation turnover (MIT) was characterised by dilution of 15N-labelled fertiliser in the N harvestThe C:N ratio and the residence time of C in the fractions decreased with particle size. The presence of a grass rhizosphere increased the decay rate of old C. Accumulation of new C in particle size fractions increased with species competitiveness and with N supply. Species competitiveness increased C turnover in the aggregated fraction, as a result of greater accumulation of new C and faster decay of old C. Fertiliser N increased N turnover and C mineralisation in the SOM. Species competitiveness decreased soil -N exchanged with the IER and increased dissolved organic C (DOC) content. The nature of the current rhizosphere is thus an important factor driving C and N transformations of the old root litter, in relation with grass species strategy. Plant competitiveness may stimulate the C and N turnover in the more evolved SOM fractions in a similar way to the mineral N supply.  相似文献   

11.
 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO2) were, respectively, 89.1 μl CO2 g–1 soil h–1 and 55.1 μl CO2 g–1 soil h–1 in the forest and 36.7 μl CO2 g–1 soil h–1 and 35.7 μl CO2 g–1 soil h–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g–1) to the grassland zone (15.9 μg g–1). A good correlation (R 2=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO2 production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000  相似文献   

12.
This study is aimed at quantifying organic carbon (C) and total nitrogen (N) dynamics associated with physically separated soil fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau. Concentrations of organic C and N of soil, free and occluded particulate organic matter (OM), and aggregate- and mineral-associated OM in different land uses are increased in the following order: 50 years cultivation < 12 years cultivation ≤ native grassland. The prolonged cropping of up to 50 years markedly affected the concentrations of free and occluded particulate OM and mineral-associated OM. After wet-sieving, 43% of native grassland soil mass was found in >1−10 mm water-stable aggregates that stored 40% of bulk soil organic C and N; only 16% and 7% of soil mass containing 16% and 7% of bulk soil organic C and N was >1−10 mm water-stable aggregates of soils cultivated for 12 years and 50 years, respectively. This indicated that losses of soil organic C and N following cultivation of native grassland would be largely related to disruption of >1–10 mm size aggregates and exposure of intra-aggregate OM to microbial attack. Organic C and N concentrations of soil aggregates were similar among aggregate size fractions (>0.05−10 mm) within each land use, suggesting that soil aggregation process of these soils did not follow the hierarchy model. The increase of the C-to-N ratio of free and occluded particulate fractions in the cultivated soils compared to the grassland soil indicated a greater loss of N than C.  相似文献   

13.
The aim of this study was to determine the effect of land‐use and forest cover depletion on the distribution of soil organic carbon (SOC) within particle‐size fractions in a volcanic soil. Emphasis was given to the thermal properties of soils. Six representative sites in Mexico were selected in an area dominated by Andosols: a grassland site, four forested sites with different levels of degradation and an agricultural site. Soils were fractionated using ultrasonic energy until complete dispersion was achieved. The particle‐size fractions were coarse sand, fine sand, silt, clay and particulate organic matter from the coarse sand sized fraction (POM‐CS) and fine sand (POM‐FS). Soil organic carbon decreased by 70% after forest conversion to cropland and long‐term cultivation; forest cover loss resulted in a decrease in SOC of up to 60%. The grassland soil contained 45% more SOC than the cropland one. Soil organic carbon was mainly associated with the silt‐size fraction; the most sensitive fractions to land‐use change and forest cover depletion were POM followed by SOC associated with the silt and clay‐sized fractions. Particulate organic matter can be used as an early indicator of SOC loss. The C lost from the clay and silt‐sized fractions was thermally labile; therefore, the SOC stored in the more degraded forest soils was more recalcitrant (thermally resistant). Only the transformation of forest to agricultural land produced a similar loss of thermally stable C associated with the silt‐sized fraction.  相似文献   

14.
The Grain to Green Program in China which began in 1999 led to the conversion of 0.64 million ha of cropland to grassland on steep sloping landscapes. However, the pattern of natural vegetation succession following cropland has not been well represented in previous regional syntheses of land use change effects on soil organic carbon (SOC). A chronosequence study focusing on the vegetation succession and soil carbon stocks was conducted in the center of the Loess Plateau. The chronosequence included fields of 0, 2, 5, 8, 9, 10, 12, 15 and 25 years of self‐restoration after cropland abandonment, as well as a natural grassland reference. Plant coverage, species richness and plant biomass increased significantly with time of cropland abandonment. Over time, the species composition more nearly resembled a natural grasslands community. Cropland abandonment replenished SOC stocks by 3.6 kg C m−2 during the 25‐year self‐restoration, but the SOC accumulation was restricted to the upper soil profiles (0–60 cm). SOC accumulation rate was 88 g C m−2 y−1 in 0–30 cm and 55 g C m−2 y−1 in 30–60 cm soil depth, respectively. These carbon stocks were still significantly lower than those found in the natural grassland soil. Our results suggest that the recovery of plant communities and SOC stocks appears to be slow in this semiarid environment without revegetation effort along with appropriate field management, although the post‐agricultural soils have a high potential for carbon sequestration. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

15.
Aggregate formation is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of particulate organic matter (POM) and its interactions with mineral particles. C stabilization processes lead to 13C fractionation and consequently to various δ13C values of soil organic matter (SOM) fractions. Differences in δ13C within the aggregates and fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to heavier δ13C in remaining C. We hypothesized that: 1) 13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) 13C fractionation (but not the δ13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, minimal differences between Δ13C of SOM fractions between various ecosystems correspond to maximal probability of the SOM formation pathways.We tested these hypotheses on three soils formed from cover loam during 45 years of growth of coniferous or deciduous forests or arable crops. Organic C pools in large macroaggregates, small macroaggregates, and microaggregates were fractionated sequentially for four density fractions to obtain free POM with ρ < 1.6 g cm−3, occluded POM with two densities (ρ < 1.6 and 1.6–2.0 g cm−3), and mineral fraction (ρ > 2.0 g cm−3).The density fractions were 13C enriched in the order: free POM < light occluded POM < heavy occluded POM < mineral fraction. This, as well as their C/N ratios confirmed that free POM was close to initial plant material, whereas the mineral fraction was the most microbially processed. To evaluate the successive steps of SOM formation, the Δ13C values between δ13C of SOM fractions and δ13C of bulk SOM were calculated. The Δ13C indicated that, parallel with biochemical transformations, the physical disintegration strongly contributed to the formation of free and occluded light POM. In contrast, biochemical transformations were more important than physical disintegration for formation of heavy occluded POM from light occluded POM. This was confirmed by review of 70 Δ13C values from other studies analyzed Δ13C depending on the density of SOM fractions. Accordingly, the successive steps of SOM formation were: fLF<1.6 = oLF<1.6 → oDF1.6–2.0 = MF>2.0. The obtained steps of C stabilization were independent on the initial precursors (litter of coniferous forest, deciduous forest or grasses).To test the second hypothesis, we proposed an extended scheme of C flows between the 3 aggregate size classes and 4 SOM fractions. Δ13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. This confirmed the earlier concept of SOM turnover in aggregates, but for the first time quantified the C flows within the aggregates and SOM density fractions based on δ13C values. So, the new 13C natural abundance approach is suitable for analysis of C pathways by SOM formation under steady state without 13C or 14C labeling.  相似文献   

16.
Andosols are characterised by high organic matter (OM) content throughout the soil profile, which is mainly due to the stabilisation of soil organic matter (SOM) by mineral interactions. The aim of the study was to examine whether there were differences in the chemical composition of mineral-associated SOM and free OM in the top A horizon and in the subsoil (horizons below the A11 horizon). Our experimental approach included the replicated sampling of a fulvic and an umbic Andosol under pine and laurel forest located on the island of Tenerife with a Mediterranean sub-humid climate. We determined the extent of the organo-mineral interactions by comparing the sizes of the light (free) and heavy (dense) soil fractions obtained by physical separation through flotation in a liquid with a density of 1.9 g cm–3. We determined the elemental and isotopic composition of both fractions and analysed their chemical composition by analytical pyrolysis. The elemental and isotopic composition showed similar values with depth despite the different vegetation and climatic conditions prevailing at the two sites. Carbon (C) stabilised by mineral interactions increased with depth and represented 80–90% of the total C in the lowest horizons. The heavy fractions mainly released N-containing compounds upon analytical pyrolysis, whereas lignin-derived and alkyl compounds were the principal pyrolysis products released from the light fractions of the top- and subsoil horizons. Principal component analysis showed that the chemical composition of OM stabilised by mineral interaction differs in the different horizons of the soil profile. In the A horizons, the chemical composition of this OM was similar to those of the light fractions, i.e. litter input. There was a gradual change in the bulk molecular composition from a higher contribution of plant-derived molecules in the light and heavy fractions of the A horizon to more microbial-derived molecules as well as black C-derived molecules at depth. We conclude that transport processes in addition to decomposition and possibly in situ ageing affect the chemical composition of mineral-associated OM in subsoils.  相似文献   

17.
《Soil Use and Management》2018,34(2):187-196
The objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM ), rather than SOM per se , as indicators of soil physical quality (SPQ ) based on their effect on aggregate stability (AS ). Chemically extracted humic and fulvic acids (HA and FA ) were used as chemical fractions, and heavy and light fractions (HF and LF ) obtained by density separation as physical fractions. The analyses were conducted on medium‐textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC ), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se . In both geographical regions, soils under cropland showed the smallest content of SOC , HA and carbon concentration in LF and HF , and the largest HF /LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA /SOC (r  = −0.83**), HA /FA (r  = 0.58**), carbon concentration of LF (r  = 0.69**) and HF (r  = 0.70**) and HF /LF ratio (r  = 0.80**), cropland showed lowest AS . These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.  相似文献   

18.
High-yield (HY) areas of an agricultural cropland were characterized by different positions on a slope and lower silt and clay contents, compared to low-yield (LY) areas, and this was associated with differences in water regime and C and N turnover. To understand differences in N flows of HY and LY areas, a combination of 15N tracer techniques and physical fractionation procedures was applied. Within 570 d after application of 15N labelled mustard litter to an agricultural cropland, the distribution of 15N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 d, only 2.5% of the initial 15N amount was found in POM fractions, with higher amounts in POM occluded in aggregates than in free POM. After this period, stabilization of the initial 15N in fine silt- and clay-sized fractions amounts to 10% in HY, but 20% in LY soils. 70% to 85% of the added 15N were lost. Initial decomposition of labelled material was faster in HY than in LY areas during the first year, but the remaining 15N amounts in POM fractions of the different areas were similar after 570 d. 15N amounts and concentrations in mineral-associated fractions increased within 160 d after application. From 160 to 570 d, HY and LY areas showed different 15N dynamics, resulting in a decline of 15N amounts in HY, but constant 15N amounts in LY soils. The results indicate faster decomposition processes in HY than in LY areas, due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote N stabilization in fine mineral fractions. As a whole, N flows were higher in HY compared to LY areas, thus supporting higher yields and accelerated organic matter degradation due to higher N supply.  相似文献   

19.
The impact of rising atmospheric carbon dioxide (CO2) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O3) or vary based on species-specific growth responses to elevated CO2. To understand how projected increases in atmospheric CO2 and O3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO2-O3 Enrichment (FACE) experiment. Tracer amounts of 15NH4+ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO2 and O3 treatments. The 15N tracer and strongly depleted 13C-CO2 were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO2 relative to ambient CO2. As main effects, neither CO2 nor O3 significantly altered 15N recovery in SOM. Elevated CO2 significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO2 has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O3 had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO2 can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.  相似文献   

20.
Forest soils contain about 30% of terrestrial carbon (C) and so knowledge of the influence of forest management on stability of soil C pools is important for understanding the global C cycle. Here we present the changes of soil C pools in the 0-5 cm layer in two second-rotation Pinus radiata (D.Don) plantations which were subjected to three contrasting harvest residue management treatments in New Zealand. These treatments included whole-tree harvest plus forest floor removal (defined as forest floor removal hereafter), whole-tree, and stem-only harvest. Soil samples were collected 5, 10 and 15 years after tree planting at Kinleith Forest (on sandy loam soils) and 4, 12 and 20 years after tree planting at Woodhill Forest (on sandy soils). These soils were then physically divided into light (labile) and heavy (stable) pools based on density fractionation (1.70 g cm−3). At Woodhill, soil C mass in the heavy fraction was significantly greater in the whole-tree and stem-only harvest plots than the forest floor removal plots in all sampling years. At Kinleith, the soil C mass in the heavy fraction was also greater in the stem-only harvest plots than the forest floor removal plots at year 15. The larger stable soil C pools with increased residue return was supported by analyses of the chemical composition and plant biomarkers in the soil organic matter (SOM) heavy fractions using NMR and GC/MS. At Woodhill, alkyl C, cutin-, suberin- and lignin-derived C contents in the SOM heavy fraction were significantly greater in the whole-tree and stem-only harvest plots than in the forest floor removal plots in all sampling years. At Kinleith, alkyl C (year 15), cutin-derived C (year 5 and 15) and lignin-derived C (Year 5 and 10) contents in the SOM heavy fraction were significantly greater in stem-only harvest plots than in plots where the forest floor was removed. The analyses of plant C biomarkers and soil δ13C in the light and heavy fractions of SOM indicate that the increased stable soil C in the heavy fraction with increased residue return might be derived from a greater input of recalcitrant C in the residue substrate.  相似文献   

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