Light Fraction Soil Organic Matter Research Articles

Soil fertility in silvopastoral systems integrating tree legumes with signalgrass (Urochloa decumbens Stapf. R. Webster)

Silvopastoral Systems (SPS) can increase overall productivity and generate continuous income in order to stimulate simultaneous growth and development of trees, forage and livestock. Moreover, the SPS with tree legumes would be important for add nutrients to the system, mainly N, and ensure the soil health and quality. Soil properties were assessed in two SPS, implanted in 2011, using tree legumes and Urochloa decumbens Stapf. R. Webster (Signalgrass). Treatments were Signalgrass + Mimosa caesalpiniifolia Benth (Sabia) and Signalgrass + Gliricidia sepium (Jacq.) Kunth ex Walp. (Gliricidia), and they were allocated in a randomized complete block design, with three replications. Soil was sampled in 2013, 2017, and 2018, at 0, 4, and 8 m along transects perpendicular to tree double rows, from 0- to 20- and 20- to 40-cm layers. Soil chemical properties included pH, P, K+, Ca2+, Mg2+, Al3+, H++Al3+, cation exchange capacity (CEC), and base saturation. In addition, light fraction of soil organic matter (LF-SOM), soil basal respiration (SBR), and natural abundance of 13C of the respired CO2 (δ13C-CO2) were analyzed. Soil pH (5.3, 5.2, 5.1), P (11.3, 7.2, 3.6 mg dm-3), and CECeffective (5.8, 5.1, 5.0 cmolc dm-3) decreased (P < 0.05) along the years 2013, 2017, and 2018, respectively. In 2018, the LF-SOM and δ13C-CO2 was greater in Sabia (1.1 g kg-1 and -16.4‰) compared to Gliricidia (0.7 g kg-1 and -18.2‰). Silvopastoral systems reduced soil fertility regardless of the tree legume species used as result of biomass nutrient stock, without maintenance fertilization. Sabia had greater deposition of LF-SOM, without increasing SBR, providing potential for microbial C use efficiency. Enriched C-CO2 isotope composition shows an efficient SOM oxidize in SPS with Gliricidia or Sabia. This information can contribute to the assessments related to CO2 balance and C retention. Both SPS contribute to C sequestration.

Molecular composition and possible transformations of labile soil organic matter fractions in Mediterranean arable soils: Relevance and implications

With the increased global interest in sequestering carbon in soil, it is necessary to understand the composition of different pools of soil organic matter (SOM) that cycle over suitably short timeframes. To explore in detail the chemical composition of agroecologically relevant yet distinct fractions of SOM, the light fraction of SOM (LFOM), the 53-μm particulate organic matter (POM), and the mobile humic acid (MHA) fractions were sequentially extracted from agricultural soils and characterized using both 13C cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy and also Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The NMR results showed a decrease in the O-alkyl C region assigned to carbohydrates (51–110 ppm) and an increase in the aromatic region (111–161 ppm) proceeding from the LFOM to the POM and then to the MHA fraction. Similarly, based on the thousands of molecular formulae assigned to the peaks detected by FT-ICR-MS, condensed hydrocarbons were dominant only in the MHA, while aliphatic formulae were abundant in the POM and LFOM fractions. The molecular formulae of the LFOM and POM were mainly grouped in the high H/C lipid-like and aliphatic space, whereas a portion of the MHA compounds showed an extremely high (17–33, average of 25) double bond equivalent (DBE) values, corresponding to low H/C values of 0.3–0.6, representative of condensed hydrocarbons. The labile components appeared most pronounced in the POM (93% of formulae have H/C ≥ 1.5) similar to the LFOM (89% of formulae have H/C ≥ 1.5) but in contrast to the MHA (74% of formulae have H/C ≥ 1.5). The presence of both labile and recalcitrant components in the MHA fraction suggests that the stability and persistence of soil organic matter is influenced by a complex interaction of physical, chemical, and biological factors in soil. Understanding the composition and distribution of different SOM fractions can provide valuable insights into the processes that govern carbon cycling in soils, which can help inform strategies for sustainable land management and climate change mitigation.

Molecular diversity and the fate of biochemical fractions of eucalypt tissues in soil

The molecular diversity of the source substrate has been regarded as a significant controller of the proportion of plant material that is either mineralized or incorporated into soil organic matter (SOM). However, quantitative parameters to express substrate molecular diversity remain elusive. In this research, we fractionated leaves, twigs, bark, and root tissues of 13C-enriched eucalypt seedlings into hot water extractables (HWE), total solvent (acetone) extractables (TSE), a cellulosic fraction (CF), and the acid unhydrolyzable residue (AUR). We used 13C NMR spectroscopy to obtain a molecular diversity index (MDI) based on the relative abundance of carbohydrate, protein, lignin, lipid, and carbonyl functional groups within the biochemical fractions. Subsequently, we obtained artificial plant organs containing fixed proportions (25%) of their respective biochemical fractions to be incubated with soil material obtained from a Haplic Ferralsol for 200-days, under controlled temperature (25 ± 1 °C) and moisture adjusted to 70–80% of the soil water holding capacity. Our experimental design was a randomized complete block design, arranged according to a factorial scheme including 4 plant organs, 4 biochemical fractions, and 3 blocks as replicates. During the incubation, we assessed the evolution of CO2 from the microcosms after 1, 2, 3, 4, 7, 10, 13, 21, 28, 38, 45, 70, 80, 92, 112, 148, 178 and 200 days from the start of the incubation. After the incubation, soil subsamples were submitted to a density fractionation to separate the light fraction of SOM (LFOM) i.e., with density <1.8 g cm−3. The heavy fraction remaining was submitted to wet-sieving yielding the sand-sized SOM (SSOM) and the mineral-associated SOM (MAOM), with particle-size greater and smaller than 53 µm, respectively. We found that HWE and AUR exhibited comparatively higher MDIs than the TSE and CF. During the incubation, HWE and CF were the primary sources of 13C-CO2 from all plant organs and after 92 days, the respiration of the TSE of bark and roots increased. Otherwise, the AUR contributed the least for the release of 13C-CO2. There were no significant relationships between the MDI and the amount of 13C transferred into the LFOM or SSOM. Otherwise, the transfer of 13C into the MAOM increased as a linear-quadratic function of MDI, which in turn was negatively correlated with the total 13C-CO2 loss. Overall, the MDI exerted a stronger control on the 13C-labeled MAOM than on 13C-CO2 emissions, highlighting the need to improve our ability to distinguish and quantify direct plant inputs from those of microbial origin entering soil C pools.

Deadwood affects the soil organic matter fractions and enzyme activity of soils in altitude gradient of temperate forests

The main objective of our study has been to determine the role of deadwood in the shaping of the amount of soil organic matter fractions in mountain forest soils. For this purpose, a climosequence approach comprising north (N) and south (S) exposure along the altitudinal gradient (600, 800, 1000 and 1200 ​m a.s.l.) was set up. By comparing the properties of decomposing deadwood and those of the soils located directly beneath the decaying wood we drew conclusions about the role of deadwood in the shaping of soil organic matter fractions and soil carbon storage in different climate conditions. The basic properties, enzymatic activity and fractions of soil organic matter (SOM) were determined in deadwood and affected directly by the components released from decaying wood. Heavily decomposed deadwood impacts soil organic matter stabilization more strongly than the less decayed deadwood and the light fraction of SOM is more sensitive to deadwood effects than the heavy fraction regardless of the location in the altitude gradient. Increase in SOM mineral-associated fraction C content is more pronounced in soils under the influence of deadwood located in lower locations of warmer exposure. Nutrients released from decaying wood stimulate the enzymatic activity of soils that are within the range of deadwood influence.

Soil Organic Matter Fractions in Relation to Root Characteristics of Different Tree Species in Altitude Gradient of Temperate Forest in Carpathian Mountains

The roots are a key functional component of belowground systems and one of the main factors influencing the quality and quantity of soil organic matter. Our research aimed to determine the fractional composition of the soil organic matter (SOM) in soils under various tree species on an altitude gradient. In our research, we related the SOM fractions with the root characteristics. There is a lack of information on the relationship between the SOM fractions and the root properties. We assessed labile and heavy fractions of SOM content in forest mountain soils with a climosequence approach. The study plots were established at 600, 800, and 1000 m above sea level in a beech stand (Fagus sylvatica L.) and a fir stand (Abies alba Mill.). In this case, three research plots with beech and fir were designated in each altitude variant. Forest stands growing in the same soil conditions were selected for the study. The research used stands of similar age with the same tree canopy density. The basic physicochemical properties (pH, hydrolytic acidity, carbon and nitrogen content, base cations content) and the fractional composition of the SOM were determined from soil samples. In addition, we determined the basic characteristics of the roots (diameter, length, biomass, decomposition, production). The correlation between soil organic matter fractions and root characteristics was recorded. This study confirmed the importance of climatic conditions in shaping the fractional composition of forest soils. In the highest locations, characterized by lower temperatures, the light fraction of the SOM exhibited the highest C and N content, which is the effect of slower decomposition processes. Apart from climatic conditions, the stabilization of SOM is influenced by the tree species composition of a forest stand. Beech forest stands lead to a larger accumulation of a heavy fraction of SOM. This study indicates a positive correlation between the light fraction of SOM, root biomass, and decomposition rate of roots. Our research shows that avoiding single-species coniferous stands and introducing admixtures of deciduous species, which increase the heavy SOM fraction, is justified in forest management.

Variation of soil organic matter depends on light-fraction organic matter under long-term monocropping of different crops

Cultivating crops influences soil organic matter (SOM), but the effect of different crops remains unclear, particularly under long-term monocropping. The objective of this study was to identify how different crops influence the content and chemical structures of SOM under long-term monocropping. Here, soils were sampled (0–20 cm) under 27-year soybean and maize monocropping and separated into different physical fractions. The content and chemical structures of SOM in all fractions were determined. SOM contents were higher under soybean than maize in bulk soil and macroaggregates and their light-fractions instead of microaggregates and silt and clay. The difference in SOM chemical structure was observed in aggregates and density fractions rather than bulk soils and supported by the result of principal component analysis. The proportion of O-alkyl C in macro- and microaggregates and all free light fractions and that of aromatic C in mineral-associated fractions were higher, while that of carbonyl C was lower under maize than soybean. These results demonstrated that different crops monocropping influences the content and chemical structures of SOM, and the variations were mainly in the light-fraction SOM and highlight a higher sensitivity of physical fractions than bulk soil to different crops.

Agrosilvopastoral Systems and Well-Managed Pastures Increase Soil Carbon Stocks in the Brazilian Cerrado

Agrosilvopastoral systems have been promoted as sustainable models that combine crops, livestock grazing, and forestry in the same area. We hypothesize that agrosilvopastoral systems can improve soil C and N stocks over time. Therefore, in this study, we aimed to evaluate the changes in soil C and N stocks after conversion of low-productivity pasture into well-managed pasture and agrosilvopastoral land in the Brazilian Cerrado. Soil samples were collected in 2016 and 2018 at 0–5, 5–10, 10–20, and 20–30 cm depths from the following areas: integrated crop-livestock-forest (ICLF), marandu grass (Brachiaria brizantha) monoculture (MAR), low productivity pasture of signal grass (Brachiaria decumbens) monoculture (PAST), and native vegetation (NV; “Cerrado”). The C and N content and stocks, 13C natural abundance, and C contents in the physical and chemical fractions of soil organic matter (SOM) were measured. The ICLF and MAR systems promoted faster recovery of soil C and N stocks at all soil depths compared with PAST. The C content of the free light fraction of SOM under the ICLF and MAR systems increased, reaching values similar to NV up to 20 cm. The C content was higher in the humin fraction at all depths in all areas, and for this measurement, the ICLF system performed remarkably compared with PAST at a depth of 5−10 cm. Our findings support the hypothesis that conversion of low-productivity pasture into agrosilvopastoral and actively managed pasture systems leads to improvements in soil quality and C and N stocks in the Brazilian Cerrado.

Soils on the southern border of the cryolithozone of European part of Russia (the Subpolar Urals) and their soil organic matter fractions and rheological behavior

Soil organic matter (SOM) plays an important role in the functioning of mountain ecosystems. This article contains the results of a study on SOM of high-latitude soils of the Subpolar Urals and its influence on the deformation behavior of soils. The first approach of this study was to define the soils of the Subpolar region to identify the main soil types and their morphological and chemical properties. We investigated soil types of mountain-forest, subalpine and alpine tundra vegetation zones, as well as soil types with permafrost. We investigated SOM using the method of physical fractionation, which isolates three fractions of SOM: free particulate organic matter (fPOM<1.6), occluded particulate organic matter (oPOM<1.6) and heavy mineral-associated fraction (MaOM>1.6). The deformation behavior of soils was studied using rheometry. Based on the results, SOM mainly consisted of heavy-fraction MaOM<1.6, accounting for 57–99%. The light fractions of free fPOM<1.6 and occluded oPOM<1.6 organic matter were considerably smaller (0.14–23.9%). However, the direct dependence of the organic carbon concentration on the content of light fractions fPOM<1.6 (0.89, p < 0.05) and oPOM<1.6 (0.84, p < 0.05) for the first mineral horizons was revealed. There also was a significant negative correlation between the carbon content and the heavy-fraction MaOM>1.6 (−0.86, p < 0.05). Rheological studies showed that the stability, resilience and interparticle bonds in soils also depend on the contents of light fractions of SOM: fPOM<1.6 (0.52–0.85, p < 0.05) and oPOM<1.6 (0.46–0.60, p < 0.05).

Soil biochemical properties and stabilisation of soil organic matter in relation to deadwood of different species.

Despite the increasing number of studies on deadwood, we still have limited knowledge of its dynamics. The aim of this study was to examine the effect of deadwood on the biochemical properties of soil and stabilisation of soil organic matter (SOM). The investigation was carried out in the Czarna Rózga Reserve in Central Poland. The logs of four tree species at different stages of decomposition (III, IV and V) were selected for the analysis. Three replicate logs were sampled for each combination of decay classes, and the soil samples were collected from directly under the logs and from 1 m away from the logs. In this way, changes to the chemical and biochemical properties of the wood were determined. The SOM was physically fractioned. As the rate of deadwood decomposition increases, its biochemical activity increases and its chemical properties change. The biochemical activity, especially the soil's enzyme activity, was stimulated under highly decayed deadwood. The effects of deadwood are visible in SOM fractions, particularly in the content of the light fraction of SOM.

Soil and Root Attributes in Pastures Managed under Different Stocking Rates and Nitrogen Fertilization Levels

Core Ideas Grassland is a widespread agroecosystem in the globe; however, not enough attention has been given to link grazing management to soil health. Grazing management practices greatly impact soil quality. Moderate stocking rate are more adequate to keep grassland stability. Pasture management affects soil physical–chemical properties and root depth distribution. Adjusting grazing pressure and N fertilization are key management tools affecting these responses. This study evaluated the effect of N fertilization and stocking rate (SR) on soil physical and chemical attributes and root depth distribution in grazed elephantgrass (Pennisetum purpureum Schum. cv. 381) pastures (2009 and 2010). Treatments consisted of three SR (2.0, 3.9, and 5.8 AU ha−1; 1 AU = animal unit = 450 kg body weight) and three N doses (0, 150, and 300 kg N ha−1 yr−1) on elephantgrass pastures. Response variables included soil bulk density (BD), particle density (PD), total porosity (TP), aggregate stability (AS), water infiltration rate (WIR), light fraction of soil organic matter (LF‐SOM), soil fertility, root length density (RLD), and root biomass. Increased SR resulted in greater BD. Soil bulk density ranged from 1.42 to 1.58 g cm−3 for SR ranging from 2.0 to 5.8 AU ha−1, respectively. The C concentration of the LF‐SOM decreased with increasing SR, ranging from 383 to 174 mg C kg−1 soil. Soil fertility, RLD, and the stock of C and N in the root system varied among soil layers. At the end of 2 yr, SR and N fertilization did not affect TP, WIR, and AS nor influenced soil fertility and root system distribution. Overgrazing led to soil compaction and reduced LF‐SOM, indicating a trend of pasture degradation. Moderate SR resulted in greater LF‐SOM, indicating a positive trend for C accumulation in the long term.

DNA occurrence in organic matter fractions isolated from amended, agricultural soils

The persistence of soil organic matter (SOM) constituents as a function of their recalcitrance has been recently questioned, with several papers showing the influence of the combined action of physical protection and chemical stabilization mechanisms as the main factors affecting SOM mean residence time. Using a physical fractionation method, SOM located between aggregates (FR), occluded within macro- (MA) and micro-aggregates (MI), and associated with the mineral fractions (MIN) were isolated from an agricultural soil differently amended (using compost, sewage sludge and biochar), and the occurrence of (total, bacterial and plant) DNA in these SOM pools was investigated. Following physical fractionation, total DNA (tDNA) was recovered from all SOM pools and from all treatments. Independently from the amendment, most tDNA accumulates in the FR fraction (30–70%), followed by the MIN pool (25–55%). The positive correlation between tDNA contents and C/N ratios (both tending to decrease following the order FR, MA and MI) observed for the light SOM fractions, together with the opposite pattern characterizing the heavy, MIN fraction (characterized by relatively high tDNA contents and low C/N ratios), suggests a different origin of the latter SOM pool, i.e., new molecules resulting from microbial transformations rather than highly degraded litter inputs. Therefore, tDNA may represent a promising proxy of organic matter dynamics in mineral soils. Finally, the MI fraction shows the highest number of microbial taxa and diversity, and seems to constitute a separate microbial niche in which different bacterial communities carry out their activity.

Distribution and thermal stability of physically and chemically protected organic matter fractions in soils across different ecosystems

Accrual of carbon (C) and nitrogen (N) in soil is a significant and realizable management option to mitigate climate change; thus, a clear understanding of the mechanisms controlling the persistence of C and N in soil organic matter (SOM) across different ecosystems has never been more needed. Here, we investigated SOM distribution between physically and chemically stabilized fractions in soils from a variety of ecosystems (i.e., coniferous and broadleaved forest soils, grassland soils, technosols, and agricultural soils). Using elemental and thermal analyses, we examined changes in the quantity and quality of physically fractionated SOM pools characterized by different mechanisms of protection from decomposition. Independently of the ecosystem type, most of the organic C and total N were found in the mineral-associated SOM pool, known to be protected mainly by chemical mechanisms. Indexes of thermal stability and C/N ratio of this heavy SOM fraction were lower (especially in agricultural soils) compared to light SOM fractions found free or occluded in aggregates, and suggested a marked presence of inherently labile compounds. Our results confirm that the association of labile organic molecules with soil minerals is a major stabilization mechanism of SOM, and demonstrate that this is a generalizable finding occurring across different mineral soils and ecosystems.

An investigation of the distribution of phosphorus between free and mineral associated soil organic matter, using density fractionation

We investigated whether density fractionation can be used to determine the distribution of organic phosphorus (OP) between free and mineral-associated soil organic matter (SOM). We performed density fractionations using sodium polytungstate solution (specific gravity 1.6 g cm−3) on 20 soils from UK semi-natural and pasture ecosystems, to obtain a light fraction (LF) and a heavy fraction (HF) for each soil. The fractions were quantified by weight, and analysed for organic carbon (OC), total N (TN), total P (TP), inorganic P (IP), and OP (by difference). Good recoveries of soil mass (96%), OC and TN (both ~ 90%) were obtained, but recovery of OP only averaged 56%. The average P:C ratio of HF SOM exceeded that of LF SOM by a factor of six, greater than the factor of two obtained for TN:OC. For the soils studied, the elements of SOM were predominantly in the HF, with averages of 75% for C, 82% for N, and 90% for P. The incomplete recovery of OP demands further work. Nonetheless, the results show that HF SOM is much richer in P than LF SOM.

Nitrogen fertilization decreases the decomposition of soil organic matter and plant residues in planted soils

Nitrogen fertilization may affect the decomposition of soil organic matter (SOM) and plant residues in soil, but this effect is still very uncertain and depends on living plants. We investigated the effects of mineral N (Nmin) availability on SOM and plant residue decomposition in wheat (Triticum aestivum L.) growing soils in a pot experiment. Five treatments were assessed: (1) Control [no maize (Zea Mays L.) residues and no N fertilization]; (2) 15N-urea addition; (3) 15N maize leaves; (4) 15N maize leaves + urea; and (5) 15N-urea + maize straw. The decomposition of SOM and plant residues was traced by the changes of N and C in the light fraction (density < 1.80 g cm-3) during the 127 days. Urea fertilization decreased the decomposition of SOM and maize residues, as indicated by remaining N and C in the light fraction compared to soil without urea. The C decomposition was tightly coupled to that of N in the light fraction SOM. In soils with maize residues, both maize- and SOM-derived light fractions decomposed slowly with N fertilization. Soil microbial biomass N content was increased by maize residues but was unaffected by urea addition. Under low soil Nmin levels, microbes met their N demand by increasing an acquisition from accelerated decomposition of organic sources. To mine N in the Nmin limited soils, soil microbes might have directly taken up more N-containing organics and thus facilitated SOM decomposition. For such an acceleration of SOM decomposition, the presence of N uptake by living plants was especially important, which decreased the Nmin in soil and so, increased N limitation for microorganisms. We concluded that N fertilization decreases SOM decomposition and increases the efficiency of C sequestration in soil through higher portion of un-decomposed crop residues.

ORGANIC MATTER FRACTIONS IN A QUARTZIPSAMMENT UNDER CULTIVATION OF IRRIGATED MANGO IN THE LOWER SÃO FRANCISCO VALLEY REGION, BRAZIL

Improper land use has lead to deterioration and depletion of natural resources, as well as a significant decline in agricultural production, due to decreased soil quality. Removal of native vegetation to make way for agricultural crops, often managed inadequately, results in soil disruption, decreased nutrient availability, and decomposition of soil organic matter, making sustainable agricultural production unviable. Thus, the aim of the present study was to evaluate the impact of growing irrigated mango (over a 20 year period) on the organic carbon (OC) stocks and on the fractions of soil organic matter (SOM) in relation to the native caatinga (xeric shrubland) vegetation in the Lower São Francisco Valley region, Brazil. The study was carried out on the Boa Esperança Farm located in Petrolina, Pernambuco, Brazil. In areas under irrigated mango and native caatinga, soil samples were collected at the 0-10 and 10-20 cm depths. After preparing the soil samples, we determined the OC stocks, carbon of humic substances (fulvic acid fractions, humic acid fractions, and humin fractions), and the light and heavy SOM fractions. Growing irrigated mango resulted in higher OC stocks; higher C stocks in the fulvic acid, humic acid, and humin fractions; and higher C stocks in the heavy and light SOM fraction in comparison to nativecaatinga, especially in the uppermost soil layer.

Carbon Accumulation in the Bulk Soil and Different Soil Fractions During the Rehabilitation of Desertified Grassland in Horqin Sandy Land (Northern China)

Desertification, which affects more than two-thirds of the world's arid and semi-arid regions, is a significant global ecological and environmental problem. There is a strong link between desertification of the drylands and emission of CO2 from soil and vegetation to the atmosphere. The Horqin Sandy Land is a severely desertified area in China's agro-pastoral ecotone due to its fragile ecology, combined with unsustainable land management. We estimated changes of organic carbon content in the bulk soil (0–5 cm), in the light-fraction of soil organic matter (based on density fractionation), and in the various particle-size fractions in areas with mobile sand dunes after implementing grazing exclusion (12 and 27 years) and tree and shrub planting (22 and 24 years). Carbon stocks in the bulk soil and all soil density and particle-size fractions increased significantly in the exclosure and plantation plots. The average rates of carbon accumulation in the bulk soil in the exclosure and plantation plots were 16.0 and 17.8 g m-2 y-1, respectively, versus corresponding values of 2.3 and 7.1 g m-2 y-1 for the light fraction, 4.3 and 8.0 g m-2 y-1 for the coarse fraction, 5.0 and 3.4 g m-2 y-1 for the fine sand, 4.5 and 4.2 g m-2 y-1 for the very fine sand, and 1.8 and 1.8 g m-2 y-1 for the silt clay fraction. The older the exclosure and plantation, the more carbon accumulated in the bulk soil and in each fraction. The carbon pool exceeded the level in non-desertified grasslands after 27 years of grazing exclosure and 24 years of the shrub plantation. Our results suggest that both grazing exclusion and planting trees and shrubs can restore desertified grassland, creating a high potential for sequestering soil carbon, but that the plantations appeared to accumulate soil carbon faster than the exclosures.

Short-term bioavailability of carbon in soil organic matter fractions of different particle sizes and densities in grassland ecosystems

The quality, stability and availability of organic carbon (OC) in soil organic matter (SOM) can vary widely between differently managed ecosystems. Several approaches have been developed for isolating SOM fractions to examine their ecological roles, but links between the bioavailability of the OC of size–density fractions and soil microbial communities have not been previously explored. Thus, in the presented laboratory study we investigated the potential bioavailability of OC and the structure of associated microbial communities in different particle-size and density fractions of SOM. For this we used samples from four grassland ecosystems with contrasting management intensity regimes and two soil types: a Haplic Cambisol and a typical Chernozem. A combined size–density fractionation protocol was applied to separate clay-associated SOM fractions (CF1, <1μm; CF2, 1–2μm) from light SOM fractions (LF1, <1.8gcm−3; LF2, 1.8–2.0gcm−3). These fractions were used as carbon sources in a respiration experiment to determine their potential bioavailability. Measured CO2-release was used as an index of substrate accessibility and linked to the soil microbial community structure, as determined by phospholipid fatty acids (PLFA) analysis.Several key factors controlling decomposition processes, and thus the potential bioavailability of OC, were identified: management intensity and the plant community composition of the grasslands (both of which affect the chemical composition and turnover of OC) and specific properties of individual SOM fractions. The PLFA patterns highlighted differences in the composition of microbial communities associated with the examined grasslands, and SOM fractions, providing the first broad insights into their active microbial communities. From observed interactions between abiotic and biotic factors affecting the decomposition of SOM fractions we demonstrate that increasing management intensity could enhance the potential bioavailability of OC, not only in the active and intermediate SOM pools, but also in the passive pool.

Surface Soil Organic Matter Qualities of Three Distinct Canadian Arctic Sites

Cryosolic soils store large amounts of carbon (C) because soil organic matter (SOM) decomposition is slower than plant growth. The response of arctic SOM to climate change is likely to depend not only on temperature, but also upon complex interactions between soil properties and SOM chemistry. We hypothesized that organic surface soils (>17% carbon) have more labile SOM than mineral surface soils (<17% carbon). Furthermore, we hypothesized that high arctic soils have more labile SOM than soils from the Low Arctic and subarctic. This study was conducted in 3 arctic ecosystems: subarctic (Churchill, Manitoba; n = 138), Low Arctic (Daring Lake, Northwest Territories;n = 60), and High Arctic (Truelove Lowlands, Nunavut; n = 54). The 0–10 cm depth of several different Cryosolic soils was sampled. The results from density fractionation and solid-state 13C cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy showed that organic surface soils contained relatively more labile C than mineral surface soils. Organic soils contained about 13% more O-Alkyl-C and 30% less Aromatic-C than mineral soils. Furthermore, for Churchill, Daring Lake, and Truelove organic soils, 53, 73, and 20% of the C was included in the light fraction of SOM [LF (LF < 1.55 g mL-1)], whereas 24, 19, and 14% of the C was included in the LF of mineral soils, respectively. Organic surface soils of subarctic and low arctic sites contained relatively more labile C than the high arctic site. Results showed that the subarctic and low arctic sites store about 15% more O-alkyl-C and 35% less Aromatic-C than high arctic organic soils (P < 0.001).

Effects of Grazing Exclusion on Soil Properties and on Ecosystem Carbon and Nitrogen Storage in a Sandy Rangeland of Inner Mongolia, Northern China

The Horqin sandy rangeland of northern China is a seriously desertified region with a fragile ecology. The sandy alluvial and aeolian sediments have a coarse texture and loose structure and are therefore vulnerable to damage caused by grazing animals and wind erosion. We investigated whether grazing exclusion could enhance ecosystem carbon (C) and nitrogen (N) storage and thereby improve overall soil quality. We compared soil properties, C and N storage in biomass (aboveground and below-ground), and the total and light fraction soil organic matter between adjacent areas with continuous grazing and a 12-year grazing exclosure. The soil silt+clay content, organic C, total Kjeldahl N, available N and K, and cation-exchange capacity were significantly (P<0.05) greater in the exclosure. We found that to a depth of 100cm, the exclosure plots had greater light fraction C storage (by 267.2gm(-2)=73.3%), light fraction N storage (by 16.6gm(-2)=105.7%), total soil C storage (by 1174.4gm(-2)=43.9%), and total N storage (by 91.1gm(-2)=31.3%). Biomass C and N storage were also 205.0 and 8.0gm(-2) greater (154.8 and 181.8%, respectively). The increase was greatest in the light fraction organic matter and biomass and decreased with increasing depth in the soil. The results suggest that light fraction C and N respond more rapidly than total soil C and N to grazing exclusion and that vegetation recovers faster than soil. Our results confirmed that the degraded sandy rangeland is recovering and sequestering C after the removal of grazing pressure.

Mongolian pine plantations enhance soil physico-chemical properties and carbon and nitrogen capacities in semi-arid degraded sandy land in China

Horqin Sandy Land is a seriously desertified and an ecologically fragile region of China. Soil degradation and desertification in this region are the result of several decades of overgrazing, non-manure cropping with short fallow, and arbitrary land use and management. We investigated whether the afforestation of active sand dunes with Mongolian pine (Pinus sylvestris var. mongolica Litv.) enhances the carbon (C) and nitrogen (N) storages and the overall soil quality. We compared soil physico-chemical properties, and C and N storages in the total and light fraction soil organic matter of active sand dunes, and of 25- and 35-year-old plantations. Soil water holding capacity, fine particle content and nutrients were significantly higher in plantations than in active sand dunes, with greater improvements occurring in the top 5cm than in the 5–15cm layer of the soil profile. Soil C and total N storages were 6.1 and 3.7 folds in 25-year-old, and 10.2 and 5.3 folds in 35-year-old plantations compared to active sand dunes, respectively. Carbon and N storages in the light fraction were 20.9 and 15.8 folds in 25-year-old, and 36.1 and 25.0 folds in 35-year-old plantations, respectively, relative to active sand dunes. The light fraction played an important role in soil C sequestration and its amount is an indicator of the effects of afforestation on C storage in sandy soil. The upper 15cm of the soil profile in the 35-year-old plantations of Mongolian pine has the capacity to sequester significant amount of C in the region, potentially offsetting part of the carbon that has been lost due to desertification over the past century in the Horqin Sandy Land. Our results conclude that afforestation with Mongolian pine has had positive impacts on soil quality and has increased the capacity for soil C sequestration in semi-arid degraded areas.