Content and composition of free and occluded particulate organic matter in a differently textured arable Cambisol as revealed by solid‐state 13C NMR spectroscopy

The composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse‐grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS ¹³C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O‐alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O‐alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O‐alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O‐alkyl C ratio.     

Does Particulate Organic Matter Fraction Meet the Criteria for a Model Soil Organic Matter Pool?

There is a well-recognized need for improved fractionation methods to partition soil organic matter into functional pools. Physical separation based on particle size is widely used, yielding particulate organic matter(POM, i.e., free or "uncomplexed" organic matter 50 μm) as the most labile fraction. To evaluate whether POM meets criteria for an ideal model pool, we examined whether it is:1) unique, i.e., found only in the 50 μm fraction and 2) homogeneous, rather than a composite of different subfractions. Following ultrasonic dispersion, sand( 50 μm) along with coarse(20–50 μm) and fine(5–20 μm) silt fractions were isolated from a silt loam soil under long-term pasture at Lincoln, New Zealand. The sand and silt fractions contained 20% and 21% of total soil C, respectively.We adopted a sequential density separation procedure using sodium polytungstate with density increasing step-wise from 1.7 to 2.4 g cm~(-3) to recover organic matter(light fractions) from the sand and silt fractions. Almost all(ca. 90%) the organic matter in the sand fraction and a large proportion(ca. 60%–70%) in the silt fractions was recovered by sequential density separation. The results suggested that POM is a composite of organo-mineral complexes with varying proportions of organic and mineral materials. Part of the organic matter associated with the silt fractions shared features in common with POM. In a laboratory bio-assay, biodegradability of POM varied depending on land use(pasture arable cropping). We concluded that POM is neither homogeneous nor unique.     

Characterization of water soluble organic matter in soils by size exclusion chromatography and fractionation with polyvinylpyrrolidone

Water extracts were obtained from four types of soils (Brown Lowland soil, Yellow soil with manure application for 6 years, non-allophanic Andosol, and allophanic Andosol), and the organic matter in the water extracts was fractionated according to the solubility in acid and adsorption onto polyvinylpyrrolidone (PVP). For the water extracts and their fractions, the amounts of organic C, total N, and anthrone-reactive C (ARC) were analyzed, and high performance size exclusion chromatography (HPSEC) was carried out. The PVP-non-adsorbed fulvic acid (FA) fraction accounted for the largest proportion of the total water-soluble organic C, ranging from 52% for the Yellow soil to 96% for the allophanic Amdosol, followed by the PVP-adsorbed FA and humic acid (HA) fractions. The water extract of the allophanic Andosol was characterized by the lack of HA fraction and a small proportion of PVP-adsorbed FA fraction. For all the water extract samples, more than 90% of the ARC was recovered in the PVP-non-adsorbed FA fraction. The proportion of ARC in the total organic C in the fraction was also highest in the PVP-non-adsorbed FA fraction. The molecular weight (MW) of the humic substances (HS) at peak maximum was estimated at 1,300 Da for the water extracts and their fractions from the Brown Lowland soil and non-allophanic Andosol samples by HPSEC using polyethylene glycols as MW standards. Manure application increased the MW of HS in the HA and PVP-adsorbed FA fractions. On the other hand, only a small amount of HS was found in the water extract of the allophanic Andosol by HPSEC.     

Influence of land use on the characteristics of humic substances in some tropical soils of Nigeria

In highly weathered tropical conditions, soil organic matter is important for soil quality and productivity. We evaluated the effects of deforestation and subsequent arable cropping on the qualitative and quantitative transformation of the humic pool of the soil at three locations in Nigeria. Cultivation reduced the humic pool in the order: acetone‐soluble hydrophobic fraction (HE) > humic acid (HA) > humin (HU) > fulvic acid (FA), but not to the same degree at all three sites. The C and N contents, as well as the C/N ratios of humic extracts, were large and not substantially influenced by land use. The δ¹³C values of the humic extracts were invariably more negative in forested soils thereby showing a dilution of δ¹³C signature with cultivation from C3 to C4 plants. The δ¹³C values of apolar HE fractions were generally more negative, indicating a reduced sensitivity compared with other humic fractions to turnover of crop residues. The contents of hydrophobic constituents (alkyl and aromatic C), as revealed by cross‐polarization magic angle spinning (CPMAS) ¹³C‐NMR spectroscopy, in HA, FA and HU were generally < 50%, with the exception of larger hydrophobicity in HU in the forested soil at Nsukka and HA in that at Umudike. The HE fraction contained significantly more apolar constituents, and consequently had a larger intrinsic hydrophobicity than the other humic fractions. The larger reduction of apolar humic constituents than of the less hydrophobic humic fractions, when these soils were deforested for cultivation, indicates that at those sites the stability of accumulated organic matter is to be ascribed mainly to the selective preservation of hydrophobic compounds.     

Transformation of organic matter from maize residues into labile and humic fractions of three European soils as revealed by 13C distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fractions of soil organic matter have yet to be clarified in terms of chemical structures and mechanisms involved. We studied by ¹³C‐dilution analysis and CPMAS‐¹³C‐NMR spectroscopy the distribution of organic carbon from mixed or mulched maize residues into specific defined fractions such as carbohydrates and humic fractions isolated by selective extractants in a year‐long incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative δ¹³C values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS‐¹³C‐NMR spectra of humic (HA) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic materials, whereas spectra of organic matter extracted with an acetone solution (HE) indicated solubilization of an aliphatic‐rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of ¹³C showed that all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone solution allowed a subsequent extraction of HA and, especially, FA still containing much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retained in HA and FA, hydrophobic components in the HE fraction had chemical features similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mainly in the hydrophilic fraction of humic substances which are, in turn, stabilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the accumulation of new organic matter in soils.     

Labile pools of organic matter and microbial biomass in the surface soils under shifting cultivation in northern Thailand

In order to analyze the N mineralization process under shifting cultivation in northern Thailand, labile pools of soil organic matter were studied, which were considered to be the factors contributing to the N mineralization process. Organic C, (organic + NH₄ ⁺)-N, and hexose-C were extracted from fresh soils in the surface 0–5 cm layers with a 0.5 M K₂S₀. solution at 110°C in an autoclave (fraction A) or at room temperature with a reciprocal shaker (fraction B), and analyzed as labile pools of organic matter. In the traditional shifting cultivation system, the content of organic C in fraction A in the fallow fields for 8 to 15 y was 3,710 mg kg^-1 while that in the fallow fields for 1 y and 3 to 5 y was 2,640 and 2,600 mg kg^-1, respectively. A high correlation was observed between the contents of the labile pool in fraction A and total soil organic matter. The ratio of the pool in fraction A to total soil organic matter apparently remained constant through the input-output balance in the pool. The content of the labile pool in fraction B was the highest among the fields cultivated for 1 y after the slash and burn practice and it decreased in the course of the fallow period. The content of organic C was 548 mg kg^-1 in the fields cultivated for 1 y and 235 mg kg^-1 in the fallow fields for 8-15 y, respectively. There was a reverse relation between the contents of the pool in fraction B and microbial biomass. Therefore, the origin of the pool in fraction B was attributed to the microbial debris associated mainly with a decrease in the soil moisture content in the dry season. On the other hand, in the relatively intensive cultivation system, there was no significant difference in the contents of the labile pools both in fractions A and B among the land use stages, suggesting that the preservation mechanism of these pools, which was observed in the traditional cultivation system, did not operate well in the intensive system. In alternative farming systems in future, it will be essential to apply organic materials to soils to supply organic matter and to maintain the microbial biomass.     

Soil particulate organic matter effects on nitrogen availability after afforestation with Eucalyptus globulus

We examined the hypothesis that changes in the quality and/or quantity of soil particulate organic matter (POM) after afforestation of pasture land with Eucalyptus globulus Labill. plantations caused increased nitrogen (N) immobilization and a decline in N availability. The quantity of POM was measured on soils from 10 paired pasture/plantation sites in south-western Australia. Net mineralization of C and N were measured over a 14-day incubation of POM, whole soil, and a mix of POM (33%) and whole soil (67%) at 25 °C and optimal moisture content (matric potential of 25 kPa). There was no significant difference in total organic C between pasture and plantation. However, the POM fraction C was higher in plantation soils (75%) than under pasture (62%), reflecting the coarser nature of organic inputs under plantation. Total soil N concentration was 20% lower under plantation compared to pasture, and the proportion in the POM was higher (74% compared to 57% for pasture soil). The C:N ratios in POM under both pasture and plantation, and in the whole soil under plantation were around 19, but C:N ratios of whole soil under pasture was 17. Average C mineralization was 13% lower in plantation relative to that in pasture soil. The isolated POM fraction had 18% higher C mineralization rate than that in whole soil. The change in net N mineralization with afforestation was marked, with 50% lower net N mineralization in plantation than pasture whole soils. Net N mineralization in the isolated POM fraction was also about 50% of that in the whole soil for both pasture and plantation soils. Although, the pasture and plantation POM had similar C:N ratios, the net N mineralization was 2-fold greater in pasture POM than in plantation POM, suggesting that biochemical characteristics other than the C:N ratio had the main influence on net N mineralization rates. The POM fraction did not significantly immobilize N from the whole soil when placed in a mixture of POM and whole soil, suggesting that N immobilization was not the main mechanism for POM to influence N availability in these soils.     

Effect of Long-Term Soil Management on the Mutual Interaction Among Soil Organic Matter, Microbial Activity and Aggregate Stability in a Vineyard

Vineyard management practices to enhance soil conservation principally focus on increasing carbon (C) input, whereas mitigating impacts of disturbance through reduced tillage has been rarely considered. Furthermore, information is lacking on the effects of soil management practices adopted in the under-vine zone on soil conservation. In this work, we evaluated the long-term effects (22 years) of alley with a sown cover crop and no-tillage (S + NT), alley with a sown cover crop and tillage (S + T), and under-vine zone with no vegetation and tillage (UV) on soil organic matter (SOM), microbial activity, aggregate stability, and their mutual interactions in a California vineyard in USA. Vegetation biomass, microbial biomass and activity, organic C and nitrogen (N) pools, and SOM size fractionation and aggregate stability were analysed. Soil characteristics only partially reflected the differences in vegetation biomass input. Organic C and N pools and microbial biomass/activity in S + NT were higher than those in S + T, while the values in UV were intermediate between the other two treatments. Furthermore, S + NT also exhibited higher particulate organic matter C in soil. No differences were found in POM C between S + T and UV, but the POM fraction in S + T was characterized by fresher material. Aggregate stability was decreased in the order: S + NT > UV > S + T. Tillage, even if shallow and performed infrequently, had a negative effect on organic C and N pools and aggregate stability. Consequently, the combination of a sown cover crop and reduced tillage still limited SOM accumulation and reduced aggregate stability in the surface soil layer of vineyards, suggesting relatively lower resistance of soils to erosion compared to no-till systems.     

Impact of tillage intensity on carbon and nitrogen pools in surface and sub‐surface soils of three long‐term field experiments

Management options such as the intensity of tillage are known to influence the turnover dynamics of soil organic matter. However, less information is available about the influence of the tillage intensity on individual soil organic matter pools with different turnover dynamics in surface as compared with sub‐surface soils. This study aimed to analyse the impact of no tillage (NT), reduced tillage (RT) and conventional tillage (CT) on labile, intermediate and stable carbon (C) and nitrogen (N) pools in surface and sub‐surface soils. We took surface and sub‐surface soil samples from the three tillage systems in three long‐term field experiments in Germany. The labile, intermediate and stable C and N pool sizes were determined by using the combined application of a decomposition experiment and a physical‐chemical separation procedure. For the surface soils, we found larger stocks of the labile C and N pool under NT and RT (C, 1.7 and 1.3 t ha^?1; N, 180 and 160 kg ha^?1) than with CT (C, 0.5 t ha^?1; N, 60 kg ha^?1). In contrast, we found significantly larger stocks of the labile C pool under CT (2.7 t ha^?1) than with NT and RT (2 t ha^?1) for the sub‐surface soils. The intermediate pool accounted for 75–84% of the soil organic C and total N stocks. However, the stocks of the intermediate N and C pools were only distinctly larger for NT than for CT in the surface soils. The stocks of the stable C and N pools were not affected by the tillage intensity but were positively correlated with the stocks of the clay‐size fraction and oxalate soluble aluminum, indicating a strong influence of site‐specific mineral characteristics on the size of these pools. Our results indicate soil depth‐specific variations in the response of organic matter pools to tillage of different intensity. This means that the potential benefits of decreasing tillage intensity with respect to soil functions that are closely related to organic matter dynamics have to be evaluated separately for surface and sub‐surface soils.     

Modelling nitrogen mineralization from manures: representing quality aspects by varying C:N ratio of sub-pools

The mineralization/immobilization of nitrogen when organic sources are added to soil is represented in many simulation models as the outcome of decomposition of the added material and synthesis of soil organic matter. These models are able to capture the pattern of N release that is attributable to the N concentration of plant materials, or more generally the C:N ratio of the organic input. However, the models are unable to simulate the more complex pattern of N release that has been observed for some animal manures, notably materials that exhibit initial immobilization of N even when the C:N of the material suggests it should mineralize N. The APSIM SoilN module was modified so that the three pools that constitute added organic matter could be specified in terms of both the fraction of carbon in each pool and also their C:N ratios (previously it has been assumed that all pools have the same C:N ratio). It is shown that the revised model is better able to simulate the general patterns on N mineralized that has been reported for various organic sources. By associating the model parameters with measured properties (the pool that decomposes most rapidly equates with water-soluble C and N; the pool that decomposes slowest equates with lignin-C) the model performed better than the unmodified model in simulating the N mineralization from a range of feeds and faecal materials measured in an incubation experiment.     

Carbon and nitrogen mineralization of added particulate and macroorganic matter

Particulate organic matter (POM) is more sensitive than total SOM to changes in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine the effects of added POM (75 to 250 μm size fraction), or macroorganic matter (MOM, 250 to 2000 μm size fraction) on C and N mineralization and microbial C and N content. A 1 kg composite made from 16 predominantly silt loam soils was amended with 10 g of POM, MOM or MOM ground to a reduced size of 75 to 250 μm (GMOM). The MOM amendment equaled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 weeks was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 μm did not affect mineralization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N added. Contribution of POM to total mineralized N from soil organic matter (SOM) in the composite soil was proportional to the POM content in SOM or approximately 12%. Amended soils had 25 to 42% more biomass-C than the control soil 2 weeks after amendment application. After 8 weeks, the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soil. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respiration and microbial biomass-C and N, which also are suggested indicators of soil quality. Therefore, POM may be a suitable soil quality indicator that provides similar information as soil respiration or microbial biomass determinations.     

Density separation of soil organic matter across three land uses in calcareous soils of Iran

ABSTRACT

The aim of this study was to examine the usefulness of physical and chemical fractionation in quantifying soil organic matter (SOM) in different stabilized fraction pools. Soil samples from three land use types in Lorestan province, Southwest Iran were examined to account for the amount of organic carbon and nitrogen in different SOM fractions. Size/density separation and chemical oxidation methods were applied to separate the SOM fractions including particulate organic matter (POM), Si + C (silt and clay), DOC (dissolved organic C), rSOM (oxidation-resistant organic carbon and nitrogen) and S + SA (sand and stable aggregates). The values obtained for TOC, TN, and HWC were highest in forest lands followed by the range and agricultural lands. Among the SOM fractions, S + SA showed the highest values (5.75, 5.77 and 20.6 g kg^?1 for agriculture, range and forest lands respectively) followed by POM, Si + C, rSOM, and DOC. The concentrations of C and N in the labile fractions obtained the higher values than in the stabilized fractions. Forest lands had the highest amounts of organic C and N among all fractions whereas agricultural lands showed highest values for inorganic C content of soils in different fractions.     

Long-term effect of chemical fertilizer,straw, and manure on labile organic matter fractions in a paddy soil

To assess the effect of long-term fertilization on labile organic matter fractions, we analyzed the C and N mineralization and C and N content in soil, particulate organic matter (POM), light fraction organic matter (LFOM), and microbial biomass. Results showed that fertilizer N decreased or did not affect the C and N amounts in soil fractions, except N mineralization and soil total N. The C and N amounts in soil and its fractions increased with the application of fertilizer PK and rice straw. Generally, there was no significant difference between fertilizer PK and rice straw. Furthermore, application of manure was most effective in maintaining soil organic matter and labile organic matter fractions. Soils treated with manure alone had the highest microbial biomass C and C and N mineralization. A significant correlation was observed between the C content and N content in soil, POM, LFOM, microbial biomass, or the readily mineralized organic matter. The amounts of POM–N, LFOM–N, POM–C, and LFOM–C closely correlated with soil organic C or total N content. Microbial biomass N was closely related to the amounts of POM–N, LFOM–N, POM–C, and LFOM–C, while microbial biomass C was closely related to the amounts of POM–N, POM–C, and soil total N. These results suggested that microbial biomass C and N closely correlated with POM rather than SOM. Carbon mineralization was closely related to the amounts of POM–N, POM–C, microbial biomass C, and soil organic C, but no significant correlation was detected between N mineralization with C or N amounts in soil and its fractions.     

Carbohydrates as Chemical Constituents of Biowaste Composts and their Humic and Fulvic Acids

The decomposition of organic matter of source-separated biowaste during composting was followed during 18 months. Compost samples were fractionated into three parts: (i) hot water soluble extract (HWE) (ii) bitumen fraction and (iii) humic substances (humic acids (HA) and fulvic acids (FA)). Original compost samples and the HA and FA fractions were hydrolyzed with sulfuric acid for hexoses and pentoses. Quantitative spectrophotometric and qualitative GC/MS analyses of monosaccharides as trimethylsilyl ethers of the corresponding alditols were carried out.

During composting, the amount of HA in the organic matter of the compost increased, the amounts of HWE and bitumen decreased and the amount of the FA fraction changed only a little. Carbohydrates were found to be important constituents of biowaste composts and their HA and FA fractions. Elemental analysis (C, N and H) of compost and HA samples showed an increase in the C:H ratio and in unsaturation of compounds during composting. The decrease in the C:N ratio was marginal.

The amounts of hexoses and pentoses in original compost samples and the HA and FA fractions decreased during composting. The sugar alcohols erythritol, xylitol, L-arabitol, ribitol, L-rhamnitol, L-fucitol, D-mannitol, D-glucitol and galactitol were identified in both the HA and FA fractions. 2-Deoxy-D-erythro-pentitol was identified in one HA fraction and inositol in two FA fractions. An analysis of gas chromatographic data for relative abundances showed that, in every sample except one and in every stage of composting D-glucitol was the main sugar alcohol. In general, the relative amount of D-glucitol decreased during composting, while the relative amounts of all other sugar alcohols increased.

As chemical indicators of compost maturity, carbohydrates would appear to be a important group of compounds. Most informative as a general indicator would be the ratio of the amount of HA to the amount of organic matter in the total compost samples.

According to our studies, the carbohydrates in composts are covalently bound to the structures of FA and HA. Carbohydrate determination clearly deserves more attention in the structural elucidation of FA and HA.     

Short-term effects of tillage on mineralization of nitrogen and carbon in soil

Tillage is known to decrease soil organic nitrogen (N) and carbon (C) pools with negative consequences for soil quality. This decrease is thought partly to be caused by exposure of protected organic matter to microbial degradation by the disturbance of soil structure. Little is known, however, about the short-term effects of tillage on mineralization of N and C, and microbial activity. We studied the short-term effects of two types of tillage (conventional plough- and a non-inverting-tillage) on mineralization and microbial N and C pools in a sandy loam under organic plough-tillage management. The release of active and protected (inactive) N by tillage was further studied in the laboratory by use of ¹⁵N labelling of the active pool of soil N followed by simulation of tillage by sieving through a 2 mm sieve. Results showed that the two types of tillage as well as the simulation of tillage had very few effects on mineralization and microbial pools. The simulation of tillage caused, however, a small release of N from a pool which was otherwise protected against microbial degradation. The use of soil crushing for disruption of larger macroaggregates (>425 μm) and chloroform fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. The relative contribution from the protected N pool was, however, similar in the three treatments (22-27%), thus the pools subjected to mineralization were characterised by similar degree of protection. On the basis of isotopic composition the pools of N mineralised were indistinguishable. This suggests that the released N originated from the same pool, that is the soil microbial biomass. The study points to the microbial pool as the main source of labile N which may be released by tillage, and thus to its importance for sustained soil fertility in agricultural systems.     

Labile,recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.     

Land use effects on soil carbon fractions in the southeastern United States. I. Management-intensive versus extensive grazing

Changes in grassland management intended to increase productivity can lead to sequestration of substantial amounts of atmospheric C in soils. Management-intensive grazing (MiG) can increase forage production in mesic pastures, but potential impacts on soil C have not been evaluated. We sampled four pastures (to 50 cm depth) in Virginia, USA, under MiG and neighboring pastures that were extensively grazed or hayed to evaluate impacts of grazing management on total soil organic C and N pools, and soil C fractions. Total organic soil C averaged 8.4 Mg C ha^–1 (22%) greater under MiG; differences were significant at three of the four sites examined while total soil N was greater for two sites. Surface (0–10 cm) particulate organic matter (POM) C increased at two sites; POM C for the entire depth increment (0–50 cm) did not differ significantly between grazing treatments at any of the sites. Mineral-associated C was related to silt plus clay content and tended to be greater under MiG. Neither soil C:N ratios, POM C, or POM C:total C ratios were accurate indicators of differences in total soil C between grazing treatments, though differences in total soil C between treatments attributable to changes in POM C (43%) were larger than expected based on POM C as a percentage of total C (24.5%). Soil C sequestration rates, estimated by calculating total organic soil C differences between treatments (assuming they arose from changing grazing management and can be achieved elsewhere) and dividing by duration of treatment, averaged 0.41 Mg C ha^–1 year^–1 across the four sites.     

Glomalin-related soil protein contains non-mycorrhizal-related heat-stable proteins, lipids and humic materials

Glomalin is reportedly a stable and persistent protein produced in copious quantities by mycorrhizal fungi and may be an important pool of organic N in soil. Glomalin-related soil protein (GRSP), however, is only operationally defined by its extraction method, and has been only poorly characterized at best. The goal of this study was to characterize the molecular structures within GRSP. Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and pyrolysis field-ionization mass spectrometry (Py-FIMS) revealed that GRSP contains a consortium of proteins along with many impurities. Employing proteomic techniques, we found that glomalin itself may be a thioredoxin-containing chaperone; however, no homologies with proteins or DNA of mycorrhizal origin were detected. Proteomics techniques further revealed that this fraction contains large amounts of soil-related heat-stable proteins and proteins of non-mycorrhizal origin. Results of this research show that the current extraction procedure that defines GRSP yields a mixture of compounds and thereby overestimates glomalin stocks when quantified using the Bradford assay. The chemical nature of glomalin has yet to be conclusively determined; it is unlikely that the chemical structure of glomalin can be elucidated from the mixture extracted as GRSP. Instead, an investigation into the specific biochemistry of immunoreactive assays currently used to define GRSP, followed by proteomic characterization of monoxenic mycorrhizal cultures may be required to advance our understanding of the chemical nature and agronomic significance of GRSP in soils.     

Role of allophanes in the accumulation of glomalin‐related soil protein in tropical soils (Martinique,French West Indies)

Thermo‐stable, operationally defined soil protein, known as glomalin, may make an important contribution to carbon storage in soils. The term glomalin is used because this putative protein, or group of proteins, was originally thought to be produced only by Glomus fungi. There is currently little information on the glomalin‐related soil protein (GRSP) content of tropical soils, particularly allophanic soils that are known to have different carbon dynamics to temperate climate soils. We have measured the Bradford‐reactive GRSP content of soils sampled from forests and grasslands on the tropical island of Martinique and compared the observations with soil composition. Two operationally defined fractions of GRSP were measured, namely easily‐extractable and total GRSP. The contents of GRSP in moist soils were in the range of 2–36 g kg^?1, accounting for about 8% of soil organic carbon, and were greater in topsoils than in corresponding subsoils. Both the GRSP contents and the fraction of soil organic carbon attributed to GRSP were greater than those reported for temperate climate soils. Both total and easily extractable GRSP contents were positively correlated to soil organic carbon content. The fraction of soil organic carbon that could be attributed to soil protein decreased with increasing allophane content for allophanic soils. No other trends of GRSP content with soil properties or land use were found. GRSP extraction was decreased about seven‐fold by air‐drying of soils, confirming the irreversible change in the soil microstructure of allophanic soils. Total and easily extractable GRSP were correlated and we conclude that both are good probes of thermo‐stable soil protein content for these soils. No attempt was made to verify the fungal origin of the protein detected.     

红壤侵蚀区植被恢复过程中土壤有机碳组分变化

为了解土壤有机碳组分在植被恢复过程中的变化规律,选取了红壤区本底条件基本一致的不同恢复年限马尾松林为研究对象,以未治理的侵蚀裸地(CK1)和恢复后的次生林(CK2)为对照,采用物理化学分组法,将土壤有机碳分为由溶解性有机碳(DOC)和颗粒有机碳(POM)组成的活性碳库、物理保护态的团聚体与粉粒和黏粒组合成的缓效性碳库以及化学结构稳定的惰性碳库。结果表明:在植被恢复过程中(0~30年)活性碳库储量及其分配比例在植被恢复7~10年显著提高(P0.05),并在植被恢复27~30年保持较稳定水平,缓效性碳库储量及其分配比例在27~30年呈显著变化(P0.05),而活性碳库分配比例有所降低,且POM、DOC与缓效性碳库均达显著相关(P0.01),说明活性碳库在恢复7~10年后逐渐向缓效性碳库转化;惰性碳库储量随恢复年限不断增加,但其分配比例保持较稳定水平。相关性分析显示,恢复年限、不同组分与不同碳库均达显著相关(P0.01),且缓效性碳库随植被恢复最敏感,说明在马尾松恢复过程中土壤有机碳以活性碳库积累逐渐转化为缓效性碳库积累为主,进而影响惰性碳库的积累,有利于土壤有机碳的长期保持。