Stable Organic Matter Fractions Research Articles

Comparative analysis of soil organic matter fractions, lability, stability ratios, and carbon management index in various land use types within bharatpur catchment, Chitwan District, Nepal

BackgroundLand use and land cover changes have a significant impact on the dynamics of soil organic matter (SOM) and its fractions, as well as on overall soil health. This study conducted in Bharatpur Catchment, Chitwan District, Nepal, aimed to assess and quantify variations in total soil organic matter (TSOMC), labile organic matter fraction (CL), stable organic matter fraction (CS), stability ratio (SR), and carbon management index (CMI) across seven land use types: pastureland, forestland, fruit orchards, small-scale conventional agricultural land, large-scale conventional agricultural land, large-scale alternative fallow and conventional agricultural land, and organic farming agricultural land. The study also explored the potential use of the Carbon Management Index (CMI) and stability ratio (SR) as indicators of soil degradation or improvement in response to land use changes.ResultsThe findings revealed significant differences in mean values of TSOMC, CL, and CS among the different land use types. Forestland and organic farming exhibited significantly higher TSOMC (3.24%, 3.12%) compared to fruit orchard lands (2.62%), small scale conventional farming (2.22%), alternative fallow and conventional farming (2.06%), large scale conventional farming (1.84%) and pastureland (1.20%). Organic farming and Forestland also had significantly higher CL (1.85%, 1.84%) and CS (1.27%, 1.39%) compared to all other land use types. Forest and organic farming lands showed higher CMI values, while pastures and forests exhibited higher SR values compared to the rest of the land use types.ConclusionsThis study highlights the influence of various land use types on soil organic matter pools and demonstrates the potential of CMI and SR as indicators for assessing soil degradation or improvement in response to land use and land cover changes.

Sorption of Organic Contaminants by Stable Organic Matter Fraction in Soil.

Soil organic matter (SOM) and its heterogeneous nature constitutes the main factor determining the fate and transformation of organic chemicals (OCs). Thus, the aim of thus research was to analyze the influence of the molecular chemodiversity of a stable SOM (S-SOM) on the sorption potential of different groups of OCs (organochloride pesticides-OCPs, and non-chlorinated pesticides-NCPs, polycyclic aromatic hydrocarbons-PAHs). The research was conducted as a batch experiment. For this purpose, a S-SOM was separated from six soils (TOC = 15.0-58.7 gkg-1; TN = 1.4-6.6 gkg-1, pH in KCl = 6.4-7.4 and WRB taxonomy: fluvisols, luviosols, leptosols) by alkaline urea and dimethylsulphoxide with sulfuric acid. Isolated S-SOM fraction was evaluated by UV-VIS, FT-IR and EEM spectroscopy to describe molecular diversity, which allowed the assessment of its potential sorption properties regarding OCs. In order to directly evaluate the sorption affinity of individual OCs to S-SOM, the mixture of the 3 deuterated contaminants: chrysene (PAHs), 4,4'DDT (OCPs) atrazine (NCPs) were applied. The sorption experiment was carried out according to the 106 OECD Guidelines. The OCs concentration was analyzed by gas chromatography triple mass spectrometry (GC-MS/MS). OCs were characterized by different sorption rates to S-SOM fractions according to the overall trend: atrazine (87.5-99.9%) > 4,4'DDT (64-81.6%) > chrysene (35.2-79.8%). Moreover, atrazine exhibited the highest saturation dynamic with fast bounding time amounting to 6 h of contact with S-SOM. Proportionally, the chrysene showed the slowest binding time achieving an average of 55% sorption after 78 h. Therefore, S-SOM isolated from different soils demonstrated varying binding capacity to OCs (CoV = 21%, 27% and 33% for atrazine, DDT and chrysene, respectively). Results indicate that each sample contains S-SOM with different degrees of transformation and sorption properties that affect the OCs availability in soil. Spectroscopic analyses have shown that the main component of S-SOM are biopolymers at various stages of transformation that contain numerous aromatic-aliphatic groups with mostly hydrophilic substituents.

Phytoremediation of dredged marine sediment: Monitoring of chemical and biochemical processes contributing to sediment reclamation

In this study, a pilot phytoremediation experiment was performed to treat about 80 m3 of silty saline sediments contaminated by heavy metals and organic compounds. After preliminary mixing with a sandy soil and green compost application, three different plant treatments [Paspalum vaginatum (P); P. vaginatum + Spartium junceum (P + S); P. vaginatum + Tamarix gallica (P + T)] were compared to each other and to an unplanted control (C) in order to evaluate the plant efficiency in remediating and ameliorating agronomical and functional sediment properties. The experiment was monitored for one year after planting by taking sediment samples at two depths and performing several chemical and biochemical analyses. After one year, the increase in hydrolytic enzyme and dehydrogenase activities indicated the stimulation of sediment functionality. Additionally, the availability of energy sources derived from organic matter application and plant-root activity promoted the formation of a stable organic matter fraction. Finally, P + S and P + T were also effective in decontaminating polluted marine sediments from both organic (total petroleum hydrocarbons, TPH) and inorganic (heavy metal) pollutants.

Microclimate affects soil chemical and mineralogical properties of cold alpine soils of the Altai Mountains (Russia)

The present work focuses on cold alpine soils of the Altai Mountains (Siberia, Russia). Permafrost is widespread and often occurs at a depth of about 100 cm. The area is characterised by extremely cold winters and cool summers: the aim was consequently to find out whether weathering could be more intense on thermally less unfavoured conditions or whether the abundance of water could be a more important factor. We investigated 10 soils in a very small area close to a local glacier tongue. Five of the investigated soils were south-facing and the other five north-facing. The soils have the same parent material (mica-rich till), altitude, topography and soil age. The vegetation is alpine grassland that is partially intersected with some juniper and mosses. Soil chemical properties such as organic C, N, soil organic matter quality (using DRIFT), pH value, (oxy)hydroxides, total elemental contents (XRF) and soil micromorphology and mineralogy (using diagnostic treatments and XRD) were determined. The age constraint of the site was given by geomorphic studies together with 14C dating of a nearby peat bog and the stable organic matter fraction of the soils. The soils have a Holocene age. The results showed astonishingly clearly—similarly to the European Alps—that the north-facing soils have a higher weathering state. This is expressed by lower pH values, higher oxalate and dithionite extractable Fe, Al, Mn and Si contents, higher C and N concentrations and stocks when compared to the south-facing sites. No statistically significant differences with respect to weathering indexes could be detected. The geochemical evolution of the soils seems to be enhanced at north-facing sites, even though very severe climatic conditions prevail. Furthermore, biodegradation seems to be less pronounced on north-facing compared to south-facing sites as poorly degraded organic matter is accumulated. This gives rise to more organic ligands that promote metal binding and their subsequent eluviation along the soil profile. We consequently must assume that weathering is not limited by low temperatures in the active layer but is rather controlled by soil moisture that seems to be higher during the warmer period in the north-facing soils.

Charcoal and stable soil organic matter as indicators of fire frequency, climate and past vegetation in volcanic soils of Mt. Etna, Sicily

Charcoal fragments in soils are useful to reconstruct past vegetation because the level of preservation is often good enough to determine the tree genus. All forest ecosystems have the potential to burn as a result of naturally occurring or human-induced fires. Forest fires are coupled to climate and are a not-negligible factor of pedogenesis in Mediterranean areas, where they occur frequently. Furthermore, soil organic matter (SOM) is prone to undergo peculiar changes due to forest fires, both in terms of quantity and quality. A soil sequence along an elevational gradient ranging from Mediterranean to subalpine climate zones on slopes of Mt. Etna (Sicily, Italy) was investigated in respect of soil organic C and charcoal. The amount of charcoal and the identification of charred species gave an indication of the fire frequency and vegetation changes that have occurred in the past. The distribution into labile and stable organic fractions provided insight into the stabilisation and turnover mechanisms of SOM. The stable organic matter fraction was measured as the residue of a H2O2 treatment. The soils along the altitudinal sequence are variations of Vitric Andosols that developed on a single trachy-basaltic lava flow having an age of 10–15 ky BP. Maquis vegetation dominates at the lower sites of the toposequence, followed by oak- and chestnut-forests at mid elevations, and pine-forest at the highest-elevated sites. Charcoals are older at higher elevations (ages of up to 1.5 ky cal BP). Here, the vegetation type has not changed over the last > 1000 years, as all charcoal pieces were identified as Pinus nigra. Charred material at the lower sites could be identified as particles of deciduous shrubs, Quercus, Castanea sativa, Lonicera implexa and Cytisus spp. with mostly a modern age up to about 300 y cal BP. A similar finding was obtained for the stable (H2O2 resistant) SOM. Very high ages for this fraction were found at the highest elevations where it had an age of up to 8.2 ky BP — an age that is close to the start of soil formation. At the lower sites, where frequent bush fires often destroyed a part of the stable fraction, the stable SOM fraction had a maximum age of 1 ky. The studied soils have recorded the signals of the interrelated factors fire frequency, climatic effects and vegetation whose role cannot always be clearly distinguished. With decreasing altitude and with a warmer climate, vegetation changes and fire frequency, org. C and especially nitrogen abundance and the amount of labile SOM increases. At the lower sites, charcoal particles reflect the more recent vegetation probably because the repeated fires here hindered their preservation. Our findings hence suggest that a high fire frequency is a powerful rejuvenating factor for soil organic matter, removing part of the old SOM and promoting plant recolonisation that is a source of young SOM. Fire frequency and intensity on Mt. Etna is, however, moderate enough even at the lowest altitudes for the organic matter pool to be high and not depleted.

Determination of Humic Acids Content in Composts by Means of Near- and Mid-Infrared Spectroscopy and Partial Least Squares Regression Models

Humic acids are part of the stable organic matter fraction in soils and composts. Due to their favorable properties for soils and plants, and their role in carbon sequestration, they are considered a quality criterion of composts. Time-consuming chemical extraction of humic acids and the inherent source of errors require alternative approaches for humic acids quantification. Different measurement techniques in the mid-infrared (MIR: KBr pellet technique) and near-infrared (NIR: fiber probe as well as an integrating sphere with a sample rotator) regions were applied. Partial least squares regression (PLSR) models based on infrared spectra were developed to determine humic acids contents in composts. As the wavenumber regions used (NIR: 6105-5380 cm(-1) and 4360-4220 cm(-1), MIR: 1745-1685 cm(-1) and 1610-1567 cm(-1)) represent different molecular vibrations, the importance of the methylene-group-derived vibrations for the NIR models is discussed. The correlation coefficients obtained for the KBr pellet technique, the NIR fiber probe technique, and the NIR integrating sphere (r = 0.94, 0.93, and 0.94) and the root mean square errors of cross-validation (RMSECV = 2.2% organic dry matter (ODM), 2.5% ODM, and 2.2% ODM) make the models appropriate for application in composting practice.

Sodium hypochlorite separates an older soil organic matter fraction than acid hydrolysis

Different chemical treatments can be used to isolate an old and chemically resistant soil organic matter fraction from soils. Here, we compared the residues after acid hydrolysis (6 N HCl) with those obtained after NaOCl treatment (6 wt%) of the silt + clay fractions from 48 soil samples. The samples were taken at sites differing in climate and land use across Switzerland. To determine the influence of the two treatments on soil organic matter and mineral structures, we examined infrared spectra and isotopic signatures of carbon ( 14C and δ 13C) of the residues. Treatment with NaOCl removed more (63 to 91%) organic carbon (OC) than did treatment with HCl (35 to 66%), and it had no effect on mineral structures, whereas treatment with HCl converted crystalline minerals to more amorphous ones. Increases in specific soil surface area (SSA) did not correlate with the amount of OC removed. The amount of OC removed by each treatment was not (NaOCl) or only weakly (HCl) related to the initial OC content of the silt + clay fraction, suggesting the presence of a relatively constant fraction of chemically resistant OC. 14C activities of NaOCl-resistant residues were lower than those of HCl residues, indicating that soil organic matter residues isolated by NaOCl treatment were older than the residues obtained by acid hydrolysis. It is concluded that oxidation with NaOCl is the better way than hydrolysis with HCl to obtain an operationally-defined stable organic matter fraction from soils.

Free Atmospheric CO2 Enrichment (FACE) Increased Labile and Total Carbon in the Mineral Soil of a Short Rotation Poplar Plantation

The global net terrestrial carbon sink was estimated to range between 0.5 and 0.7 Pg C y−1 for the early 1990s. FACE (free atmospheric CO2 enrichment) studies conducted at the whole-tree and community scale indicate that there is a marked increase of primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. During the first rotation of the POP/EuroFACE experiment in a short-rotation Poplar plantation, total soil C content increased more under ambient CO2 treatment than under FACE, while under FACE more new C was incorporated than under ambient CO2. These unexpected and opposite effects may have been caused by a priming effect, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates. In order to gain insight into these processes affecting SOM decomposition, we obtained the labile, refractory and stable pools of soil C and N by chemical fractionation (acid hydrolysis) and measured rates of N-mineralization. Results of the first 2 years of the second rotation show a larger increase of total soil C% under FACE than under ambient CO2. In contrast to the first rotation, total C% is now increasing faster under FACE than under ambient CO2. Based on these observations we infer that the priming effect ceased during the second rotation. FACE treatment increased the labile C fraction at 0–10 cm depth, which is in agreement with the larger input of plant litter and root exudates under FACE. N-mineralization rates were not affected by FACE. We infer that the system switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation). Our results on N-mineralization (second rotation) are in agreement with observations made at three forest FACE sites (Duke Forest, Oak Ridge, and Rhinelander), but our finding of increasing mineral soil C content contrasted with results at the Duke Forest where no significant increase in C content of the mineral soil occurred. However, the FACE induced increase in total C content occurred within the fraction with the shortest turnover time, i.e. the labile fraction. The refractory and stable fractions were not affected. The question remains whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C storage in refractory and stable organic matter fractions.

BIOCHEMICALLY PROTECTED SOIL ORGANIC CARBON AT THE NORTH APPALACHIAN EXPERIMENTAL WATERSHED

Land use and soil management affect the balance between labile and stable organic matter fractions in surface soils. This study was conducted to quantify the contribution of nonhydrolyzable carbon (NHC) to total soil organic carbon (SOC) and to identify the role of biochemical protection mechanisms in SOC sequestration by comparing the NHC fraction associated with aggregates from 0 to 5-, 5 to 10-, and 10 to 20 cm depths under forest, meadow, no-till (NT) and conventional tillage (CT) treatments. The NHC contribution to SOC declined from 53% under forest to 37% under CT and 39% under NT, implying that the conversion from forest to cultivation led mainly to a reduction in the NHC. Aggregate-associated NHC concentration increased with aggregate size (except for CT treatment). Conversion from CT to NT enriched NHC in all aggregate fractions, but even more so in the >250-μm fraction, underscoring the importance of macroaggregate fractions in encapsulating and thus protecting SOC from microbial processes. The formation of macroaggregates is coupled with the depletion of microaggregates, which can be quantitatively described by functions (for each aggregate class) expressed in terms of the aggregate mass and the proportion NHC/SOC. Both the NHC fraction and the nonhydrolyzable C:N ratio increased with the increase in SOC concentration in soils under meadow and forest, suggesting a large potential for SOC sequestration through biochemical protection mechanisms.

Accumulation of organic matter fractions in a gravel-bed constructed wetland

The function of a gravel-bed wetland in treating wastewaters is dependent on the turn-over rate of organic matter (OM) fractions in accumulated solids. Organic deposits from a gravel-bed planted (Schoenoplectus tabernaemontani) wetland, which had experienced pore clogging after 5 years of receiving farm dairy wastewater were therefore collected and determined for labile (water-soluble) and stable (humic acid, fulvic acid and humin) OM fractions, total carbon (C), microbial biomass and microbial respiration rate. Over 90% of the accumulated organic solids was present as stable fractions, with humic compounds at least 2-fold higher in surface deposits and the top 100 mm of the gravel bed than the lower gravel substratum. Clogging of the gravel pore spaces over a 5-year wetland operation was probably due to the accumulation of refractory (stable) organic solids, particularly in the top 100 mm of the gravel bed. Microbial respiration rate and microbial biomass were significantly correlated with stable OM fractions, suggesting that these microbial parameters may be used to predict the nature of accumulated OM fractions. Further research is required to evaluate the use of these parameters as indicators of labile and stable fractions in wetlands with a range of OM loadings and accumulation.

Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters

Organic matter (OM) composition, microbial biomass and microbial activity in a planted ( Schoenoplectus tabernaemontani), gravel-bed wetland receiving cumulative OM (determined as volatile suspended solids) loadings over 5 years from farm dairy wastewater (8.2 kg OM m −2) and in situ plant residues (8.4 kg OM m −2) were investigated. Organic deposits above and within the gravel stratum (0–100- and 100–400 mm depths) were collected from six sites (with three transverse points per site) along the wetland channel. They were sequentially extracted for labile and stable OM fractions and determined for total carbon (C), total nitrogen (N), biomass C, biomass N, and microbial activity (respiration rate). Over 90% of the OM accumulated in the wetland was present as stable OM fractions. Humic acid, fulvic acid and humin were the predominant stable OM fractions, accounting for ∼63–96% of total C in surface deposits and the gravel substratum. The predominance of stable OM fractions in the wetland was attributed to the refractory nature of OM inputs (lignocellulose and humic compounds) from wetland plant litter and the applied dairy wastewater. Clogging of the gravel pore spaces over a 5-year wetland operation was therefore a result of the accumulation of refractory organic solids, originating from plant litter and applied dairy wastewater. Mineral aluminium (Al) and iron (Fe) compounds did not play a major role in OM stabilisation and accumulation in the wetland since organic C associated with Al and Fe accounted for <2% of the total C in surface deposits and the gravel substratum. Humic compounds were at least 2-fold higher in surface deposits and the top 100 mm of the gravel-bed than the lower gravel substratum, suggesting that pore clogging by these compounds was more prominent in the top layer of the gravel-bed. Microbial respiration rate and microbial biomass were significantly ( P≤0.001) correlated with sediment OM fractions, suggesting that these microbial parameters may be used to predict changes in the labile and stable fractions of OM accumulation in the wetland. Both quantitative assessment of OM content and qualitative determination of labile and stable OM fractions in surface deposits and the gravel substratum are equally important in the understanding of OM accumulation-decomposition and pore clogging in gravel-bed wetlands.

Decomposition of 14C-labelled melanoid fungal residues in a marginally sodic soil

Three melanoid fungi, namely Alternaria alternata, Curvularia lunata and Drechslera australiensis were grown on 14C-glucose and then fractionated into cell wall, cytoplasm and melanin. The decomposition of these fractions and their contribution to the stable organic matter fraction was studied in a marginally sodic soil. The rate of decomposition of fungal melanins was less than that of the cell wall or cytoplasm which had the highest rate of decomposition. However, the contribution of these fractions to the humic acid fraction was very low. Most of the 14C-activity was recovered in the humin fraction.