Particulate Organic Matter Fractions Research Articles

Tree Mortality From Insect Infestation Enhances Carbon Stabilization in Southern Appalachian Forest Soils

AbstractForest insect and pathogen outbreaks may exacerbate anthropogenic climate change if they accelerate soil carbon loss to the atmosphere. We quantified soil respiration and carbon content for nearly a decade after girdling or natural infestation of hemlock (Tsuga canadensis L. Carr., a codominant species in southern Appalachian forests) by hemlock woolly adelgid (Adelges tsugae) to improve understanding of soil carbon response to disturbance from forest insect and pathogens. From 2005 to 2013, net soil respiration was similar among hemlock mortality (~50% basal area reduction) and reference hardwood plots, but both girdled and hemlock woolly adelgid‐infested plots showed greater activities of β‐glucosidase (a cellulose‐hydrolyzing extracellular enzyme), decreased O‐horizon, and decreased fine root biomass. During this period, mineral soil carbon accumulated at a higher rate in hemlock mortality plots than in reference plots in both surface (0–10 cm) and subsurface (10–30 cm) soils, driven by increases in the mineral‐associated fraction of the soil organic matter. In contrast, particulate organic matter (POM) carbon accrued slowly in surface soils and declined in the subsurface of girdled plots. δ13C values of the POM fraction demonstrate increased microbial processing of surface soil organic matter over time, suggesting enhanced decomposition of organic matter in this pool. These findings indicate that hemlock mortality in this system has led to enhanced soil carbon stabilization through the transformation and translocation of carbon from detrital and POM pools to the mineral‐associated organic matter pool. Accelerated responses in the girdled versus naturally infested treatments highlight limitations associated with using girdling to simulate natural mortality.

Transport of organic-walled dinoflagellate cysts in nepheloid layers off Cape Blanc (N-W Africa)

Many aspects of to the vertical and lateral transport of organic matter particles produced in the upper ocean are still not clear. This study documents the production, vertical and lateral displacement of the particulate organic matter fraction formed by dinoflagellate cysts in nepheloid layers present off Cape Blanc in November 2015. The position and extent of cyst production in the upper water column has been assessed by collecting the export rain of cysts with floating sediment traps. This has been compared to cyst associations in different intermediate nepheloid layers in the water column along two onshore-offshore transects.We document that highest cyst export production took place at the rim of a newly formed upwelling eddy/filament. We observed lateral transport up to 130 km off the shelf break into the deeper ocean in the bottom nepheloid layers (BNL) and in intermediate nepheloid layer (INL). The depth of the INL gradually increased from 600 m closest to the shelf break to 1300 m at the most offshore station. Transport in the BNL was observed in the more onshore part between 750 and 2300 m water depth. Cyst associations in both layers were comparable, indicating a common source. The large abundance of living cysts in these layers suggest that this source had a recent origin rather than being the result of resuspension of older sediment. The cyst associations of these samples differed from those of a second INL deeper in the water column as well as from samples from the more offshore part of the BNL. This indicates that the particles in this deeper INL and the offshore part of the BNL had a different origin. This origin could not be identified therefore no conclusions were drawn with respect to the extent to which these particles have been transported vertically or laterally.The cyst associations in the surface sediments differed from those of the nepholoid layers and the upper water column and did not reflect the effects of lateral transport as registered from the water column. The surface sediments contained a higher proportion of cysts that are highly resistant to aerobic degradation than the samples from the water column. This indicates that the former were modified predominantly by species-specific post depositional degradation. Selective degradation altering the cyst associations was not observed for the nepheloid layers.

The efficiency of Cd phytoextraction by S. plumbizincicola increased with the addition of rice straw to polluted soils: the role of particulate organic matter

The addition of organic amendments to soils can significantly increase soil organic matter (SOM) content as well as the mass of large (e.g. 2000–250 μm) particulate organic matter (POM) fractions. Here we address whether the addition of organic amendments might affect both POM pools and the phytoextraction efficiency of Sedum plumbizincicola (S. plumbizincicola) in cadmium (Cd) polluted soils. We carried out a 442-days long pot experiment and measured Cd uptake by S. plumbizincicola for four successive plant crops in two polluted paddy soils subjected to rice straw (RS) amendment. We also measured soil pH, nutrient availability, pore water dissolved organic carbon (DOC) and Cd concentrations across different soil particle-size fractions. We found that RS amendments had no significant effects on soil pH, DOC or Cd concentrations in bulk soils during repeated phytoextraction. However, RS amendments were associated with significant (1) increases (i.e. 14.3 to 20.7%) in Cd uptake by S. plumbizincicola plants from the first crop harvest, (2) increases in available soil nutrients content, (3) increases in the percentage of oxidizable Cd fractions and the mass of soil POM fractions, and (4) decreases in POM Cd concentrations after repeated phytoextraction. These findings suggest that POM fractions may represent important accessible pools of Cd for uptake by S. plumbizincicola plants. RS amendments enhance both Cd bioavailability within soil POM pools and Cd uptake by the hyperaccumulator S. plumbizincicola thus providing an effective management practice to improve Cd removal from polluted soils through phytoextraction.

Effects of mineral and organic fertilizers on soil organic carbon fractions in agricultural chestnut soil

Soil organic matter can be analyzed on the basis of the different fractions. Changes in the levels of organic matter, caused by land use, can be better understood by alterations in the different fractions. Therefore in order to discover tendency of soil fertility sustainability it is significant to research on stable and labile form fractions of soil organic carbon by advanced methodology and modern technique. Our research work aimedto evaluate the effect of mineral and organic fertilizers on the labile and stable organic carbon of the chestnut soil in Mongolia. The soils samples used in this study we collected from variants of mineral (N60P40K40), organic (biohumus 1t / hec.) Fertilizer and their combination of the Long-term fertilizers experiments of Plant and Agriculture Institute Changes in soil organic C by land use for agricultural purposes occurred mainly in the fraction of particulate organic matter (> 20 μm). The clay and silt fractions were quatified with a Mastersizer S after distruction organic substances and carbonates using H2O2 and HCI and the sand fraction was determined by wet sieving. According to our research, the stable form of organic carbon in chestnut soil is 39, 0-40,1% of the total fine particle size and 59, 9-61,0% of the active form fraction. On the other hand, variants with fertilizer tend to have increased stabile composition of soil organic carbon. It indicates that soil fertility protection and increased stability are possible in the country’s agricultural technology if use mineral and organic fertilizers.

Mechanisms for a nutrient-conserving carbon pump in a seasonally stratified, temperate continental shelf sea

Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015 as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions.

Soil organic carbon stocks in topsoil and subsoil controlled by parent material, carbon input in the rhizosphere, and microbial-derived compounds

Despite a large body of studies investigating soil organic carbon (SOC) stocks and potential influencing factors, the impact of contrasting parent material, particularly in the subsoil, has received little attention. To reveal potential effects varying parent materials exert on SOC stocks, we investigated chemical (14C content and overall chemical composition via 13C NMR spectroscopy) and plant/microbial related parameters (root mass, amino sugars) of bulk soil and soil organic matter fractions from topsoil, subsoil, and rhizosphere soil at three European beech stands (Fagus sylvatica L.) only differing in parent material (Tertiary sand, Quaternary loess, and Tertiary basalt).The results suggest that the clay fraction, its amount being largely dependent on the respective parent material, took a central role in shaping differences in SOC stocks among the investigated sites by affecting soil organic matter stabilization via organo-mineral association and aggregation. This fraction was particularly relevant in the subsoil, where it accounted for up to 80% of the bulk soil SOC stocks that decreased with decreasing amounts of the clay fraction (basalt > loess > sand site). Determining the soil's nutrient composition, parent material likely also indirectly affected SOC stocks by changing rhizosphere traits (such as fine root density or mortality) and by attracting root growth (and thus organic matter inputs) to subsoil with higher nutrient contents, where in situ root inputs in the form of rhizodeposits were likely the prime source of plant-derived SOC. However, root inputs also contributed in large part to topsoil SOC stocks and were associated with higher abundance of microbial compounds (amino sugars), whose relative importance increased with increasing soil depth.Independent of soil depth and site, amino sugars and the amount of the clay fraction, combined with parameters related to the input of organic matter (root mass and amount of the particulate organic matter fraction) explained more than 90% of the variability in SOC stocks, indicating a key role of these measures in impacting SOC stocks. Because parent material directly or indirectly influenced these parameters, we demonstrate the necessity to consider differences in parent material when estimating and predicting SOC stocks.

Variability in copepod trophic levels and feeding selectivity based on stable isotope analysis in Gwangyang Bay of the southern coast of the Korean Peninsula

Abstract. Trophic preference (i.e., food resources and trophic levels) of different copepod groups was assessed along a salinity gradient in the temperate estuarine Gwangyang Bay of Korea, based on seasonal investigation of taxonomic results in 2015 and stable isotope analysis incorporating multiple linear regression models. The δ13C and δ15N values of copepods in the bay displayed significant spatial heterogeneity as well as seasonal variations, which were indicated by their significant relationships with salinity and temperature, respectively. Both spatial and temporal variations reflected those in isotopic values of food sources. The major calanoid groups (marine calanoids and brackish water calanoids) had a mean trophic level of 2.2 relative to nanoplankton as the basal food source, similar to the bulk copepod assemblage; however, they had dissimilar food sources based on the different δ13C values. Calanoid isotopic values indicated a mixture of different genera including species with high δ15N values (e.g., Labidocera, Sinocalanus, and Tortanus), moderate values (Calanus sinicus, Centropages, Paracalanus, and Acartia), and relatively low δ15N values (Eurytemora pacifica and Pseudodiaptomus). Feeding preferences of different copepods probably explain these seasonal and spatial patterns of the community trophic niche. Bayesian mixing model calculations based on source materials of two size fractions of particulate organic matter (nanoplankton at < 20 µm vs. microplankton at 20–200 µm) indicated that Acartia and Centropages preferred large particles; Paracalanus, Calanus, Eurytemora, and Pseudodiaptomus apparently preferred small particles. Tortanus was typically carnivorous with low selectivity on different copepods. Labidocera preferred marine calanoids Acartia, Centropages, and harpacticoids; on the other hand, Sinocalanus and Corycaeus preferred brackish calanoids Paracalanus and Pseudodiaptomus. Overall, our results depict a simple energy flow of the planktonic food web of Gwangyang Bay: from primary producers (nanoplankton) and a mixture of primary producers and herbivores (microplankton) through omnivores (Acartia, Calanus, Centropages, and Paracalanus) and detritivores (Pseudodiaptomus, Eurytemora, and harpacticoids) to carnivores (Corycaeus, Tortanus, Labidocera, and Sinocalanus).

Microbial/biochemical indicators showing perceptible deterioration in the topsoil due to deforestation

Microbial parameters have been used to monitor changes in the topsoil under different land covers. Soils from five land cover systems common in northern Iran, including virgin natural forest, Alnus subcordata C.A.M. plantation, Quercus castaneifolia C. A. Mey plantation, Cupressus sempervirens var. horizontalis plantation and degraded natural forest were used to assess the effects of land cover changes on soil. We tested the following hypotheses: (i) deforestation would markedly influence topsoil C, N and P microbial ratios which make it distinct from the natural forest and plantations in the long-term, (ii) virgin natural forest and the establishment of plantations promote soil enzyme activities and the fractions of particulate organic matter (POC and PON) and dissolved organic matter (DOC and DON), 30 years after planting. Sixteen samples per land cover were taken from top 15 cm soil and characterized with respect to pH, organic C, total N, N mineralization, NH4+, NO3−, available P, fine root biomass, enzymes activities (urease, acid phosphomonoesterase, arylsulfatase and invertase), basal respiration (BR), substrate inducted respiration (SIR), microbial biomass-C, -N and -P (MBC, MBN and MBP), POC, PON, DOC and DON. There were no significant differences in BR/MBC, MBC/Corg, BR/SIR and MBC/MBN microbial indices among the plantations, virgin and degraded natural forests. The values of soil MBC/MBP were increased, more than one and half-fold, following deforestation (12.82) compared to virgin natural forest (8.53) site. The natural forest and establishment of plantations reduced the ratio of MBN/MBP, while the greater amount (1.14) was observed under degraded natural forest. With replacement in the type of tree species from Carpinus betulus and Parrotia persica in the natural forest to Quercus castaneifolia plantation, almost 30 years after planting, POC/PON increased significantly in the topsoil (13.29), about two-three fold, compared to virgin (7.13) and deforested (3.96) natural forests. The soil DOC/DON ratio varied significantly and it increased by about one and half-fold, following the growth of Quercus castaneifolia (2.87) and Cupressus sempervirens (2.68) trees compared to the other ecosystems. Based on PCA, virgin natural forest and Alnus subcordata plantation presented a good condition of soil fertility, more microbial and enzyme activities with a low ratio of microbial indicators. In general, microbial/biochemical indicators showed perceptible deterioration in the topsoil due to deforestation. As a conclusion, soil enzyme activity, MBC/MBP, MBN/MBP, POC/PON and DOC/DON are robust and sensitive indicators that can be used to measure long-term variations in the topsoil due to changed land cover.

Effects of nutrient enrichment and grazing by an invasive filter feeder on phytoplankton biomass in a South West Atlantic coastal lagoon

Invasive species can exert strong effects on structure and function of marine ecosystem. In estuaries, nutrient enhancement due to anthropogenic activities can increase microalgal biomass but invading filter-feeders can eventually regulate their abundance. This is what we hypothesized that is happening in a SW Atlantic coastal lagoon (Mar Chiquita, 37°40′S, 57°23′W, Argentina) invaded by the polychaete Ficopomatus enigmaticus. Here, we experimentally evaluated this hypothesis in a mesocosms experiment. Four treatments were performed: (T1) with nutrients (NO3, PO4 and NH4), (T2) with reefs and nutrients, (T3) with reefs, and (T4) without reefs or nutrients. Water samples were obtained to determine in vivo chlorophyll a (Chla) and nutrients concentration. Additionally, to evaluate the trophic position and particle selectivity by the polychaete, analysis of stable isotopes of particulate organic matter (POM) of the water and individuals were performed. Stable isotopes analysis showed that the main resource used by F. enigmaticus was the fraction of POM between 62 and 250 µm, showing particle selectivity and suggesting that the reefs have the potential to promote shifts in size, composition and biomass of local food source. Mesocosms experiments showed that nutrient supply increased the Chla concentration, but when reefs were added, the Chla decreased. Thus, nutrients increased the phytoplankton biomass but grazing by the reefs counteracted these increments. This indicates that an invader such as F. enigmaticus might modulate an anthropogenic impact via suspension feeding, and therefore its role in mitigate the consequences of eutrophication may be highly important.

Biochemical stabilization of soil organic matter in straw-amended, anaerobic and aerobic soils

Crop straw incorporation is a useful approach for increasing the quantity and changing the chemical composition of soil organic matter (SOM). This process is influenced by soil aeration. The present study investigated the stability of whole SOM, particulate organic matter (POM) and mineral-associated organic matter (MinOM) fractions with wheat straw amendment under aerobic and anaerobic conditions over a 12-month incubation period. Solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to analyze the chemical composition of whole SOM, POM and MinOM fractions. The decomposition rate of wheat straw was lower under anaerobic than under aerobic conditions (0.014 vs. 0.020day−1). Wheat straw incorporation increased the original soil organic carbon content (7.4g kg−1) under both aerobic (up to 10.2g·kg−1) and anaerobic (up to 10.3g·kg−1) conditions, but the content of mineral-associated organic carbon (MinOC) under aerobic condition (7.0g·kg−1) was significantly larger than that under anaerobic condition (4.9g·kg−1). The proportion of alkyl carbon (C) in SOM, POM and MinOM fractions was greater under anaerobic than under aerobic conditions, while the opposite was true for the proportion of O-alkyl C of SOM and POM and MinOM fractions. A/O-A indices (i.e., the ratio of alkyl C to O-alkyl C) of whole SOM, POM and MinOM were higher under anaerobic than under aerobic conditions. We conclude that wheat straw incorporation resulted in the enrichment of alkyl C in the POM and MinOM fractions under anaerobic conditions, and thus improved the stability of SOM. In this way, the decomposition of crop residue influenced SOM structural chemistry at the molecular level.

Amount and incorporation of plant residue inputs modify residue stabilisation dynamics in soil organic matter fractions

Carbon sequestration in agricultural soils has been promoted as a means to reduce atmospheric concentrations of greenhouse gases (GHG) whilst improving soil productivity. Although there is broad agreement on practices that increase carbon (C) stocks, uncertainty remains on how agricultural management affects the stability of these gains. The fate of above-ground residue into soil organic matter (SOM) was tracked using isotopically labelled (13C and 15N) residue over 12 months in a pasture soil in sub-tropical Australia. Agricultural residue management was simulated by (1) altering the rate of residue input and (2) incorporating residue with topsoil or leaving on soil surface. Increased input and incorporation of residue increased residue-derived SOM content, with the majority of residue-derived SOM accumulating as particulate organic matter (POM) (65%) with more modest gains in mineral-associated fractions. Rapid accumulation of residue-derived SOM in the mineral-associated fractions in the initial stages of decomposition, coinciding with a high loss of labile residue components, indicate an important role for soluble OM inputs in providing an immediate and long-term sink for C and N. However, this must be considered alongside high rates of accumulation in the more readily mineralised POM fraction, particularly when a soil is approaching saturation, which is likely to lead to greater mineralisation of SOM.

Influence of Manure Type and Bedding Material on Carbon Content of Particulate Organic Matter in Feedlot Amendments Using 13C NMR-DPMAS

ABSTRACTFeedlots in southern Alberta apply composted (CM) or stockpiled (SM) manure with straw (ST) or wood-chip (WD) bedding to cropland, but few studies have examined the effect of manure type and bedding material on carbon composition of these feedlot amendments prior to land application using solid-state 13C NMR-DPMAS (nuclear magnetic resonance-direct polarization, magic angle spinning). The particulate organic matter fraction was extracted from four amendment treatments (CM-ST, CM-WD, SM-ST, SM-WD) to avoid possible paramagnetic interference from considerable mineral soil in the manure from the unpaved feedlot. The hypothesis was that O-alkyl C of POM should be lower for more decomposed manure types (CM than SM) and bedding materials (ST than WD), and that alkyl C, aromatic C, carboxyl C, aromaticity, and alkyl:O-alkyl (A:O-A) ratio should be greater for the more decomposed amendments. The C composition of all feedlot amendments was dominated by aromatic C (8%–14%) and O-alkyl (7%–14%) C and had considerable less contribution from carboxyl (2%–4%) and alkyl C (1%–3%). The manure type hypothesis was supported for O-alkyl C (but not for the other three C groups), aromaticity, but not the A:O-A ratio. The bedding hypothesis was supported for O-alkyl C, aromaticity, and A:O-A ratio, but not for alkyl C, aromatic C, and carboxyl C. A decrease in O-alkyl C, increase in aromaticity, and increase in A:O-A ratio (bedding only) with more decomposed manure types or bedding materials suggested that these 13C NMR parameters may have potential to evaluate the maturity and stability of composted feedlot manures.

Cloud condensation nuclei activity and hygroscopicity of fresh and aged cooking organic aerosol

Cooking organic aerosol (COA) is potentially a significant fraction of organic particulate matter in urban areas. COA chemical aging experiments, using aerosol produced by grilling hamburgers, took place in a smog chamber in the presence of UV light or excess ozone. The water solubility distributions, cloud condensation nuclei (CCN) activity, and corresponding hygroscopicity of fresh and aged COA were measured. The average mobility equivalent activation diameter of the fresh particles at 0.4% supersaturation ranged from 87 to 126 nm and decreased for aged particles, ranging from 65 to 88 nm. Most of the fresh COA had water solubility less than 0.1 g L−1, even though the corresponding particles were quite CCN active. After aging, the COA fraction with water solubility greater than 0.1 g L−1 increased more than 2 times. Using the extended Köhler theory for multiple partially soluble components in order to predict the measured activation diameters, the COA solubility distribution alone could not explain the CCN activity. Surface tensions less than 30 dyn cm−1 were required to explain the measured activation diameters. In addition, COA particles appear to not be spherical, which can introduce uncertainties into the corresponding calculations.

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

Catchments impacted by wildfire typically experience elevated rates of post-fire erosion and formation and deposition of pyrogenic carbon (PyC). To better understand the role of erosion in post-fire soil carbon dynamics, we determined distribution of soil organic carbon in different chemical fractions before and after the Gondola fire in South Lake Tahoe, CA. We analyzed soil samples from eroding and depositional landform positions in control and burned plots pre- and post-wildfire (in 2002, 2003, and 10-years post-fire in 2013). We determined elemental concentrations, stable isotope compositions, and biochemical composition of organic matter (OM) using mid-infrared (MIR) spectroscopy for all of the samples. A subset of samples was analyzed by 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (CPMAS 13C-NMR). We combined the MIR and CPMAS 13C-NMR data in the Soil Carbon Research Program partial least squares regression model to predict distribution of soil carbon into three different fractions: 1) particulate, humic, and resistant organic matter fractions representing relatively fresh larger pieces of OM, 2) fine, decomposed OM, and 3) pyrogenic C, respectively. Samples from the post-fire eroding landform position showed no major difference in soil organic carbon (SOC) fractions one year post-fire. The depositional samples, however, had increased concentrations of all SOC fractions, particularly the fraction that resembles PyC, one year post-fire (2002), which had a mean of 160 g/kg compared with burned hillslope soils, which had 84 g/kg. The increase in all SOC fractions in the post-fire depositional landform position one year post-fire indicates significant lateral mobilization of the eroded PyC. In addition, our NMR analyses revealed a post-fire increase in both the aryl and O-aryl carbon compounds in the soils from the depositional landform position, indicating increases in soil PyC concentrations post-fire. After 10 years, the C concentration from all three fractions declined in the depositional landform position to below pre-fire levels likely due to further erosion or elevated rates of decomposition. Thus, we found, at this site, that both fire and erosion exert significant influence on the distribution of PyC throughout a landscape and its long-term fate in the soil system.

Assessment of Soil Aggradation through Soil Aggregation and Particulate Organic Matter by Riparian Switchgrass Buffers

The restoration of riparian zones has been an important issue in many regions for the recovery of ecosystem functions. The objective of this study was to assess soil aggradation in a 7-year established riparian switchgrass buffer (SGB) and in a non-buffered riparian zone with an annual row crop (ARC). We measured the aggregate size distribution and stability of macroaggregates, aggregate-associated soil organic carbon, soil organic matter fractions and the chemical composition of light particulate organic matter to monitor soil aggregation in a riparian soil following the conversion of agricultural row crops to switchgrass filters. Aggregate size fractions were separated by wet sieving using the aggregate size-stability protocol. The proportion of soil and total organic C was quantified for each aggregate size class. Soil organic matter fractions were isolated by size and density into light particulate organic matter and heavy particulate organic matter and mineral fraction organic matter. The categorization of aggregates by size and water stability (slaking resistance) showed a significantly larger (p < 0.001) proportion of water-unstable large macroaggregates (>2000 µm) under SGB (34%) compared to that under ARC (29%), while the proportion of water-unstable small macroaggregates (250–2000 µm) was significantly higher under ARC (14%) than under SGB (10%). Our results showed that the amounts of light and heavy particulate organic matter did not change in the short-term (7 years) after SGB establishment. It appears that the lower soil stabilization and soil organic C storage under SGB is related to (i) the large number of coarse roots; (ii) lower inputs of light and heavy particulate organic matter; (iii) no changes in the alkyl-C/O-alkyl-C ratio over time; and (iv) light particulate organic matter with a high C/N ratio.

Infrared spectroscopy prediction of organic carbon and total nitrogen in soil and particulate organic matter from diverse Canadian agricultural regions

Infrared spectroscopy has the capacity to predict soil organic carbon (SOC) and total nitrogen (TN) at local/regional scales, but no studies have been conducted to evaluate this technique at a large (cross-regional) scale in Canada. In this paper, mid-infrared (MIR) and near-infrared (NIR) spectroscopies in combination with partial least-squares regression (PLSr) were used to predict SOC and TN in whole soil and in particulate organic matter (POM) fractions on cross-regional, regional, and local scales. Both MIR- and NIR-PLSr models have well estimated SOC [coefficient of determination (R2) = 0.79–0.92, residual prediction deviation/ratio of prediction to deviation (RPD) = 2.19–3.47], TN (R2 = 0.70–0.92; RPD = 1.83–3.50), POM-C (R2 = 0.76–0.96; RPD = 2.04–5.25), and POM-N (R2 = 0.70–0.97; RPD = 1.83–5.78). The prediction efficiency of cross-regional models (R2 = 0.90–0.96; RPD = 3.13–5.49) was similar to or better than the prediction of regional (R2 = 0.70–0.97; RPD = 1.83–5.78) and local models (R2 = 0.7...

Correspondence of measured soil carbon fractions and RothC pools for equilibrium and non-equilibrium states

The link between carbon turnover model pools and measurable carbon fractions is of key interest for initial parameterisation and subsequent validation of dynamic soil carbon models. In this study we performed the established particle-size fractionation of soils from 54 intensively monitored sites in Germany and from archived samples from 5 other long-term experiments in Germany and the United Kingdom. The Rothamsted carbon (RothC) model was then used to compare the measured soil C fractionation from the 54 intensively monitored sites against modelled pools using spin-up equilibrium runs whilst dynamic (non-equilibrium) model runs were performed when comparing data from the long-term experiments. We detected good agreement between measured soil C fractions and modelled pools, indicated by correlation coefficients of 0.73 and 0.81 for the resistant plant material pool (RPM) and 0.91 and 0.94 for the humus pool (HUM) for the intensively monitored and the long-term sites, respectively. Slightly larger errors were detected for the intensively monitored sites together with a bias in the relationship between the RPM pool and particulate organic matter fraction. This bias detected for the intensively monitored sites indicated that the equilibrium assumption for arable agricultural sites, even though under crop cover for at least 50years, might not be entirely valid. From the relative mean absolute error of 11% for the HUM pool and 26% for the RPM pool of the combined data set we conform that the measured fractions can be used to estimate the RothC model pools in arable soils. Given the magnitude of these errors, however, we rather suggest to apply the fractionation approach instead of using an equilibrium assumption for the RothC initialisation of arable sites.

Effects of organic matter fraction and compositional changes on distribution of cadmium and zinc in long-term polluted paddy soils

Soil particulate organic matter (POM) has rapid turnover and metal enrichment, but the interactions between organic matter (OM) and metals have not been well studied. The present study aimed to investigate changes in the OM concentration and composition of the POM fraction and their corresponding effects on metal distribution and extractability in long-term polluted paddy soils. Soil 2000–53 μm POM size fractions had higher contents of C–H and C=O bonds, C–H/C=O ratios and concentrations of fulvic acid (FA), humic acid (HA), cadmium (Cd) and zinc (Zn) than the bulk soils. Cadmium and Zn stocks in soil POM fractions were 24.5–27.9% and 7.12–16.7%, respectively, and were more readily EDTA-extractable. Compared with the control soil, the 2000–250 μm POM size fractions had higher organic carbon concentrations and C/N ratios in the polluted soils. However, there were no significant differences in the contents in C–H and C=O bonds or C–H/C=O ratios of POM fractions among the control, slightly and highly polluted soils. In accordance with the lower contents of C=O bonds and FA and HA concentrations, the Cd and Zn concentrations in 250–53 μm POM size fractions were lower than those in 2000–250 μm POM size fractions. Enrichment of Cd in POM fractions increased with increasing soil pollution level. These results support the view that changes in the OM concentration and the size and composition of POM fractions play a key role in determining the distribution of Cd and Zn in paddy soils.

Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

Abstract. Recent Arctic studies suggest that sea ice decline and permafrost thawing will affect phytoplankton dynamics and stimulate heterotrophic communities. However, in what way the plankton composition will change as the warming proceeds remains elusive. Here we investigate the chemical signature of the plankton-dominated fraction of particulate organic matter (POM) collected along the Siberian Shelf. POM (> 10 µm) samples were analysed using molecular biomarkers (CuO oxidation and IP25) and dual-carbon isotopes (δ13C and Δ14C). In addition, surface water chemical properties were integrated with the POM (> 10 µm) dataset to understand the link between plankton composition and environmental conditions. δ13C and Δ14C exhibited a large variability in the POM (> 10 µm) distribution while the content of terrestrial biomarkers in the POM was negligible. In the Laptev Sea (LS), δ13C and Δ14C of POM (> 10 µm) suggested a heterotrophic environment in which dissolved organic carbon (DOC) from the Lena River was the primary source of metabolisable carbon. Within the Lena plume, terrestrial DOC probably became part of the food web via bacteria uptake and subsequently transferred to relatively other heterotrophic communities (e.g. dinoflagellates). Moving eastwards toward the sea-ice-dominated East Siberian Sea (ESS), the system became progressively more autotrophic. Comparison between δ13C of POM (> 10 µm) samples and CO2aq concentrations revealed that the carbon isotope fractionation increased moving towards the easternmost and most productive stations. In a warming scenario characterised by enhanced terrestrial DOC release (thawing permafrost) and progressive sea ice decline, heterotrophic conditions might persist in the LS while the nutrient-rich Pacific inflow will likely stimulate greater primary productivity in the ESS. The contrasting trophic conditions will result in a sharp gradient in δ13C between the LS and ESS, similar to what is documented in our semi-synoptic study.

Carbon saturation drives spatial patterns of soil organic matter losses under long-term bare fallow

Spatial controls of soil organic carbon (SOC) turnover are not well understood. We hypothesized that spatial patterns of SOC turnover are related to carbon (C) saturation rather than to the size of measurable SOC-pools such as particulate organic matter (POM), determined as SOC in particle-size fractions. Therefore, we repeatedly grid-sampled a field after one, three, seven, and eleven years under bare fallow management, which revealed a spatial gradient from high to low degrees of C saturation. We measured the contents of SOC and the contents of SOC in coarse sand-size (2000–250μm, POM1), fine sand-size (250–53μm, POM2), silt-size (53–20μm, POM3), and fine silt to clay-size fractions (nonPOM, <20μm), calculated the degree of C saturation from textural properties and nonPOM contents, and related these parameters to SOC losses. In the first year of bare fallow, the soil contained on average 12.1g SOC kg−1, of which 0.6gkg−1, 1.7gkg−1, and 2.1gkg−1 were lost after three, seven, and eleven years of bare fallow, respectively. The SOC losses within eleven years were spatially variable and varied between 1% and 46% relative to the initial SOC content. In support of our hypothesis, SOC losses were largest at subsites with largest degrees of C saturation (R2=0.83). Although the POM fractions declined most drastically, they only comprised 4 to 9% of bulk SOC, and they did neither correlate with nor explain spatial patterns of SOC losses. We conclude that the concept of C saturation is superior to conventional physical fractionation approaches for predicting spatio-temporal patterns of SOC turnover at sites with a high degree of C saturation.