Occluded Fraction Research Articles

Changes in land use practices influence soil sulfur fractions and their bioavailability

Changes in land use practices may affect the distribution of soil sulfur (S) fractions and their bioavailability. Therefore, this study was undertaken to assess the influence of different land use changes on the distribution of soil S fractions and their bioavailability for plant nutrition. Soil samples from farmers’ fields with different land use practices such as rice-mustard-jute (R-M-J), rice-lentil-jute (R-L-J), rice-lentil-sesame (R-L-S), rice-vegetable-jute (R-V-J), and rice-potato-green gram (R-P-G) were collected and analyzed for different fractions of S. The bioavailability of S was assessed by extracting the soil with six different extractants (acidic, neutral, and alkaline) with different extraction modes and chemistry. The results showed that changes in land use practices could influence the distribution of soil S fractions and their bioavailability. Organic S was the dominant fraction, accounting for 93.5% of total S across land use practices. The inorganic S fraction (water-soluble, sorbed, and occluded) varied significantly among the land use practices. Among the inorganic fractions, the water-soluble fraction was the dominant fraction across the land use practices. The bioavailability of S, as assessed by different chemical extractants, was in the following order: sodium hydrogen carbonate (NaHCO3) > mehlich-3 > ammonium bicarbonate-diethylenetriamine penta-acetic acid (AB-DTPA) > ammonium acetate-acetic acid (NH4Ac-HOAc) > calcium dihydrogen phosphate {Ca(H2PO4)2} > calcium chloride (CaCl2). By establishing relationships between extractable S and soil S fractions, it was observed that all the extractants could obtain S from the water-soluble, sorbed, and organic S fractions, with little extractability from the occluded fraction. Among the extractants tested, mehlich-3 extracted a similar amount of S corresponding to the inorganic fraction across the land use practices. It also maintained positive relationships with different fractions of S, and as a multi-nutrient extractant, its use in routine soil testing can be recommended.

Applied phosphorus is maintained in labile and moderately occluded fractions in a typical meadow steppe with the addition of multiple nutrients

Phosphorus (P) is a limiting nutrient second only to nitrogen (N) in the drylands of the world. Most previous studies have focused on N transformation processes in grassland ecosystems, particularly under artificial fertilization with N and atmospheric N deposition. However, P cycling processes under natural conditions and when P is applied as an inorganic P fertilizer have been understudied. Therefore, it is essential to examine the fate of applied P in grassland ecosystems that have experienced long-term grazing and, under certain circumstances, continuous hay harvest. We conducted a 3-year field experiment with the addition of multiple nutrient elements in a typical meadow steppe to investigate the fate of the applied P in various fractions of P pools in the top soil. We found that the addition of multiple nutrients significantly increased P concentrations in the labile inorganic P (Lab-Pi) and moderately occluded inorganic P (Mod-Pi) fractions but not in the recalcitrant inorganic P (Rec-Pi) fraction. An increase in the concentration of total inorganic P was found only when P and N were applied together. However, the addition of other nutrients did not change P concentrations in any fraction of the mineral soil. The addition of P and N significantly increased the total amount of P taken up by the aboveground plants but had no effect on the levels of organic and microbial P in the soil. Together, our results indicate that the P applied in this grassland ecosystem is taken up by plants, leaving most of the unutilized P as Lab-Pi and Mod-Pi rather than being immobilized in Rec-Pi or by microbial biomass. This implies that the grassland ecosystem that we studied has a relatively low P adsorption capacity, and the application of inorganic P to replenish soil P deficiency in degraded grasslands due to long-term grazing of livestock or continuous harvest of forage in the region could be a practical management strategy to maintain soil P fertility.

Climatic controls on soil and saprock nitrogen distribution and persistence in the Sierra Nevada

AbstractBackgroundNitrogen (N) is an essential nutrient in soil that regulates plant growth, terrestrial sequestration of atmospheric carbon dioxide, and persistence of organic compounds. However, major knowledge gaps remain about how climate change may impact N accumulation and persistence, especially in deep soil and saprock (friable weakly weathered bedrock).AimsOur objective was to understand how climate impacts the accumulation of N in soil and saprock and how climate impacts soil N distribution, persistence, and mechanisms of N persistence.MethodsWe investigated N concentration in bulk soil and density fractions. We estimated N persistence along a bio‐climatic sequence—sites range from a low‐elevation oak savannah, mid‐elevation pine‐oak/mixed‐conifer forest, to a high‐elevation subalpine forest—in the southern Sierra Nevada in California. A combination of radiocarbon and elemental composition measurements along with a first‐order kinetic model was used.ResultsThe N concentration in the bulk soil and density fractions declined with depth, and there was a relatively greater mineral‐associated heavy fraction (HF) N in deeper samples. The cooler/wetter mixed conifer site held 37% of profile N in saprock, which was greater than that of the entire soil profile at the drier/hotter oak savannah. The majority of N in soil, which was in the HF, was not influenced by climate proxies tested. However, both unprotected and occluded fractions of N were strongly influenced by climate. Soil N mean residence time (MRT) showed that drier/hotter climates have a shorter MRT, compared to mid‐elevation sites with cooler/wetter climates.ConclusionsThe effect of climate on deep saprock N storage might be indirect, primarily through climatic influence on the thickness of saprock. Overall, our findings suggest the mineral‐associated HF N pool will not be vulnerable to changes in climate and will continue to contribute to the persistent soil N pool. The amount of topsoil and subsoil unprotected and occluded N can be explained by gross primary productivity and mean annual precipitation indicating that changes in climate can influence N partitioning. N stored in deep soil and saprock may be less vulnerable to climate than N stored in drier/hotter climates with less deeply stored N. It is critical to dig deeper to understand terrestrial ecosystems’ response to climate.

Biodegradation of occluded hydrocarbons and kerogen macromolecules of the Permian Lucaogou shales, Junggar Basin, NW China

Three kerogen samples (JJZG-1, JJZG-2 and JJZG-3) isolated from the Permian Lucaogou shales of varying biodegradation levels (BLs ≈ 0, 3 and 7, respectively) were subjected to sequential stepwise pyrolysis combined with on-line detection of gas chromatography-mass spectrometry (GC-MS). Occluded fractions (bitumen II) released at low-temperature steps (≤410 °C) show consistent biodegradative signatures with that reported for solvent-extracted fractions (bitumen I) of the original shales, e.g., broad range of abundant n-alkanes, isoprenoids and regular hopanes for the non-biodegraded JJZG-1; trace n-alkanes and abundant hopanes for the moderately biodegraded JJZG-2; and no n-alkanes but still prominent hopanes including the microbially produced 25-nohopanes for the severely biodegraded JJZG-3. This consistency between bitumen II and bitumen I fractions indicates the biodegradability of the kerogen-occluded bitumen II with limited protection from host kerogen. A minor level of protection was suggested by the trace distribution of n-alkanes in the bitumen II of JJZG-2, whereas the bitumen I had no n-alkanes. The kerogen itself was more resistant to biodegradation as reflected by the persistence of high abundances of both n-alkanes and hopanes in the high temperature (≥460 °C) products of all three kerogen samples. However, the relative abundances of these product groups did show some evidence of biodegradation alteration, e.g., ratios of n-C15 alkene/C27 hop-17(21)-ene at 510 °C pyrolysis decreased by order of magnitude from the non-biodegraded (JJZG-1 = 27.4) to highly biodegraded (0.3 for JJZG-3) samples. The reduced biodegradation impact on the kerogen fraction (Cf. bitumen fractions) was also evident by the absence of 25-norhopanes in the high-temperature analysis of the JJZG-3 kerogen.

The changes in the chemical forms of thallium, cobalt and manganese with air-drying of soils

ABSTRACT Thallium (Tl) is a highly toxic metal that induces pathological changes in organs. Many Tl-contaminated soils have been reported worldwide due to the releases of Tl from mineral weathering and mining industry. There is, however, limited information related to the dynamics and analysis of Tl in soil, since Tl is one of the emerging soil contaminants. Tl and cobalt (Co) are specifically adsorbed by manganese (Mn) oxide. On the other hand, air-drying of soils causes the dissolution of Mn oxide and the elution of heavy metals such as Co occluded by Mn oxide. Thus, the Mn oxide dissolution with air-drying is expected to affect the chemical forms of Tl with high sorption affinity to Mn oxide. We conducted the sequential extraction method using moist and air-dried soils. Air-drying caused the increases in Mn and Co in the exchangeable and acid soluble fractions, respectively, and the decreases in those in the Mn oxide occluded fraction, which is consistent with previous reports. Our research revealed that the exchangeable Tl increased and sum of the acid-soluble and Mn oxide occluded Tl decreased due to air-drying. The increments in the exchangeable fraction and the decrements in acid-soluble and Mn oxide occluded fractions were almost the same. Consequently, the increase in exchangeable Tl could be attributed to the other two fractions, indicating that soil Tl is sensitive to the soil drying.

Effects of recovery models on organic carbon pathways: A method using 13C natural abundance

The rehabilitation of disaster-prone areas can enhance the stability of soil structure and is a common way to increase organic carbon storage. The response of soil carbon sequestration pathways to different recovery modes is not clear, especially in mountain soils. After 11 years of recovery, we evaluated soil organic carbon (SOC) sequestration pathways in plantations (dominated by Olea europaea ‘Leccino’), croplands [Zea mays (L.)] natural shrublands (Lycium chinense Mill), and natural grasslands [Setaria viridis (L.) Beauv]. The physical and chemical properties of the soil and the 13C natural abundance of each aggregate and its density components were studied. The results showed that, during the restoration process, the soil organic carbon content of natural grassland increased the most, while the δ13C value of soil of natural shrubs was the highest. The natural abundance of 13C was used to reveal the pathway of C flow in soil organic matter (SOM), as follows: free light fractions (ρ < 1.6 g cm−3)→mineral fractions (ρ > 2.0 g cm−3)→dense occluded fractions (ρ from 1.6 to 2.0 g cm−3) (in plantation, natural shrubland, and grassland). However, in cropland soil, C flowed as follows: mineral fractions→free light fractions→dense occluded fractions. Specifically, the SOC content decreased with aggregate particle size, and after entering the soil, plant litter was first stored in large aggregates and then decomposed into the free light fraction. The study revealed the mechanism of organic carbon sequestration in the restoration area, emphasizing that artificial restoration treatment can change the carbon conversion pathway, and reduced the sequestration of organic carbon.

Long‐term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland

Burning has commonly been used to increase forage production and nutrients cycling in grasslands. However, its long-term effects on soil organic carbon (SOC) and nitrogen (N) pools within the aggregates and the relation between aggregates-associated SOC and soil CO2 emissions need further appraisal. This study evaluated the effects of 64years of annual burning on SOC and N dynamics compared to annual mowing and undisturbed treatments in a grassland experiment established in 1950. Soils were sampled from four depths representing the upper 30cm layer and fractionated into macroaggregates, microaggregates and silt+clay fractions. The macroaggregates were further fractionated into three occluded fractions. The SOC in the bulk soil and aggregates were correlated to soil CO2 effluxes measured under field conditions. Compared to the undisturbed treatment, annual burning decreased aggregates stability, SOC and N in the upper 30cm layer by 8%, 5% and 12%, respectively. Grassland mowing induced greater aggregates stability than burning only in the upper 5cm. Burning also decreased SOC in the large macroaggregates (e.g., 0-5cm) compared to mowing and the undisturbed grasslands but proportionally increased the microaggregates and their associated SOC. Soil N associated with aggregates decreased largely following grassland burning, for example, by 8.8-fold in the microaggregates within the large macroaggregates at 20-30cm compared to the undisturbed grassland. Burning also increased soil CO2 emissions by 33 and 16% compared to undisturbed and mowing, respectively. The combustion of fresh C and soil organic matter by fire is likely responsible for the low soil aggregation, high SOC and N losses under burned grassland. These results suggested a direct link between grass burning and SOC losses, a key component for escalating climate change severity. Therefore, less frequent burning or a rotation of burning and mowing should be investigated for sustainable grasslands management.

Changes in the Fractional Composition of Organic Matter in the Soils of the Forest–Steppe Zone during Their Postagrogenic Evolution

The postagrogenic dynamics of organic carbon (Corg), total nitrogen (Ntot), and density fractions of organic matter (OM) in the dark gray soil (Haplic Phaeozem, Belgorod oblast, Les na Vorskle (Forest on Vorskla) Reserve) and migrational–mycellary Chernozem (Haplic Chernozem, Kursk oblast, Streletskaya Steppe Reserve) were examined. The most significant postagrogenic changes in all the studied soil properties were observed in the top 0–5 cm layer. The contents of Corg and Ntot in this layer of the Phaeozem chronosequence increased by 1.2–1.3 times over 45 years after cessation of farming. The contents of Corg and Ntot in the top 0–5 cm layer of Chernozem chronosequence increased by 1.9–2.0 times over 60 years of the postagrogenic development. The accumulation of Corg took place in all density fractions. In both chronosequences, the free fraction of organic matter (density <1.6 g cm–3) increased, while the occluded fraction (density <2.0 g cm–3) virtually did not change. In 60 years after the cessation of farming on Chernozem, all the studied parameters became close to those in the Chernozem under natural steppe. In the Phaeozem, these parameters recovered by no more than 60% of the level typical for the natural soil over 45 years after the cessation of farming. Thus, the postagrogenic restoration of organic matter and all its fractions in the forest-steppe zone was much faster in the Chernozem in comparison with the Phaeozem.

Effects of Organic Wastes on Soil Organic Carbon and Surface Charge Properties in Primary Saline-alkali Soil

High salinity and low fertility have restricted crop production in primary saline-alkali soils. Soil organic carbon (SOC) and surface charge characteristics affect the soil fertility and soil colloid characteristics of primary saline-alkali soils, respectively. In this paper, the SOC and surface charge properties of primary saline-alkaline soil under organic wastes applications were assessed. Five treatments were involved in this experiment: chemical fertilizer combined with sheep manure (SM), corn straw (CS), fodder grass (FG), and granular corn straw (GS), while chemical fertilizer only was used as control (CK). The content of SOC was significantly different under different organic wastes application (p &lt; 0.05). Treatment GS recorded the highest content of SOC compared with the other treatments. In addition, the content of each SOC density fraction increased after the application of organic wastes. Similarly, the application of organic wastes, increased the proportion of organic carbon in free light fraction (Fr-FLOC) and organic carbon in occluded fraction (Oc-FLOC) in the soil however the proportion of organic carbon in heavy fraction (HFOC) decreased. In this study, we found that treatment GS has a greater impact on soil surface charge properties than other treatments, and through redundancy analysis (RDA) the content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were identified as the main factors affecting the surface charge properties of soil organic carbon. In conclusion, GS is the recommended organic waste for ameliorating primary saline-alkali soil, as compared to the other organic waste treatments.

Quantifying the relationships between soil fraction mass, fraction carbon, and total soil carbon to assess mechanisms of physical protection

Relationships between soil fractions (their mass or carbon (C)) and soil organic carbon (SOC) have been used to develop central ideas in SOC research. However, few attempts have been made to quantify the relationship between SOC and all soil fractions, despite the potential for such an effort to address SOC stabilization processes. We identified 41 published studies that used diverse management techniques to cause a change in SOC concentration and disrupted soil into macroaggregates (>250 μm), free microaggregates (53–250 μm) and free silt + clay (<53 μm), subsequently disrupting macroaggregates into constituent fractions (coarse particulate organic matter [cPOM] > 250 μm, occluded microaggregates, and occluded silt + clay). We used linear hierarchical models to quantify relationships between mass, C concentration and total C of fractions and SOC. Soil mass redistribution toward macroaggregates was associated with SOC accumulation, however total microaggregate mass (free + occluded) did not increase with macroaggregate mass, as would be expected given de novo microaggregate formation within macroaggregates. Instead, high SOC soils exhibited a greater percent of total microaggregates occluded in macroaggregates. Occlusion in macroaggregates was also associated with increased C concentrations of microaggregates (35% higher, SE = 3.2) and silt + clay (30% higher, SE = 3.9) relative to their free counterparts. Taken together, these relationships suggest reduced macroaggregate turnover promotes SOC accumulation via the stabilization of C into occluded fractions. Rates of SOC increase with silt + clay C concentrations failed to increase with mean site-level SOC concentration, indicating of the studied soils (median SOC concentration = 14 g kg−1; max 68), SOC accumulation appears unlikely to be limited by C storage capacity in the silt + clay fraction. For each unit SOC gain, macroaggregates accounted for 83% (95% CI = 74, 91), and occluded microaggregates for 43% (95% CI = 33, 52), consistent relationships that have potential to be used as benchmarks for fraction-based SOC models.

Impact of Manure and Fertilizers on Chemical Fractions of Zn and Cu in Soil under Rice-Wheat Cropping System

The present study was conducted in a long-term field experiment (in progress since kharif 2009–10) at Department of Soil Science, Punjab Agricultural University, Ludhiana. The objective of the study was to investigate the impact of organic manure (biogas slurry) and fertilizer application on chemical fractions of zinc (Zn) and copper (Cu) under rice-wheat system. Treatments applied to rice-wheat system were arranged in a split-plot design with three replications. Main plot treatments applied to rice were four combinations of manure and fertilizers viz., nitrogen (N) @ 120 kg ha−1 without manure and phosphorus (P) fertilizer (M0N120P0), N @ 80 kg ha−1, P @ 30 kg ha−1 and manure @ 6 t ha−1 (M6N80P30), N @ 120 kg ha−1, P @ 30 kg ha−1 and no manure (M0N120P30) and N @ 80 kg ha−1, manure @ 6 t ha−1 without P-fertilizer (M6N80P0). Three sub-plot treatments applied in wheat included N @ 120 kg ha−1, K @ 30 kg ha−1 and three levels of Pfertilizer viz., P @ 30 kg ha−1 (P30), P @ 60 kg ha−1 (P60) and P @ 0 kg ha−1 (P0). Results revealed that the concentration of Zn and Cu was higher under the treatments where biogas slurry was applied along with fertilizers as compared to the sole application of fertilizers. The increase in water soluble and exchangeable (WSEX) fractions in organically treated plots indicated their mobilization from occluded fractions viz., crystalline iron oxide (CFeOX) and amorphous iron oxide (AFeOX) bound fractions. The residual (RES) fraction was the dominant portion of total Zn and Cu content of soil; whereas the contribution of WSEX fraction was limited. Organic matter bound fractions of Zn and Cu were the most important contributing towards their uptake by both rice and wheat crops.

Phosphorus cycling within soil aggregate fractions of a highly weathered tropical soil: A conceptual model

Effective use of soil phosphorus (P) for crop production requires an understanding of how P pools are stabilized and cycled within soil aggregates, rather than assuming that P dynamics, particularly organic P, closely follow those of C. The main goal of this study was to compare C and N cycling with P dynamics in soil aggregate fractions under two distinct crop species, maize (Zea mays) and pigeon pea (Cajanus cajan) in a highly-weathered Lixisol. We found that while C and N follow an open cycle, whereby C and N are mineralized from microaggregates during macroaggregate turnover and partially exit the soil system as gas and leachate, P has a relatively closed cycle, where most of the mineralized and solubilized P from microaggregates is lost from the plant-available pool via sorption to the unaggregated silt and clay-sized particles (<53 μm). While the above postulated P cycling mechanisms were the same for maize and pigeon pea, P loss from microaggregates and subsequent enrichment of the silt and clay particles was significantly higher in soils under maize compared to pigeon pea (320 and 331 mg P kg−1 lost from occluded microaggregates and gained by free silt and clay particles, respectively, compared to 129 and 97 mg P kg−1 under pigeon pea). This is attributed to the significantly increased soil aggregation under pigeon pea, which led to greater accumulation of P, particularly organic P, in the free microaggregates (77 mg P kg−1 compared to 29 mg P kg−1) and slower rates of macroaggregate turnover. We conclude that increasing soil aggregation can substantially reduce organic P losses from aggregate occluded fractions and its subsequent sorption as inorganic P to silt and clay particles. Thus, P cycling can be improved in tropical cropping systems on highly-weathered soils by introducing crop species that enhance the occlusion of organic P into aggregates.

Chemical Fractions of Mn in Acidic Soils and Selection of Suitable Soil Extractants for Assessing Mn Availability to Maize (Zea Mays L.)

ABSTRACTTwenty surface (0–15 cm) samples of acidic soils were analyzed for water soluble (WS), exchangeable (EX), lead displaceable (Pb-disp.), acid soluble (AS), manganese (Mn) oxide occluded (MnOX), organically bound (OB), amorphous Fe oxide occluded (AFeOX), crystalline iron (Fe) oxide occluded (CFeOX) and residual (RES) fractions of Mn, and also for extractable Mn in some common soil extractants: (diethylenetriaminepentaacetic acid (DTPA) (pH 7.3), DTPA (pH 5.3), AB-DTPA (pH 7.6), Mehlich-3 (pH 2.0), Modified Olsen, 0.005 M calcium chloride (CaCl2), 1 M magnesium chloride (MgCl2) and ion exchange resins. The WS-Mn fraction showed a significant and positive correlation with Mn extractable in DTPA (pH 5.3) and AB-DTPA (pH 7.6), while both WS-Mn and EX-Mn fractions correlated significantly and positively with Mn concentration and uptake by maize plants grown in these soils. The AB-DTPA (pH 7.6) and DTPA (pH 5.3) appeared suitable to assess the availability of Mn in acidic soils.

Fallow associated with autumn-plough favors structure stability and storage of soil organic carbon compared to continuous maize cropping in Mollisols

Aggregate formation and stability of soil organic carbon (SOC) differ in different farming systems, probably due to differences in effects of tillage and residue management. This study used a 24-year field experiment to compare the effects of continuous maize cropping and natural fallow on aggregate formation and SOC storage in various aggregate-size classes and density fractions of a Chinese Mollisol. Soils collected from the upper 0.2-m layer were wet-sieved into four aggregate-size classes (>2, 0.25–2, 0.053–0.25 and 0.25 mm) and increased that of micro-aggregates (<0.25 mm) compared to the initial value while the opposite was observed in the natural fallow system. The fallow system generally had greater SOC concentration in the occluded fraction, higher proportion of newly-derived C as % total SOC in the light fraction and greater contribution of total residue C to new C in macro-aggregates and light fractions compared to the continuous maize system. Furthermore, the fallow system resulted in shorter turnover time of SOC than the continuous maize system. Natural fallow associated with autumn-plough improved soil structural stability and SOC storage while continuous maize cropping with residue removal decreased SOC sequestration and soil aggregate stability.

Thermal Evolution of Adsorbed/Occluded Hydrocarbons inside Kerogens and Its Significance As Exemplified by One Low-Matured Kerogen from Santanghu Basin, Northwest China

Organic compounds could be adsorbed and even occluded within the macromolecular structures of kerogen. Studies concerning thermal evolution of adsorbed/occluded hydrocarbons inside kerogens will be helpful in understanding its structural characteristics and evolution features and estimating the effectiveness of adsorbed and occluded fractions. In this work, adsorbed and occluded hydrocarbons have been released from a low-matured kerogen from the upper Permian Lucaogou formation of Santanghu Basin, northwest China, and from its pyrolysis residues by solvent extraction and oxidative treatment, respectively. The results showed that some n-alkanes, terpanes, and steranes were detected from both adsorbed and occluded fractions. Series of even-carbon-numbered n-alk-(1)-enes were also determined from the occluded components. The early stage of thermal evolution showed similarities in biomarker features for both adsorbed and occluded hydrocarbons; however, variations were noted in biomarker compositions with increasing thermal levels. The obvious changes of C21–22 pregnane/C27–29 sterane ratios from adsorbed to occluded fractions at high thermal evolution suggested that high concentrations of pregnane (containing homopregnane) in soluble components in highly or overly mature stage may be mainly derived from the corresponding components covalently bound within kerogen. Occluded components exhibited stronger thermal stability than adsorbed components. This study showed that evaluation of the thermal evolution characteristics of kerogens/source rocks based on biomarkers compositions from the adsorbed components at highly to overly matured stages was difficult, while we hope this kind of assessment of kerogens/source rocks can be achieved from occluded components.

Carbon pool size and stability are affected by trees and grassland cover types within agroforestry systems of western Canada

Agroforestry systems are common land uses across Canada and could play a substantial role in sequestering carbon (C) as part of efforts to combat climate change. We studied the impact of component land cover types (forested vs. adjacent herbland) in three agroforestry systems (hedgerow, shelterbelt and silvopasture) on organic C and nitrogen (N) distribution in three density fractions of soils at the 0–10 and 10–30cm layers. The study evaluated 36 sites (12 hedgerows, 12 shelterbelts and 12 silvopastures) in central Alberta, Canada, distributed along a soil/climate gradient of increasing moisture availability. At the 0–10cm layer, soil organic C (SOC) stock in the bulk soil was significantly greater in the silvopasture system (101) than in either the hedgerow (77) or shelterbelt system (67Mg Cha−1). Soil organic C stock in both soil layers (0–10 and 10–30cm) was also significantly greater in the forested land cover (89 and 119Mg Cha−1, respectively) than in adjacent herblands (76 and 77Mg Cha−1). Across all sites, 31.5, 29.1, and 35.5% of SOC was found in the light fraction (<1.6gcm−3), occluded fraction (ultrasonic dispersion at 360W for 5min, <1.6gcm−3), and heavy fraction (>1.6gcm−3) of soils, respectively. The largest pool of SOC in the more labile light fraction of the 0–10cm layer was in the silvopasture system (50Mg Cha−1), whereas the smallest labile light fraction component of SOC was in the shelterbelt system (17Mg Cha−1). The largest pool of SOC in the more stable heavy fraction of both the 0–10 and 10–30cm depth classes was in the shelterbelt (33 and 35Mg Cha−1, respectively), while the least SOC was in the silvopasture system (26 and 20Mg Cha−1, respectively). We conclude that the presence of Populus based silvopasture system can increase C storage in surface mineral soils, and that the establishment of Picea based shelterbelts in an otherwise annually cropped agricultural landscape enhances the size of the stable SOC pool.

Pyrogenic alterations of Podzols at the North-east European part of Russia: Morphology, carbon pools, PAH content

Fires are a natural factor for taiga ecosystems of the North-European part of Russia. Pine forests on sandy soils often suffer from fires. The paper surveys in what extent fires in lichen pine forests (burnt 2, 10 and 16years ago) affect soils and soil organic matter (SOM). Ground fires in lichen pine forests fully disturb soil litters. Changes in soil morphology of pyrogenic origin are observed in the topsoil down to depth of 20–30cm. The greatest changes were found in soil 2-year fire affected site. The physicochemical soil properties of the sites affected by fire 10 and 16years ago were similar to those at the reference sites. There are no great after-fire changes in nitrogen and carbon stocks in the soils. The concentration (and stocks) of total carbon and nitrogen in soil litter decreases at the first after-fire years. Mineral soil horizons, particularly the upper mineral layer, become enriched with combustion products, i.e. pyrogenic carbon and nitrogen of free and occluded fractions. There is a Ctot. increase in the upper mineral soil horizons possibly due to black carbon or to OM absorbed at the charcoal particles. Fires increase content of Polycyclic Aromatic Hydrocarbons (PAHs) in the soils, their concentration in the upper subhorizon of forest litter exceeds that of the background by 2–9 times. This increase is due to the presence of di- and trinuclear polyarenes.

Pasture degradation decreases organic P content of tropical soils due to soil structural decline

Degradation of tropical pastures on highly weathered soils is linked to soil structural decline and associated losses of organic matter, which could both also affect soil phosphorus (P) dynamics and availability. Our aim was to elucidate these linkages by examining the effect of pasture degradation on the contents and forms of P contained within aggregate size classes and macroaggregate fractions (collectively referred to as soil structural components). We conducted a study on nine farms in a deforested part of the Colombian Amazonia, each farm with degraded and productive Brachiaria spp. pastures. Topsoil (0–10cm) samples were separated by wet-sieving into aggregate size classes, and macroaggregates were further separated into occluded fractions. Soils and structural components were analyzed for concentrations of total P, available P (extracted using anion exchange resins), NaOH–EDTA extractable organic and inorganic P, and enzyme-hydrolyzable organic P (simple monoester-like, DNA-like, myo-inositol hexakisphosphate-like) in the NaOH–EDTA extracts. Degraded pasture soils had significantly fewer large macroaggregates and more microaggregates, both with significantly lower organic P concentrations (in mgPkg−1 of structural component) than those in productive pasture soils. At the same time, total and extractable inorganic P concentrations in bulk soil and structural components did not differ between pasture types, suggesting a shift from organic to non-extractable P upon degradation. Soils under productive pastures contained 37% more organic P than degraded pasture soils (in mgPkg−1 soil), mainly in large macroaggregates. The organic P concentrations were strongly correlated with C concentrations across all soil structural components, suggesting similar stabilization mechanisms for organic P and C. In bulk soil and most structural components, around 60% of organic P was enzyme-hydrolyzable. The lower contents of all enzyme-hydrolyzable as well as enzyme-stable organic P in degraded vs. productive pasture soils indicate a reduction of all organic P classes during pasture degradation. The large macroaggregates and, in particular, microaggregates occluded within this fraction were identified as an important site for organic P storage. Our results highlight a linkage between soil structure and organic P, both of which can play an important role in maintaining the productivity of pastures established on highly weathered soils.

Factors affecting the molecular structure and mean residence time of occluded organics in a lithosequence of soils under ponderosa pine

Occluded, or intra-aggregate, soil organic matter (SOM) comprises a significant portion of the total C pool in forest soils and often has very long mean residence times (MRTs). However, occluded C characteristics vary widely among soils and the genesis and composition of the occluded organic matter pool are not well understood. This work sought to define the major controls on the composition and MRT of occluded SOM in western U.S. conifer forest soils with specific focus on the influence of soil mineral assemblage and aggregate stability. We sampled soils from a lithosequence of four parent materials (rhyolite, granite, basalt, and dolostone) under Pinus ponderosa. Three pedons were excavated to the depth of refusal at each site and sampled by genetic horizon. After density separation at 1.8 g cm−3 into free/light, occluded and mineral fractions, the chemical nature and mean residence time of organics in each fraction were compared. SOM chemistry was explored through the use of stable isotope analyses, 13C NMR, and pyrolysis GC/MS. Soil charcoal content estimates were based on 13C NMR analyses. Estimates of SOM MRT were based on steady-state modeling of SOM radiocarbon abundance measurements. Across all soils, the occluded fraction was 0.5–5 times enriched in charcoal in comparison to the bulk soil and had a substantially longer MRT than either the mineral fraction or the free/light fraction. These results suggest that charcoal from periodic burning is the primary source of occluded organics in these soils, and that the structural properties of charcoal promote its aggregation and long-term preservation. Surprisingly, aggregate stability, as measured through ultrasonic dispersion, was not correlated with occluded SOM abundance or MRT, perhaps raising questions of how well laboratory measurements of aggregate stability capture the dynamics of aggregate turnover under field conditions. Examination of the molecular characteristics of the occluded fraction was more conclusive. Occluded fraction composition did not change substantially with soil mineral assemblage, but was increasingly enriched in charcoal with depth relative to bulk SOM. Enrichment levels of 13C and 15N suggested a similar degree of microbial processing for the free/light and occluded fractions, and molecular structure of occluded and free/light fractions were also similar aside from charcoal enrichment in the occluded fraction. Results highlight the importance of both fire and aggregate formation to the long-term preservation of organics in western U.S. conifer forests which experience periodic burning, and suggest that the composition of occluded SOM in these soils is dependent on fire and the selective occlusion of charcoal.

Spatial and seasonal sediment phosphorus species and its relation with granulometry, organic matter and CaCO3in a tropical estuary

ABSTRACT Gaspar, F.L; Montes, M. de J.F; Nascimento-Filho, G.A.;Lins, I.C.; Paulo, J.G.; Longo, A.P. 2013. Spatial and seasonal sediment phosphorus species and its relation with granulometry, organic matter and CaCO3 in a tropical estuary In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) The identification of phosphorus species in sediments is a useful approach for the assessment of potential sources or sinks of phosphorus in estuarine regions, and helps to understand the biogeochemical processes that control the availability of phosphorus in tropical areas. This study aimed to identify the spatial and seasonal distribution of the occluded, bioavailable and organic phosphorus fractions, in sediments of a polluted estuary (Botafogo river), and other considered as a background area (Carrapicho river), and determine the correlations between the concentration of phosphorus in sediments with the grain size, calcium carbonate, organic matter, dissolved phosphorus and salinity. No significan...