Soil Pools Research Articles

Patterns and drivers of atmospheric nitrogen deposition retention in global forests.

Forests are the largest carbon sink in terrestrial ecosystems, and the impact of nitrogen (N) deposition on this carbon sink depends on the fate of external N inputs. However, the patterns and driving factors of N retention in different forest compartments remain elusive. In this study, we synthesized 408 observations from global forest 15N tracer experiments to reveal the variation and underlying mechanisms of 15N retention in plants and soils. The results showed that the average total ecosystem 15N retention in global forests was 63.04 ± 1.23%, with the soil pool being the main N sink (45.76 ± 1.29%). Plants absorbed 17.28 ± 0.83% of 15N, with more allocated to leaves (5.83 ± 0.63%) and roots (5.84 ± 0.44%). In subtropical and tropical forests, 15N was mainly absorbed by plants and mineral soils, while the organic soil layer in temperate forests retained more 15N. Additionally, forests retained more than , primarily due to the stronger capacity of the organic soil layer to retain . The mechanisms of 15N retention varied among ecosystem compartments, with total ecosystem 15N retention affected by N deposition. Plant 15N retention was influenced by vegetative and microbial nutrient demands, while soil 15N retention was regulated by climate factors and soil nutrient supply. Overall, this study emphasizes the importance of climate and nutrient supply and demand in regulating forest N retention and provides data to further explore the impacts of N deposition on forest carbon sequestration.

Determination of Soil Carbon Pools in Selected Soils of Abia State, Nigeria

Soil carbon pools exist in Abia State soils but variation, quantification and speciation remain unascertained. This study was carried out on soils collected from two locations (Umuahia North and Ikwuano LGAs) under three land use systems: arable land (AL), forest land (FL) and palm plantation (PP). The aim of the study was to assess carbon pools in the study soil. Laboratory data was subjected to analysis of variance (ANOVA) in Randomized Complete Block Design (RCBD). Significantly different means were separated using Fisher’s least significant difference (F-LSD) at 5% level of probability (p≤0.05). Correlation analysis was used to assess the relationship between the selected soil properties. It was observed that organic carbon increased significantly (p<0.05) 2.47 under forest land in Umuahia North and decreased to 0.97 under arable land in Ikwuano, permanganate oxidizable carbon (POC) increased significantly in Umuahia North (p<0.05) while decreasing in Ikwuano, the POC interaction on the both locations were significant, water soluble organic carbon (WSOC) and microbial biomass carbon (MBC) were neither significant in location, land use or their interaction.

Organic carbon in Mollisols of the world − A review

Mollisols represent 29 % of agricultural land and they are considered to be one of the most fertile soils in the world. Here, we compare soil organic carbon (SOC) concentrations and pools of Mollisols for the globe, the USA and Poland, and review how differences are caused by climate, land use, and key environmental factors. Globally, the mean thickness of the A horizon in Mollisols is 50 cm. At 0–30 cm the mean SOC concentration is 2.3 %, SOC pool is 84 t ha−1, and clay content calculated at 0–50 cm soil depth is 21 %. Mollisols in the USA have an A horizon thickness of 36 cm and have a mean clay content of 27 % (0–50 cm). SOC concentrations are 2.0 and 1.7 % and SOC pools are 85 and 116 t ha−1 at 0–30 and 0–50 cm soil depth, respectively. Mollisols in Poland have SOC concentrations of 1.8 and 1.5 % at 0–30 and 0–50 cm soil depth, respectively, and lower clay content (17 %) at 0–50 cm depth. The SOC pool at 0–30 cm depth is 74 t ha−1 whereas it is 106 t ha−1 at 0–50 cm. At the global scale, the highest SOC concentrations and pools are in Mollisols from Eastern Europe (including Ukraine and Western Russia) and Asia, while the lowest SOC pools are found in Mollisols from South America. The Mollisols in Western and Central Europe and North America have similar SOC pools, although Mollisols in North America have higher SOC concentrations and lower A horizon thickness. Globally, the mean pH value of Mollisols is 7.1, and the pH is slightly lower in Mollisols of the USA (6.9 ± 0.9). The SOC concentrations and pools are strongly and positively correlated with clay content. Soil moisture and temperature regimes determine SOC concentration and pools in Mollisols, and higher SOC concentrations and pools are in Mollisols with frigid and frigid-cryic soil temperature regimes as well as aquic, xeric, and aridic soil moisture regimes. Mollisols under grassland have the largest SOC pools compared to those cultivated or under forest. The important environmental factors on SOC concentrations and pools in Mollisols worldwide are soil texture, land use, and soil temperature regime.

Long-term fertilization increases soil but not plant or microbial N in a Chihuahuan Desert grassland

Abstract. Although the negative consequences of increased nitrogen (N) supply for plant communities and soil chemistry are well known, most studies have focused on mesic grasslands, and the fate of added N in arid and semi-arid ecosystems remains unclear. To study the impacts of long-term increased N deposition on ecosystem N pools, we sampled a 26-year-long fertilization (10 g N m−2 yr−1) experiment in the northern Chihuahuan Desert at the Sevilleta National Wildlife Refuge (SNWR) in New Mexico. To determine the fate of the added N, we measured multiple soil, microbial, and plant N pools in shallow soils at three time points across the 2020 growing season. We found small but significant increases with fertilization in soil-available NO3--N and NH4+-N, yet the soil microbial and plant communities do not appear to be taking advantage of the increased N availability, with no changes in biomass or N content in either community. However, there were increases in total soil N with fertilization, suggesting increases in microbial or plant N earlier in the experiment. Ultimately, the majority of the N added in this multi-decadal experiment was not found in the shallow soil or the microbial or plant community and is likely to have been lost from the ecosystem entirely.

Global pattern of organic carbon pools in forest soils.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Bio-stimulating effect of endophytic Aspergillus flavus AUMC 16068 and its respective ex-polysaccharides in lead stress tolerance of Triticum aestivum plant

Heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety. As a result, metal-induced phytotoxicity concerns require quick and urgent action to retain and maintain the physiological activities of microorganisms, the nitrogen pool of soils, and the continuous yields of wheat in a constantly worsening environment. The current study was conducted to evaluate the plant growth-promoting endophytic Aspergillus flavus AUMC 16,068 and its EPS for improvement of plant growth, phytoremediation capacity, and physiological consequences on wheat plants (Triticum aestivum) under lead stress. After 60 days of planting, the heading stage of wheat plants, data on growth metrics, physiological properties, minerals content, and lead content in wheat root, shoot, and grains were recorded. Results evoked that lead pollution reduced wheat plants’ physiological traits as well as growth at all lead stress concentrations; however, inoculation with lead tolerant endophytic A. flavus AUMC 16,068 and its respective EPS alleviated the detrimental impact of lead on the plants and promoted the growth and physiological characteristics of wheat in lead-contaminated conditions and also lowering oxidative stress through decreasing (CAT, POD, and MDA), in contrast to plants growing in the un-inoculated lead polluted dealings. In conclusion, endophytic A. flavus AUMC 16,068 spores and its EPS are regarded as eco-friendly, safe, and powerful inducers of wheat plants versus contamination with heavy metals, with a view of protecting plant, soil, and human health.

Green manuring alters reactive N losses and N pools in arable soils: A meta-regression study

Green manuring is a conservation agricultural practice that improves soil quality and crop yield. However, increasing the active nitrogen (N) and carbon (C) pools during green manure (GM) amendment may accelerate soil N transformation and stimulate N loss. Previous studies have reported the effects of cover crop incorporation on N2O emission; however, the driving mechanisms and other N losses remain unclear. Therefore, we conducted a comprehensive meta-analysis of 109 published articles (517 paired observations) to clarify the effects of GM amendment on soil reactive N (Nr) losses (N2O emissions, NH3 volatilization, and N leaching and runoff), N pools, and N cycling functional gene abundance. The results showed that green manuring increased soil microbial biomass N (MBN) and NO3−-N concentrations and stimulated N2O emission but significantly lowered N leaching and yield-scaled NH3 volatilization. Practices of green manuring made a dominant contribution to the variation in N2O emissions and NH3 volatilization after GM application. Furthermore, applying legume-based GM, using N derived from GM (GMN) as an additional input, and short-term GM amendment each stimulated N2O emissions. In contrast, adopting non-legume GM, using GMN to partially substitute mineral N, and applying GM to the soil surface or paddy field mitigated NH3 loss during GM amendment. Additionally, the variation in NH3 volatilization was positively related to soil pH and N application rate (NAR) but had a negative relationship with mean annual precipitation (MAP). This study highlighted the marked effects of green manuring on soil N retention and loss. Agricultural operations that adopt GM amendment should select suitable GM species and optimize mineral N inputs to minimize N loss.

Unlocking the Secrets of Soil Health: Exploring the Influence of Natural and Synthetic Nutrients on Soil Organic Carbon and Active & Passive Carbon Pool in Vertisols

The experiment carried out in this field study was between 2022 and 2023 at the (AICRP) research farm in the outward-bound farm of the College of Agriculture, Indore. Indore is a small city in the western part of Madhya Pradesh, Central India, situated on the Malwa Plateau. An experiment was conducted with three replication by using different combination of N, P levels and crop residues in RBD design. Experiment field was medium to black soil with pH of 7.63 and medium in organic carbon 0.58%. The soil was low in available nitrogen (208.0 kg/ha) medium in available phosphorus (21.0 kg/ha) and high in potassium (585 kg/ha) and divided into gross plot size of 10 x 7.2 m2 and a net plot size of 9 x 6.4 m2, respectively. During crop harvest four different depths were used to gather the soil samples: 0–10, 10–20, 20–30, and 30–40 cm. All the collected samples are examined separated for soil organic carbon, active (highly labile, labile SOC), and passive (non-labile, less labile SOC) carbon pools. The finding showed that maximum soil organic carbon (%) was observed with the application of treatment T6 treatment (FYM 6 t ha-1 + N20 P13. Similar trend was found with active (very labile, labile) and passive carbon (less labile, non-labile) pools of soil.

Carbon stock increase during post-agricultural succession in central France: no change of the superficial soil stock and high variability within forest stages

Forest development following agricultural abandonment concerns extensive areas including the Massif Central region of France where this study was undertaken. This land-use and land-cover change is expected to have effects on biodiversity and ecosystem services, including an increase of carbon sequestration—a major concern in the face of climate change. Nevertheless, uncertainties about carbon stock changes during successions are remaining, especially as to the total stock and the contribution of the different carbon pools. Our work contributes to this field by studying carbon stocks in multiple plots of different successional stages. We measured and estimated carbon stocks in aboveground and belowground vegetation, deadwood, litter and superficial soil, and surveyed plant communities and plot conditions (slope, aspect, soil characteristics). The average total carbon stock increased along the succession from 70.60 at stage 0 to 314.19 tC ha−1 at stage 5. However, the total carbon stocks at the young forest stage (abandoned for 74 years maximum) and the older forest stage (forested for at least 74 years) were not significantly different, and probably reflected strong local heterogeneity in the older forest stage. An increase of the carbon stock was found in all pools, except the soil pool that did not vary significantly between the successional stages. The aboveground carbon stock was found strongly related to the woody species cover, especially the macrophanerophyte cover. This case study supports the view that the succession dynamics of former agricultural plots participates in carbon sequestration, sometimes with great local variations.

Isotopic Evidence for Degradation of Particulate Black Carbon in the Ocean

AbstractBlack carbon (BC) has emerged as an integral part of the global carbon (C) cycle, constituting 12% ± 5% of the organic C pool in rivers and soils, with the potential to generate negative climate feedback. However, its ability to sequester C depends on the recalcitrant nature of BC in the environment, which is under debate. Using CTO‐375 method and by measuring concentrations and isotopic compositions of particulate BC (δ13CPBC), we explore the transformation of particulate black carbon (PBC) along the atmosphere‐river‐ocean continuum. Significantly high δ13CPBC in the ocean compared to rivers and atmospheric particulate matter indicates (a) degradation of PBC, potentially through photodegradation and leaching, and/or (b) availability of an enriched source other than fluvial or aeolian inputs. This evidence for degradation of PBC in aquatic systems warrants rethinking on its C sequestration potential and role in aquatic C biogeochemistry and further raises concerns regarding the use of sedimentary BC as a paleoenvironmental proxy.

Does microbial carbon use efficiency differ between particulate and mineral‐associated organic matter?

Abstract Microbial carbon use efficiency (CUE), a key parameter to characterize microbial carbon conversion efficiency, is assumed to be similar in soil models for different soil functional pools with varied organic matter composition and nutrient availability, that is, particulate organic matter (POM) and mineral‐associated organic matter (MAOM). However, empirical studies comparing microbial CUE in POM versus MAOM are largely lacking. It is not known whether microbial CUE variance may underpin the variant behaviour (i.e. turnover and composition) of different soil functional pools. Here we collected surface soils from 25 natural forests and grasslands with divergent edaphic properties, and compared microbial CUE in their POM versus MAOM using soil fractionation in combination with incubation using 18O‐labelled water. We also quantified edaphic properties, organic matter composition, microbial community structures and the stoichiometric imbalance of nitrogen (N) and phosphorus (P) relative to carbon based on the dissolved pool relative to microbial biomass (ImN and ImP) to investigate variables regulating microbial CUE and its variation in POM relative to MAOM (CUEPOM/CUEMAOM). In contrast to our expectation, microbial CUE did not consistently differ between POM and MAOM across sites, albeit with large inter‐sample variations in CUEPOM/CUEMAOM (from 0.3 to 4.4). Although MAOM had higher substrate quality, indicated by lower ratios of soil organic carbon to total nitrogen (C/NOM) and higher N‐compounds/aromatic ratios, and higher proportions of r‐strategists and fast‐growing bacteria in the microbial community than POM, POM and MAOM had similar degrees of N limitation (i.e. ImN), which was the best predictor of microbial CUE across all samples. Therefore, although MAOM harboured more N‐containing compounds than POM, N limitation was not necessarily lower, leading to an overall similar microbial CUE in POM and MAOM. Nevertheless, CUEPOM/CUEMAOM decreased with increasing P limitation in POM relative to MAOM. Overall, our paper presents a comprehensive, empirical study comparing microbial CUE in POM and MAOM of diverse soils, and our results support the use of similar microbial CUE for POM and MAOM in soil models, but also highlight potential contrasts in microbial CUE between soil pools under strong P limitation. Such inferences deserve attention under potentially increasing P limitation induced by N deposition. This study advances our mechanistic understanding of ecological patterns and processes from the organismic to the ecosystem scale based on soil microbial physiology and different soil functional pools. Read the free Plain Language Summary for this article on the Journal blog.

Changes in soil aggregate stability and aggregate-associated carbon under different slope positions in a karst region of Southwest China

Soil aggregates are crucial for reducing soil erosion and enhancing soil organic carbon sequestration. However, knowledge regarding influences of different slope positions on compositions and carbon content for different soil aggregates is limited. Soil samples were collected from various slope positions including dip slope, anti-dip slope and valley depression in the Longtan karst valley of Southwest China. Contents of macroaggregate (> 0.25 mm), microaggregate (0.053–0.25 mm) and silt and clay fraction (< 0.053 mm), and aggregate-associated carbon contents under the three slope positions were measured. Compared to the anti-dip slope, the mean weight diameter under the dip slope and valley depression decreased by 28.48 % and 58.79 %, respectively, while the geometric mean diameter decreased by 39.01 % and 62.57 %, respectively. The mean carbon content in silt and clay fraction was 27.59 % and 21.00 % lower than the macroaggregate- and microaggregate-associated carbon content, respectively. Under the valley depression and dip slope, soil organic carbon contents in bulk soil (37.67 % and 10.36 %, respectively), microaggregate (37.56 % and 4.95 %), and silt and clay fraction (39.99 % and 12.84 %, respectively) were significantly lower than those under the anti-dip slope. However, the difference in macroaggregate-associated carbon content among the three slope positions was not significant. The silt and clay fraction was the major contributor to soil carbon pool in bulk soil in the study area because of its high content. Compared to the anti-dip slope, contribution of macroaggregates to soil carbon pool under the dip slope and valley depression decreased by 25.53 % and 47.95 %, respectively, whereas the contribution of silt and clay fraction increased by 22.68 % and 42.66 %, respectively. These results suggested that the anti-dip slope surpassed both the dip slope and valley depression in carbon sequestration and soil and water conservation in karst regions.

Common soil history is more important than plant history for arbuscular mycorrhizal community assembly in an experimental grassland diversity gradient

The relationship between biodiversity and ecosystem functioning strengthens with ecosystem age. However, the interplay between the plant diversity - ecosystem functioning relationship and Glomeromycotinian arbuscular mycorrhizal fungi (AMF) community assembly has not yet been scrutinized in this context, despite AMF’s role in plant survival and niche exploration. We study the development of AMF communities by disentangling soil- and plant-driven effects from calendar year effects. Within a long-term grassland biodiversity experiment, the pre-existing plant communities of varying plant diversity were re-established as split plots with combinations of common plant and soil histories: split plots with neither common plant nor soil history, with only soil but no plant history, and with both common plant and soil history. We found that bulk soil AMF communities were primarily shaped by common soil history, and additional common plant history had little effect. Further, the steepness of AMF diversity and plant diversity relationship did not strengthen over time, but AMF community evenness increased with common history. Specialisation of AMF towards plant species was low throughout, giving no indication of AMF communities specialising or diversifying over time. The potential of bulk soil AMF as mediators of variation in plant and microbial biomass over time and hence as drivers of biodiversity and ecosystem relationships was low. Our results suggest that soil processes may be key for the build-up of plant community-specific mycorrhizal communities with likely feedback effects on ecosystem productivity, but the plant-available mycorrhizal pool in bulk soil itself does not explain the strengthening of biodiversity and ecosystem relationships over time.

Influence of barrens restoration treatments on soil carbon, nitrogen, and mercury pools and emissions

AbstractCurrently barrens communities only represent about 1% of their original area in the Great Lakes region. To maintain or restore barrens vegetation, prescribed fire is often applied to limit the regeneration of undesirable species and shrubs. Vegetation community response is a combination of direct fire effects on the vegetation vitality and the indirect effect of soil nitrogen (N) loss that favors nutrient‐poor adapted barren communities. In this study, we assessed forest floor and upper mineral soil (0–5 cm) pools of carbon (C), N, and mercury (Hg) before and after prescribed fire of the Moquah Barrens in northwest Wisconsin. Although we took measurements in four distinct cover types, we found no relationship between cover type and soil pools. Across all cover types, prescribed fire led to considerable emissions of C, N, and Hg in the forest floor but only Hg in the upper mineral soils (0–5 cm), presumably because maximum fire temperatures were met for Hg volatilization. We classified fire severity and soil surface temperatures at the quadrat scale, but no discernable relationships with emissions were observed. The lack of detectable relationships is likely the result of a mismatch between the scales of response variables and predictors. As a result, we calculated ecosystem‐scale fire emissions based on the total area burned because we could not discern other smaller scale predictors. Overall emissions from dormant, spring season prescribed fires at the Moquah Barrens were approximately 11,000 Mg (5.5 Mg ha−1) for C, 350 Mg for N (0.17 Mg ha−1), and 4,500 g for Hg (2.3 g ha−1).

Nitrogen fixation‐driven carbon sequestration: Brief communication

AbstractPrior to the commercial nitrogen fertilizer production in 1919, all the world's terrestrial and aquatic carbon was supported by nitrogen fixation. Annual N deposition to semi‐arid lands and temperate forests is less than 5 kg/ha‐year and 10 kg/ha, respectively. Plant and soil C/N ratios range from 9.9 to 29.8 and 9 to 14, respectively. In an equilibrium, sustainable ecosystem where N is not removed from soil pools and is only dependent on annual N inputs, maximum C sequestration rates are approximately 3.25 kg to 46 kg/ha for arid ecosystems and 23 to 101 kg/ha for forest ecosystems. Commercial N applications range from approximately 70 to 160 kg/ha‐year. Managed nitrogen fixation rates range from approximately 50 to 130 kg N/ha‐year. For each additional kg N‐entering forests, the additional C is approximately 13 kg. N‐fixing plants range from alder and lupines in the arctic, to Prosopis and Acacias in semi‐arid lands and the large trees Inga and Pentaclethra in tropical rainforests. If N‐fixing plants achieving 50 kg N/ha‐year were planted on the equivalent of a 447 km square, on six continents, the IEA target of 1.7Gt CO2 (0.72 Gt of carbon) capture capacity by 2030 https://www.ief.org/news/whats‐the‐target‐for‐carbon‐sequestration‐and‐how‐do‐we‐get‐there could be achieved.

Carbon pool and respiration of rhizosphere soils of different mangrove plant species in bangladesh sundarbans

Bangladesh Sundarbans like other mangrove ecosystems are vital carbon reservoirs in the global carbon cycle. Soil respiration, a key carbon flux, is closely linked to climate change. Despite extensive research on the Sundarbans, a gap exists in studying rhizosphere soil carbon pool (SOC) and respiration (Rs), which is crucial for understanding its role in global climate dynamics, especially the local climate. This study investigated SOC pools and Rs rates of oligohaline, mesohaline, and polyhaline zones of the Bangladesh Sundarban Mangrove Forests (SMF). The oligohaline zone exhibited the highest average SOC content (11.26 ± 5.52 t/ha), followed by the mesohaline zone (9.91 ± 3.09 t/ha) and the polyhaline zone (9.86 ± 4.16 t/ha). The Rs rate was comparatively higher in the mesohaline zone (28.19 ± 5.02 mg CO2/g soil), followed by the polyhaline zone (27.81 ± 4.38 mg CO2/g soil), and the oligohaline zone (27.63 ± 4.16 mg CO2/g soil) though the differences were not significant. Further analyses explored the influences of plant species on SOC and Rs. While rhizosphere soil of distinct plant species displayed varying SOC values, Rs did not exhibit significant differences among different plant species, and no significant relation was observed between Rs and SOC values. Mangroves were noted to store substantial amounts of organic carbon in their soils, yet they released relatively less carbon dioxide (CO2) through soil respiration compared to other tropical forests. This unique characteristic underscores the critical role of mangroves in global climate change dynamics. Conclusively, this study offers insightful information about the carbon dynamics of the Bangladesh SMF, emphasizing the significance of mangroves as carbon reservoirs with the potential to influence climate change adaptation strategies. Bangladesh J. Bot. 53(1): 131-140, 2024 (March)

The effect of repeated hurricanes on the age of organic carbon in humid tropical forest soil.

Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30-year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes ("soilR") and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0-10 cm and 35-60 cm depths relative to a modeled control soil with constant inputs over the 30-year period. Between 1988 and 2018, the occluded C stock increased and δ13C in all pools decreased, while changes in particulate and mineral-associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris-deposited C, and possible microbial scavenging of old and young C in the particulate and mineral-associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.

Organic carbon distribution between structural and process pools in the gray forest soil of different land use

The summarized data on the content of organic carbon (Corg) in the subtypes of gray forest soils occurring on the territory of Russia was presented. It was shown that the humus horizons of virgin light-gray, typical-gray, and dark-gray forest soils contain, on average, 2.16 ± 0.67, 2.42 ± 0.61, and 3.58 ± 0.95% Сorg, respectively, while the plowing layers of arable soils contain 1.36 ± 0.40, 1.71 ± 0.40, and 2.84 ± 0.86%, respectively. Structural (particulate organic matter 0.05–2 mm in size, CPOM, and mineral-associated organic matter &lt;0.05 mm in size, CMAOM) and process (potentially mineralizable organic matter, C0, and microbial biomass, Cmic) pools were isolated in the organic matter of samples from different horizons of gray forest soils (Luvic Retic Greyzemic Phaeozems (Loamic)) under small-leaved forest and barley crop. The CPOM/CMAOM ratio in the upper soil horizons under forest and arable land was 0.60 and 0.26, respectively, and this ratio decreased with depth to 0.05 under both land uses. The sizes of the CMAOM, CPOM, C0, and Cmic pools correlated with each other and depended on the depth of the soil horizon, while the effect of land use on the pool ratios was found only for the surface horizons. The contribution of CPOM and CMAOM to the potentially mineralizable pool of organic matter in gray forest soil was 20–41 and 71–87%, respectively. According to the obtained data, the size of the C0 pool was almost equal to the annual amount of the heterotrophic CO2 emission from the soil. It was emphasized that determining of the sizes and ratios of structural and process soil organic matter pools should be important in the programs of carbon monitoring and recarbonization of agroecosystems.

Prolonged drought moderates flood effects on soil nutrient pools across a rainfall gradient

Climate change is manifesting through increased intensity and frequency of extreme events such as droughts and floods. Drought causes significant stress by limiting water availability, resulting in reduced plant growth and belowground carbon (C) inputs as well as plant nutrient uptake and microbial activity, thus impacting C and nutrient cycling. Severe floods can increase leaching of nutrients, which affect nutrient availability for plants and microorganisms potentially compounding drought-induced effects on C and nutrient cycles. However, few studies have assessed the interactions between drought and floods, constraining our ability to predict ecosystem-level effects of projected climate shifts toward fewer but larger rainfall events with longer dry spells between events. We conducted a study to assess how long-term drought moderates the effect of flooding on soil C, nitrogen (N) and phosphorus (P) pools across a rainfall gradient in south-eastern Australia. We collected soil monoliths from field rainfall manipulation facilities with a 5–7 years history of simulated reduced rainfall at three sites representing a rainfall gradient (∼200–800 mm) and exposed them to a 1-in-100-year flood event under laboratory conditions. We assessed nutrients and C in soil and microbial pools before and after the flood and in flood-induced leachates. The results showed that flooding caused significantly greater losses of N in leachates, with greater effect sizes in the drought treatments compared to ambient rainfall. This response was particularly pronounced for nitrate (152 % increase on average), especially at the site with the highest mean annual precipitation (>25-fold increase). This suggest that drought may significantly reduce the capacity of ecosystems to retain N following flood events, particularly in mesic ecosystems where C and nutrient pools are greater.

Legacy Mercury Re-emission and Subsurface Migration at Contaminated Sites Constrained by Hg Isotopes and Chemical Speciation.

The re-emission and subsurface migration of legacy mercury (Hg) are not well understood due to limited knowledge of the driving processes. To investigate these processes at a decommissioned chlor-alkali plant, we used mercury stable isotopes and chemical speciation analysis. The isotopic composition of volatilized Hg(0) was lighter compared to the bulk total Hg (THg) pool in salt-sludge and adjacent surface soil with mean ε202HgHg(0)-THg values of -3.29 and -2.35‰, respectively. Hg(0) exhibited dichotomous directions (E199HgHg(0)-THg = 0.17 and -0.16‰) of mass-independent fractionation (MIF) depending on the substrate from which it was emitted. We suggest that the positive MIF enrichment during Hg(0) re-emission from salt-sludge was overall controlled by the photoreduction of Hg(II) primarily ligated by Cl- and/or the evaporation of liquid Hg(0). In contrast, O-bonded Hg(II) species were more important in the adjacent surface soils. The migration of Hg from salt-sludge to subsurface soil associated with selective Hg(II) partitioning and speciation transformation resulted in deep soils depleted in heavy isotopes (δ202Hg = -2.5‰) and slightly enriched in odd isotopes (Δ199Hg = 0.1‰). When tracing sources using Hg isotopes, it is important to exercise caution, particularly when dealing with mobilized Hg, as this fraction represents only a small portion of the sources.