Decomposable Soil Organic Matter Research Articles

A new method for estimating carbon dioxide emissions from drained peatland forest soils for the greenhouse gas inventory of Finland

Abstract. In peatlands drained for forestry, the soil carbon (C) or carbon dioxide (CO2) balance is affected by both (i) higher heterotrophic CO2-C release from faster decomposing soil organic matter (SOM) and (ii) higher plant litter C input from more vigorously growing forests. This balance and other greenhouse gas (GHG) sinks and sources in managed lands are annually reported by national GHG inventories to the United Nations Climate Change Convention. In this paper, we present a revised, fully dynamic method for reporting the CO2 balance of drained peatland forest soils in Finland. Our method can follow temporal changes in tree biomass growth, tree harvesting and climatic parameters, and it is built on empirical regression models of SOM decomposition and litter input in drained peatland forests. All major components of aboveground and belowground litter input from ground vegetation as well as live trees and trees that died naturally are included, supplemented by newly acquired turnover rates of woody plant fine roots. Annual litter input from harvesting residues is calculated using national statistics of logging and energy use of trees. Leaching, which also exports dissolved C from drained peatlands, is not included. The results are reported as time series from 1990–2021 following the practice in the GHG inventory. Our revised method produces an increasing trend of annual emissions from 0.2 to 2.1 t CO2 ha−1 yr−1 for the period 1990–2021 in Finland (equal to a trend from 1.4 to 7.9 Mt CO2 yr−1 for the entire 4.3 Mha of drained peatland forests), with a statistically significant difference between the years 1990 and 2021. Across the period 1990–2021, annual emissions are on average 1.5 t CO2 ha−1 yr−1 (3.4 Mt CO2 yr−1 for 2.2 Mha area) in warmer southern Finland and −0.14 t CO2 ha−1 yr−1 (−0.3 Mt CO2 yr−1 for 2.1 Mha area) in cooler northern Finland. When combined with data on the CO2 sink created by the growing tree stock, in 2021 the drained peatland forest ecosystems were a source of 1.0 t CO2 ha−1 yr−1 (2.3 Mt CO2 yr−1) in southern Finland and a sink of 1.2 t CO2 ha−1 yr−1 (2.5 Mt CO2 yr−1) in northern Finland. We compare these results to those produced by the semi-dynamic method used earlier in the Finnish GHG inventory and discuss the strengths and vulnerabilities of the new revised method in comparison to more static emission factors.

Carbon and Nitrogen Mineralization Kinetics in Soil Under Differential Nutrient Management Regimes in Inceptisols of Coastal India

ABSTRACT In intensive cropping systems, nutrient, and residue management techniques have a tendency to change the soil’s carbon (C) and nitrogen (N) cycle and, consequently, its potential for sequestration and loss. To understand the carbon and nitrogen mineralization kinetics, laboratory incubation studies were conducted with soils at variable depths from a long-term nutrient management experiment on cereal-vegetable-pulse cropping system. The experiment was laid out completely randomized design with 10 treatments replicated thrice. Long-term integrated application of inorganics with organics, biofertilizers, and amendment along with crop residues showed higher carbon and nitrogen mineralization potential and lower potential was observed in soils where crops were raised without application of any nutrient inputs (control) and uncultivated fallow. It was calculated that the potential for higher levels of carbon and nitrogen mineralization existed in the surface (0–15 cm) and subsurface (15–30 cm) layers, respectively, and was negligible in the deeper layer (30–45 cm). The mineralization rate was initially faster up to 42 days of incubation, indicating proper utilization of readily decomposable soil organic matter in the labile pools and thereafter found to slow down. Addition of lime along with the integrated use of organics and inorganics has found to raise the pH to a level that provides a suitable soil environment which affects the carbon and nitrogen mineralization kinetics resulting in buildup of total organic carbon and total nitrogen in the soils. The first-order kinetic model was the best fit in explaining the kinetic parameters of C and N mineralization. Soils under integrated nutrient management (INM) with farmyard manure (FYM) along with biofertilizers, amendment, and inorganic nutrients exhibited better carbon dynamics higher C mineralization (Cmin), potentially mineralizable C (C0), rate of C mineralization (dCmin/dt), and C mineralization quotient (qM), whereas soils applied with similar combinations with vermicompost was found effective for better nitrogen dynamics in terms of enhanced N mineralization (Nmin), potentially mineralizable N (N0), rate of N mineralization (dNmin/dt) and active N fraction over control and fallow soils. Integrated application of inorganic and organic inputs with crop residues over the years has shown beneficial impacts on soil carbon and nitrogen mineralization processes and their kinetics.

Increased soil carbon storage through plant diversity strengthens with time and extends into the subsoil.

Soils are important for ecosystem functioning and service provisioning. Soil communities and their functions, in turn, are strongly promoted by plant diversity, and such positive effects strengthen with time. However, plant diversity effects on soil organic matter have mostly been investigated in the topsoil, and there are only very few long-term studies. Thus, it remains unclear if plant diversity effects strengthen with time and to which depth these effects extend. Here, we repeatedly sampled soil to 1 m depth in a long-term grassland biodiversity experiment. We investigated how plant diversity impacted soil organic carbon and nitrogen concentrations and stocks and their stable isotopes 13 C and 15 N, as well as how these effects changed after 5, 10, and 14 years. We found that higher plant diversity increased carbon and nitrogen storage in the topsoil since the establishment of the experiment. Stable isotopes revealed that these increases were associated with new plant-derived inputs, resulting in less processed and less decomposed soil organic matter. In subsoils, mainly the presence of specific plant functional groups drove organic matter dynamics. For example, the presence of deep-rooting tall herbs decreased carbon concentrations, most probably through stimulating soil organic matter decomposition. Moreover, plant diversity effects on soil organic matter became stronger in topsoil over time and reached subsoil layers, while the effects of specific plant functional groups in subsoil progressively diminished over time. Our results indicate that after changing the soil system the pathways of organic matter transfer to the subsoil need time to establish. In our grassland system, organic matter storage in subsoils was driven by the redistribution of already stored soil organic matter from the topsoil to deeper soil layers, for example, via bioturbation or dissolved organic matter. Therefore, managing plant diversity may, thus, have significant implications for subsoil carbon storage and other critical ecosystem services.

Distinct roles of bacteria and fungi in mediating soil extracellular enzymes under long-term nitrogen deposition in temperate plantations

Soil microbes decompose soil organic matter by producing various extracellular enzymes, and also regulate soil carbon (C) and nutrient cycles. However, how shifts in soil microbial community under long-term nitrogen (N) deposition govern soil extracellular enzyme activities (EEAs) is not known. Here, we conducted a decade-long N fertilization experiment consisting of control, low-N (2 g N m−2 year−1), medium-N (5 g N m−2 year−1), and high-N (10 g N m−2 year−1) treatments in a N-limited temperate plantation in northern China. The diversity and community composition of fungi and bacteria were determined with bacterial 16S rRNA genes and fungal internal transcribed spacer genes sequencing. Soil EEAs involved in C, N, and phosphorus (P) cycles were also measured. The results showed that high-N addition increased fungal diversity and altered microbial community composition. In particular, rare fungal diversity and rare bacterial community composition were more sensitive to N addition. N addition improved β-1,4-glucosidase (BG) and acid phosphatase (AP) activities by altering bacterial community composition, and inhibited leucine aminopeptidase (LAP) and polyphenolic oxidase (PPO) activities by affecting fungal functional groups. Specifically, shifts in bacterial community composition caused by N addition were reflected in the significant increase in the dominant class Thermoleophilia, which could be an indicator of secreted hydrolase involved in C and P cycles. Symbiotrophic fungi were more closely associated with LAP for acquiring N and PPO. Our data provide a novel view of the link between major extracellular enzymes and specific microbial taxa under the background of N deposition.

Microbial activity evaluation and aerobic transformation of deep water offshore synthetic drilling fluids in soil: A case study of ternary mixture of synthetic ethyl esters of plants oil (Seep mixture) synthetic drilling fluid in agbami (Niger delta) deep water field

Approval for offshore discharge of cuttings drilled with synthetic-based drilling Fluids (SBFs) by environmental regulatory and protection agencies usually require detailed laboratory biodegradation analysis of the base fluid chemicals among others. This concept of using biodegradation test as an environmental fluid-compliance criterion is that a fluid that degrades readily in the laboratory will not persist for an extensive period of time in the offshore environment.Hence, this research focuses on microbial activity measurement and aerobic biodegradation of a newly developed synthetic ester fluid named “ternary mixture of synthetic ethyl esters of plants oil (SEEPmixture)” and its’ comparison with already existing synthetic deep water fluids (SHBF and SEBF) in Agbami deep water field. The chemical composition and components of each drilling fluid sample was analyzed by a GC system combined with a high resolution accurate mass (HRAM) spectrometer while a Fourier transform infra-red (FTIR) spectroscopy was used for functional group identification.The microbial activity of each soil sample that was contaminated with different base oils (drilling fluids) and that of the uncontaminated soil sample (control) was determined by trapping the carbon IV Oxide (CO2) evolved from each soil sample with a strong alkali solution of barium hydroxide (Ba(OH)2) as the microbial biomass actively decompose soil organic matter. The degradation of the drilling fluid chemicals was monitored under aerobic conditions.The soil contaminated with SEBF fluid chemical experienced a low microbial activity due to its low respiration (low amount of evolved carbon IV oxide per mg of soil) but a more pronounced adverse effect on microbial activity was experienced by soil contaminated with SHBF chemical. While microbial activities were not impaired with the SEEP mixture fluid chemical presence in the soil. Meanwhile, in regards to soil aerobic transformation, the SEEP mixture fluid took the shortest time of 13.6 days for it to decompose to half of its original concentration (5mg/kgsoil) from an initial concentration of 10 mg/kg soil. This implies that the fluid sample is readily and inherently biodegradable in soil. Whereas, the SHBF fluid sample required about 34.50 days for it to decompose to half of its original concentration. This indicates its non-readily biodegradable nature in soil. The half-life of the SEBF was 25 days and thus not readily biodegradable.

Diversity of Soil Surface Insects in Three Land Use Types in Sintang District, West Kalimantan

Soil surface insects are organisms that have an important role in the process of decomposing soil organic matter, serving as indicators of soil fertility, and maintaining ecosystem balance. This study aimed to determine the diversity and abundance of soil surface insects on three types of land, namely rubber plantations, rubber agroforestry, and secondary forests and the insect that attacks rubber stems in Sintang District, West Kalimantan. The research method used was field observation, sampling with pitfall traps and measurement of environmental conditions, identification, and data analysis on three types of land. The results showed that 649 individuals were found which were divided into 8 orders of 11 families of 14 species. The highest abundance was found in rubber agroforestry land, the highest insect diversity was in secondary forests. The types of insects found attacking the rubber tree trunks, both dead and alive, were Coptotermes curvignathus in rubber agroforestry and Reticulitermes flavipes in secondary forest and rubber plantations.

The effect of trichoderma harzianum dose and shallot population (Allium cepa L.) on chili production (Capsicum annuum L.) by intercropping system

Trichoderma harzianum is a soil fungus that plays a role in decomposing soil organic matter, and contains several components of substances such as N, P, S and Mg and nutrients needed by plants for their growth. Intercropping is one way to increase the efficiency of land use by planting several types of plants on the same land and at the same time. Meanwhile, to increase land use efficiency and reduce the risk of chili farming failure, it can be achieved by implementing an intercropping system of chili with shallots. The research aimed to determine of T.harzianum dosage and the shallots population (Allium cepa L.). The interaction between these on the chili production (Capsicum annuum L.) by intercropping system. The experimental design used was Randomized Block Design 4 × 3 factorial with three replications. Threre were interactions between T. harzianum dosage and shallots population on fruits number per plant, fruit weight per plant and fruit length per plant. The best result were found at 20 g per plant T. harzianum dosage and two shallots population on fruits number per plant (153,33 fruit), fruit weight per plant (121,55 g) and fruit length per plant (126,41 cm).

Substrate stoichiometric regulation of microbial respiration and community dynamics across four different ecosystems

Microbes decompose soil organic matter (SOM), yet it is unclear how substrate inputs (i.e., stoichiometry) directly mediate microbial activities and community dynamics. We hypothesized that C+N input has the largest effect on microbial respiration and community structure, followed by C input and N input. Soils were collected from four ecosystems (grassland, piñon-juniper, ponderosa pine, mixed conifer) and amended with NH4NO3 (N only; 100 μg g−1 wk−1), 13C-glucose (C only; 1000 μg g−1 wk−1), or C+N in a five-week laboratory incubation. We found that C+N input induced the greatest total respiration while C input induced the greatest SOM-derived respiration (i.e., priming effect) across ecosystems. Shifts in community composition were the largest with C+N input, followed by C input, and showed little response to N input. C only and C+N inputs increased both of the relative and absolute abundances of Actinobacteria and Proteobacteria (α, β, γ), but reduced the relative abundances of Verrucomicrobia and δ-Proteobacteria. C+N input increased the relative abundances of Bacillales, Rhizobiales, Burkholderiales and of 9 families, and reduced the relative abundances of Myxococcales and of 12 families, but showed little effect on the absolute abundances of these bacterial taxa. N input reduced the absolute abundances of Actinobacteria, Proteobacteria, and Verrucomicrobia but did not affect their relative abundances in the mixed conifer soil; by contrast, N input reduced relative abundances of δ-Proteobacteria and increased the relative abundances of γ-Proteobacteria but did not affect their absolute abundances in the ponderosa pine soil. We also found that substrate inputs were the main driver of SOM decomposition, microbial respiration and diversity, while soil ecosystem was the main driver of community composition and abundances of most bacterial phyla. Our work suggests that substrate stoichiometry has predictable effects on soil C cycling, microbial diversity and community composition, but has variable effects on microbial abundances, and that incorporating bacterial gene copies in abundance calculations can help more accurately estimate microbial responses across taxonomic levels and ecosystems.

Evidences of plants’ impact on land degradation and climate change: An urgent call for new multidisciplinary research

Atmospheric carbon has been stored in soils for thousands of years in the form of organic matter which associates carbon to key plant nutrients. Recent analysis reveals that when facing nutrient deficiency, plants decompose soil organic matter to mine nutrients and carbon is released into the atmosphere which causes land degradation and climate change.

Air-drying and drying-rewetting effects in Japanese Andosols subjected to long-term organic rice farming

ABSTRACT Paddy fields are generally submerged during the growing season and drained in the fallow season, and occasionally stimulating the air-drying effect and drying-rewetting effect on soil nitrogen (N) mineralization and soil carbon (C) decomposition. It is important to understand how these two effects take place in Andosol field under organic farming. To elucidate these effects, we incubated fresh, air-dried, and dried-rewetted paddy soil anaerobically under flooded condition. Soils were from the fields subjected to organic rice farming (ORF) for 4–5 years (4ORF), 8–9 years (8ORF), and 12 years (12ORF) with only rice residues and fallow weeds as organic matter applied, and cnventional rice farming (CRF). Soil samples were collected from plow layer of 0–15 cm depth after the rice harvest. The results showed that air-dried and dried-rewetted treatments could significantly increase N mineralization potential in 8ORF and 12ORF. The air-drying effects were not significantly changed in 4ORF, but significantly increased in 8ORF and 12ORF as compared to CRF. The drying-rewetting effects of C decomposition potentials were highest in 12ORF significantly. The N mineralization potential of air-drying effect showed no significant difference compared to drying-rewetting effect on conventional and organic rice farming. The ratio of C decomposition to N mineralization after 8 weeks incubation was higher in fresh soil (at 7.26–8.61) than those in air-dried (at 4.09–4.35) and dried-rewetted (at 3.94–4.26) soils. These indicated that the easily decomposable soil organic matter under the anaerobic flooded condition from fresh soil was changed by air-drying and drying-rewetting treatments. The slow increase of N mineralization and C decomposition with years of organic farming suggest the slow increase of fallow weed, the biomass of which gradually increase with years. The specific behavior of 12ORF soil suggest the transition of soil condition from a transitional state to an established state.

Unexpected high methylmercury contents related to soil organic carbon and its molecular composition in wetland soils of the Yarlung Tsangbo River, Tibet

Deciphering the role of soil organic matter on methylmercury production in wetlands is an important environmental topic in Tibet, where is the province with the largest number of international rivers, including the Yarlung Tsangbo River. In this study, total mercury (THg) and methylmercury (MeHg) contents in soils from wetlands along the YTR were detected, and roles of the molecular composition of soil organic carbon (SOC) in the formation of MeHg were explained in detail by pyrolysis–gas chromatography/mass spectrometry technology (Py-GC/MS). On average, THg and MeHg contents were 62.9 ng/g and 5.88 ng/g, respectively. About 12.6% of total Hg was in the form of MeHg. Unexpected high methylmercury levels occurred in wetland soils along the YTR. The topsoils (0–20 cm) had more THg but less MeHg contents than those in subsoils (20–48 cm). The proportion of MeHg to THg (RHg) was higher in subsoils than in topsoils independent of wetland types. Correlations between THg, MeHg, and SOC were weak in topsoils but strong in subsoils. Lignin and phenol moieties of SOC pyrolysis products were significantly and positively related to THg in topsoils, while positive or negative correlations were found between THg, MeHg, and pyrolysis products of SOC in subsoils at p < 0.05 level. C/N ratios were negatively related to RHg to a significant level in both topsoils and subsoils. Hg/C ratios correlated negatively with C/N ratios in both topsoils and subsoils at the 0.001-level, and larger coefficients in subsoils confirmed that older and high decomposed soil organic matter generally have high levels of Hg. Principle component analysis indicated that THg contents were closely related to plant-derived organic compounds, and MeHg contents were closely related to phytoplankton-derived organic compounds. Our findings suggested that molecular compositions of soil organic matter are critical to MeHg presentence in wetland soils along the YTR.

Time series and isotopic evidence for gaseous components (222Rn, CO2 and its carbon isotopes) of soil under a cool temperate deciduous forest in Hokkaido, Japan

The behavior of gaseous components of forest soil was assessed by simultaneous monitoring of 222Rn, CO2 and its carbon isotopes (δ13C, Δ14C) in soil air under a semi natural forest in northern Japan. The isotope ratio of 14C (Δ14C) in soil air CO2 was found to decrease towards late summer, suggesting gradual depletion of easily decomposed soil organic matter. There was a specific case that soil air CO2 with depleted 14C appeared suddenly under snowpack in winter. It is not always appropriate to use soil 222Rn as a tracer of CO2 efflux from the soil surface to the atmosphere.

Effect of dry-wet cycles on carbon dioxide release from two different volcanic ash soils in a Japanese temperate forest

ABSTRACT In the present study, two volcanic ash soils (soil A and B) from a temperate broad-leaved forest in eastern Japan were aerobically incubated under repeated dry-wet cycles and continuously constant moisture conditions. The primary aims were to quantify the potential for enhancement of carbon dioxide (CO2) release owing to increased water fluctuation and to examine differences in the responses of volcanic ash soils with different physicochemical properties. Soil B, rather than soil A, was a typical Andosol. During incubation at 20°C for 120 days with five dry-wet cycles, the CO2 release rate was measured periodically. Abundance of the stable carbon isotope in CO2 (δ13C-CO2) was measured to capture changes in the origin of decomposed soil organic matter (SOM) owing to the dry-wet cycles. The CO2 release rate under the dry-wet cycles was up to 49% higher than the values predicted from a parabolic relationship between CO2 release and water content during incubation under the continuously constant moisture condition. The magnitude of CO2 release enhancement was 2.7-fold higher in soil B relative to that in soil A. The δ13C-CO2 value in the dry-wet cycles was enriched by 0.3–2.3‰ compared to that during incubation under the continuously constant moisture conditions, suggesting that the decomposition of well-metabolized and/or old SOM was enhanced by the dry-wet cycles. Thus, the present study suggests that Andosols, which have been believed to have a strong SOM stabilization ability, are vulnerable to dry-wet cycles. Then, increased water fluctuation in a future warmer world would have significant potential to stimulate CO2 release from soils.

Indicators of peat soil degradation in the Biebrza valley, Poland

Abstract Peat mineralisation leads to net loss of CO2 to the atmosphere, as well as to release of other elements from the decomposed soil organic matter (SOM) to groundwater. This results in the degradation of peat soils and the ecosystems they support. Here we evaluated the practical indicatory suitability of the existing and proposed new indices for the assessment of peat soil degradation in the Biebrza river valley encompassing, unique on European scale, peatland ecosystems. We studied relationships between soil organic carbon (SOC) and total nitrogen (Ntot), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in a series of degraded peat soils in the Biebrza valley. Samples were taken from soils developed on peat deposits that varied in thickness and the degree of peat decomposition, from undegraded to highly mineralised peats. The relationships between changes in the SOC content and changes in the values of the remaining variables (SOM, Ntot, DOC, DON, C/N ratio), were statistically tested. Linear and non-linear regressions were used to establish the relationships amongst the variables examined. The losses of soil C and N occur independently and differ between stages of peat soil mineralisation. From our study, it results that the peat mineralisation intensity may be estimated based on the loss of SOC. We found that 1% loss of SOC corresponded to 1.028% loss of SOM, regardless of the degree of peat soil mineralisation, whereas SOM solubility, measured by the content of DOC, varied based on the intensity of peat soil mineralisation. The content of DOC decreased with the decrease in the SOC content, whereas the DOC/ SOC ratio increased depending on the intensity of peat decomposition. The C/N ratio is not a reliable indicator of peat mineralisation, because its values are driven not only by the nitrogen natively present in peat soils but also by nitrogen from external sources. The contents of SOC and Ntot did not decrease uniformly during peat decomposition because C and N show various mobility in the processes of SOM mineralisation. We found that the DOC/SOC ratio was most indicative of peat soil mineralisation intensity. © IOŚ-PIB

Priming alters soil carbon dynamics during forest succession

The mechanisms underlying soil organic carbon (C) dynamics during forest succession remain challenged because amount, quality, and composition of C inputs change with tree growth and species composition. Soils were collected from two stages (grassland and young secondary forest) of secondary succession after the clear-cut of primary old-growth forest due to land-use change, with a native old-growth forest (undisturbed for more than 200 years) as the reference. Soil samples were incubated for 170 days and the priming effects were quantified by one pulse addition of 13C-labeled glucose. 13C-PLFAs (phospholipid fatty acids) were analyzed to identify microbial functional groups utilizing glucose and to explore their accordance with SOM priming during succession. Soil C was primed much more strongly in young secondary forest than in grassland or old-growth forest. Priming resulted in large C losses (negative net C balance) in young-forest soil, whereas C stocks increased in grassland and old-growth forest (positive net C balance). Microbial composition assessed by PLFA and utilization of easily available organics (13C-PLFA) indicated that fungi were mainly responsible for priming in young-forest soil. Consequently, labile C released by litter decomposition and root exudation together with the availability of soil nutrients determines microbial functional groups that decompose soil organic matter during initial succession. These findings provide novel ecological connections between soil organic matter dynamics and C (de)stabilization with microbial functioning during forest succession and show that priming direction and intensity is important to distinguish soil C dynamics in young- and old-growth forests.

Fresh knowledge for an old relationship: new discoveries in molecular mycorrhizal research.

Fresh knowledge for an old relationship: new discoveries in molecular mycorrhizal research.

Effect of oxidizing and reducing agents in soil on methane production in Southeast Asian paddies

ABSTRACTMethane is one of the greenhouse gases emitted from paddy soil ecosystems and may induce global warming and climate change; therefore, mitigation options are urgently required to establish mitigation technology to reduce methane emission without affecting rice production. Methane is produced by a balance between oxidizing agents (such as iron) and reducing agents (easily decomposable soil organic matter), according to the so-called Takai theory. To evaluate options for mitigating methane production potential and to examine the applicability of the Takai theory in Southeast Asian paddy soils, 23 soil samples were collected from Thailand, Indonesia, Philippines, and Vietnam. These soil samples were anaerobically incubated to measure their methane production potential and examined to see whether their chemical properties, such as the ferrous, total iron, and organic matter contents, were correlated. We found a significant negative correlation between the methane production potential and the total iron content, and a positive correlation between the methane production potential and the hexose content, as an index for a soil’s easily decomposable organic matter content. The methane–C/CO2–C production ratio was also positively correlated with the mineralizable nitrogen/ferrous contents ratio, which indicated that the Takai theory, established for Japanese paddy soils, is also useful in Southeast Asian paddy soils and that the soil’s iron content is important to estimate the methane production potential.

Downward migration of 137Cs within the humus layer under temperate coniferous stands in the Czech Republic

This study examined the current distribution and downward migration of 137Cs in the humus horizon under temperate coniferous stands in the Czech Republic. Depth distribution profiles of lithogenic alkali metals (K, Rb, and Cs) were also traced to find any indication regarding the 137Cs dynamics within the humus horizon. Soil (bulk soil) samples were collected manually from the uppermost down to a depth of 20 cm at three points situated 10–20 m apart in three locations diversely affected by the Chernobyl accident in 1986. Humus samples (about 6 cm in thickness) were separately collected at three points adjacent to the bulk soil sampling. The humus samples were divided into three fractions (upper, middle, and lower) denoted to Hu, Hm, and Hl, respectively, depending on their depths. Activity concentration of 137Cs and the amount of alkali metals (K, Rb, and Cs) in each humus fraction were determined with gamma spectrometry and ICP spectrometry, respectively. Some properties (pH, density, SOM, and identification of clay minerals), of both bulk soils and humus horizons were also investigated. The highest activity concentration of 137Cs (Bq kg−1) appeared in the upper portion of the soil (mainly humus horizon) under three coniferous stands about 30 years after the Chernobyl accident. Increasing activity of 137Cs (Bq) was found in the lower humus fraction (Hl, 4~6-cm depth range) with no appreciable amounts of clay minerals like illite and smectite in the investigated sites. The findings suggest that the fallout 137Cs moves downward at a speed of 0.13~0.19 cm year−1 with degrading organic matter within the humus horizon. Possible association of 137Cs with alkali metals (K, Rb, and Cs) was suggested by depth distribution profiles of lithogenic alkali metals (K, Rb, and Cs) in humus. The humus horizon under temperate coniferous stands plays an important role in retaining fallout 137Cs for a long time. Comparing the depth distributions of the fallout radionuclide 137Cs with the depth distributions of lithogenic alkali metals (K, Rb, and Cs) gives valuable information for clarifying mechanism of 137Cs movement in humus. Further investigation is needed to elucidate mechanism of 137Cs migration within humus horizon by tracing 137Cs speciation and decomposing soil organic matter simultaneously.

Bamboo invasion of broadleaf forests altered soil fungal community closely linked to changes in soil organic C chemical composition and mineral N production

Soil fungi play an important role in decomposing soil organic matter and facilitating nutrient uptake by plants, however, the relationship between fungal community and soil biogeochemical cycling during plant invasion is poorly understood. The objective of this study was to investigate the effects of Moso bamboo (Phyllostachys edulis) invasion into broadleaf forests on the soil organic C (SOC) chemical composition, fungal community and mineral N production. We collected soil samples in evergreen broadleaf forests, mixed bamboo-broadleaf forests and bamboo forests. Soil fungal community and SOC chemical composition were determined. Bamboo invasion decreased alkyl C but increased O-alkyl C contents. Soil fungal abundance (18S rRNA) was decreased, while their alpha diversity was increased by bamboo invasion. Additionally, bamboo invasion enhanced net N mineralization rate but reduced gross nitrification rate. The fungal community composition strongly correlated with alkyl C content, and alkyl C content explained 32% of the variation in the fungal abundance. Fungal community composition correlated with gross nitrification rate, with 43% of the variation in gross nitrification rate attributable to soil fungal abundance. Changes in soil fungal community caused by bamboo invasion into broadleaf forests were closely linked to changed soil organic C chemical composition and decelerated nitrate production.

Modelling the influence of ectomycorrhizal decomposition on plant nutrition and soil carbon sequestration in boreal forest ecosystems.

Tree growth in boreal forests is limited by nitrogen (N) availability. Most boreal forest trees form symbiotic associations with ectomycorrhizal (ECM) fungi, which improve the uptake of inorganic N and also have the capacity to decompose soil organic matter (SOM) and to mobilize organic N ('ECM decomposition'). To study the effects of 'ECM decomposition' on ecosystem carbon (C) and N balances, we performed a sensitivity analysis on a model of C and N flows between plants, SOM, saprotrophs, ECM fungi, and inorganic N stores. The analysis indicates that C and N balances were sensitive to model parameters regulating ECM biomass and decomposition. Under low N availability, the optimal C allocation to ECM fungi, above which the symbiosis switches from mutualism to parasitism, increases with increasing relative involvement of ECM fungi in SOM decomposition. Under low N conditions, increased ECM organic N mining promotes tree growth but decreases soil C storage, leading to a negative correlation between C stores above- and below-ground. The interplay between plant production and soil C storage is sensitive to the partitioning of decomposition between ECM fungi and saprotrophs. Better understanding of interactions between functional guilds of soil fungi may significantly improve predictions of ecosystem responses to environmental change.