Soil Biogeochemical Processes Research Articles

Woody plants reduce the sensitivity of soil extracellular enzyme activity to nutrient enrichment in wetlands: A meta-analysis

Greater deposition of nitrogen (N) and phosphorus (P) is expected to increase decomposition of soil organic carbon (C) in wetlands. Soil extracellular enzyme activities (EEAs) are sensitive indicators of soil biogeochemical processes, but their response to N and/or P enrichment in wetlands needs to be assessed on a global scale. We conducted a meta-analysis with peer-reviewed papers from Web of Science and China National Knowledge Infrastructure on soil EEAs after N and/or P enrichment in wetlands. Seven soil EEAs and five soil chemical properties in the 0-20 cm depth from 31 studies analyzing N and/or P enrichment were collated, from studies that described vegetation composition in wetland. Soil EEAs were classified into C-, N-, and P- acquiring function categories. To examine influence of vegetation types on soil EEAs, we further defined two categories of wetlands with different vegetation types: moss/sedge-dominated non-woody and shrub/tree-dominated woody wetlands. We showed that soil EEAs in moss/sedge-dominated wetlands were more susceptible to nutrient enrichments than those in shrub/tree-dominated wetlands. Nitrogen enrichment increased C- and P-acquiring EEAs in both categories of wetlands. However, for P enrichment, significant responses of soil EEAs were observed in moss/sedge-dominated wetlands only: β-1,4-glucosidase activity increased 76% but phosphatase activity decreased 78%, on average. Soil EEAs in both wetland categories remained unchanged when N and P were supplied together. Woody plants may override microbial response to nutrient enrichment and mitigate the negative effect of nutrient enrichment on soil organic C stock. Further studies are needed to consider the effect of plant physiology and vegetation dynamics on soil EEAs, and integrate wetland soil EEAs modules into process-based biogeochemical model to better understand the feedback of wetland ecosystems with global change.

“Omics” Technologies for the Study of Soil Carbon Stabilization: A Review

Evidence-based decisions governing sustainable agricultural land management practices require a mechanistic understanding of soil organic matter (SOM) transformations and stabilization of carbon in soil. Large amounts of carbon from organic fertilizers, root exudates, and crop residues are input into agricultural soils. Microbes then catalyze soil biogeochemical processes including carbon extracellular transformation, mineralization, and assimilation of resources that are later returned to the soil as metabolites and necromass. A systems biology approach for a holistic study of the transformation of carbon inputs into stable SOM requires the use of soil “omics” platforms (metagenomics, metatranscriptomics, metaproteomics, and metabolomics). Linking the data derived from these various platforms will enhance our knowledge of structure and function of the microbial communities involved in soil carbon cycling and stabilization. In this review, we discuss the application, potential, and suitability of different “omics” approaches (independently and in combination) for elucidating processes involved in the transformation of stable carbon in soil. We highlight biases associated with these approaches including limitations of the methods, experimental design, and soil sampling, as well as those associated with data analysis and interpretation.

Soil gas probes for monitoring trace gas messengers of microbial activity

Soil microbes vigorously produce and consume gases that reflect active soil biogeochemical processes. Soil gas measurements are therefore a powerful tool to monitor microbial activity. Yet, the majority of soil gases lack non-disruptive subsurface measurement methods at spatiotemporal scales relevant to microbial processes and soil structure. To address this need, we developed a soil gas sampling system that uses novel diffusive soil probes and sample transfer approaches for high-resolution sampling from discrete subsurface regions. Probe sampling requires transferring soil gas samples to above-ground gas analyzers where concentrations and isotopologues are measured. Obtaining representative soil gas samples has historically required balancing disruption to soil gas composition with measurement frequency and analyzer volume demand. These considerations have limited attempts to quantify trace gas spatial concentration gradients and heterogeneity at scales relevant to the soil microbiome. Here, we describe our new flexible diffusive probe sampling system integrated with a modified, reduced volume trace gas analyzer and demonstrate its application for subsurface monitoring of biogeochemical cycling of nitrous oxide (N2O) and its site-specific isotopologues, methane, carbon dioxide, and nitric oxide in controlled soil columns. The sampling system observed reproducible responses of soil gas concentrations to manipulations of soil nutrients and redox state, providing a new window into the microbial response to these key environmental forcings. Using site-specific N2O isotopologues as indicators of microbial processes, we constrain the dynamics of in situ microbial activity. Unlocking trace gas messengers of microbial activity will complement -omics approaches, challenge subsurface models, and improve understanding of soil heterogeneity to disentangle interactive processes in the subsurface biome.

Microbial linkages to soil biogeochemical processes in a poorly drained agricultural ecosystem

Soil microorganisms mediate biogeochemical processes, but how microbial community composition influences these processes remains contested. We combined monthly sequencing of soil 16S rRNA genes and intensive measurements of nitrogen (N), carbon (C), and iron (Fe) cycling along a topographic gradient in a poorly drained intensive agricultural ecosystem (corn–soybean rotation) in the midwestern United States. Observed microbial composition changed little over time within and among years despite large differences in weather and crop type. Yet, microbial composition varied greatly with topographic location and correlated strongly with moisture, soil organic carbon (SOC), and especially pH. Microbial families, genera, and/or amplicon sequence variants often correlated significantly with measured biogeochemical processes or pools, yet different taxa within the same phylogenetic groups often responded in opposite ways, indicating a lack of ecological coherence among close relatives. Dominant phyla were generally similar across the topographic gradient but specific members showed consistent tradeoffs among locations. Ammonia oxidizing archaea and bacteria sequences varied oppositely with pH across the gradient, but their combined relative abundances remained similar, as did potential nitrification rates. Nitrospira sequences correlated positively with nitrous oxide (N2O) fluxes, suggesting a direct or indirect contribution of nitrification (or possibly comammox) to N2O production. We also found significant linkages between taxonomic groups and redox-sensitive Fe pools, indicating a role for redox variation in structuring microbial communities. Several globally dominant bacteria identified previously correlated significantly with measured biogeochemical variables, providing insights into their possible functional roles. Overall, microbial composition provided a coarse measure of several key biogeochemical functions and implicated taxa that possibly mediate these processes in a widespread agroecosystem of North America.

Soil bacterial community functions and distribution after mining disturbance

Mining disturbances alter soil edaphic factors, modifying soil biogeochemical processes and thus impacting the soil microbiome. The objectives of this study were (1) to identify the dominant edaphic factor influencing the soil bacterial functions after mining disturbance and (2) to investigate how the soil microbiome was distributed, relative to the dominant edaphic factor. We found that soil pH was the most important predictor explaining the distribution of microbial attributes, such as microbial diversity, taxonomic composition, and ecological clusters along a mining disturbance gradient. Our structural equation model (SEM) indicates that soil pH shaped the bacterial community indirectly, by altering soil nutrients and metal availability. Furthermore, the microbial functions responded to soil pH, as the soil microbiome was sensitive to changes in nutrient and metal(loid) availability. For example, the bacterial community was enriched in core functional genes associated with nutrient availability (including nitrogen and phosphorus) in soil with high pH, whereas there were more core functional genes involved in metal availability (including metal transport and resistance) in soil with low pH. We conclude that soil pH is a key controller of soil bacterial communities, due to its direct and indirect effects on the availability of nutrients and metal(loid)s, after mining disturbance.

Nitrogen isotopes in the soil-to-tree continuum - Tree rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions.

Anthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (δ15N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δ15N changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and δ15N values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive δ15N signals. Our results reveal that airborne anthropogenic N with distinct δ15N signals may directly modify the NO3- δ15N values in surface soils, but not the ones of NH4+, the preferred N form of the studied trees. Hence, the tree-ring δ15N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δ15N range, we report differences in: soil nutrient content and N transformation rates; δ15N values of NH4+, total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root δ15N values with fungal identification to infer that hydrophobic EcM fungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots, which also assimilate N directly under less acidic conditions. Despite the complexities of soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring δ15N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δ15N values as a suitable indicator for environmental research on forest N cycling.

The influence of elevated CO2 on bacterial community structure and its co-occurrence network in soils polluted with Cr2O3 nanoparticles.

Elevated CO2 (eCO2) and nanoparticles release are considered among the most noteworthy global concerns as they may impose negative effects on human health and ecosystem functioning. A mechanistic understanding of their combined impacts on soil microbiota is essential due to the profound eCO2 effect on soil biogeochemical processes. In this study, the impacts of Cr2O3 nanoparticles (nano-Cr2O3) on the activity, structure and co-occurrence networks of bacterial communities under ambient and eCO2 were compared between a clay loam and a sandy loam soil. We showed that eCO2 substantially mitigated nano-Cr2O3 toxicity, with microbial biomass, enzyme activity and bacterial alpha-diversity in clay loam soil were much higher than those in sandy loam soil. Nano-Cr2O3 addition caused an increase in alpha-diversity except for clay loam soil samples under eCO2. 16S rRNA gene profiling data found eCO2 remarkably reduced community divergences induced by nano-Cr2O3 more efficiently in clay loam soil (P<0.05). Network analyses revealed more complex co-occurrence network architectures in clay loam soil than in sandy loam soil, however, nano-Cr2O3 decreased but eCO2 increased modularity and network complexity. Rising CO2 favoured the growth of oligotrophic (Acidobacteriaceae, Bryobacteraceae) rather than the copiotrophic bacteria (Sphingomonadaceae, Caulobacteraceae, Bacteroidaceae), which may contribute to community recovery and increase available carbon utilization efficiency. Our results suggested that the degree to which eCO2 mitigates nano-Cr2O3 toxicity is soil dependent, which could be related to the variation in clay and organic matter content, resilience of the resistant bacterial taxa, and microbial network complexity in distinct soils.

Spatially Related Sampling Uncertainty in the Assessment of Labile Soil Carbon and Nitrogen in an Irish Forest Plantation

The importance of labile soil carbon (C) and nitrogen (N) in soil biogeochemical processes is now well recognized. However, the quantification of labile soil C and N in soils and the assessment of their contribution to ecosystem C and N budgets is often constrained by limited information on spatial variability. To address this, we examined spatial variability in dissolved organic carbon (DOC) and dissolved total nitrogen (DTN) in a Sitka spruce forest in central Ireland. The results showed moderate variations in the concentrations of DOC and DTN based on the mean, minimum, and maximum, as well as the coefficients of variation. Residual values of DOC and DTN were shown to have moderate spatial autocorrelations, and the nugget sill ratios were 0.09% and 0.10%, respectively. Distribution maps revealed that both DOC and DTN concentrations in the study area decreased from the southeast. The variability of both DOC and DTN increased as the sampling area expanded and could be well parameterized as a power function of the sampling area. The cokriging technique performed better than the ordinary kriging for predictions of DOC and DTN, which are highly correlated. This study provides a statistically based assessment of spatial variations in DOC and DTN and identifies the sampling effort required for their accurate quantification, leading to improved assessments of forest ecosystem C and N budgets.

Simulation of water and chemical transport of chloride from the forest ecosystem to the stream

The study evaluates the biogeochemical model ForSAFE-2D, designed to simulate water and chemical transport from the forest to the stream, by simulating the hydrology and the transport of the chemical tracer chloride (Cl−) along a forest hillslope in Northern Sweden. The simulated Cl− exports were in balance with the simulated inputs but measurements suggested a net release of Cl− from the catchment. Underestimated deposition inputs (deposition peaks and possibly dry deposition) were probably and partially responsible for this mismatch. However, we could not exclude that other soil biogeochemical processes omitted in ForSAFE-2D could also contribute to Cl− exports from the catchment. The study showed that ForSAFE-2D is a promising tool to better understand the factors that regulate the chemical export from the forest to the stream. The results also confirmed that there are limitations in using Cl− as a tracer in forest ecosystems.

Converting croplands to orchards changes soil microbial community composition and co‐occurrence patterns

AbstractSoil microorganisms are key to uncovering the mechanisms driving variation in soil biogeochemical processes associated with land‐use change. A large number of croplands have been converted to orchards on the Chinese Loess Plateau due to the increased economic benefits which result. However, the microbial community and their functional composition remain poorly understood. In this study, soil samples were collected from croplands and orchards. Soil physicochemical properties and the community (represented by 16S rRNA for bacteria and ITS for fungi) were measured, and interactions among species and the soil organic matter (SOM) degradation via microbial metabolism and its associated genes were analyzed. Croplands converted to orchards affected bacterial and fungal community structure by increasing the relative abundance of Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria, and Ascomycota, whereas it decreased the α‐diversity of bacteria and fungi. The co‐occurrence network was larger and more complex within croplands than within orchards, indicating more extensive interactions and higher community stability potential. The abundance of potential genes related to cellulose and hemicellulose metabolism in orchards was higher than that in croplands, whereas the abundance of genes related to lignin decomposition was lower than that in croplands. In addition, the abundance of saprotrophic and symbiotrophic fungi in orchards was significantly lower than that in croplands (27 and 88%, respectively), whereas the abundance of pathotrophic fungi in orchards was almost six‐times that in croplands. The soil organic carbon (SOC) and soil C:N in orchards were significantly lower than that in croplands. Converting croplands to orchards significantly altered the microbial community composition and their functionality, as well as decreased the complexity of interaction between microorganisms. The decreased SOC and increased soil C:N ratio could be attributed to these variations. Improved management practices should be implemented for the maintenance of soil biodiversity and SOC in orchards to avoid soil degradation and ensure sustainable development.

Long-term forest restoration influences succession patterns of soil bacterial communities.

Microorganisms have a major influence on soil biogeochemical processes and vegetation establishment. However, their long-term succession patterns and short-term turnover are not well-understood in artificial forest ecosystems. The aim of the present study was to investigate the effects of stand ages and seasons on soil bacterial community in a chronosequence of Chinese Pinus massoniana plantations, in 3, 19, and 58-year-old plots. Soil physicochemical properties were measured in three stand ages between two seasons (dry-rainy). The soil bacterial community composition was determined by 16S rRNA Illumina HiSeq sequencing. The results showed that soil bacterial community diversity and structure significantly differed among three stand ages, but was not different between two seasons. The diversity of soil bacterial community increased with an increase in stand age. Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla in the three stands. The soil bacterial community structure in all the stands was influenced by soil pH, available phosphorus content, and litter phosphorus content. With the accumulation of available phosphorus, the relative abundance of Acidobacteria decreased, while that of Proteobacteria increased. These shifts suggested that dominant microbial communities transitioned from oligotrophic to copiotrophic with increasing stand age. Extending rotation periods could increase soil bacterial diversity, and in turn help improving soil quality of P. massoniana plantations.

Soil Biogeochemical Cycle Couplings Inferred from a Function-Taxon Network.

Soil biogeochemical cycles and their interconnections play a critical role in regulating functions and services of environmental systems. However, the coupling of soil biogeochemical processes with their mediating microbes remains poorly understood. Here, we identified key microbial taxa regulating soil biogeochemical processes by exploring biomarker genes and taxa of contigs assembled from metagenomes of forest soils collected along a latitudinal transect (18° N to 48° N) in eastern China. Among environmental and soil factors, soil pH was a sensitive indicator for functional gene composition and diversity. A function-taxon bipartite network inferred from metagenomic contigs identified the microbial taxa regulating coupled biogeochemical cycles between carbon and phosphorus, nitrogen and sulfur, and nitrogen and iron. Our results provide novel evidence for the coupling of soil biogeochemical cycles, identify key regulating microbes, and demonstrate the efficacy of a new approach to investigate the processes and microbial taxa regulating soil ecosystem functions.

Bacterial contributions of bio-crusts and litter crusts to nutrient cycling in the Mu Us Sandy Land

Desertification has become an important issue for the sustainable development of human society at global scale and has led to the changes in soil properties and vegetation cover. Biocrusts and litter crusts play roles in improving the soil microhabitat of sandy ecosystems. Soil microbial communities mediate ecosystem functions in various ecosystems, e.g., soil biogeochemical processes. However, limited information is available about how the underlying processes of bio-crusts/litter crusts restoration are driven by soil bacterial communities in sandy land. Here, we investigated the changes in soil bacteria from three groups (sandy land, bio-crusts, and litter crusts) and three soil layers (0–2 cm, 2–5 cm, 5–10 cm) with nine replicates each collected in July 2019 utilized high-throughput pyrosequencing of the V4-V5 rRNA gene region. Most soil nutrients (SOM, AP, AK, and TN) and enzyme activities (BG and DHA) had differences among the three groups and three soil layers. OTU richness and diversity of bacteria were positively correlated with most soil variables. The constructed co-occurrence networks between soil variables and bacterial communities, and within bacterial communities showed that bacterial taxa had closer relationships with all soil variables in crusts than sandy land and varied among the three sand groups (sandy land, bio-crusts, and litter crusts). The result showed that the composition of bacterial community was regulated mainly by soil variables and crust types. Compared with sandy land, more predictors in nutrient cycling were found in crust types. They played major roles in nutrient cycling in desert ecosystem restoration on the basis of random forest modeling. Our findings indicate some bacterial taxa may played the predominant roles in connecting with soil variables and other bacterial taxa across crusts types, and litter crusts and bio-crusts drive the nutrient cycling by mediating the restoration of bacterial taxa in sandy ecosystems.

Salinity and nutrient modulate soil bacterial communities in the coastal wetland of the Yellow River Delta, China.

The Yellow River Delta is the largest and youngest estuarine and coastal wetland in China and is experiencing the most active interactions of seawater and freshwater in the world. Bacteria played multifaceted influence on soil biogeochemical processes, and it was necessary to investigate the intermodulation between the soil factors and bacterial communities. Soil samples were collected at sites with different salinity degree, vegetations, and interference. The sequences of bacilli were tested using 16S rRNA sequencing method and operational taxonomic units were classified with 97% similarity. The soil was highly salinized and oligotrophic, and the wetland was nitrogen-restricted. Redundancy analysis suggested that factors related with seawater erosion were principal to drive the changes of soil bacterial communities and then the nutrient level and human disturbance. A broader implication was that, in the early succession stages of the coastal ecosystem, seawater erosion was the key driver of the variations of marine oligotrophic bacterial communities, while the increasing nutrient availability may enhance in the abundance of the riverine copiotrophs in the late stages. This study provided new insights on the characteristics of soil bacterial communities in estuarine and coastal wetlands.

Soil prokaryotic community structure and co-occurrence patterns on the fragmented Chinese Loess Plateau: Effects of topographic units of a soil eroding catena

Soil prokaryotes composition and diversity are the key to uncovering the mechanisms that drive variations in soil biogeochemical processes. Soil erosion is a primary factor that affects the spatial distribution of the soil prokaryotic community, but how soil prokaryotes in soil-eroding catena respond to environmental factors related to topography remains largely unclear. In this study, topsoils were sampled from three typical erosion geomorphic units (autonomous, transitional and depositional zones) in 2018 to identify the soil prokaryotic community and interactions among the species on the Chinese Loess Plateau. The alpha-diversity was greater but the beta-diversity was lower in the autonomous and transitional zones than in the depositional zone. Gammaproteobacteria and Bacteroidetes were 73% and 68% lower in the autonomous and transitional zones than in the depositional zone. In addition, Deltaproteobacteria, Acidobacteria, Firmicutes, Actinobacteria, Chloroflexi, Gemmatimonadetes and Nitrospirae were significantly higher in the autonomous and transitional zones. A less clustered network and fewer co-occurrences within the prokaryotic community and functional groups of processes were found in the depositional zone than in the autonomous and transitional zones. The alpha-diversity index was significantly negatively correlated with clay particles, soil water content, soil organic carbon (SOC), and ratio of SOC and nitrogen (C/N) but positively correlated with total nitrogen (TN). The higher relative abundances of copiotrophic groups (including Gammaproteobacteria, Bacteroidetes, etc.) in the depositional zone was mainly due to the increased SOC caused by the deposition of SOC-rich clay. Reassembly of the soil physico-chemistry characteristics among the topographic units significantly altered the soil prokaryotic community along the soil-eroding catena.

How Do Plants and Climatic Conditions Control Soil Properties in Hypersaline Tidal Flats?

Hypersaline tidal flats (HTF) are ecotones associated with mangrove ecosystems in arid and semiarid coasts. They are predominantly vegetated by halophytes which are related to environmental stabilization and fauna protection. Some plants thrive in HTF by modifying soil biogeochemical conditions at their rhizospheres, expanding across barren soils. Thus, we aimed to study rhizospheric and the adjacent bulk soils of the three most abundant plant species in HTF under a seasonal semiarid climate of northeastern Brazil. We analyzed both rhizospheric and bulk soils of vegetation patches in wet and dry seasons. We found that HTF soils are a heterogeneous system highly influenced by water availability and plant activity. The soils were mostly sandy textured, containing low C and N contents, and hypersaline conditions. Comparing bulk and rhizospheric soils, we concluded that plants changed their own rhizosphere by creating nutrient pools to improve survival under saline conditions. Seasonal changes also affected soil biogeochemical processes in HTF, mainly the bulk soils by changing water availability. Understanding rhizospheric changes by halophytes, their expansion over barren soils, and the amelioration of soil physicochemical conditions, are fundamental to provide support for preservation and management of coastal ecosystems, including HTF.

Salinity controls soil microbial community structure and function in coastal estuarine wetlands.

Soil salinity acts as a critical environmental filter on microbial communities, but the consequences for microbial diversity and biogeochemical processes are poorly understood. Here, we characterized soil bacterial communities and microbial functional genes in a coastal estuarine wetland ecosystem across a gradient (~5 km) ranging from oligohaline to hypersaline habitats by applying the PCR-amplified 16S rRNA (rRNA) genes sequencing and microarray-based GeoChip 5.0 respectively. Results showed that saline soils in marine intertidal and supratidal zone exhibited higher bacterial richness and Faith's phylogenetic diversity than that in the freshwater-affected habitats. The relative abundance of taxa assigned to Gammaproteobacteria, Bacteroidetes and Firmicutes was higher with increasing salinity, while those affiliated with Acidobacteria, Chloroflexi and Cyanobacteria were more prevalent in wetland soils with low salinity. The phylogenetic inferences demonstrated the deterministic role of salinity filtering on the bacterial community assembly processes. The abundance of most functional genes involved in carbon degradation and nitrogen cycling correlated negatively with salinity, except for the hzo gene, suggesting a critical role of the anammox process in tidal affected zones. Overall, the salinity filtering effect shapes the soil bacterial community composition, and soil salinity act as a critical inhibitor in the soil biogeochemical processes in estuary ecosystems.

Investigation of carbon dynamics in rhizosphere by synchrotron radiation-based Fourier transform infrared combined with two dimensional correlation spectroscopy

Rhizosphere, formed via the input of root exudates, is one of the most dynamic biological interfaces on earth. Investigation of carbon dynamics in rhizosphere is thus crucial for the understanding of soil biogeochemical processes. Herein, synchrotron radiation-based Fourier transform infrared (SR-FTIR) combined with two dimensional correlation spectroscopy (2D-COS) was used to probe and identify the changes of chemical constituents and functional groups of organic carbon on the root/soil interface in rhizosphere of two plants [Leptochloa chinensis (L.) Nees and Cyperus rotundus L.]. The SR-FTIR results showed obviously heterogeneous distributions of functional groups in rhizosphere at microscale. Specifically, regardless of plant species, about 20–30 μm regions in rhizosphere can be affected by root activities. The peak area ratios of organic-OH and aliphatic-C to clay-OH on the root/soil interface in rhizosphere were 4.04–8.48 times higher than that in bulk soil, providing direct evidence of the organic carbon storage due to root activities. 2D-COS analysis suggested that the root activities induced the first adsorption or sequestration of newly organics (3350 cm−1) on the root/soil interface, followed by the destruction of clay-OH (3621 or 860 cm−1), leading to the release of mineral associated organics and nutrients (e.g., 1510 and 1150 cm−1) from the soil. These results can enlarge our knowledge on the concentration, distribution, and dynamics of organic carbon in rhizosphere at the microscale level and also the environmental behaviors and fate of other elements and contaminants that associated with organic carbon in rhizosphere. CapsuleSR-FTIR combined with 2D-COS can explore the distribution and dynamics of organic carbon on the root/soil interface in rhizosphere.

Emergent properties of downed woody debris in canopy gaps: A response of the soil ecosystem to manipulation of forest structure

Natural forest disturbance events can influence soil biogeochemical processes in two ways - by creating downed woody debris (DWD; fallen tree boles or branches) and by creating canopy gaps that alter forest microclimate. DWD represents a substrate for microbial growth and a persistent store of carbon and nutrients, but microbial activity is also sensitive to temperature and moisture. We studied the potential interaction of DWD and canopy gaps on soil microbial processes, and wondered if microclimatic conditions resulting from the manipulation of forest structure would be enough to inhibit production, thereby altering a critical ecosystem process. Gaps and DWD (>10 cm diameter) were added to a maturing, even-aged, second-growth northern hardwood forest (the Flambeau Experiment; N Wisconsin, USA) to enhance structural complexity and promote key ecosystem processes typically associated with late-successional forests. We investigated the influence of DWD and gaps on soil microbial community composition, extracellular enzyme activity and soil characteristics. Soils were sampled near intermediately and highly decayed DWD and 2 m away from DWD (control) in gaps and closed canopy a decade after manipulation.DWD decomposition influenced the surrounding soil differentially depending on decay class and canopy condition. Mean C- and P-potential extracellular enzyme activities (BG, BX and AP) were enhanced near highly decayed DWD in gaps. The relative abundance of bacteria (actinomycete, anaerobic, gram-negative and gram-positive) remained constant in gaps but decreased from May to August in closed canopy. In gaps, soil total exchangeable cations increased by 34.6%, available phosphorus by 152% and fungal to bacterial ratios by 23.3% but temperatures decreased by 3.42% suggesting that canopy condition continues to affect soil properties and microbial processes a decade after gap creation. These results highlight the contribution of DWD to the forest floor and the influence of decaying wood characteristics on belowground ecosystems critical to future forest productivity. Retaining or adding heterogeneously distributed DWD of varying decay status may be essential to maintain ecosystem functions associated with nutrient cycling and microbial community dynamics in managed forests.

Effects of Sea Animal Activities on Tundra Soil Denitrification and nirS- and nirK-Encoding Denitrifier Community in Maritime Antarctica.

In maritime Antarctica, sea animals, such as penguins or seals, provide a large amount of external nitrogen input into tundra soils, which greatly impact nitrogen cycle in tundra ecosystems. Denitrification, which is closely related with the denitrifiers, is a key step in nitrogen cycle. However, effects of sea animal activities on tundra soil denitrification and denitrifier community structures still have received little attention. Here, the abundance, activity, and diversity of nirS‐ and nirK-encoding denitrifiers were investigated in penguin and seal colonies, and animal-lacking tundra in maritime Antarctica. Sea animal activities increased the abundances of nirS and nirK genes, and the abundances of nirS genes were significantly higher than those of nirK genes (p < 0.05) in all tundra soils. Soil denitrification rates were significantly higher (p < 0.05) in animal colonies than in animal-lacking tundra, and they were significantly positively correlated (p < 0.05) with nirS gene abundances instead of nirK gene abundances, indicating that nirS-encoding denitrifiers dominated the denitrification in tundra soils. The diversity of nirS-encoding denitrifiers was higher in animal colonies than in animal-lacking tundra, but the diversity of nirK-encoding denitrifiers was lower. Both the compositions of nirS‐ and nirK-encoding denitrifiers were similar in penguin or seal colony soils. Canonical correspondence analysis indicated that the community structures of nirS‐ and nirK-encoding denitrifiers were closely related to tundra soil biogeochemical processes associated with penguin or seal activities: the supply of nitrate and ammonium from penguin guano or seal excreta, and low C:N ratios. In addition, the animal activity-induced vegetation presence or absence had an important effect on tundra soil denitrifier activities and nirK-encoding denitrifier diversities. This study significantly enhanced our understanding of the compositions and dynamics of denitrifier community in tundra ecosystems of maritime Antarctica.