Soil Biogeochemical Processes Research Articles

Soil Slope Exposure Affects Physico-Chemical and Microbiological Properties in Soil Aggregate Size Fractions

Slope exposure is known to affect soil biogeochemical processes in mountainous forest ecosystems, but little attention has yet been paid to its influence at a soil aggregate scale. Therefore, we evaluated the effects of slope exposure (north- vs south-facing slope) on the physico-chemical and microbiological properties of bulk soil and dry-sieved and water-stable aggregate size fractions in both organic (OF) and mineral (AE) horizons in an Italian alpine forest. The changes in organic carbon (OC) and nitrogen (ON) fractions were assessed together with a battery of thirteen enzyme activities involved in the main nutrient cycles. In addition, soil biological properties including microbial biomass (estimated as double-stranded DNA content), and microbial activity (assessed as the ratio between the extra-(exDNA) and intracellular (iDNA) fractions of the total soil DNA pool) were determined. The OF horizon at the north-facing slope was enriched in recalcitrant and insoluble OC and ON fractions and characterized by a lower microbial activity, as indicated by the higher exDNA/iDNA ratio with respect to the south-facing slope. On the contrary, exDNA and iDNA contents, microbial biomass, as well as most of the enzyme activities, reached higher levels at the southern exposure in the AE horizon. These exposure-effects were bulk soil- and aggregate size fraction-specific. Overall, lower values of the chemical and microbiological parameters were found in the water-stable fraction. Our findings indicate that slope exposure (and thus topography), soil horizon, and aggregate size distinctly influence soil OC dynamics in mountain ecosystems.

Effects of Forest Gaps on the Structure and Diversity of Soil Bacterial Communities in Weeping Cypress Forest Plantations.

The decline in forest ecological function caused by pure forest plantations planted in the Yangtze River basin is becoming increasingly serious. To investigate this problem, we selected the local low-efficiency weeping cypress plantations for forest gap transformation. Three forest gap sizes, specifically large, medium, and small gaps, were established, and the effects of gap sizes on soil bacterial community structure and diversity in winter and summer were studied compared to no gaps (CK; control). Compared to CK, forest gaps had a significant effect on soil organic carbon (SOC) and soil total nitrogen (TN), and the highest values of SOC and soil TN under two seasons occurred in large forest gaps. The interactions of forest gap sizes and seasons had significant effects on pH, SOC, TN, and alpha diversity indices, including Simpson, Chao1, and ACE indices. Compared to winter, forest gaps significantly increased the soil bacterial community diversity indices in summer. Forest gap sizes significantly affected the composition of the bacterial community, but the composition of the dominant bacteria at the phyla and genera levels was similar. Linear discriminant effect size (LEfSe) analysis showed that there were 32 indicator bacterial species in two seasons. Co-occurrence network analysis revealed that the relationship of the soil bacterial community at the phyla level was complex, and there was a significant positive correlation among bacterial species. Soil bulk density (BD) and soil moisture (SM) significantly affected the soil bacterial alpha diversity indices. The composition of the dominant bacteria at the phyla level was significantly affected by soil microbial carbon (MBC), whereas the composition of dominant bacteria at the genera level was affected by soil hydrolysable nitrogen (AN) and the carbon/nitrogen (C/N) ratio. In this study, compared to the other forest gaps, large forest gaps were more conducive to the accumulation of soil nutrients, thus improving the structure of the soil bacterial community. Importantly, changes in the soil bacterial community structure due to gap formation may have profound effects on soil biogeochemical processes in weeping cypress forest plantations.

Resource landscape, microbial activity, and community composition under wintering crane activities in the Demilitarized Zone, South Korea.

Endangered cranes migrate to the Demilitarized Zone in Korea in search for habitat and food during winter. While cranes have the potential to influence soil biogeochemical processes via dropping, foraging, and walking, few studies have investigated ecological roles of migrating birds in the new habitat. Here, we explored how cranes alter resource landscape (the amount and quality of carbon) and microbial community in soil. We set up control (fenced, no crane access) and treatment (free crane activities) plots (n = 6, respectively) in a rice paddy, and collected soils at 0–15 cm three months after the crane migration. Soils were tested for total carbon, total nitrogen, water extractable organic carbon, and Diffuse Reflectance Infrared Fourier Transform Spectroscopy, along with microbial parameters (biomass, respiration, community composition). The wintering crane activity significantly increased total carbon and nitrogen contents, but decreased the ratio of CH (aliphatic) to COO (carboxylic) in soil. Also, both microbial biomass and respiration was greater in soils under crane activities. Bacterial and fungal community composition differed with or without crane activities, with treatment soils harboring more diverse microbial communities. Our results demonstrate that crane migration created a distinct system with altered resource landscape and microbial community, highlighting beneficial effects of migratory cranes on the soil biogeochemical processes in rice paddies. This study may help encourage more farmers, local governments, and the public to participate in crane conservation campaigns targeted at rice fields.

Short-Term Responses of Soil Organic Carbon Pool and Crop Performance to Different Fertilizer Applications

Some intensive farmers tend to expect short-term beneficial effects by applying soil amendments, but inconsistent fertilization practices are often conducted, causing economic losses and environmental problems. This study aimed at investigating the short-term application effects of different soil amendments on soil organic carbon (SOC) fractions, biogeochemical properties, and crop performance for finding the best land management approach using one-year field trial growing Chinese cabbages. This filed experiment was conducted in 2020 and included eight fertilizer treatments: control (w/o fertilizers), chemical fertilizer (CF), manure compost (MC), double MC amount (2MC), CF + MC, CF + rice husk (RH), MC + RH, and CF + MC + RH. As a result, the concentrations of recalcitrant to labile C forms, including Loss-On-Ignition C (LOIC), Walkley-Black C, permanganate oxidizable C (POXC), and microbial biomass C, were the highest in a mixture of MC and RH and 2MC. Additionally, the treatment with the largest difference from the control in key soil parameters was 2MC: bulk density (10%), total N (30%), available P (186%), and CO2 (433%) and N2O (825%) emissions, followed by MC + RH. Moreover, more than 20% higher fresh weight (FW) of cabbage was found in 2MC and MC + RH than in the control. Therefore, these two organic amendments appeared to benefit SOC storage and overall soil biogeochemical processes, contributing to higher biomass crop production. Moreover, LOIC significantly correlated to bulk density, available P and K, and FW, while POXC significantly correlated to N concentration in plants, indicating the short-term fertilization effects on the status of SOC fractions and the qualities of soil and plant by applying soil amendments. Overall, our findings suggest that applying MC + RH would be an alternative to replace the conventional farming practices for promoting soil quality and crop performance, but further studies to sustain the application effects of this amendment should be monitored for longer durations.

The Right Place at the Right Time: Seasonal Variation of Bacterial Communities in Arid Avicennia marina Soils in the Red Sea Is Specific to Its Position in the Intertidal

Mangrove forests play an important role in facilitating biogeochemical pathways and cycling acting as blue carbon sinks. These services are primarily regulated by the activity of the soil microbiome. However, there is still limited research into spatial and temporal variation patterns of bacterial community assemblages in mangrove soils. This study investigated important ecological scales of microprocesses that govern microbial communities in an arid mangrove ecosystem. Shifts in microbial community composition were influenced by fluctuations in environmental factors within the mangrove forests of the Red Sea influenced by seasonal changes in sea level. Notably, in summer microbial communities in shrub sites differed significantly from the fringe and the winter samples, with lower alpha diversity yet a higher dominance of specialized species capable of surviving in extreme conditions. The onset of dispersal limitation and heterogenous selection and the reduction of drift are likely the main forces shaping community assemblages. Specifically, in summer lower mean tidal levels eliminate tidal inundation creating a harsh high salinity and high temperature environment with no tidal connection thereby influencing the onset of dispersal limitation. An increased understanding of the spatial and temporal variation of bacterial communities is critical when assessing delivery of ecosystem services and their role in soil biogeochemical processes.

A Longitudinal Study of the Microbial Basis of Nitrous Oxide Emissions Within a Long-Term Agricultural Experiment

Much of the global nitrous oxide emissions are derived from agricultural management driving microbial N transformations. Crop rotation, no-till, and cover cropping are feasible conservation agronomic strategies used to prevent N losses to the environment, though their effect on soil microbial N cycling at the field scale remains relatively unknown. Our goal was to determine the effect of crop rotation (continuous corn [Zea mays L.], CCC; and continuous soybean [Glycine max (L.) Merr.], SSS), tillage (no-till, NT; and chisel tillage, T), and cover crops (cover crop mixture, CC; and no cover crop, NCC) on the quantification of functional genes related to the N cycle from different times throughout the growing season. The study was conducted during the growing season of the cash crops following the first season of cover crops introduced after 23 years of management. Using quantitative polymerase chain reaction (qPCR) techniques, we quantified nifH (N2 fixation), amoA (nitrification) and nirK, nirS, and nosZ (denitrification). Our results show that CCC increased nitrous oxide emissions by 44% compared to SSS and reduced soil pH by nearly 1 unit. The reduction in soil pH, coupled with an increase in fertilizer-derived ammonium, caused ammonia-oxidizing bacteria (AOB) and nirK copy numbers to increase. The SSS rotation showed opposite results. Bacterial denitrification via the nirK pathway was likely the N cycle mechanism behind nitrous oxide emissions in CCC. The cover crop mixture of cereal rye [Secale cereale L.] and hairy vetch [Vicia villosa Roth] reduced soil nitrate levels, though they did increase nitrous oxide emissions, likely due to priming and the inclusion of a legume in the cover crop mixture. Nitrous oxide emissions were affected by sampling date, crop rotation, and cover crop use, suggesting management factors that add abundantly available N alter the microbial N cycle directly or indirectly. Chisel tillage increased the abundance of all N cycle genes compared to no-till. Together, our work adds further insight into the microbial N cycle, especially nitrous oxide evolution, from three common conservation agricultural management practices, contributing to our understanding of key soil biogeochemical processes.

Microplastics as an Emerging Environmental Pollutant in Agricultural Soils: Effects on Ecosystems and Human Health

Microplastics are <5 mm in size, made up of diverse chemical components, and come from multiple sources. Due to extensive use and unreasonable disposal of plastics, microplastics have become a global environmental issue and have aroused widespread concern about their potential ecological risks. This review introduces the sources, distribution and migration of microplastics in agricultural soil ecosystems. The effects of microplastics on soil physicochemical properties and nutrient cycling are also discussed. Microplastics can alter a series of key soil biogeochemical processes by changing their characteristics, resulting in multiple effects on the activities and functions of soil microorganisms. The effects of microplastics on soil animals and plants, the combined effects of microplastics and coexisting pollutants (organic pollutants and heavy metals), and their potential risks to human health are also discussed. Finally, prevention and control strategies of microplastic pollution in agricultural soil ecosystems are put forward, and knowledge gaps and future research suggestions about microplastic pollution are given. This review improves the understanding of environmental behavior of microplastics in agricultural soil ecosystems, and provides a theoretical reference for a better assessment of the ecological and environmental risks of microplastics.

Effect of elevated atmospheric CO2 concentration and temperature on volatile halogenated organic compound content in soils of Schima superba and Cunninghamia lanceolata seedlings.

Global changes caused by the increases of atmospheric CO2 concentration and temperature have important effects on soil biogeochemical processes. The synthesis and release of volatile halogenated organic compounds (VOXs) is an important pathway for soil to participate in the global material cycle and energy flow. In this study, Schima superba and Cunninghamia lanceolata seedlings in the southern subtropics were selected as the research objects. Four treatments, including control (CK), elevated CO2 concentration (EC), elevated temperature (ET) and elevated both factors (EC+ET) were set up. The effects of EC and ET on soil VOXs formation were studied by an open-top chamber system coupled with a purging and trapping gas chromatography/mass spectrometry. The results showed that VOXs content in the soil of S. superba seedlings was 0.065-0.252 ng·g-1, which was higher than that of C. lanceolata (0.038-0.136 ng·g-1). At the EC, ET and EC+ET treatments, VOXs contents were reduced in soils of both species. The effect of ET was the most significant, with the decrease rates of 74.2% and 72.1% in both soils, respectively. The change of VOXs content with increasing temperature mainly attributed to the changes of soil moisture and nitrogen content. The content of VOXs in the soils of S. superba seedlings decreased more than that of C. lanceolata under different treatments. In CK, EC, ET and EC+ET treatment, bromodichloromethane (BDCM) (27.5%, 36.7%, 32.9%, 32.6%) and tetrachloromethane (TCM) (9.0%, 16.8%, 22.7%, 15.8%) were the main VOXs in the soil of S. superba seedlings, respectively, while BDCM and dibromomethane (DBM) were the main VOXs in the soil of C. lanceolata seedlings. BDCM accounted for 31.9%, 38.2%, 40.9% and 37.2% of the VOXs content in each treatment, and DBM accounted for 17.9%, 16.5%, 19.2% and 16.0% of the VOXs content, respectively. Simulating elevated atmospheric CO2 concentration and temperature was conducive to more comprehensive reflection of the ecological effect of global climate change, and it could provide data support for improving the VOCs flux model.

Spartina alterniflora invasion has a greater impact than non-native species, Phragmites australis and Kandelia obovata, on the bacterial community assemblages in an estuarine wetland

The invasion of Spartina alterniflora poses a serious threat to the sustainability of native ecosystems worldwide. However, compared with other non-native plants (e.g., Phragmites australis and Kandelia obovata), how Spartina alterniflora invasion influences the community structure of bacteria and their assembly processes and functionality remains elusive. Here, we characterized the diversity, community structure, assembly processes and functional guilds of bacteria underneath five plant species and a bare tidal flat at three soil depths in an estuarine wetland. We found that plant species played a more important role than soil depth in mediating the bacterial community structure. Compared with bare tidal flats, the native species Cyperus malaccensis, rather than Scirpus triqueter, significantly changed the bacterial community structure. However, S. alterniflora invasion increased bacterial alpha diversity and significantly altered the bacterial community structure by enriching Chloroflexi, Bacteroidetes and Firmicutes while reducing Acidobacteria, Nitrospirae and Gemmatimonadetes. The invasion of P. australis and translocation of K. obovata had less pronounced effects on the bacterial community structure. Total carbon, total nitrogen and salinity were the key environmental factors mediating the bacterial community structure. Overall of all the non-native plant species, the invasion of S. alterniflora increased the relative importance of stochastic processes in the assembly of bacterial communities, and shifted the bacterial functional profiles by stimulating sulfur cycling groups and suppressing nitrogen cycling groups. Altogether, our results suggest that S. alterniflora invasion has a greater effect than P. australis invasion or K. obovata translocation on the profiles and assembly processes of the bacterial communities, with important implications for soil biogeochemical processes in coastal wetlands.

Microbial biomass in forest soils under altered moisture conditions: A review

AbstractMicrobial biomass is known to decrease with soil drying and to increase after rewetting due to physiological assimilation and substrate limitation under fluctuating moisture conditions, but how the effects of changing moisture conditions vary between dry and wet environments is unclear. Here, we conducted a meta‐analysis to assess the effects of elevated and reduced soil moisture on microbial biomass C (MBC) and microbial biomass N (MBN) across a broad range of forest sites between dry and wet regions. We found that the influence of both elevated and reduced soil moisture on MBC and MBN concentrations in forest soils was greater in dry than in wet regions. The influence of altered soil moisture on MBC and MBN concentrations increased significantly with the manipulation intensity but decreased with the length of experimental period, with a dramatic increase observed under a very short‐term precipitation pulse. Moisture effect did not differ between coarse‐textured and fine‐textured soils. Precipitation intensity, experimental duration, and site standardized precipitation index (dry or wet climate) were more important than edaphic factors (i.e., initial water content, bulk density, and clay content) in determining microbial biomass in response to altered moisture in forest soils. Different responses of microbial biomass in forest soils between dry and wet regions should be incorporated into models to evaluate how changes in the amount, timing, and intensity of precipitation affect soil biogeochemical processes.

Soil enzyme stoichiometry revealed the changes of soil microbial carbon and phosphorus limitation along an elevational gradient in a Pinus taiwanensis forest of Wuyi Mountains, Southeast China

Understanding changes in soil enzyme activities and ecoenzymatic stoichiometry is important for assessing soil nutrient availability and microbial nutrient limitation in mountain ecosystems. However, the variations of soil microbial nutrient limitation across elevational gradients and its driving factors in subtropical mountain forests are still unclear. In this study, we measured soil properties, microbial biomass, and enzyme activities related to carbon (C), nitrogen (N), and phosphorus (P) cycling in Pinus taiwanensis forests at different altitudes of Wuyi Mountains. By analyzing the enzyme stoichiometric ratio, vector length (VL), and vector angle (VA), the relative energy and nutrient limitation of soil microorganisms and its key regulatory factors were explored. The results showed that β-glucosaminidase (BG) activities increased along the elevational gradient, while the activities of β-N-acetyl glucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (AcP) and (NAG+LAP)/microbial biomass carbon (MBC) and AcP/MBC showed the opposite trend. Enzyme C/N, enzyme C/P, enzyme N/P, and VL were enhanced with increasing elevation, while VA decreased, indicating a higher degree of microbial P limitation at low elevation and higher C limitation at high elevation. In addition, our results suggested that dissolved organic carbon and microbial biomass phosphorus are critical factors affecting the relative energy and nutrient limitation of soil microorganisms at different elevations. The results would provide a theoretical basis for the responses of soil carbon, nitrogen, and phosphorus availability as well as the relative limitation of microbial energy and nutrition to elevational gradients, and improve our understanding of soil biogeochemical cycle process in subtropical montane forest ecosystems.

Enhanced Microbial Ferrihydrite Reduction by Pyrogenic Carbon: Impact of Graphitic Structures.

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial (Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H2 atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H2 treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Acidification of soil due to forestation at the global scale

Forestation is a key strategy to mitigate climate change caused by anthropogenic carbon dioxide emissions. However, the impacts of forestation on soil pH remain unclear, despite critical roles of soil pH in regulating key soil biogeochemical processes. Here, we collected a global dataset of soil pH change after forestation, which included 1082 observations from 171 published papers. Results showed that soil pH declined significantly by 0.23 after forestation over the globe. Soil pH consistently declined after forestation, no matter the forest was established naturally or by planting, on croplands or grasslands. The decline of pH after forestation was generally larger in neutral soils (pH 6–7) than in acidic soils (pH < 6) and alkaline soils (pH > 7), and larger in boreal and temperate forests than in tropical forests. Soil pH decreased significantly in humid areas but not in arid regions. Random forest analysis showed that climate was the most important regulatory factor to influence soil pH change after forestation. Mean annual temperature and precipitation probably affected soil pH both directly and indirectly via altering soil physiochemical properties. Given vital roles of soil pH in regulating carbon and nutrient dynamics, our findings have important implications for the long-term impacts of forestation on carbon and nutrient dynamics.

Soil freezing-thawing induces immediate shifts in microbial and resource stoichiometry in Luvisol soils along a postmining agricultural chronosequence in Western Germany

Freeze-thaw (FT) events exert a great physiological stress on soil microorganisms and hence impact biogeochemical processes in soils. As numerous environmental factors affect microbial and chemical responses to FT, a better understanding of the leverage factors that regulate the responses to FT events is required. To date, FT-induced shifts and transformations in microbial and resource stoichiometry have received particularly little attention. We exposed fifteen Luvisol soils with different time after restoration and corresponding differences in soil organic C contents from a postmining agricultural chronosequence to a single FT event and analysed changes in soil chemistry and microbial stoichiometry one hour and eighteen hours after thawing. FT considerably altered soil biochemical attributes within the first hours of thawing. Microbial biomass C declined substantially after FT, and its relative losses were positively correlated with enhanced dissolved organic C contents. Thus, microbial cell lysis likely led to the significant increase of dissolved organic C. Moreover, microbial biomass C losses were disproportionally higher in C-rich soils, suggesting that soil microorganisms in high-C soils might be particularly prone to FT stress. Microbial biomass N marginally decreased one hour after thawing, yet returned to initial levels eighteen hours after thawing. The alternating responses of microbial biomass C and N caused a strong stoichiometric reduction of the microbial C:N ratio. The resulting microbial oversaturation with N relative to C is likely the first step in the chain of processes that generally lead to the high N losses commonly recorded in agricultural soils in the aftermath of FT events. Metabolic activity of the soil microbial community increased with the relative decline of the microbial biomass C:N ratio eighteen hours after thawing, suggesting increased levels of microbial metabolic expenditure due to stoichiometric shifts. The strength of the FT-driven biochemical responses was strongly dependent on soil organic C content, indicating that high-C soils might be especially vulnerable to initial C and N losses due to shifts in microbial stoichiometry.

Evolution of the transport properties of soil aggregates and their relationship with soil organic carbon following land use changes

Aggregates are functional units to describe the impact of soil structural changes on physical and biogeochemical processes in soil. Both incubation and field experiments have shown that changing agricultural practices could reshape the intra-aggregate structure in a matter of days, but most such data were obtained from a single time-point and it is hence impossible to interpret that such a change was just a temporal transition or the new equilibria towards which the aggregates had evolved following the management changes. Understanding this is indispensable as intra-aggregate structure and its ability to transport substrates modulate all biogeochemical processes involved in soil carbon and nutrient cycle. This paper investigates this using soil samples archived from a reversion experiment initiated in 2008 at Rothamsted Research (UK), where parts of a plot that had been fallow since the 1950 s were converted to wheat or grass in 2008. We used X-ray Computed Tomography images, acquired at voxel size 1.5 µm, of aggregates in the archived soils to investigate the evolution of transport property of the aggregates over time, as well as its relationship with soil organic carbon (SOC). We also evaluated the development of pore connectedness following the conversion. The results show that the transport ability of the aggregates explains the SOC change much better than the porosity, and that noticeable changes in porosity of the connected pores and their ability to transport substrates did not emerge until the sixth year after the conversion. Ten years after the conversion, there was still no sign of the porosity of the connected pores and the bulk diffusion coefficient to plateau. In addition, we found the conversion to grass changed the intra-aggregate pore geometry significantly in that the bulk diffusion coefficients of their aggregates trends with their porosities in a way differing significantly from those for the bare fallow and arable treatments. All these suggest that the intra-aggregate reconfiguration following the conversion is a slow process, and that the ability of pore space to transport substrates is more important than the habitat they provide in SOC stabilisation.

“Triple locks” on soil organic carbon exerted by sphagnum acid in wetlands

Sphagnum acid, a major phenolic metabolite of Sphagnum mosses, is considered to play a key role in the remarkable accumulation of organic matter in Sphagnum-dominated wetlands. However, while previous studies have mainly focused on the antimicrobial property of Sphagnum metabolites, the effects of sphagnum acid on other soil biogeochemical processes related to soil organic carbon (SOC) stabilization are poorly investigated. Here we employ a series of incubation experiments involving sphagnum acid amendment into natural wetland soils and artificial reaction systems to comprehensively evaluate potential pathways driving SOC stabilization by sphagnum acid. We show that sphagnum acid greatly enhances the reductive dissolution of ferric iron [Fe(III)] (hydr)oxides in both wetland soils and synthetic Fe(III)-organic matter complexes, induces the formation of amorphous and poorly-crystalline Fe(III) and increases Fe-bound organic carbon. Furthermore, sphagnum acid quenches hydroxyl radicals produced by Fenton reactions during redox oscillations and indirectly suppresses the activity of phenol oxidase and β-glucosidase by inhibiting microbial activity, acidification and increasing enzyme sorption to newly formed Fe(III). Hence, sphagnum acid exerts “triple locks” on SOC through (i) Fe protection of organic matter, (ii) radical quenching, and (iii) microbial suppression. These important but under-investigated pathways are all closely related to Fe transformation, highlighting the key role of Sphagnum metabolite–Fe–organic matter interactions in regulating wetland soil carbon sinks. Changing distribution of Sphagnum under climate change and human activity will have cascading effects on soil Fe species, microbial activity and SOC stabilization, which warrants further investigation in the attempt to protect and increase wetland soil carbon stocks.

Metagenomic Analyses of Plant Growth-Promoting and Carbon-Cycling Genes in Maize Rhizosphere Soils with Distinct Land-Use and Management Histories.

Many studies have shown that the maize rhizosphere comprises several plant growth-promoting microbes, but there is little or no study on the effects of land-use and management histories on microbial functional gene diversity in the maize rhizosphere soils in Africa. Analyzing microbial genes in the rhizosphere of plants, especially those associated with plant growth promotion and carbon cycling, is important for improving soil fertility and crop productivity. Here, we provide a comparative analysis of microbial genes present in the rhizosphere samples of two maize fields with different agricultural histories using shotgun metagenomics. Genes involved in the nutrient mobilization, including nifA, fixJ, norB, pstA, kefA and B, and ktrB were significantly more abundant (α = 0.05) in former grassland (F1) rhizosphere soils. Among the carbon-cycling genes, the abundance of 12 genes, including all those involved in the degradation of methane were more significant (α = 0.05) in the F1 soils, whereas only five genes were significantly more abundant in the F2 soils. α-diversity indices were different across the samples and significant differences were observed in the β diversity of plant growth-promoting and carbon-cycling genes between the fields (ANOSIM, p = 0.01 and R = 0.52). Nitrate-nitrogen (N-NO3) was the most influential physicochemical parameter (p = 0.05 and contribution = 31.3%) that affected the distribution of the functional genes across the samples. The results indicate that land-use and management histories impact the composition and diversity of plant growth-promoting and carbon-cycling genes in the plant rhizosphere. The study widens our understanding of the effects of anthropogenic activities on plant health and major biogeochemical processes in soils.

Combined selective gamma irradiation and pulverized soil inoculation for ecologically relevant soil microfauna studies

The separate and collective role of soil nematodes and protists at community level in soil biogeochemical processes remains poorly quantified due to the lack of appropriate methodologies to study them independently from other soil biota under conditions emulating natural soil. Here we present a methodological exploration for the selective removal of nematodes and protists using gamma irradiation and the subsequent reinoculation with metazoan-free soil powder inoculum that allows reconstructing a complex microbial food web comprising the native microflora and soil protist communities, without other fauna. Nematodes, and culturable flagellates and ciliates were removed from 6 kGy onwards, while a small fraction of the amoebae survived (8% of the entire protist population) at 16 kGy but were totally removed at 27 kGy. Reinoculation with pulverized soil powder successfully established a protist community of similar size and composition as the control soil as assessed by both amplicon sequencing-based detection and microscopic observation after cultivation. A viable native microflora community remained from 6 to 16 kGy doses, but microbial activity was completely inhibited at 27 kGy. We suggest that, in ecological relevant experiments where selective removal of nematodes or other fauna is the purpose and protist community is meant to be kept intact, gamma irradiation needs to be followed routinely by soil powder inoculation in order to restore the suppressed protist community. This experiment also opens up new perspectives for studying the ecological roles of the entire protist community or specific groups thereof (flagellates and ciliates) in real soil and in the presence of a viable native microflora community.

Distinct and dynamic distributions of multiple elements and their species in the rice rhizosphere

AimsThe biogeochemical cycles of elements from soils to plants are mainly governed by their rhizosphere processes. Understanding these processes is challenging and remains largely unresolved due to the complex interrelationships among different elements and due to a lack of appropriate techniques for simultaneous spatiotemporal monitoring.MethodsThis study employed an updated In-situ Porewater Iterative (IPI) sampler to collect porewater across the rice rhizosphere at a spatial resolution of 1.7 mm and a time interval of 3–10 days. An IPI sampler array (0–22 mm measurement distance every 1.7 mm) was adopted to capture the in situ spatiotemporal dynamics of ten elements (Fe, Mn, As, P, S, Cr, Co, Zn, Sb and Cd) in the paddy rhizosphere to examine their covarying changes in time and space dimensions, with an emphasis on As and Cd.ResultsThe findings revealed that the solute-phase concentration of most elements, other than Sb and Cd, increased to a peak after 30 days of paddy soil flooding and then decreased. Additionally, Sb and Cd continuously decreased during flooding. Fe (−52%), Mn (−17%), P (−43%), Co (−11%), and As species (−74%) were substantially immobilized within a 10 mm zone around the roots, while Zn (28%) and Cd (41%) increased. Almost all arsenite-oxidizing genes were significantly promoted in the rhizosphere.ConclusionsOur study showed most sampled elements covaried with Fe both in time and space in the rhizosphere, but the elements are temporally and spatially determined by multiple biogeochemical processes in soils as well as exudates from plant roots.Graphical abstract

Changes in soil enzymatic activity in a P-limited Mediterranean shrubland subject to experimental nitrogen deposition

N deposition is a key driver of global environmental change (GEC). The effects of N addition on soil enzymes which play a key role in biogeochemical cycles and functioning of terrestrial ecosystems remain unclear, especially in vulnerable ecosystems such as P- or/and water-limited Mediterranean ecosystems. Here, we studied seasonal effects of N deposition on the activities of five soil extracellular enzymes in a semi-arid, P-deficient Mediterranean shrubland, and the association with physical and nutritional soil variables. Over the study period N deposition caused direct effects on soil enzymatic activities due to N-accumulation that increased its availability and triggered ecosystem mobilization of less available nutrients, such as P, in accordance with optimum allocation theory. N deposition intensified P limitation, enhanced the activity of phosphatases and reduced the activity of N- and C-mobilizing enzymes; it is likely this could lead to a longer term increase in soil C storage, due to a reduction in soil organic matter decomposition. The prompt response of soil enzyme activity to experimental N deposition indicates the high vulnerability of soil biogeochemical processes in this Mediterranean shrubland if submitted to prolonged N deposition.