Soil Redox Potential Research Articles

Aquatic macrophytes mitigate the short-term negative effects of silver nanoparticles on denitrification and greenhouse gas emissions in riparian soils

Silver nanoparticles (AgNPs) are increasingly released into the aquatic environments because of their extensive use in consumer products and industrial applications. Some researchers have explored the toxicity of AgNPs to nitrogen (N) and carbon (C) cycles, but little is known about the role of aquatic plants in regulating the impact of AgNPs on these biogeochemical processes and related microorganisms. Here, two 90-day pot experiments were conducted to determine the effect of AgNPs on denitrification rates and greenhouse gas emissions in riparian wetland soils, with or without emergent plants (Typha minima Funck). As a comparison, the toxicity of equal concentration of AgNO3 was also determined. The results showed that AgNPs released a great quantity of free Ag+, most of which was accumulated in soils, while little (less than 2%) was absorbed by plant shoots and roots. Both AgNPs and AgNO3 could increase the soil redox potential and affect the growth and nutrient (N and phosphorus) uptake of plants. In soils with plants, there was no significant difference in denitrification rates and emissions of N2O and CH4 between control and AgNPs or AgNO3 treatments at all tested concentrations (0.5, 1 and 10 mg kg−1). However, low levels of AgNPs (0.5 mg kg−1) significantly enhanced CO2 emission throughout the experiment. Interestingly, in the absence of plants, a high dosage (10 mg kg−1) of AgNPs generally inhibited soil denitrification and stimulated the emissions of CO2, CH4 and N2O in the short-term. Meanwhile, the abundance of key denitrifying genes (nirS and nirK) was significantly increased by exposure to 10 mg kg−1 AgNPs or AgNO3. Our results suggest that emergent plants can alleviate the short-term negative effects of AgNPs on N and C cycling processes in wetland soils through different pathways.

Soil aeration and redox potential as function of pore connectivity unravelled by X‐ray microtomography imaging

AbstractPlatinum (Pt)‐tipped electrodes are frequently employed to measure the soil redox potential (EH). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air‐filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, EH readings behave non‐uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O2) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah‐horizon, Calcaric Gleysol). We combined physio‐chemical methods with non‐destructive X‐ray computed microtomography (μCT) and 3D‐image analysis. μCT scans were conducted at three‐time points, that is, reducing conditions with EH < −100 mV (CT‐1), the transition from reducing towards oxidising conditions with an EH increase > 5 mV h−1 (CT‐2), and oxidising conditions with EH > 300 mV (CT‐3). We observed that the shift from reducing towards oxidising conditions took place at an air‐filled porosity (εCT) of ~0.03 cm3 cm−3, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm3 cm−3. Besides the relation of EH and ε, image analysis revealed that a connected εCT (εCT_conn) of ~0.02 cm3 cm−3 is needed to enable enhanced O2 diffusion from the soil surface towards the Pt surface and facilitate a straightforward EH response. We conclude that εCT_conn is a critical parameter to assess aeration processes in temporarily water‐saturated soils to characterise a switch in redox conditions.Highlights Usually, soil redox dynamics are related to the air‐filled porosity (εCT) but here its connected portion (εCT_conn) was found more relevant. 3D X‐ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process. Connected εCT (εCT_conn) of ~0.02 cm3 cm−3 facilitated oxidising soil conditions. εCT_conn is a critical parameter to assess the aeration process in temporarily water‐saturated soils.

Influence of tetracycline on arsenic mobilization and biotransformation in flooded soils

This study examined the effect of tetracycline addition on arsenic (As) mobilization and biotransformation in two contrasting soils (upland soil and paddy soil) under flooded conditions. The soils with added tetracycline (0–50 mg kg−1) were incubated for 30 days, and soil properties and microbial functional genes over time were quantified. Tetracycline significantly promoted As reduction and As release into porewater in both soils. The enhancement had resulted from an increase in the concentration of dissolved organic carbon and a decrease in soil redox potential. Tetracycline also increased the abundances of As-reducing genes (arsC and arrA) and the relative abundances of As-reducing bacteria Streptomyces, Bacillus, Burkholderia, Clostridium and Rhodococcus, all of which have been found resistant to tetracycline. These genera play a key part in stimulating As reduction in the presence of tetracycline. The study indicated the significance of tetracycline in the biochemical behavior of As in flooded soils and provided new insights into the potential effects of tetracycline on the quality and safety of agricultural products in the future.

Influence mechanisms of iron, aluminum and manganese oxides on the mineralization of organic matter in paddy soil

The mineralization of soil organic matter (SOM) is closely related to the emission of greenhouse gas into atmosphere and the stability of organic carbon in soil. The influence of minerals on SOM mineralization in the specific soil received very few attentions. The influence characteristics and potential mechanisms of oxides on the mineralization of SOM in the paddy soil were observed in this study by incubating soil with the addition (dosage: 10 g kg−1) of prepared gibbsite, goethite, ferrihydrite or birnessite for 60 days. A sequence control treatment (753 mg CO2–C kg−1) > goethite treatment (656 mg CO2–C kg−1) ≈ gibbsite treatment (649 mg CO2–C kg−1) > birnessite treatment (529 mg CO2–C kg−1) > ferrihydrite treatment (441 mg CO2–C kg −1) was found in the cumulative amount of released CO2 in 60 days of incubation. Oxides especially ferrihydrite significantly decreased the content of dissolved organic matter (DOM) but tended to increase the content of microbial biomass carbon (MBC). The molecular structure of DOM in the paddy soil was simplified by gibbsite, ferrihydrite and birnessite after the incubation. Oxides especially birnessite and ferrihydrite reduced soil pH and the content of soil available N but increased soil redox potential (Eh). All examined oxides especially Fe oxides enhanced soil bacterial abundance but only birnessite significantly affected bacterial composition at phyla level. The stimulation on the immobilization and/or microbial assimilation of labile organic carbon, the modulation on soil basic properties (available N, pH, Eh), and the decrease of the relative abundance of some decomposing bacteria phyla such as Actinobacteria were the potential pathways of oxides in decreasing SOM mineralization.

Divergent responses of wetland methane emissions to elevated atmospheric CO2 dependent on water table

Elevated atmospheric CO2 may have consequences for methane (CH4) emissions from wetlands, yet the magnitude and direction remain unpredictable, because the associated mechanisms have not been fully investigated. Here, we established an in situ macrocosm experiment to compare the effects of elevated CO2 (700 ppm) on the CH4 emissions from two wetlands: an intermittently inundated Calamagrostis angustifolia marsh and a permanently inundated Carex lasiocarpa marsh. The elevated CO2 increased CH4 emissions by 27.6–57.6% in the C. angustifolia marsh, compared to a reduction of 18.7–23.5% in the C. lasiocarpa marsh. The CO2-induced increase in CH4 emissions from the C. angustifolia marsh was paralleled with (1) increased dissolved organic carbon (DOC) released from plant photosynthesis and (2) reduced (rate of) CH4 oxidation due to a putative shift in methanotrophic community composition. In contrast, the CO2-induced decrease in CH4 emissions from the C. lasiocarpa marsh was associated with the increases in soil redox potential and pmoA gene abundance. We synthesized data from worldwide wetland ecosystems, and found that the responses of CH4 emissions to elevated CO2 was determined by the wetland water table levels and associated plant oxygen secretion capacity. In conditions with elevated CO2, plants with a high oxygen secretion capacity suppress CH4 emissions while plants with low oxygen secretion capacity stimulate CH4 emissions; both effects are mediated via a feedback loop involving shifts in activities of methanogens and methanotrophs.

Paddy ponding water management to reduce methane emission based on observations of methane fluxes and soil redox potential in the Red River Delta, Vietnam*

AbstractReducing methane (CH4) emissions from paddy fields that contribute to the greenhouse effect has been addressed recently through the application of the alternate wetting and drying irrigation method. However, in poorly drained areas, such as the Red River Delta in Vietnam, the soil cannot be dried immediately, and so CH4 can continue to be produced unintentionally. Therefore, the purpose of this case study was to identify the optimal ponding water management schedule to reduce CH4 emissions by using the measured data of ponding depth, soil redox potential (Eh), and CH4 fluxes from field experiments and to show its effects on CH4 emission and water conservation. Observations in the winter–spring cropping season showed that the non‐flooding period of 3–8 days suppressed CH4 emission, and the continuous flooding period of 14–22 days caused CH4 re‐emission. Information regarding the non‐flooding period to be maintained and the flooding period to be avoided to suppress CH4 emission was not obtained for the summer–autumn cropping season due to abundant rainfall. The proposed schedule could suppress CH4 emission by 27%–85% and increase the amount of conserved water by up to 18% compared with traditional flooding protocols, but it may increase irrigation water due to the frequency and the amount of re‐flooding.

Silicon-rich soil amendments impact microbial community composition and the composition of arsM bearing microbes

Arsenic (As) cycling in flooded rice paddies is driven by soil microbes which among other transformations can cause conversion between inorganic and organic As species. Silicon (Si)-rich soil amendments cause increased methylated As species, particularly DMA, in grain likely because they influence the microbial community responsible for As methylation, but the mechanism remains unclear. To investigate how Si-rich amendments influenced the microbial community, we sequenced the 16S rRNA and arsM genes from rhizosphere soil collected at grain ripening from unamended rice paddy mesocosms or those amended with Si-rich rice husk, charred husk, or calcium silicate, and paired these data with geochemistry and As speciation in grain. We found that Si amendments influenced the 16S rRNA and arsM community composition. Increased C storage from calcium silicate amendment drove differences in the 16S rRNA community, whereas low soil redox potential drove differences in the arsM community. Differences in grain As were observed independent of Si-rich amendments, and did not correspond to differences in either the 16S rRNA or arsM community. Instead, methane flux and soil redox potential correlated with differences in grain DMA. Si-rich amendments drove changes in the microbial community composition and the subset of arsM-bearing organisms, but higher grain DMA levels were not directly caused by Si-rich amendments. Our findings imply that microbes active at lower soil redox potentials where As is mobilized are likely involved in DMA production, and future work should focus on linking the active community with DMA production.

Response of Mungbean (cvs. Celera II-AU and Jade-AU) and Blackgram (cv. Onyx-AU) to Transient Waterlogging.

Mungbean [Vigna radiata (L.) Wilczek] and blackgram [Vigna mungo (L.) Hepper] are important crops for smallholder farmers in tropical and subtropical regions. Production of both crops is affected by unexpected and increasingly frequent extreme precipitation events, which result in transient soil waterlogging. This study aimed to compare the waterlogging tolerance of mungbean and blackgram genotypes under the varying duration of waterlogging stress at germination and seedling stages. We evaluated the responses to different durations of transient waterlogging in a sandy clay loam under temperature-controlled glasshouse conditions. Waterlogging durations were 0, 1, 2, 3, 4, 5, 6, 7, and 8 days during germination and 0, 2, 4, 8, and 16 days during the seedling stage. We used two mungbean genotypes (green testa), Celera II-AU (small-seeded), and Jade-AU (large-seeded), contrasting in seed size and hypocotyl pigmentation, and a blackgram genotype (black testa), Onyx-AU. Waterlogging reduced soil redox potential, delayed or even prevented germination, decreased seedling establishment, and affected shoot and root development. In the seedlings waterlogged (WL) at 15 days after sowing (DAS), adventitious root formation and crown nodulation varied between the genotypes, and 16 days of waterlogging substantially reduced growth but did not result in plant death. Plants in soil with waterlogging for 8–16 days followed by drainage and sampling at 39 DAS had reduced shoot and root dry mass by 60–65% in mungbean and 40% in blackgram compared with continuously drained controls, due at least in part to fewer lateral roots. Soil plant analysis development (SPAD) chlorophyll content was also reduced. Onyx-AU, a blackgram genotype, was more tolerant to transient waterlogging than Jade-AU and Celera II-AU in both growth stages. Of the two mungbean genotypes, Celera II-AU had a greater seedling establishment than Jade-AU post waterlogging imposed at sowing. In contrast, Jade-AU had more plant biomass and greater recovery growth than Celera II-AU after waterlogging and recovery during the seedling stage. Both species were delayed in emergence in response to the shorter periods of transient waterlogging at germination, and with the longer waterlogging germination and emergence failed, whereas at the seedling stage both showed adaptation by the formation of adventitious roots.

Immobilization of heavy metal(loid)s in acid paddy soil by soil replacement-biochar amendment technology under normal wet condition.

The remediation of agricultural soil contaminated by acid mine drainages (AMD) with extreme acidity and elevated concentrations of metal(loid)s still remains to be solved. In the present study, the combination of soil replacement-biochar (BC) amendment was adopted in 270-day incubation experiments to evaluate the effect on the metal(loids) (As, Pb, Cu, Cd, and Zn) immobilization and soil properties (pH, dissolved organic carbon (DOC), redox potential (Eh), and soil water holding capacity (SWC)). The incubation study showed that soil replacement-biochar amendment improved soil health by changing soil properties, which in turn exhibited significant effects on CaCl2-extracted metal(loid)s. The combination of soil replacement and biochar amendment exhibited positive effect on the immobilization of Pb, Cu, Cd, and Zn, while, the risk of As and Cd mobility induced by biochar in the ageing process should be paid attention. Further laboratory seed germination study suggested that soil replacement-biochar amendment could effectively alleviate the stress of metal(loid)s, with the treatment of S50BC achieving the best remediation results. The results of this study suggested that soil replacement-biochar amendment was a promising remediation technology for agricultural soil contaminated by AMD. Graphical abstract.

Tree Canopies Influence Ground Level Atmospheric Electrical and Biogeochemical Variability

Static electric fields in the atmosphere are increasingly recognized as interacting with various organisms over several levels of biological organization. Recently, a link between atmospheric electrical variations and biogeochemical processes has been established in the context of open fields, yet biological structures like trees produce substantial alterations in atmospheric electric properties. Here, we assess whether these structural changes affect the dynamics of the electrical landscape and its relation to geochemical processes. To this end, we theoretically assess how trees alter their surrounding electric fields and empirically compare the temporal dynamics of atmospheric potential gradients, positive ions in the near-ground level atmosphere and soil electrochemical properties in an open field and under a tree. The developed model of electric fields around trees provides insight into the extent to which trees shield the underlying electric landscape, revealing that a substantial increase in atmospheric potential gradient only marginally affects the electric field under the canopy. We further show that soil electrochemical properties are tied to the temporal dynamics of positive ion in the near-ground level atmosphere, and that the presence of a tree reduces the temporal variability in both ground level positive ion concentrations and soil redox potential. This suggests that a tree can alter the temporal variability in atmospheric electricity and soil electro-chemistry, thereby likely indirectly influencing soil microorganisms and processes as well as electro-sensitive organisms that perceive and utilize atmospheric electric fields.

Geoelectrical and Physicochemical Evaluation of Soil Corrosivity on Metallic Pipelines: A Case Study

Geoelectrical sounding and physicochemical analyses were conducted on the topsoil underlying Osupa area in Ogbomoso, south western Nigeria to evaluate the soil corrosivity on the metallic water pipelines across the area. Schlumberger electrical resistivity soundings were conducted at 24 stations with electrode spacing varied from 1 to 100 m. The resistivity data were interpreted by using partial curve matching and computer-aided 1D inversion. Physicochemical analyses were also conducted on soil samples collected from about 1 m depth in test pits dug at points coincident with the sounding stations, following the BS/AWWA/ANSI Standards for Corrosivity testing to determine the soil pH, redox potential, moisture content and chloride content. The soil corrosivity was evaluated based on soil resistivity alone and the combined effect of soil pH and resistivity. The studied soils have resistivity ranging from 10 Ωm to 492 Ωm and thickness varying from 0.5 m to 4.6 m. The pH, moisture content, redox potential and chloride content range from 4.22 to 8.41, 14.33% to 29.09%, +50 mV to +97 mV and 102 ppm to 196 ppm respectively. The corrosivity intensity, based on the combined effect of soil pH and resistivity is essentially Medium-to- Medium-High being Medium at 10 locations, Medium-High at 8 locations, and High, Medium-Low, and Low at 2 locations each. More reliable information can be obtained about soil corrosivity toward buried metallic structures if the combined effect of the soil parameters affecting soil corrosion is considered.

Growing Rice (Oryza sativa) Aerobically Reduces Phytotoxicity, Uptake, and Transformation of CeO2 Nanoparticles.

This study compared the impact and uptake of root-administered CeO2 nanoparticles (NPs) in rice growing under flooded and aerobic soil conditions, which are two water regimes commonly used for rice cultivation. CeO2 NPs at 100 mg/kg improved photosynthesis and plant growth by reducing the oxidative damage and enhancing plant tolerance to stress, while a higher concentration (500 mg/kg) of CeO2 NPs negatively affected plant growth. More significant effects were observed under the flooded condition than under the aerobic condition. CeO2 NPs of 100 and 500 mg/kg resulted in 78% and 70% higher accumulation of Ce in shoots under the flooded condition compared to the aerobic condition. CeO2 NPs partially transformed to Ce(III) species in soils and plants under both conditions. A higher extent of transformation under the flooded condition, which was partly attributed to the lower soil pH and redox potential under the flooded condition, leads to higher plant uptake of Ce. A higher extent of transformation in rhizosphere soil was observed. A higher plant transpiration rate (TR) under flooded conditions resulted in a higher accumulation of CeO2 species in shoots. This study, for the first time, reported that water regimes influenced the biotransformation of CeO2 NPs and their uptake and impact in rice plants.

Soil acidification enhances the mobilization of phosphorus under anoxic conditions in an agricultural soil: Investigating the potential for loss of phosphorus to water and the associated environmental risk

Soil redox potential (EH) and pH are key parameters regulating the solubility and fate of phosphorus (P). However, the impact of soil acidification on the redox-induced mobilization and speciation of P in soils under a wide range of EH values has not been extensively studied. Here, we investigated the mobilization and speciation of P in an acidified agricultural soil at two different pH values (e.g., highly acidic soil; pH = 5.6 and slightly acidic soil; pH = 6.1) compared to the un-acidified soil (control soil; pH = 7.3) under a wide range of EH condition (+459 to −281 mV). The impacts of EH/pH-dependent changes of Fe-Mn oxides, and dissolved organic (DOC) and inorganic (DIC) carbon on P mobilization and speciation were also investigated using geochemical and spectroscopic (X-ray absorption near edge structure) techniques. The concentrations of dissolved P under anoxic conditions increased up to 69.3% in the highly acidic soil compared with the control soil. The decrease of the Fe-P fraction, the decrease of Ferrihydrite-Pads speciation, and the strong linear correlation between the dissolved P and Fe2+ (R2 > 0.85) supports the finding that enhanced P mobilization under anoxic conditions may be attributed to Fe reduction in the highly acidic soil. The concentration of dissolved Fe and P remained low until pH dropped below 6.35 for P and 6.28 for Fe, while a liner increase was found in dissolved Mn accompanying a general trend of pH decrease. This result suggests that the dissolution of reducible Mn under acidic soil conditions was an important factor for enhancing mobilization of dissolved P under anoxic conditions. This trend was due to the low amount of Mn, indirectly speeding up Fe reduction. These results can help to develop management practices to effectively mitigate P export and protect water resources from diffuse P pollution.

Alternate wetting and drying enhanced the yield of a triple-cropping rice paddy of the Mekong Delta

ABSTRACT In the Mekong Delta, triple rice cropping is widely spread. The rice cultivation is characterized by a short cropping period and nearly year-round flooding. Putting the soil under an oxidative condition through changing the water regimes during cropping periods might have a potential to increase the rice yield. To test this potential, an experiment was conducted at a typical triple-cropping farmer’s alluvial paddy in a central area of the delta for five years, 15 consecutive cropping seasons. We adopted an intermittent irrigation technique known as alternate wetting and drying (AWD). Under AWD, we irrigated to a depth of 5 cm above the soil surface only when the field water level dropped to 15 cm below the soil surface, with some exceptions. As a control, we prepared a treatment under which the paddy was continuously flooded (CF) except 2 weeks before expected harvest. We also prepared three straw treatments: all raw straw return, all straw-ash return, and no straw return to the paddy. For other field management, we adopted locally conventional methods. Results show that the straw treatment effects on the rice yield were not significant. For water treatments, the soil redox potential under AWD remained higher, in most cases, than that under CF; and the rough rice yield, harvest index, and percentage of ripened grains under AWD respectively became 8.9%, 4.4%, and 3.5% higher (p < 0.01) than those under CF, with lower N contents in rice grains and 43% less irrigation water use. The increase in rice yield by AWD was found with an increase in harvest index and a decrease in aboveground biomass and nitrogen content, indicating that it was due to the promotion of nutrient translocation in rice by AWD. The effect of AWD was enhanced particularly in the winter–spring cropping season, during which the nighttime temperature was the lowest. This piece of evidence that the rice yield of the delta can be increased simply by introducing AWD has the potential to encourage farmers to take voluntary actions that will also lead to a reduction in methane emission from rice paddies.

Indicator of redox in soil (IRIS) films as a water management tool for rice farmers

Rice is a crucial part of the world's food supply but is also susceptible to uptake of contaminants including arsenic (As) and cadmium (Cd) depending on the soil redox potential. Careful control of soil redox state by implementing alternate wetting and drying (AWD) water management can decrease mobility of soil As and Cd, but can be difficult to manage. Indicators of reduction in soil (IRIS) tubes and films have been studied by pedologists for wetland delineation; here, we explore the use of the IRIS film technology as passive samplers of soil redox potential in rice paddies. The goal of this study was to test the response time of IRIS films under different water management (i.e., variable soil redox potentials). After paddy soils were exposed to severe or safe AWD, where rice paddies were allowed to dry to >30 cm below the soil surface and 15 cm below the soil surface, respectively, IRIS films, coated with Fe oxide or Mn oxide paint, were installed. Immediately following IRIS film installation, soils were reflooded, and percent removal of Fe or Mn oxides were monitored on films that were removed every 12 h for Fe films, and every 6 h for Mn films. Porewater was collected at installation and every 12 h during the studies to observe correlations between IRIS film paint removal and porewater chemistry. We observed quicker paint removal for Mn films than Fe films, and paint removal varied due to growing season and water management. Moreover, correlations between porewater chemistry and Mn paint removal were observed. While further work is still needed to understand kinetics of IRIS paint removal as it relates to porewater parameters, this work illustrates that IRIS films are a low-cost tool that rice farmers can use to better manage water and we highlight considerations for possible implementation strategies for the future.

Cycling of reduced phosphorus compounds in soil and potential impacts of climate change

AbstractSoil phosphorus (P) remains an ever‐increasing topic of importance, notably for its key role as a nutrient for driving food production but with parallel concerns over damaging water quality, all against a backdrop of uncertainty of long‐term rock phosphate supplies. Soil is a key interface that holds P and regulates its onward flow to plants or leakage to waters. Often overlooked are a ubiquitous group of P compounds that exist in alternative oxidation states to that of phosphate (+5). Redox cycling, and the behaviour that chemically reduced P compounds exhibit in soils, introduces alternative routes of cycling P that may become more important as the soil system itself alters, especially due to the external pressures of climate change, bringing about critical dynamics in rainfall and runoff and also wetting and drying. All of these factors are known to affect soil redox potential and consequently the oxidation state of soil P. This review considers the chemically reduced species in the P cycle, exploring their sources and sinks, while considering their importance within the primary global biogeochemical cycling of P and how this may be impacted by climate change in the temperate climate of the northern hemisphere.Highlights This paper addresses how climate change will affect soil phosphorus cycling. This review is novel as it considers the reduced phosphorus forms when discussing P transfer in soils. Climate change is likely to increase prevalence of reduced P and free phosphate in northern hemisphere temperate soils. Phosphorus cycling relies partly on redox P processes. Their importance will increase as climate alters soils.

Differences in oxalate–carbonate pathway of Brosimum alicastrum in karst homegarden and forest soils

AbstractThe oxalate‐carbonate pathway (OCP) allows atmospheric carbon dioxide (CO2atm) sequestration through deposition of CaCO3 in soils. The participation of Brosimum alicastrum Sw. (Moracea) in the OCP has been demonstrated in traditional Maya agroforesty systems (homegardens) in Yucatan, Mexico, but remains unstudied in wild vegetation. To compare the OCP role of B. alicastrum trees associated to homegardens and forest, the calcium oxalate (CaC2O4) content of leaves, bark, roots, and adjacent soil of 40 trees was measured, as well as the abundance of soil and rizospheric oxalothropic bacteria. The highest CaC2O4 contents were present in leaves (45.37 g kg–1) and adjacent soil (576.93 mg kg–1) of wild trees. Oxalotrophic bacteria abundance was higher in the rhizospheric soil associated to wild trees. Soil CaC2O4 content was positively correlated to soil Ca (rs = .911; p &lt; .01) and CaCO3 (rs = .885; p &lt; .01) contents in wild trees and negatively correlated to soil redox potential (rs = –.625; p = .02). The CaC2O4 contents and oxalotrophic bacteria abundance in soils indicate an active participation of B. alicastrum trees in the OCP in both systems, allowing the capture of CO2atm. The lower potential to sequester CO2 in homegardens is mainly due to the use of the B. alicastrum as forage, which prevents the addition of litter to soil. If trees in homegardens were not defoliated, the amount of CO2atm captured by B. alicastrum in these systems would be greater than those in forest because the tree density in homegardens can be up to 18 times greater than in forest.

Control of paddy soil redox condition on gross and net ammonium fixation and defixation

Abiotic fixation and defixation of ammonium (NH4+) in silicate interlayers are common processes in paddy soils, owing to their often high levels of 2:1 type clay minerals. Fixed NH4+ hence forms a buffer during the supply and loss of plant-available N. The soil redox potential (Eh) is suspected to influence NH4+ (de)fixation by its impact on the negative charge of Fe3+-containing clay minerals, yet experimental confirmations are scarce. We examined the effect of a fluctuating or generally increased Eh on dynamics of NH4+ (de)fixation in a Bangladeshi paddy soil (total N: 1.9 g kg−1 | CEC: 43 cmol kg−1) during two incubation experiments. In those, we either 15N-labelled the fixed NH4+ pool (with initially 87 mg kg−1 exchangeable versus 393 mg kg−1 fixed NH4+-N) or the exchangeable pool (with initially 141 mg kg−1 exchangeable versus 249 mg kg−1 fixed NH4+-N), and this under three Eh-controlling treatments: continuous flooding (CF), CF with extra MnO2 added (3.5 g kg−1 Mn4+) (CF-MnO2), and alternate wetting and drying (AWD). The CF treatment overall led to somewhat higher net NH4+ fixation than AWD and, less clearly, also compared to the CF-MnO2 treatment, resulting from higher gross NH4+ fixation as well as lower gross defixation. These fluxes always led to a dynamic equilibrium between the exchangeable and fixed NH4+ pool within a few days of incubation, and this equilibrium was the same for both experiments but depended on the Eh treatment. However, by altering other NH4+ transformation processes, Eh treatments also indirectly impacted NH4+ (de)fixation fluxes, for which the equilibrium ratio between exchangeable versus fixed NH4+ turns out to be the determining factor.

Distinct patterns of abundant and rare subcommunities in paddy soil during wetting–drying cycles

Wetting–drying cycles typically result in a wide range of soil moistures and redox potentials (Eh) that significantly affect the soil microbial community. Although numerous studies have addressed the effects of soil moisture on soil microbial community structure and composition, the response of active microbes to the fluctuation in soil Eh is still largely unknown; this is especially true for the ecological roles of abundant and rare taxa. To explore the dynamics of active and total microbial communities in response to wetting–drying cycles, we conducted a microcosm experiment based on three wetting–drying cycles and 16S rRNA transcript (active) and 16S rRNA gene (total) amplicon sequencing. We found that both active and total microbial communities during three wetting–drying cycles were clustered according to the number of wetting–drying cycles (temporal factor) rather than soil moisture or Eh. Dynamics of the active microbial community, however, were redox dependent during the first wetting–drying cycle. In addition, rare taxa in the active microbial community exhibited more obvious differences than abundant ones during three wetting–drying cycles. Species turnover of abundant and rare taxa of total and active microbes, rather than species richness, explained the highest percentage of community variation. Rare taxa exhibited the most marked temporal turnover during three wetting–drying cycles. Members of Rhodospirillaceae were the major contributor to the resilience of abundant taxa of active microbes during the first wetting–drying cycle. Overall, these findings expand our current understanding of underlying assembly mechanisms of soil microbial communities responding to wetting–drying cycles.

Waterlogging tolerance of the common reedsPhragmites mauritianusandP. australis

AbstractThis study compared the responses ofPhragmites mauritianusandP. australisto waterlogging stress for 5 months. Plants were subjected to a 2 × 4 factorial experiment with two species and four soil moisture regimes: well drained (WD), half flooded (½F), flooded/drained at weekly intervals (F/D) and continuously flooded (F). Soil redox potential (Eh) was high in the WD (+138 to +126 mV) and low in the F treatments (−360 to −401 mV). Flooding increased plant height, number of culms, shoot elongation and total dry biomass accumulation in both species. In the flooded treatments, there was greater biomass allocation to above ground than below ground components, which contributed to increases in above‐ground/below‐ground ratios. In the flooded treatments, specific gravity of culms decreased by 18% inP. australisand by 10% inP. mauritianusindicating greater aerenchyma with waterlogging. In both reeds, flooding induced numerous adventitious roots from the base of the culms. In the F treatments, plant height, number of culms, total dry biomass accumulation and aerenchyma development were greater inP. australisthan inP. mauritianus. These data suggest thatP. australishas a competitive edge overP. mauritianusunder waterlogged conditions.