Reduced Soil Conditions Research Articles

Ornamental Grasses Tolerate Cyclical Flood, Drought, and Submergence During Stormwater Management, but Conditions and Survival Vary by Species

Perennial ornamental grasses are often recommended for rain gardens, but few data support their use. We conducted two experiments to evaluate the ability of ornamental grass cultivars to grow while subjected to cyclical flooding, submergence, and drought typical of rain gardens. Our objectives were to determine the effects of cyclical flood and drought (Expt. 1) and submergence depth and duration (Expt. 2) on grass growth and survival. Seven cultivars were evaluated during greenhouse trials, including Pixie Fountain tufted hairgrass [Deschampsia cespitosa (L.) P. Beauv.], Northwind switchgrass (Panicum virgatum L.), Red October big bluestem (Andropogon gerardii Vitman), Purpurascens Chinese silvergrass (Miscanthus sinensis Andersson), Blue Heaven® little bluestem [Schizachyrium scoparium (Michx.) Nash], Blonde Ambition blue grama grass [Bouteloua gracilis (Kunth) Lag. ex Griffiths], and Karl Foerster feather reed grass [Calamagrostis ×acutiflora (Schrad.) DC]. During Expt. 1, grasses underwent four cycles of flooding duration (2 days or 7 days) followed by drought (drying to volumetric soil water contents of 0.14 or 0.07 cm3·cm−3). During Expt. 2, grasses were cyclically submerged at 15 or 30 cm above the soil surface for 2, 4, or 7 days, followed by floodwater removal and drainage for 2 days before being resubmerged. Cyclical submergence continued until the 7-day submergence treatments completed four cycles. Both experiments were replicated in a full factorial randomized complete block design. Controls were included in both experiments. Plants were measured to determine plant height, shoot count, visual damage rating, shoot dry weight, and root dry weight. Floodwater chemistry and soil reducing conditions were measured during Expt. 2. Chinese silvergrass and switchgrass survived cyclical soil flooding/drought and submergence for 7 days at a depth of 30 cm while maintaining acceptable foliar damage. All grasses survived cyclical flood and drought when the soil volumetric water content was maintained at 14%, suggesting they can withstand periodic soil flooding as long as the water is not too deep. As water depth and duration increased from 4 days to 7 days, little bluestem, blue grama grass, and feather reed grass experienced significant foliar damage. Tufted hair grass and big bluestem experienced significant foliar damage when submerged for 2 days. Our results showed that perennial ornamental grasses can tolerate cyclical flood and drought and periodic submergence, but that plant conditions and survival vary, which can inform strategic plant placement within rain gardens, bioretention basins, and other stormwater management systems.

Iron Oxyhydroxide Transformation in a Flooded Rice Paddy Field and the Effect of Adsorbed Phosphate.

The mobility and bioavailability of phosphate in paddy soils are closely coupled to redox-driven Fe-mineral dynamics. However, the role of phosphate during Fe-mineral dissolution and transformations in soils remains unclear. Here, we investigated the transformations of ferrihydrite and lepidocrocite and the effects of phosphate pre-adsorbed to ferrihydrite during a 16-week field incubation in a flooded sandy rice paddy soil in Thailand. For the deployment of the synthetic Fe-minerals in the soil, the minerals were contained in mesh bags either in pure form or after mixing with soil material. In the latter case, the Fe-minerals were labeled with 57Fe to allow the tracing of minerals in the soil matrix with 57Fe Mössbauer spectroscopy. Porewater geochemical conditions were monitored, and changes in the Fe-mineral composition were analyzed using 57Fe Mössbauer spectroscopy and/or X-ray diffraction analysis. Reductive dissolution of ferrihydrite and lepidocrocite played a minor role in the pure mineral mesh bags, while in the 57Fe-mineral-soil mixes more than half of the minerals was dissolved. The pure ferrihydrite was transformed largely to goethite (82-85%), while ferrihydrite mixed with soil only resulted in 32% of all remaining 57Fe present as goethite after 16 weeks. In contrast, lepidocrocite was only transformed to 12% goethite when not mixed with soil, but 31% of all remaining 57Fe was found in goethite when it was mixed with soil. Adsorbed phosphate strongly hindered ferrihydrite transformation to other minerals, regardless of whether it was mixed with soil. Our results clearly demonstrate the influence of the complex soil matrix on Fe-mineral transformations in soils under field conditions and how phosphate can impact Fe oxyhydroxide dynamics under Fe reducing soil conditions.

Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in an Anoxic Soil.

It is known that the association of soil organic matter (SOM) with iron minerals limits carbon mobilization and degradation in aerobic soils and sediments. However, the efficacy of iron mineral protection mechanisms under reducing soil conditions, where Fe(III)-bearing minerals may be used as terminal electron acceptors, is poorly understood. Here, we quantified the extent to which iron mineral protection inhibits mineralization of organic carbon in reduced soils by adding dissolved 13C-glucuronic acid, a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate, or pure 57Fe-ferrihydrite to anoxic soil slurries. In tracking the re-partitioning and transformation of 13C-glucuronic acid and native SOM, we find that coprecipitation suppresses mineralization of 13C-glucuronic acid by 56% after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks, owing to ongoing reductive dissolution of the coprecipitated 57Fe-ferrihydrite. Addition of both dissolved and coprecipitated 13C-glucuronic acid resulted in increased native SOM mineralization, but the reduced bioavailability of the coprecipitated versus dissolved 13C-glucuronic acid decreased the priming effect by 35%. In contrast, the addition of pure 57Fe-ferrihydrite resulted in negligible changes in native SOM mineralization. Our results show that iron mineral protection mechanisms are relevant for understanding the mobilization and degradation of SOM under reducing soil conditions.

Enhanced dissolution of arsenic in anaerobic soils upon organic amendment application: acid detergent-soluble organic matter as a potential indicator

Application of organic amendments (OAMs) often enhances arsenic (As) dissolution in paddy soils. Therefore, understanding the properties of OAMs that determine the extent of As dissolution is essential for appropriate soil management. Since As dissolution increases with decrease in soil redox potential caused by microbial respiration, the decomposability of OAMs might be a critical factor controlling As dissolution in amended soils. We hypothesized that contents of acid detergent-soluble organic matter (ADSOM, mainly composed of non-fiber organic matter and hemicellulose) in OAMs can help estimate the potential of OAMs in accelerating As dissolution in soils with added OAMs. Therefore, two contrasting soil types, Andosol and Fluvisol, were mixed with 24 different OAMs and subjected to anaerobic incubation for 14 weeks. Changes in soil Eh and dissolved As contents were monitored throughout the incubation period, and As species in solid phases and ferrous iron (Fe(II)) contents in soils were measured after 2 and 6 weeks of incubation. The higher the ADSOM content in soils with OAMs, the higher the dissolved As contents in soils and the lower the Eh values. Dissolved As also positively correlated with the proportion of As(III) in solid phases and Fe(II) content after 2 and 6 weeks of incubation, indicating that decomposition of ADSOM led to reducing soil conditions, thereby promoting the reduction of As(V) and As-bearing Fe oxides and subsequent As dissolution. The results were consistent between the two types of soils, despite dissolved As content in the Andosol being two orders lower than that in Fluvisol. This is the first study to demonstrate that ADSOM can be a prominent indicator of the potential of OAMs, for promoting As dissolution, when applied to paddy soils.

Redoxtrons – An experimental system to study redox processes within the capillary fringe

AbstractSpatiotemporal characterisation of the soil redox status within the capillary fringe (CF) is a challenging task. Air‐filled porosities (ε), oxygen concentration (O2) and soil redox potential (EH) are interrelated soil variables within active biogeochemical domains such as the CF. We investigated the impact of water table (WT) rise and drainage in an undisturbed topsoil and subsoil sample taken from a Calcaric Gleysol for a period of 46 days. We merged 1D (EH and matric potential) and 2D (O2) systems to monitor at high spatiotemporal resolution redox dynamics within self‐constructed redoxtron housings and complemented the data set by a 3D pore network characterization using X‐ray microtomography (X‐ray μCT). Depletion of O2 was faster in the organic matter‐ and clay‐rich aggregated topsoil and the CF extended >10 cm above the artificial WT. The homogeneous and less‐aggregated subsoil extended only 4 cm above the WT as indicated by ε–O2–EH data during saturation. After drainage, 2D O2 imaging revealed a fast aeration towards the lower depths of the topsoil, which agrees with the connected ε derived by X‐ray μCT (εCT_conn) of 14.9% of the total porosity. However, small‐scaled anoxic domains with O2 saturation <5% were apparent even after lowering the WT (down to 0.25 cm2 in size) for 23 days. These domains remained a nucleus for reducing soil conditions (EH < −100 mV), which made it challenging to characterise the soil redox status in the CF. In contrast, the subsoil aeration reached O2 saturation after 8 days for the complete soil volume. Values of εCT_conn around zero in the subsoil highlighted that soil aeration was independent of this parameter suggesting that other variables such as microbial activity must be considered when predicting the soil redox status from ε alone. The use of redoxtrons in combination with localised redox‐measurements and image based pore space analysis resulted in a better 2D/3D characterisation of the pore system and related O2 transport properties. This allowed us to analyse the distribution and activity of microbiological niches highly associated with the spatiotemporal variable redox dynamics in soil environments.Highlights The time needed to turn from reducing to oxidising (period where all platinum electrodes feature EH > 300 mV) condition differ for two samples with contrasting soil structure. The subsoil with presumably low O2 consumption rates aerated considerably faster than the topsoil and exclusively by O2 diffusion through medium‐ and fine‐sized pores. To derive the soil redox status based upon the triplet ε–O2–EH is challenging at present in heterogeneous soil domains and larger soil volumes than 250 cm3. Undisturbed soil sampling along with 2D/3D redox measurement systems (e.g., redoxtrons) improve our understanding of redox dynamics within the capillary fringe.

Birnessite films are sensitive indicators of microbial manganese reduction in soil.

Manganese (Mn) and Fe indicator of reduction in soils (IRIS) devices are low-cost, convenient tools for identifying reducing conditions in soils. Because Mn is reduced at similar redox potentials as nitrate, there is considerable interest in using Mn IRIS tools for understanding microbial reduction of Mn as a surrogate for processes such as denitrification. However, the sensitivity of these devices to differences in Mn-reducing capacity has not been empirically investigated. Here we have found that the rate of birnessite paint removal from Mn IRIS films exposed to a twofold dilution series of the Mn-reducing bacterium Shewanella oneidensis is directly proportional to the number of S. oneidensis cells added. Thus, regularly monitored birnessite IRIS sensors are capable of indicating twofold differences in Mn reduction in soil and can be used to measure relative Mn reduction rates over time in a single location or compare and contrast Mn reduction rates across soil types.

Low pe+pH induces inhibition of cadmium sulfide precipitation by methanogenesis in paddy soil

Soil flooding is associated with the formation of cadmium (Cd)-sulfide, which is known to decrease Cd solubility and extractability. However, the threshold for Cd-sulfide precipitation is largely unknown, particularly because carbon can capture electrons for methanogenesis during sulfate reduction in highly reducing soil conditions. Using soil microcosms with different water regimens applied, we analyzed the electrochemical and spectroscopic properties and revealed a key mechanism controlling Cd stabilization that is dependent on pe+pH (a comprehensive indicator of soil redox status). The extent of Cd-sulfide precipitation was limited, with the proportion decreasing from 58.5% to 49.6% under flooding conditions (corresponding to a decrease in pe+pH from 3.28 to 2.82). Our data suggest that the increase in Cd mobilization in highly reducing soil is due to methanogenesis outcompeting sulfate reduction for available electrons. Although glucose supply could reduce the competition between oxidized carbon and sulfur in the soil for seizing electrons, the role of glucose as an electron donor/shuttle became weaker when soil was more anaerobic. The optimal soil reductive environment for maximum Cd-sulfide precipitation was observed when pe+pH was between 4.45 and 6.58. Overall, this study provides a quantitative and mechanistic understanding of how redox status (pe+pH), sulfate reduction, and methanogenesis are coupled with Cd remobilization in over-reductive paddy soil.

Manganese‐coated IRIS to document reducing soil conditions

AbstractIron‐coated indicatorof reduction in soils (IRIS) devices have been used for nearly two decades to help assess and document reducing conditions in soils, and official guidance has been approved for interpreting these data. Interest in manganese (Mn)‐coated IRIS devices has increased because Mn oxides are reduced under more moderately reducing conditions than iron (Fe) oxides (which require strongly reducing conditions), such that they are expected to be better proxies for some important ecosystem services like denitrification. However, only recently has the necessary technology become available to produce Mn‐coated IRIS, and the need is now emerging for guidance in interpreting data derived from Mn IRIS. Ninety‐six data sets collected over a 2‐yr period from 40 plots at 18 study sites among eight states were used to compare the performance of Mn‐coated IRIS with Fe‐coated IRIS and to assess the effect of duration of saturation and soil temperature as environmental drivers on the reduction and removal of the oxide coating. It appears that the current threshold prescribed by the National Technical Committee for Hydric Soils for Fe‐coated IRIS is appropriate for periods when soil temperatures are warmer (>11 °C), but is unnecessarily conservative when soil temperatures are cooler (5–11 °C). In contrast, Mn‐coated devices are particularly useful early in the growing season when soil temperatures are cool. Our data show that when using a threshold of 30% removal of Mn oxide coatings there is essentially 100% confidence of the presence of reducing soil conditions under cool (<11 °C) conditions.

Cattle biogas effluent application with multiple drainage mitigates methane and nitrous oxide emissions from a lowland rice paddy in the Mekong Delta, Vietnam

Household biogas production from livestock manure is popular in Vietnam. Effluent from the biogas production can be effectively used as organic fertilizer for rice cultivation. However, there is concern about enhanced methane (CH4) emission from the flooded soil under reductive conditions because the effluent contains labile organic carbon, the substrate for microbial CH4 production. We therefore combined multiple drainage with “cattle biogas effluent” (BE) application to mitigate CH4 emission by ameliorating the reductive soil conditions while obtaining grain yields comparable to the conventional practice, a combination of synthetic fertilizers (SY) application and continuous flooding (CF). We performed a 1-year experiment on a lowland (0 m a.s.l.) paddy field in the Mekong Delta, Vietnam, under a triple cropping system. We examined the effects of fertilizer type (SY and BE) and water management (CF, alternate wetting and drying [AWD], and midseason drainage followed by intermittent irrigation [MiDi]) on rice growth and yield and on CH4 and nitrous oxide (N2O) emissions. For AWD, a surface water level of −15 cm was used as the threshold for re-irrigation after natural drainage, whereas MiDi used a fixed irrigation schedule with one-time forced drainage during the midseason drainage period. Surface water level did not drop sufficiently in either AWD or MiDi due to the lowland conditions, and both practices caused similar seasonal changes in water level except for the forced drainage in MiDi. A significant (P = 0.05) interaction between fertilizer type and water management on grain yield was found only in dry season (winter–spring) with the highest yield level. Despite the poorly drained conditions, CH4 emission was significantly reduced by AWD and MiDi (by 27–30%), and both practices offset the significant CH4 emission enhancement by BE application. N2O emission was significantly reduced by BE application, whereas it was not affected by water management because flooded conditions were maintained during every nitrogen fertilizer application. Our results indicate that the combination of BE application and multiple drainage can mitigate CH4 and N2O emissions from a lowland rice paddy in the Mekong Delta without yield loss.

Experimental comparison of two methods to study barley responses to partial submergence

BackgroundCrop yield is dependent on climate conditions, which are becoming both more variable and extreme in some areas of the world as a consequence of global climate change. Increased precipitation and flooding events are the cause of important yield losses due to waterlogging or (partial) submergence of crops in the field. Our ability to screen efficiently and quickly for varieties that have increased tolerance to waterlogging or (partial) submergence is important. Barley, a staple crop worldwide, is particularly sensitive to waterlogging. Screening for waterlogging tolerant barley varieties has been ongoing for many years, but methods used to screen vary greatly, from the type of soil used to the time at which the treatment is applied. This variation makes it difficult to cross-compare results.ResultsHere, we have devised a scoring system to assess barley tolerance to waterlogging and compare two different methods when partial submergence is applied with either water or a starch solution at an early developmental stage, which is particularly sensitive to waterlogging or partial submergence. The use of a starch solution has been previously shown to result in more reducing soil conditions and has been used to screen for waterlogging tolerance.ConclusionsOur results show that the two methods provide similar results to qualitatively rank varieties as tolerant or sensitive, while also affecting plants differently, in that application of a starch solution results in stronger and earlier symptoms than applying partial submergence with water.

Ghosts of landuse past: legacy effects of milldams for riparian nitrogen (N) processing and water quality functions

Milldams and their legacies have significantly influenced fluvial processes and geomorphology. However, less is known about their effects on riparian zone hydrology, biogeochemistry, and water quality. Here, we discuss the potential effects of existing and breached milldams on riparian nitrogen (N) processing through multiple competing hypotheses and observations from complementary studies. Competing hypotheses characterize riparian zone processes that remove (sink) or release (source) N. Elevated groundwater levels and reducing soil conditions upstream of milldams suggest that riparian zones above dams could be hotspots for N removal via denitrification and plant N uptake. On the other hand, dam removals and subsequent drops in stream and riparian groundwater levels result in drained, oxic soils which could increase soil nitrification and decrease riparian plant uptake due to groundwater bypassing the root zone. Whether dam removals would result in a net increase or decrease of N in riparian groundwaters is unknown and needs to be investigated. While nitrification, denitrification, and plant N uptake have typically received the most attention in riparian studies, other N cycle processes such as dissimilatory nitrate reduction to ammonium (DNRA) need to be considered. We also propose a novel concept of riparian discontinuum, which highlights the hydrologic and biogeochemical discontinuities introduced in riparian zones by anthropogenic structures such as milldams. Understanding and quantifying how milldams and similar structures influence the net source or sink behavior of riparian zones is urgently needed for guiding watershed management practices and for informed decision making with regard to dam removals.

Camera illustration of Indicator of Reduction in Soils (IRIS) reduction dynamics

AbstractIndicator of Reduction in Soils (IRIS) tubes or films are used with coatings of iron or manganese oxide to observe depth or occurrence of reducing conditions, with coating removal often assessed weekly. We evaluated the use of a rhizosphere camera to capture iron and manganese reduction (coating removal) at high temporal resolution. A rhizosphere tube was coated with iron and manganese oxide (two sections of each oxide) and inserted into a saturated column filled with a surface horizon from a wet soil (Fluvaquent). Images were taken hourly over 28 d and compared with Eh and pH data. Reducing conditions were observed for manganese and iron after 1 and 4 d, respectively. This technology builds upon an existing approach and could be used to evaluate real‐time reducing soil conditions with IRIS as well as to improve oxide coating composition and tube/film development (e.g., coating thickness).

Genetic regulation of root traits for soil flooding tolerance in genus Zea.

Flooding stress caused by excessive precipitation and poor drainage threatens upland crop production and food sustainability, so new upland crop cultivars are needed with greater tolerance to soil flooding (waterlogging). So far, however, there have been no reports of highly flooding-tolerant upland crop cultivars, including maize, because of the lack of flooding-tolerant germplasm and the presence of a large number of traits affecting flooding tolerance. To achieve the goal of breeding flooding-tolerant maize cultivars by overcoming these difficulties, we chose highly flooding-tolerant teosinte germplasm. These flooding-tolerance-related traits were separately assessed by establishing a method for the accurate evaluation of each one, followed by performing quantitative trait locus (QTL) analyses for each trait using maize × teosinte mapping populations, developing introgression lines (ILs) or near-isogenic lines (NILs) containing QTLs and pyramiding useful traits. We have identified QTLs for flooding-tolerance-related root traits, including the capacity to form aerenchyma, formation of radial oxygen loss barriers, tolerance to flooded reducing soil conditions, flooding-induced adventitious root formation and shallow root angle. In addition, we have developed several ILs and NILs with flooding-tolerance-related QTLs and are currently developing pyramided lines. These lines should be valuable for practical maize breeding programs focused on flooding tolerance.

Organic Matter Clogging Results in Undeveloped Hardpan and Soil Mineral Leakage in the Rice Terraces in the Philippine Cordilleras

Rice terraces in Cordillera, Philippines, a world cultural heritage site, are threatened by the risk of collapse. It is crucial to manage these rice terraces for their conservation, while simultaneously practicing traditional farming. We examined the soil environment and investigated its effects on rice terrace conservation, by focusing on the hardpan condition; infiltration process, which is related to the collapse of rice terraces; and soil nutrition conditions in these sites. Field survey and soil analysis revealed that in areas where the hardpan was not sufficiently developed and water infiltration was effectively suppressed, organic matter content was significantly high, suggesting organic matter clogging. In these rice terraces, the amounts of P, K, Ca, and Mn were significantly low, showing the mineral leaching under reductive soil conditions. Therefore, hardpan formation, rather than organic matter clogging, is essential for the suppression of infiltration and prevention of potential terrace collapse. Because hardpan formation or organic matter clogging cannot be identified from the surface of flooded rice paddies, it is difficult to identify the influencing factor. Thus, we suggest that the hard soil layer should be checked before the planting season and drainage is allowed after the cropping season in the rainy season.

Iron monosulfide identification: Field techniques to provide evidence of reducing conditions in soils

AbstractThe accurate and timely identification of soil morphological indicators of anaerobic conditions is critical for the proper documentation of hydric soils and prolonged soil saturation. Iron monosulfide (FeS) forms under anaerobic conditions following complexation of Fe and reduced S to form insoluble, black to very dark gray (value ≤ 4, chroma ≤ 1) concentrations and/or soil coatings. These features are observable as soft masses or pore linings or are dispersed throughout the soil matrix in the form of concentrated zones of FeS. Variation in soil and environmental conditions result in a wide range of FeS expression from <1 to 100% coverage of the soil matrix. We seek to explain the environmental conditions required for FeS formation and describe diagnostic methods to document FeS in a field setting. Field identification of FeS can be performed through an oxidized color change test (either ambient air or the application of 3% H2O2) and via the evolution of H2S after the application of 1 M HCl. The use of Indicator of Reduction in Soil (IRIS) devices provides additional supporting evidence of S reducing conditions in soils and thus environmental conditions conducive to FeS formation when other necessary constituents are present. The concepts and techniques outlined in this review can serve as useful resources to inform our understanding of belowground redox chemistry and facilitate the accurate identification of FeS in wet soil environments.

Climate change and redoximorphosis in a soil with stagnic properties

In soils with perched water tables, frequent changes between reducing and oxidizing conditions result in the formation of redoximorphic features (RMF). Perched water tables are linked to the presence of a low water-permeable soil horizon and high water input by precipitation. Precipitation, however, is strongly affected by climate change. Hence, we postulate that soils with a perched water table are particularly vulnerable to climate change. To verify this hypothesis, we set up a monitoring campaign from April 2014 to October 2019 at a forested Planosol in North Rhine-Westphalia, Germany. On an hourly basis, the redox potential (EH) and air-filled pore volume (ε) were measured in four soil depths. Furthermore, we calculated the CWB and employed two future climate scenarios for the years 2071–2100 to discuss the consequences for RMF formation. Reducing conditions were evident in the moist hydrological winter of 2015 and 2016, while oxidizing soil conditions (EH > 300 mV at pH 7) prevailed throughout the drier years (2017, 2018, 2019) for the whole soil profile. Concomitant with an increase in ε during spring, we observed a switch from reducing to oxidizing soil conditions at low ε of 0.01–0.03 cm3 cm−3 in the temporarily water-saturated soil horizons. At present, reducing soil conditions are limited during early spring during which manganese and iron can be mobilized from their oxides. Thus, RMF actively form but the CWB forecast until 2100 indicates a lower water input, which will diminish the period for redox-induced mineral dissolution. This study provided the essential information that soils with a perched water table are specifically vulnerable under progressing climate change. Presumably, RMF become more and more relictic that needs to be validated in the future.

Oxidation of flooded paddy soil through irrigation with water containing bulk oxygen nanobubbles

Nanobubbles (NBs) with a diameter of <1 μm have unique properties, including a long lifetime in water. Our previous study showed that irrigation with water containing bulk oxygen NBs (oxygen NB water) reduced seasonal methane (CH4) emissions from flooded rice cultivation by 21%. We therefore hypothesize that the CH4 emission reduction attributed to soil oxidation. To test this hypothesis, we carried out three sequential soil-column experiments under flooded and rice-unplanted conditions. They shared the same experimental settings, except for the lengths of soil storage before experiment (2–3 years) and of aerobic soil preincubation (7–13 days). The columns were manually irrigated with aerated tap water (control) or oxygen NB water. A tubing pump was operated to constantly drain leaching water from the column bottom. During the 57-day experimental periods, we regularly measured dissolved greenhouse gases in the drainage and leached heavy metals. The greater amount of leached manganese than of leached iron indicates that the reductive soil conditions were relatively weak in the three experiments. Oxygen NB water reduced the total dissolved CH4 emissions by 20–28% compared to control water. However, a significant difference (p < 0.05) was found only when the CH4 emission was greatest in the experiment with the shortest length of soil storage due to a high availability of soil labile carbon. Soil oxygen profiling using a microelectrode revealed that oxygen depletion at shallow depths (4–15 mm from the soil surface) was ameliorated by oxygen NB water on day 35 of experiment when the CH4 emission was smaller due to a limited soil labile carbon. The results confirm that irrigation with oxygen NB water reduces CH4 production in a flooded paddy soil through the oxidation of shallow soil.

Improved synergistic dechlorination of PCP in flooded soil microcosms with supplementary electron donors, as revealed by strengthened connections of functional microbial interactome

The enhancement of in situ reductive dechlorination is typically accomplished through the provision of various exogenous electron donors. However, in mixed microbial communities, such as prevail in soils, the addition of electron donors may stimulate the activity of indigenous dechlorinating microorganisms, but they also inevitably affect naturally-coexisting soil redox processes and stimulate co-occurring populations that compete for such donors. In this study, we examined the chemical and microbial interactions between typical soil redox processes and PCP degradation in naturally flooded paddy soils amended with different electron donors (including formate, acetate, pyruvate and lactate) in two different reducing conditions, viz. methanogenic and sulfate-reducing conditions. Among the electron donors tested, pyruvate was most effective in facilitating PCP dechlorination. Together with the Illumina sequencing data on bacterial and archaeal responses in the soil, a co-occurrence network analysis was applied to study the impact of different electron donors on the bacterial and archaeal interactome processing PCP dechlorination in different soil-reducing conditions. The addition of exogenous fermentative electron donors facilitated change in the bacterial and archaeal communities by increasing the relative abundance, and thus strengthening associated connections amongst the core functional microbial community, assembled by fermenting Clostridia, Anaerosolibacter and Alkaliphilus, Desulfosporosinus and Desulfuromonadaceae, and methanogenic Methanosarcina. This consortia might be likely involved in dechlorination and soil redox processes such as iron/sulfate reduction and methanogenesis. Our results provide a comprehensive and new perspective on optimum regulating and facilitating strategies in the degradation of PCP in anaerobic environments.

Effects of soil redox potential (Eh) and pH on growth of sunflower and wheat

ABSTRACTSoil redox potential (Eh) and pH are fundamentals parameters for plants growth. Measuring soil Eh is essential but complex due to the lack of measurement reliability resulting from high temporal variability and metrological challenges. This paper proposes practical advancements for measuring Eh in aerobic soils using combined electrodes (improvements in methodology specific to cleaning electrodes, measurement time, cleaning the dataset). The study of soil Eh and pH on sunflower and wheat in pot experiments has highlighted the relationship between Ehcumul (Eh associated with a dimension of time) and the portion of porosity that is accessible to air. For reduced soil conditions, sunflower positively reacts to better aeration. Strong correlations exist between the duration of each potential range and the growth of sunflower. The study of sunflower growth in soil reveals extremely harmful impact resulting from too high and/or too low Eh values.

Effects of Anaerobic Soil Disinfestation on Sclerotinia sclerotiorum in Paraguay

The effects of anaerobic soil disinfestation (ASD) on Sclerotinia sclerotiorum sclerotia viability was tested in four field trials in Paraguay. Plots were amended with wheat bran (20.2 Mg/ha), molasses (10.1 Mg/ha), or wheat bran (20.2 Mg/ha) plus molasses (10.1 Mg/ha), saturated with water, and covered with black plastic mulch for 3 weeks. Control plots were not amended but were saturated and either covered (anaerobic control) or maintained uncovered (aerobic control). Tubes painted with iron oxide paint were placed in soils to assess soil reducing conditions. Sclerotia were buried 6 cm deep in treated and control soils along with temperature data loggers. After 3 weeks, the viability of sclerotia was significantly lower in all ASD-treated soils (4 to 52%) compared with the aerobic control soil (100%), regardless of the carbon source used. Sclerotial viability was also significantly reduced compared with anaerobic controls at three sites, depending on the carbon source used. A significant negative correlation between soil reducing conditions and sclerotia viability was observed at all sites. Wheat bran and molasses are widely available and inexpensive in Paraguay, and ASD with these carbon sources provides smallholder South American vegetable farmers with a new option for sustainable management of Sclerotinia and potentially other soilborne pathogens.