Soil Redox Potential Research Articles

Release of cadmium in contaminated paddy soil amended with NPK fertilizer and lime under water management

Agricultural soils contaminated with cadmium (Cd) pose a risk to receiving surface water via drainage or runoff. A 90-day laboratory incubation experiment was conducted to investigate the release characteristics and transformation of Cd from contaminated paddy soil amended with agrochemical (NPK fertilizer) and lime (L) under water management regimes of continuous flooding (F) and drying-wetting cycles (DW). The result showed that the dissolved Cd concentrations in overlying water of the fertilizer treatment under flooding (NPK+F) and drying-wetting (NPK+DW) reached up to 81.0 μg/L and 276 μg/L, and were much higher than that from the corresponding controls without NPK fertilizer addition at the end of experiment. The Cd concentration showed significantly negative correlation with overlying water pH, but positive correlation with soil redox potential and concentrations of dissolved total nitrogen, sulfate and manganese in overlying water (P < 0.05), indicating that drying-wetting cycles and N fertilizer addition may enhance soil Cd release. The Cd concentrations in overlying water from all treatments except NPK+L+F treatment exceeded the Cd threshold limit of Chinese Environmental Quality Standards for Surface Water (10 μg/L Grade V) and poses potential risk to surface water quality. Meanwhile, the proportion of Cd in the acid-soluble fraction from all incubated soil except NPK+L+F treatment increased compared to before incubation. The results indicated that continuous flooding was a reasonable water management candidate coupled with lime addition for immobilizing soil Cd.

Controlling the process of denitrification in flooded rice soils by using microbial fuel cell applications

Controlling the denitrification rate could help reduce the losses of applied nitrogen (N) fertilizer in fields. The applicability of microbial fuel cell (MFC) for controlling denitrification in flooded rice soils was investigated based on MFC theory coupled with redox changes. Because the soil about 10–20 cm beneath water and the soil near the water surface are anaerobic and aerobic, respectively, gradients of electric potentials could be generated between them, upon connecting them through insulated wires. Electrons released through the oxidation of organic matter during microbial metabolism can be utilized by this set-up, generating electricity. This can reduce the availability of electrons for reductive half-reactions of nitrate, to suppress denitrification. We studied the N losses in soil using planting pots with gas chamber experiment under three conditions: MFC systems, MFC systems with an externally applied voltage, and non-MFC systems as a control. Each system was set in triplicate, supplied with the same N fertilizer amounts, and flooded with automatic irrigation. Soil redox potential, N2O flux, and inorganic nitrogen concentration in soil pore water were periodically monitored. The redox potentials of both MFC systems and MFC systems with externally applied voltage were significantly higher than that of non-MFC systems, while N2O flux levels were significantly lower than that of non-MFC systems. The rice reproductive stage was the most effective on suppressing N2O flux with MFC application. However, the effect of externally applied voltage on suppressing N2O flux remains unclear. Inorganic nitrogen retention efficiencies in pore water were higher in MFC systems, which is consistent with the N2O flux difference. While the proportion of denitrified N estimated for MFC systems was 2.3%, that of non-MFC systems was 6.6%. We confirmed the applicability of MFCs to control soil redox potential and thereby suppress the denitrification based on planting pot experiments.

Effect of Biochar Amendment on Methane Emissions from Paddy Field under Water-Saving Irrigation

Biochar has been proposed as a new countermeasure to mitigate climate change because of its potential in inhibiting greenhouse gas emissions from farmlands. A field experiment was conducted in Taihu Lake region in China to assess the effects of rice-straw biochar amendment on methane (CH4) emissions from paddy fields under water-saving irrigation using three treatments, namely, control with no amendment (C0), 20 t ha−1 (C20), and 40 t ha−1 rice-straw biochar amendments (C40). Results showed that biochar application significantly decreased CH4 emissions by 29.7% and 15.6% at C20 and C40 biochar addition level, respectively. C20 significantly increased soil dissolved organic carbon, total nitrogen, and NH4+-N by 79.5, 24.5, and 47.7%, respectively, and decreased NO3−-N by 30.4% compared with C0. On the other hand, no significant difference was observed in soil pH and soil organic carbon in all treatments. C20 and C40 significantly increased and decreased soil oxidation-reduction potential, respectively. Compared with C0, rice yield and irrigation water productivity significantly increased by 24.0% and 33.4% and 36.3% and 42.5% for C20 and C40, respectively. Thus, rice-straw biochar amendment and water-saving irrigation technology can inhibit CH4 emissions while increasing rice yield and irrigation water productivity. The effects of increasing rice yield and irrigation water productivity were more remarkable for C40, but C20 was more effective in mitigating CH4 emission.

Impact of irrigation management on paddy soil N supply and depth distribution of abiotic drivers

In rice production, water-saving irrigation management is expanding and likely alters depth profiles of soil moisture, redox potential (Eh) and microbial activity. It is, however, unclear how such conditions then impact net soil N-release and availability to the rice crop, because we do not know well enough how water-saving irrigation management shapes depth-distribution of Eh and reductive processes, and microbial activity. A field experiment with rice was laid out on a typical young floodplain paddy soil of Bangladesh with three irrigation schemes, viz. continuous flooding (CF), safe alternate wetting and drying (AWD) and direct seeded rice (DSR), with 120 kg N ha−1 (N120) or without (N0) urea application. We evaluated changes in soil mineral N and plant N uptake, CH4 and CO2 emissions and soil pH, and at multiple depths soil Eh and temperature, dissolved C, Fe and Mn throughout 2015 dry (Boro) season (Jan–Apr). Eh stayed at or above ∼+300 mV except for sudden drops to ∼−200 mV with irrigation events in DSR. Eh quickly dropped to methanogenic conditions, under both AWD and CF; rises to ∼+200 mV were observed during AWD-drainage events but were restricted to upper 5.5 or 12.5 cm depths. Throughout the growing season there was a pronounced increase in reductive dissolution of Fe and Mn (hydro-) oxides, buildup of dissolved C, and CH4 effluxes under AWD and CF but not DSR, likely at least partially driven by the gradual soil warming from ∼20 °C till 28 °C. Predominant aerobic conditions under DSR lead to a nearly doubled C-emissions (CO2 + CH4) compared to AWD and CF, suggesting more soil organic matter (OM) degradation in the former case, while soil mineral N plus plant N build-up rate followed an opposite order. Urea application did not raise soil exchangeable N levels, even prior to significant plant uptake from 28 DAT (days after transplanting), and we forward temporal abiotic NH4+-fixation and N-removal processes as explanations. We conclude that regardless of some distinctions in temporal evolutions of puddle layer Eh, solution C, Fe and Mn, and CH4-emission, soil N-supply was quite comparable under AWD and CF, as was rice yield. In the context of N availability, AWD could be safely adopted for rice growth in the Bangladeshi Boro season. The eventual fertilizer N recovery efficiency was higher for CF (42%) than for AWD (32%), but AWD saved 12% irrigation water. While DSR saved 45% water there was a large yield penalty, likely due to drought stress but also by poor germination caused by cold night temperatures in mid-January, while seedling transplantation in CF and AWD plots was only later on 28 January. Further research should be conducted to investigate the fast and pronounced removal of exchangeable inorganic N after initial N buildup by soil OM mineralization, especially in CF and AWD. At this moment most likely candidate processes appear clay-NH4+ fixation and anaerobic NH4+-oxidation.

Metatranscriptomic insights into microbial consortia driving methane metabolism in paddy soils

ABSTRACTFlooded paddy soils are global sources of the greenhouse gas methane. Net methane fluxes from paddy fields reflect the sum of methane generation in the anoxic zone and methane consumption in the oxic zone, which are driven by methanogenic archaea and methanotrophic bacteria, respectively. Furthermore, methanogenic archaea produce methane utilizing hydrogen or acetate generated by hydrogen-producing bacteria or acetogenic bacteria, respectively. Therefore, methane emissions are regulated and orchestrated by hydrogen production, acetogenesis, methanogenesis, and methane oxidation. However, a comprehensive understanding of the microbial consortia that regulate methane emissions has yet to be achieved owing to the lack of simultaneous assessments of microbial drivers involved in these four processes, especially the production of hydrogen and acetate, which are primary substrates for methanogenesis. In this study, the transcriptional profiles of genes encoding the enzymes that catalyze each process were comprehensively investigated by shotgun RNA sequencing analysis (metatranscriptomics). Seasonal and spatial transitions in soil redox potentials and the transcriptional activities of methanogens indicated active methane metabolism in paddy soils 5 weeks after waterlogging, when the soil redox conditions of the surface (0–1 cm depth; S layer) and subsurface (5–7 cm depth; D layer) were the most divergent. Deep metatranscriptomics focusing on such soils suggested that: (1) in the anoxic D layer, rather than in the oxic S layer, Deltaproteobacteria, Acidobacteria, and Planctomycetes actively generate hydrogen; (2) Deltaproteobacteria, Betaproteobacteria, Acidobacteria, and Alphaproteobacteria generate acetate; (3) utilizing these products as substrates for methanogenesis, the archaea Methanocella, Methanoregula, and Methanosaeta actively produce methane; (4) concurrently, in the oxic S layer, methanotrophs related to Methylocystis and Methylogaea oxidize methane. The present study represents the first comprehensive report of the community structure and dynamics of the microbial consortia underpinning the methane emissions from paddy fields.

Effects of Extreme Events on Arsenic Cycling in Salt Marshes

AbstractExtreme events such as storm surges, intense precipitation, and supermoons cause anomalous and large fluctuations in water level in tidal salt marshes, which impacts the sediment biogeochemistry that dictates arsenic (As) cycling. In addition to changes in water level, which impacts soil redox potential, these extreme events may also change salinity due to freshwater inputs from precipitation or saltwater inputs due to surge. It is currently unknown how As mobility in tidal salt marshes will be impacted by extreme events, as fluctuations in salinity and redox potential may act synergistically to mobilize As. To investigate impacts of extreme events on As cycling in tidal salt marshes, we conducted a combined laboratory and field investigation. We monitored pore water and soil samples before, during, and after two extreme events: a supermoon lunar eclipse followed by a storm surge and precipitation induced by Hurricane Joaquin in fall 2015 at the St. Jones Reserve in Dover, Delaware, a representative tidal salt marsh in the Mid‐Atlantic United States. We also conducted soil incubations of marsh sediments in batch and in flow‐through experiments in which redox potential and/or salinity were manipulated. Field investigations showed that pore water As was inversely proportional to redox potential. During the extreme events, a distinct pulse of As was observed in the pore water with maximum salinity. Combined field and laboratory investigations revealed that this As pulse is likely due to rapid changes in salinity. These results have implications for As mobility in the face of extreme weather variability.

Soil Methane Production, Anaerobic and Aerobic Oxidation in Porewater of Wetland Soils of the Minjiang River Estuarine, China

Wetlands are important sources of methane emission. Anaerobic oxidation, aerobic oxidation and production of methane as well as dissolved methane are important processes of methane metabolism. We studied methane metabolism and the soil influencing factors. Potential soil methane production, anaerobic oxidation and aerobic oxidation rates, and dissolved methane in soil porewater changed seasonally and the annual average was 21.1 ± 5.1 μg g−1d−1, 11.0 ± 3.9 μg g−1d−1, 20.9 ± 5.8 μg g−1d−1, and 62.9 ± 20.6 μmol l−1, respectively. Potential soil methane production and anaerobic and aerobic oxidation were positively correlated among themselves and with soil pH and negatively correlated with soil redox potential (Eh). Potential soil methane production and aerobic and anaerobic oxidation rates were negatively related to pore soil methane concentration. Thus, the more water-saturated the soil (the lower Eh), the higher its capacity to produce methane. The potential soil capacity for methane oxidation was higher both in the same anaerobic circumstances and when the soil was suddenly subjected to aerobic conditions. The results of this study suggested a buffer effect in the methane balance in wetland areas. The environmental circumstances favoring methane production are also favorable for anaerobic methane oxidation.

Wetland drying increases the temperature sensitivity of soil respiration

Hydrological changes during wetland drying alter soil oxidation-reduction (redox) conditions such that soil organic carbon (SOC) decomposition is affected, which may influence global climate-carbon feedbacks. This study aimed to explore whether the temperature sensitivity of soil respiration is affected by changes in soil redox status. We created a gradient of redox potential in wetland soils by adjusting soil moisture content and O2 concentration in microcosms, and incubated these soils under changing temperature to estimate the temperature sensitivity (indicated by Q10) of soil respiration. Our results showed that Q10 values under aerobic conditions were 1.5–2.5 times higher than those under anaerobic incubations, with a significant positive correlation between Q10 and soil redox potential. This study suggested that SOC decomposition through soil respiration during progressive wetland drying may be increasingly sensitive to global warming, which may accelerate SOC loss from wetlands in the future.

Effects of elevated ozone concentrations on enzyme activities and organic acids content in wheat rhizospheric soil.

The elevated concentration of tropospheric ozone (O3) is an important global climate change driver, with adverse impacts on soil ecological environment and crop growth. In this study, a pot experiment was carried out in an open top chamber (OTC), to investigate the effects of elevated ozone concentration on soil enzyme activities (catalase, polyphenol oxidase, dehydrogenase and invertase), organic acids contents (oxalic acid, citric acid and malic acid) at different growth stages (tillering, jointing, heading and ripening stages) of wheat, and combined with the rhizospheric soil physicochemical properties and plant root characteristics to analyze the underlying reasons. The results showed that, elevated ozone concentration increased soil catalase, polyphenol oxidase, dehydrogenase and invertase activities at wheat ripening period to different degrees, with the effects on the activities of catalase and polyphenol oxidase being statistically significant. At the heading stage, activities of dehydrogenase and invertase were significantly increased by up to 76.7%. At the ripening stage, elevated ozone concentration significantly increased the content of citric acid and malic acid and redox potential (Eh) in rhizospheric soil, but reduced soil pH, electrical conductivity, total carbon and nitrogen. For root characteristics, elevated ozone concentrations significantly reduced the wheat root biomass, total root length and root surface area but increased the average root diameter.

Changes in mycorrhizal colonization and overall fungal communities across upland and wetland Acer forests of the Virginia Coastal Plain

Arbuscular mycorrhiza (AM) are common in the roots of most plants and provide benefits such as enhanced uptake of soil nutrients and water when the environment limits these resources. Two primary factors proposed in the literature as limiting the extent of mycorrhizal colonization of wetland plants are: (1) the low oxygen environment; and (2) the level of available phosphorus. The present study evaluated AM colonization of maple (Acer) roots and the fungal community structure in paired upland/wetland locations in the Virginia Coastal Plain to determine if wetland conditions affected the root fungal community. The range of observed AM colonization, based on 60 samples collected on 11 dates covering 3 years, was from 4 to 55%. Overall, soil redox potential and available phosphorus concentration were not significant in explaining the difference in AM colonization of Acer roots. In fact, the site with the greatest percent colonization in this study was a wetland site. A comparison of fungal diversity between the pooled wetland site communities and the pooled upland site communities was conducted and the difference was significant (p = 0.003), but the upland sites exhibited the lower diversity. Communities from all sites were dissimilar. Geographic location exerted a greater effect over community structure than did upland versus wetland status. This study concludes that the forested wetland environments studied here neither limited AM colonization nor reduced the overall fungal community and in fact may present a more favorable environment for fungal diversity.

A major QTL controlling the tolerance to manganese toxicity in barley (Hordeum vulgare L.)

Waterlogging stress disturbs plant metabolism through increased ion (manganese and iron) toxicity resulting from the changes in the soil redox potential under hypoxic conditions. Our previous study found a significant correlation between the tolerance to Mn2+ toxicity and waterlogging stress tolerance in barley, suggesting that waterlogging tolerance could be increased by improving the tolerance to Mn2+ toxicity. In this study, a doubled-haploid (DH) population from the cross between barley varieties Yerong and Franklin (waterlogging-tolerant and -sensitive, respectively) was used to identify QTL controlling tolerance to Mn2+ toxicity based on chlorophyll content and plant survival as selection criteria. Four significant QTL for plant survival under Mn2+ stress (QSur.yf.1H, QSur.yf.3H, QSur.yf.4H, and QSur.yf.6H) were identified in this population at the seedling stage. Two significant QTL (QLC.yf.3H and QLC.yf.6H) controlling leaf chlorosis under Mn2+ stress were identified on chromosomes 3H and 6H close to QSur.yf.3H and QSur.yf.6H. The major QTL QSur.yf.3H, located near the marker Bmag0013, explained 21% of the phenotypic variation. The major QTL for plant survival on 3H was validated in a different DH population (TX9425/Naso Nijo). This major QTL could potentially be used in breeding programmes to enhance tolerance to both manganese toxicity and waterlogging.

Characterizing Redox Potential Effects on Greenhouse Gas Emissions Induced by Water‐Level Changes

Core Ideas Dynamics of redox potential were induced by water‐table changes in a lysimeter. The redox potential measurements well reflected the different GHG emission sources. Redox potential monitoring is a viable tool for better understanding of GHG emissions. Soil greenhouse gas (GHG) emissions contribute to global warming. To support mitigation measures against global warming, it is important to understand the controlling processes of GHG emissions. Previous studies focusing mainly on paddy rice fields or wetlands showed a strong relationship between soil redox potential and GHG emission (e.g., N2O). However, the interpretation of redox potentials for the understanding of the controlling factors of GHG emission is limited due to the low number of continuous redox measurements in most ecosystems. Recent sensor developments open the possibility for the long‐term monitoring of field‐scale soil redox potential changes. We performed laboratory lysimeter experiments to investigate how changes in the redox potential, induced by changes in the water level, affect GHG emissions from agricultural soil. Under our experimental conditions, we found that N2O emissions followed closely the changes in redox potential. The dynamics of redox potential were induced by changing the water‐table depth in a laboratory lysimeter. Before fertilization during saturated conditions, we found a clear negative correlation between redox potentials and N2O emission rates. After switching from saturated to unsaturated conditions, N2O emission quickly decreased, indicating denitrification as the main source of N2O. In contrast, the emissions of CO2 increased with increasing soil redox potentials. After fertilization, N2O emission peaked at high redox potential, suggesting nitrification as the main production pathway, which was confirmed by isotope analysis of N2O. We propose that redox potential measurements are a viable method for better understanding of the controlling factors of GHG emissions, for the differentiation between different source processes, and for the improvement of process‐based GHG models.

PROCESS OF DENITRIFICATION IN FLOODED RICE SOILS

Denitrification, which converts soil nitrogen to nitrogen gas, involves a great loss of nitrogen fertilizer. Controlling the process of denitrification rate could help reducing the losses of applied nitrogen fertilizer in crop fields. Fertilizer nitrogen is one of the major concerns due to the high energy demand for its production. Since flooded rice soils are known to have strong denitrifying activity, innovative techniques are emerging as needs to improve nitrogen fertilizer retention efficiency in flooded rice soils. Nitrous oxide (N2O) emissions are not considered as significant, compared to that of methane (CH4) emissions from flooded rice soils, most probably due to its complete denitrification process with prolonged submergence that favors the production of nitrogen (N2) gas. This is due to a further reduction of N2O to N2. However, efficient use of nitrogen in the rice soils could have the benefits of saving energy costs that are being spent on nitrogen fertilizers in many developing countries. While conventional approaches are less efficient in controlling denitrification, technical solutions are emerging to solve this need. Technical solutions based on soil redox potential (Eh) for the control of denitrification have not been adequately covered. We propose that applicability of microbial fuel cell based on soil redox chemistry would be more promising for the control of denitrification in flooded rice soils.

Potential for Iron Reduction Increases with Rainfall in Montane Basaltic Soils of Hawaii

Core Ideas In upland soils, the potential for iron reduction to occur increases with rainfall. Bulk soil anoxia is not a requirement for soil iron reduction. Soil iron reduction likely occurs at most sites that experience periodically high soil moisture. Microbe‐mediated Fe reduction modulates the role of Fe‐bearing minerals, and can occur without saturation, in upland soils. Quantifying this Fe reduction is difficult, but critical for identifying climates in which Fe reduction plays a role in soil biogeochemistry. We measured potential for Fe reduction in upland soils along a rainfall gradient in Maui, Hawaii (2200 to 4400 mm yr–1 mean annual precipitation [MAP]), hypothesizing that potential for Fe reduction correlates with MAP. We determined the potential for Fe reduction by removal of Fe coating from (a) Fe oxide‐coated polyvinyl chloride (PVC) tubes (Indicator of Reduction in Soils; “PVC IRIS”) and (b) uniformly rusted steel rods (“Steel IRIS probes”) at 7, 11, and 14 d after installation. We measured soil redox potential (Eh) and pH at each site. Some coating was removed from all PVC and Steel IRIS probes, and fraction of Fe removed from Steel IRIS at 14 d (0.13 to 0.67) correlated with MAP (r2 = 0.66, p = 0.016). However, bulk soil Eh remained high (∼900 to 650 mV), except at the subsurface (45 cm) depth of the 4200 mm MAP site, suggesting overall oxidizing conditions. We also conducted in‐laboratory experiments to constrain the conditions of Fe removal. These laboratory experiments indicated (i) Fe reduction drives Fe coating removal, (ii) Fe coating removal (Fe reduction) initiates in unsaturated soils (iii) coating removal increases with increasing soil moisture. Our findings demonstrate that rainfall increases the likelihood for Fe reduction in otherwise oxic soils, and suggests that Fe redox influences the biogeochemistry of many upland soils.

Redox potential change of light grey forest surface-gleyed soils depending on the long-term anthropogenic impact

Influence of prolonged application of various fertilizer systems and periodic liming on the dynamics of redox potential (ROP) of light grey forest surface-gleyed soils under different crop rotations is presented in the research results. The obtained ROP indexes showed significant variability in variants of experiment depending on the fertilizer, lime and terms of determination in both fields of winter wheat and spring barley. The highest ROP values in variants with organo-mineral and mineral fertilizer systems with background of liming were observed in spring. They decreased during the period of maximum growth and development of winter wheat and spring barley together with the growth of microbiological activity and the intensity of CO2 release, and slightly increased before harvesting. It was established that the application of organo-mineral and mineral fertilizer systems with the background of liming with CaCO3 dose, calculated according to hydrolytic acidity, to the greatest extent increase ROP, especially during the period of intensive growth and development of plants. Compared to similar fertilizer system with the background of liming with a CaCO3 dose, calculated according to the pH- buffer capacity, it contributes to one-sided growth of oxidation processes and additional mineralization of humus. Thus, the organo-mineral fertilizer system with the application of 10 tons of manure per hectare of crop rotation and one norm of mineral fertilizers (N65P68K68) with the background of liming with CaCO3 dose, calculated according to the pH-buffer capacity (2.5 t/ha), contributes to rational use, preservation and fertility protection of light grey forest surface-gleyed soils, optimizing the redox potential during whole growing season of crops, and provides high yields. Studies have shown that during whole growing season of winter wheat and spring barley the lowest values of oxidation were obtained with the long-term use (for 50 years) of mineral fertilizer system with the introduction of one norm of mineral fertilizers only. It led to a decrease in the actual soil acidity to 4,03 pHKCl units and the increase of hydrolytic acidity to 5.11 mg-eq/100 g of soil. This once again emphasizes the importance of periodic liming of acid light grey forest surface-gleyed soils as a factor weakening the negative influence of mineral fertilizers. Key words: light grey forest surface-gleyed soils, redox potential, mineral fertilizers, manure, lime.

Efficacy of Anaerobic Soil Disinfestation for Control of Prunus Replant Disease.

Prunus replant disease (PRD) is an important soilborne complex that suppresses growth and productivity of replanted stone fruit and nut orchards. It is effectively managed with preplant soil fumigation but, due to regulatory challenges, nonfumigant-based control strategies for PRD and other soilborne disease problems may become increasingly important, especially in California. We examined the potential of preplant anaerobic soil disinfestation (ASD) for control of PRD in four repeated orchard replant trials on sandy loam soil near Parlier, CA. After removal of the old orchard trees, alternative ASD treatments, all using rice bran as the main carbon source, were implemented, starting in late September. The alternative treatments incorporated rice bran at (i) 20 t ha-1, alone, in 3.0-m-wide row strips; (ii) 20 t ha-1, preceded by incorporation of a sudangrass cover crop and followed by drip application of molasses (10 t ha-1), in 3.0-m-wide row strips; (iii) 20 t ha-1, alone, in 1.8-m-wide strips; or (iv) 12 t ha-1, alone, in 1.8-m-wide strips. All ASD-treated areas were covered with clear tarp and drip irrigated with 25 cm of water. Tarps remained for 6 weeks, during which the soil moisture level was kept at or above field capacity by drip irrigation. All trials included nontreated control and fumigated standard treatments. ASD raised temperature and reduced redox potential in soil at 15- and 46-cm depths for 6 weeks. Fumigation and ASD treatments both nearly eradicated bioassay inoculum of Pythium ultimum in the soil before almond trees were replanted and significantly affected almond tree root communities of fungi and oomycetes after planting. Fumigation treatments and ASD treatments with rice bran at 20 t ha-1 in 3.0-m strips increased tree growth significantly and by similar magnitudes. Among repeated experiments, mean increases in trunk cross-sectional area growth due to fumigation ranged from 137 to 264%, while the increases due to ASD at 20 t ha-1 in 3.0-m strips ranged from 148 to 214%, compared with controls. ASD offers effective control of PRD and is worthy of further optimization and testing for management of PRD and additional orchard replant problems.

Flooding-Induced Mobilization of Potentially Toxic Trace Elements from Uncontaminated, Calcareous, Agricultural Soils

Enhanced mobilization of potentially toxic trace elements (PTTE) is well documented for contaminated floodplains, wetlands, and rice paddies. Limited information is available on flooding-induced PTTE release from uncontaminated agricultural soils from temperate regions. We conducted an incubation study with simulated flooding using calcareous, uncontaminated agricultural soils to assess the release dynamics of a few PTTE and to identify the controlling factors. Packed soils were flooded, and soil redox potential (Eh), pH, total dissolved metals, and sulfur (S) concentrations in pore water and floodwater were measured for 8 wk. Pore water arsenic (As) and nickel (Ni) concentrations were initially low, but significantly increased with flooding by 12- and 6-fold, respectively. Copper (Cu) concentration declined with flooding, whereas zinc (Zn) showed no consistent trend. Arsenic and Ni concentrations correlated negatively with Eh, and positively with other redox-sensitive elements, whereas Cu behaved in a ma...

Conservation Agriculture systems alter the electrical characteristics (Eh, pH and EC) of four soil types in France

The impacts of Conservation Agriculture (CA) practices on soil characteristics have been largely studied. However, changes in soil electrical properties induced by CA cropping systems have been rarely studied for soil electrical conductivity (EC), and have never been reported for soil redox potential (Eh). Electrical conductivity is a key parameter and is largely used in precision agriculture to characterize various soil properties. Conversely, soil redox potential (Eh) has been overlooked by agronomists despite its being able to provide essential information to characterize soil conditions. This study set out to characterize the changes in soil electrical properties (Eh, pH and EC) induced by CA practices. For four contrasting soil types, varying in texture, pH and organic matter content, we showed that CA alters soil electrical activity (characterized by Eh corrected to pH7 or pe+pH) and soil electrical conductivity. Fields under CA or conventional practices displayed reversed soil profiles for electrical activity: surface soil (0–5cm) was found to be more oxidized under conventional systems than under CA practices, while the lower horizon (15–25cm) was more oxidized in CA systems than in conventional systems. CA systems also decreased electrical conductivity when it was initially high, and increased it when it was low, especially in the surface horizon (0–5cm). We discuss the possible mechanisms explaining such changes, the possible implications in the global functioning of soil/plant/microorganism system, and the interests and opportunities to use soil Eh-pH-EC as an indicator of soil health.

Phosphorus Dynamics Near Bald Cypress Roots in a Restored Wetland

Core Ideas Previous tree exposure to saturated conditions limited root death after ponding. Root growth and death had no apparent effect on concentrations of Fe2+, DOC, or DTP. Concentrations of Fe2+ were related to water table levels and redox status. Phosphorus concentrations were controlled by iron reduction and oxidation. Phosphorus (P) dissolution occurs commonly in wetland soils restored from agricultural land. Associated with P release are high concentrations of dissolved organic carbon (DOC) and Fe2+. This field study evaluated the effect of a fluctuating water table on the root dynamics of bald cypress (Taxodium distichum L. Rich.) to determine whether root death created soil reduction microsites, potentially contributing to P dissolution. The study site is a restored Carolina bay wetland with organic soils. Root growth and death were monitored on 16 6‐yr‐old bald cypress using minirhizotrons. Root dynamics, water table levels, and soil porewater chemistry and redox potential in the root zone were monitored for 2 yr. Soil solution samples were analyzed for Fe2+, pH, DOC, and P. High rates of root growth occurred during dry conditions, whereas root death occurred during sustained periods of saturation, particularly within 20 cm of the surface. Cyclic changes in concentrations of Fe2+, DOC, and dissolved total P (DTP) were related to water table position but not to changes in root numbers. After sustained periods of saturated conditions, redox potential decreased to 0 mV, Fe2+ increased to 1.75 mg Fe2+ L–1, and DOC increased to 350 mg L–1, resulting in peak DTP concentrations of 750 μg L–1, compared with 100 μg L–1 during dry periods. This study showed that in these high‐C soils (∼20% organic C) rooting dynamics had minimal impact on changes in P concentrations and that P dissolution was largely controlled by Fe reduction processes occurring within the C‐rich soil matrix.

Changes in soil redox potential in response to flood irrigation with waste water in central Mexico

SummaryIrrigation with untreated sewage water adds fresh organic matter to the soil. When it is applied by flooding, as in the Mezquital Valley of Mexico, many of the pores in the soil become temporarily waterlogged and depleted of oxygen, and reduction generates nitrous oxide (N2O) and methane (CH4). We monitored the redox potential, Eh, in the soil at two sites in the Mezquital Valley to discover whether the short‐term gaseous emissions matched the changes in Eh. One site is irrigated periodically by flooding with waste water and has alfalfa, rye grass and maize grown in succession; the other site grows maize with water from summer rain only. Each electrode buried in the soil for the purpose produced sequences of measurements that were correlated in time. We modelled the correlation and took it into account to analyse the effects of the irrigation by residual maximum likelihood (reml). After each flooding, the redox potential under alfalfa and rye grass decreased by 150–200 mV from a norm of about 450 mV for 2 days, after which it returned to its norm. The short‐term response to flooding under maize was similar, but the redox potential did not recover completely; instead there was a decrease from one irrigation event to another, in particular as a result of heavy rain in September that saturated the soil. The soil under rain‐fed maize was slower to respond, partly, we believe, because infiltration into the less aggregated clay soil and drainage from it were also slow. Irrigation with untreated sewage water caused a sharp decrease in Eh lasting 1–2 days. It seems to have depleted the soil of oxygen, causing reduction in microhabitats rich in carbon and nitrogen and generation of N2O, which was captured in the closed chambers that we had installed.Highlights Land in the Mezquital Valley of Mexico emits much greenhouse gas when irrigated with sewage water. The soil's redox potential was monitored in situ as crops were irrigated or depended solely on rain. Each irrigation caused a decrease in the soil's redox potential of 150–200 mV for 2 days. The decrease in redox potential corresponded in time with emission of nitrous oxide.