Soil Redox Potential Research Articles

Influence of waterlogging on yield of wheat (Triticum aestivum), redox potentials, and concentrations of microelements in different soils in India and Australia

Effects of waterlogging relative to drained conditions on grain yield were studied in relation to soil redox potentials and microelements (Fe and Mn) in soils from India and Western Australia, using waterlogging intolerant and tolerant varieties of wheat (Triticum aestivum L.) The grain yield of wheat decreased significantly with increasing duration of waterlogging in sodic soils. In Indian soils, soil redox potentials decreased sharply after waterlogging and were 150 and 210 mV at 10 days after waterlogging in alkali soil at pH 8.5 and pH 9.2, respectively. Two Australian soils were similarly reduced in redox potential with values of ~200 mV at 10 days after waterlogging, and redox potentials were further reduced to 100 mV and –50 mV for soils without and with added glucose, respectively, after 40 days of waterlogging. The Indian soils tended to be 2–10 times higher in DTPA-Mn than the Australian soils, whereas the Australian soils were up to 10 times higher in DTPA-Fe than the Indian soils. These increases were up to 10 and 60 times higher, respectively, than reported critical concentrations for wheat. After 21 days of waterlogging, the Indian soils were drained, and the re-aeration resulted in an increase in redox potential and a decrease in DTPA-Fe and -Mn in soil solutions, but this occurred slowly, taking 15–25 days. The results support the hypothesis that waterlogging tolerance is a product of tolerance to anoxia and microelement toxicities, and that these are both key factors limiting plant growth during and after waterlogging. These factors may also contribute to the large differences in screening wheat varieties for waterlogging tolerance in different soils.

Dynamics of methanogenesis and methanotrophy in tropical paddy soils as influenced by elevated CO2 and temperature interaction

Response of methanogenesis and methanotrophy to elevated carbon dioxide (CO2) could be affected by changes in soil moisture content and temperature. In soil microcosms contained in glass bottles and incubated under laboratory conditions, we assessed the impact of elevated CO2 and temperature interactions on methanogenesis and methanotrophy in alluvial and laterite paddy soils of tropical origin. Soil samples were incubated at ambient (370 μmol mol−1) and elevated (600 μmol mol−1) CO2 concentrations at 25, 35 and 45 °C under non-flooded and flooded conditions for 60 d. Under flooded condition, elevated CO2 significantly increased methane (CH4) production while under non-flooded condition, only marginal increase in CH4 production was observed in both the soils studied and the increase was significantly enhanced by further rise in temperature. Increased methanogenesis as a result of elevated CO2 and temperature interaction was mostly attributed to decreased soil redox potential, increased readily mineralizable carbon, and also noticeable stimulation of methanogenic bacterial population. In contrast to CH4 production, CH4 oxidation was consistently low under elevated CO2 concentration and the decrease was significant with rise in temperature. The low affinity and high affinity CH4 oxidation were faster under non-flooded condition as compared to flooded condition. Admittedly, decreased low and high affinity CH4 oxidation as a result of elevated CO2 and temperature interaction was related to unfavorable lower redox status of soil and the inhibition of CH4-oxidizing bacterial population.

Biodegradation of explosives mixture in soil under different water-content conditions

Soil redox potential plays a key role in the rates and pathways of explosives degradation, and is highly influenced by water content and microbial activity. Soil redox potential can vary significantly both temporally and spatially in micro-sites. In this study, when soil water content increased, the redox potential decreased, and there was significant enhancement in the biodegradation of a mixture of three explosives. Whereas TNT degradation occurred under both aerobic and anaerobic conditions, RDX and HMX degradation occurred only when water content conditions resulted in a prolonged period of negative redox potential. Moreover, under unsaturated conditions, which are more representative of real environmental conditions, the low redox potential, even when measured for temporary periods, was sufficient to facilitate anaerobic degradation. Our results clearly indicate a negative influence of TNT on the biodegradation of RDX and HMX, but this effect was less pronounced than that found in previous slurry batch experiments: this can be explained by a masking effect of the soil in the canisters. Fully or partially saturated soils can promote the existence of micro-niches that differ considerably in their explosives concentration, microbial community and redox conditions.

Flooding effects on phosphorus dynamics in an Amazonian mangrove forest, Northern Brazil

We examined porewater salinity, soil redox potential (Eh), soil extractable phosphate (extr.-P), leaf phosphorus (leaf-P) and plant growth in relation to inundation frequency (IF) and mangrove species distributions along a 600 m transect in the Braganca Peninsula, North Brazil. The forest species composition changed across the tidal zone with Avicennia germinans dominating (99.1%) the high intertidal (HI) zone where the IF was 41–67 d.y−1, Rhizophora mangle, Laguncularia racemosa and A. germinans co-occured in the mid intertidal (MI), and a mixed R. mangle (47.1%) - A, germinans (41.2%) stand occupied the low intertidal (LI) zone with an IF of 124–162 d.y−1. Low IF resulted in high Eh levels (200 mV) in the HI zone relative to the LI where Eh ranged from 0–100 mV. The IF showed a significant positive correlation with extr.-P (r = 0,89; p = 0.05) and a negative association with Eh (r = −0,75; p = 0.05). An ANCOVA confirmed that Eh and extr.P were influenced by flooding. Variations in these factors were reflected in patterns of P leaf tissue concentrations across the gradient; however, a MANCOVA showed that leaf-P was not related to tree height, tree volume or basal area. Water-logging conditions, porewater salinity, and P dynamics in the sediment appear to influence the forest structure. We suggest that P availability plays an important role in controling mangrove species distributions but not their growth.

Fe2+ and Mn2+, potential agents to induce suppression of Fusarium oxysporum for biological soil disinfestation

Here Fusarium oxysporum was killed in exudates obtained from soil biologically disinfested with ethanol, indicating that physical interaction with soil microorganisms was not essential. Because acetic acid was confirmed to accumulated during the treatment, we evaluated the effect of acetic acid amendment against the pathogen in plastic containers. A drop in the soil redox potential seemed to be correlated with the fungicidal efficacy of acetic acid. Under reductive soil conditions, metal ions such as Mn2+ and Fe2+ formed, and the pathogen was effectively suppressed in Mn2+ and Fe2+ solution. Therefore, Fe2+ and Mn2+ may be the agents that induce suppression of the pathogen during biological soil disinfestation.

Nitrous oxide emissions from paddy fields under different water managements in southeast China

Water management is recognized as one of the most important factors in regulating nitrous oxide (N2O) emissions from paddy fields. In China, controlled irrigation (CI) is widely applied because it has been proved highly effective in saving water. During the rice-growing season, the soil in CI paddy fields remains dry 60–80% of the time compared with soil irrigated by traditional methods. This study aims to assess N2O emissions from paddy fields under CI, with traditional irrigation (TI) as the control. The cumulative N2O emission from CI paddy fields was 2.5 kg N ha−1, which was significantly greater than that from TI paddy fields (1.0 kg N ha−1) (P < 0.05). Soil drying caused substantial N2O emissions. The majority (73.9%) of the cumulative N2O emission from CI paddy fields was observed during the drying phase, whereas no substantial N2O emissions were observed when the soil was re-wetted after the drying phase. More and significantly higher peaks of N2O emissions from CI paddy fields (P < 0.05) were also detected. These peaks were observed ~8 days after fertilizer application at water-filled pore spaces (WFPS) ranging from 78.0 to 83.5%, soil temperature ranging from 29.1 to 29.4°C, and soil redox potential (Eh) values ranging from +207.5 to +256.7 mV. The highest N2O emission was measured 8 days after the application of base fertilizer at a WFPS of 79.0%, soil temperature of 29.1°C, and soil Eh value of +207.5 mV. These results suggest that N2O emissions may be reduced obviously by keeping the WFPS higher than 83.5% within 10 days after each fertilizer application, especially when the soil temperature is suitable.

Wetland development in a previously mined landscape of East Texas, USA

We studied wetland development in a chronosequence of created wetlands in a reclaimed landscape in east Texas seasonally for 1 year. The purpose of the study was to identify features (i.e., indicators) that best reflected changes in wetland ecosystem state through time and could serve as indicators of “maturity” for bond-release. Features considered included surface water nutrients, soil nutrients, soil redox potential, vegetative biomass and diversity, and benthic invertebrate biomass and diversity. Our sampling focused on nine wetlands representing three different-age classes (n = 3 for each) as a surrogate for time. All wetland sites were created with the same homogenized mine spoil and had similar hydrology and climate. Age-specific changes in all parameters were observed, except for surface water nutrients. The oldest wetlands (i.e., “mature”) exhibited highest soil concentrations of N, C, K, P, and Ca. Soil redox potential was significantly lower in the mature wetlands, in addition to within-wetland (lowest in deepest sampling zones) and intra-annual variability (i.e., lowest during the summer). Mature created wetlands supported the highest vegetative biomass and species richness and highest densities of invertebrates; however, taxa richness was similar across all age groups. Of all parameters we measured, vegetation metrics were among the simplest and most cost-effective measures used to track the early development of mitigated wetlands. This study provides the basis from which to track the development of these reclaimed ecosystems in a more rigorous and easily replicated manner. With further validation, select use of these parameter sets in east Texas and other similar landscapes could aid both in determining compliance for regulatory purposes as well as tracking success of ecological mitigation.

Ubiquitous Net Volatilization of Polycyclic Aromatic Hydrocarbons from Soils and Parameters Influencing Their Soil−Air Partitioning

Soils are a major reservoir of organic pollutants, and soil-air partitioning and exchange are key processes controlling the regional fate of pollutants. Here, we report and discuss the soil concentrations of polycyclic aromatic hydrocarbons (PAHs), their soil fugacities, the soil-air partition coefficients (K(SA)) and soil-air gradients for rural and semirural soils, in background areas of N-NE Spain and N-NW England. Different sampling campaigns were carried out to assess seasonal variability and differences between sampling sites. K(SA) values were dependent on soil temperature and soil organic quantity and type. Soil fugacities of phenanthrene and its alkyl homologues were 1-2 orders of magnitude higher than their ambient air fugacities for all sampling sites and periods. The soil to air fugacity ratio was correlated with soil temperature and soil redox potential. Similar trends for other PAHs were found but with lower fugacity ratios. The ubiquitous source of PAHs from background soils to the atmosphere found in all temperate regions in different seasons provides an indirect evidence of potential in situ generation of two to four ring PAHs and their alkyl homologues in the surface soil. We discuss this hypothetical biogenic source and other potential processes that could drive the high soil to air fugacity ratios of some PAHs.

Spatial variability of CH4 and N2O fluxes in alpine ecosystems on the Qinghai–Tibetan Plateau

Abstract The intra- and inter-site spatial variability of methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were investigated in three alpine ecosystems at Haibei station on the Qinghai–Tibetan Plateau (QTP) in summer 2005 and in sixteen alpine ecosystems with various vegetations across the QTP in summer 2006, respectively. The magnitude of average CH 4 emissions from wetlands was at least 100 times larger than average CH 4 uptake by grassland, suggesting that the entire QTP is likely to be a source of methane in summertime because of a significant fractional area of wetlands (∼ 2.2%) on the plateau. Intra-site investigation, with sixteen chambers, revealed a significant negative relationship of CH 4 emissions with the C/N ratio of aboveground biomass and soil pH in the alpine wetland when all chambers were considered. Moreover, soil oxidation–reduction potential (ORP) had a remarkably strong influence on CH 4 emissions for nine chambers above the water level, resulting in a negative exponential relationship. For N 2 O flux in alpine meadows, a negative relationship with both soil pH and livestock dung biomass was observed. Aboveground plant biomass and soil pH were important variables overall in wetlands. Inter-site investigation found positive and negative relationships between CH 4 flux and soil biomass to 5 cm depth in nine grassland and seven wetland sites, respectively. N 2 O flux showed a moderately strong negative exponential relationship with the C/N ratio of surface soil in the grassland sites. In the wetlands, soil pH was negatively correlated with N 2 O flux, perhaps due to both reduced N 2 O release from suppressed nitrification and denitrification.

Effects of green manure crops and mulching technology on reduction in herbicide and fertilizer use during rice cultivation in Korea

Green manure crops are primarily used in environmentally friendly agricultural practices to reduce the application of chemical fertilizer and herbicide. In this study, a two-year field experiment was conducted to evaluate the effects of paper and plastic mulching with hairy vetch alone or in combination with barley on weed control and rice yield. In addition, treatment effects on soil redox potential (Eh) and the concentration of ammonium (NH 4 + ) in rice paddy were investigated. The results showed that plastic film (10 or 20 μm) and paper mulching with hairy vetch alone had no significant effects on weed density and rice yield when compared with conventional practice (herbicide and fertilizer application) during the first year. However, during the second year, plastic film (20 μm) with partial tillage of hairy vetch alone increased rice yield and decreased weed occurence; but barley and hairy vetch mixture showed opposite trends. Plastic film mulching led to a decrease in soil redox potential, mainly due to the decomposition of soil organic matter. In addition, plastic film mulching increased NH 4 + -N contents in rice paddy soil. These results suggest that the combination of plastic film with hairy vetch and barley mixture can be used in rice fields to reduce the use of chemical fertilizer and herbicide. Keywords: Green manure, hairy vetch, mulching, rice, weed con

질산화작용 억제 처리가 논토양의 인산 가용화와 벼의 양분흡수 및 생육에 미치는 영향

환원상태가 발달된 담수상태의 토양이나 습지생태계에서 <TEX>${NO_3}^-$</TEX>는 환원상태의 진전을 지연시키는 완충역할을 할 수 있다. 논토양에서 <TEX>${NO_3}^-$</TEX>가 Fe(III) 환원과 그에 따른 P의 가용화에 미치는 영향과 함께 질산화작용억제가 벼의 N, P 흡수 및 생육에 미치는 영향을 조사하였다. 담수 후 10 cm 깊이 토양의 산화환원전위 변화는 N 비료처리별로 현저하게 달리 나타났으며, 질산화작용이 억제된 요소+N-serve 처리에서는 -100 mV 이하로 낮아졌으나 <TEX>$KNO_3$</TEX>처리의 경우에는 0 mV 이상으로 유지되었다. 이러한 현상은 질소비료 처리별로 <TEX>${NO_3}^-$</TEX>에 의한 redox buffer 작용 유무에 따라서 결정되는 것이다. N-serve 처리를 통하여 질산화작용을 억제시키면 <TEX>${NO_3}^-$</TEX>에 의한 redox buffer 작용이 없어지므로 토양의 환원현상이 크게 촉진될 수 있는 것이다. 따라서 요소+N-serve 처리에서는 다른 처리에 비하여 Fe(III)의 환원과 함께 토양 용액의 <TEX>${PO_4}^{3-}$</TEX> 함량이 현저히 증가하였다. 질산화저해제와 함께 요소를 처리한 경우 토양 용액중의 N 및 P 함량과 함께 벼 유묘 지상부의 N과 P 함량이 가장 높았음에도 불구하고 그 생장은 가장 불량하였다. 이와 같이 요소+N-serve 처리에서 나타난 벼 유묘 생장 저해 현상은 과도한 Fe(II)의 용출과 그에 따른 벼 유묘의 Fe 과잉흡수에 기인하는 것으로 판단된다. In a pot experiment, we studied the effect of nitrification inhibition on Fe reduction and P release in paddy soil and growth and nutrient uptake of rice plant. Recommended level of fertilizers, 6 kg N, 5 kg <TEX>$P_2O_5$</TEX> and 4 kg <TEX>$K_2O$</TEX> per 10a, were applied, and for N fertilizer urea, urea+N-serve, and <TEX>$KNO_3$</TEX> were included. Four 30-day-old seedlings were transplanted in a waterlogged 9 L pot filled with Yuga series soil, and 3 pots were prepared in each N fertilizer treatment. Changes of soil redox potential and concentration of <TEX>${NH_4}^-$</TEX>, <TEX>${NO_3}^-$</TEX>, <TEX>$Fe^{2+}$</TEX> and <TEX>${PO_4}^{3-}$</TEX> in soil solution at 10 cm depth were monitored, and also the growth and nutrient uptake of rice plants were measured. Concentration of <TEX>${NH_4}^+$</TEX> in soil solution was highest in urea+N-serve treatment, and followed by urea and <TEX>$KNO_3$</TEX> treatments. Addition of N-serve could effectively inhibit nitrification in the soil. In the treatment of <TEX>$KNO_3$</TEX>, relatively higher <TEX>${NO_3}^-$</TEX> concentration was found at 10 cm depth soil. In urea+N-serve treatment redox potential was lower than -100 mV during the experiment, but in the treatment of <TEX>$KNO_3$</TEX> the potential was maintained above 0 mV until <TEX>${NO_3}^-$</TEX> remaining in soil solution. Reduction of Fe(III) and solubilization of P were highly correlated with redox potential changes in the three N fertilizer treatments. Concentrations of Fe(II) and <TEX>${PO_4}^{3-}$</TEX> in soil solution at 10 cm depth were much higher in the urea+N-serve treatment. The most vigorous rice seedling growth was found in the urea treatment. Although the availability of N and P in soil was enhanced in the urea+N-serve treatment through the suppression of nitrification, excessive solubilization of Fe could limit the growth of rice plants.

Mercury concentrations in oligohaline wetland vegetation and associated soil biogeochemistry

Concentrations of mercury were determined in above- and below-ground tissues of dominant plant species, as well as soils, in the wetlands of Lake Maurepas, Louisiana. Indicators of wetland soil biogeochemical status, such as soil redox potential, pore-water nutrient concentrations, and pore-water total sulfides, were also determined. Total mercury concentrations in plant tissues were within the typical range for vegetation not exposed to mercury contamination. Similarly, total mercury concentrations in soils were typical of uncontaminated wetlands within this geographic region. Soil methyl mercury levels in this study are slightly lower than those reported in other studies of nearby wetlands. This may reflect the less extensive geographic sampling in this study, or the low water levels in the Lake Maurepas system immediately prior to and during this study, which would have altered soil biogeochemical status. This is corroborated by measurements of soil redox potential and soil pore-water nitrogen and sulfur constituents conducted during this study that suggest minimal sulfate reduction was occurring in surficial soils. This study indicates that the wetlands surrounding Lake Maurepas are typical of many uncontaminated oligohaline wetlands in the southeastern U.S. in regard to mercury concentrations.

Effects of rice straw ash amendment on Cu solubility and distribution in flooded rice paddy soils

Rice straw burning is a common post-harvest practice on rice paddy land, and it leads to the accumulation of rice straw ash (RSA) in paddy soil. To understand the role of RSA in determining the mobility and bioavailability of metal contaminants, this study investigated the effects of RSA amendment on the solubility and distribution of Cu in contaminated rice paddy soils with flooding incubation. The addition of RSA to the soils suppressed the release of Cu into the soil solutions, which was primarily attributed to the metal-binding capacity of the RSA. Additionally, after the soils were flooded, the increase in soil pH and decrease in redox potential resulted in the transformation of Cu into less soluble forms. The RSA amendment appeared to enhance the changes in pH and redox potential of the flooded soils and, consequently, the immobilization of Cu in the soils. The results suggest that the RSA can retard the bioavailability and movement of the metal in contaminated soils and, thus, lower the potential environmental risk of Cu toxicity.

Effects of water management and organic fertilization with SRI crop practices on hybrid rice performance and rhizosphere dynamics

A field experiment was conducted to investigate the effects of intermittent versus continuous irrigation, together with different degrees of organic fertilization, on the growth and yield of hybrid rice, looking also at the functioning of the rhizosphere as this is a key element affecting crop performance. The crop management practices employed generally followed the recommendations of the System of Rice Intensification (SRI). The aim of the research was to learn how water management and organic fertilization together would affect crop outcomes. Under intermittent water application as recommended with SRI management (aerobic irrigation, AI), grain yield increased by 10.5–11.3%, compared to standard irrigation practice (continuous flooding, CF). The factor that contributed most to higher yield was increased number of grains per panicle. It was seen that under the range of organic fertilization treatments evaluated, intermittent irrigation compared with CF promoted greater dry matter production and higher leaf area index (LAI) during the main growth stages. Also, the combination of intermittent irrigation and organic material applications significantly increased soil redox potential (Eh), compared with CF, and also the numbers of actinomycetes in the rhizosphere soil. Actinomycetes were evaluated in this study as an indicator of aerobic soil biota. It was seen that with intermittent irrigation, the application of organic material improved the functioning of the rhizosphere and increased yield. However, these results based on 2 years of study reflect relatively short-term effects. The effects of longer-term water management and soil fertilization regimes should be also examined, to know whether these effects continue and, if they do, whether they become greater or less.

Constraints on Salt Marsh Development Following Managed Coastal Realignment: Dispersal Limitation or Environmental Tolerance?

Salt marshes restored through managed coastal realignment (MR) often develop slowly and show persistent differences in vegetation from natural marshes. Development might be constrained by the availability of propagules or poor suitability of the abiotic environment for their establishment. To distinguish between these factors, we compared vegetation colonization and environmental conditions at a salt marsh created by MR at Brancaster, Norfolk, UK, with five reference marshes, varying in age from 30 to circa 6,000 years. After 5 years, plant communities of the MR site remained different from those in mature reference marshes. In contrast, the communities of the youngest reference marsh were not significantly different from mature reference marshes. At the MR site, abundance of perennial and later‐successional species was low and large areas remained unvegetated. These differences are unlikely to be due to dispersal limitation, because 76% of the species from the local species pool colonized the site within 5 years. Although the annuals Salicornia europaea and Suaeda maritima were abundant by year 2, they were not ubiquitous until the end of the study. Tidal elevations of the MR site were suitable for vegetation development, but soil redox potentials were lower than that at the reference sites. Reducing conditions in the MR site appear to be the major cause of vegetation differences from the reference marshes, as they are associated with an abundance of bare ground and a small range of vegetation clusters. Measures to avoid low sediment redox potentials may have a great benefit in some salt marsh restoration projects.

Mobilization of Zn upon waterlogging riparian Spodosols is related to reductive dissolution of Fe minerals

Contaminated riparian soils can release metals to surface water. Periodic waterlogging affects metal mobility but the processes and soil factors governing net trends are not well understood. Experiments were combined with geochemical modelling to identify processes explaining the dynamics of zinc (Zn) in contaminated soils following waterlogging. Samples were collected from 12 Spodosols near streams in a metal‐contaminated area and four similar but uncontaminated soils were sampled in a reference area. Air‐dried samples were submerged and incubated under N2. The soil redox potential decreased from 470 mV initially to approximately 30 mV over 2 months. The pore‐water Zn concentrations surprisingly increased over the same period by, on average, a factor of 18 (range 0.6–80; immobilization in one soil only) despite an increase in pH of 1.8 units, on average. Dissolved organic matter (DOM) in the soil solution increased during waterlogging but the observed increase in Zn solubility could not be explained by increased complexation with DOM, because the estimated Zn2+ activity also increased by a factor of 18 on average (range 0.2–82). Speciation modelling suggests that Zn mobilization during waterlogging results from Fe2+ displacing sorbed Zn2+ from particulate organic matter and from dissolution of Zn‐bearing Fe/Mn oxyhydroxides. This hypothesis is supported by the significant positive correlation (r = 0.87, n = 13) between the factor change in pore‐water Zn concentration and the ratio of dithionite‐extractable Fe to organic carbon content. These results show that Fe dynamics are important for predicting the fate of trace metals in anoxic soils.

Arsenic release and speciation in a degraded fen as affected by soil redox potential at varied moisture regime

Soil surveys have demonstrated arsenic (As) contents of up to 1600 mg kg − 1 in surface horizons of degraded fens in the Bavarian Molasse basin, Germany. Ground water from the Tertiary aquifer seems to be the primary source of As. Yet, the cause of its accumulation in the topsoil is unclear. Focussing the influence of redox processes on As redistribution, we conducted soil column experiments with the A (716 mg As kg − 1 ), Ag (293 mg As kg − 1 ) and 2Ag (37 mg As kg − 1 ) horizons of a Mollic Gleysol (pH 7.2). The fixed beds were equipped with redox electrodes and suction cups and subjected to a saturation–drainage–saturation cycle. Water table fluctuations were simulated by defined pressure heads applied to the lower column boundaries via a digitally controlled vacuum system. After water saturation, the redox potential (E H) dropped to minimum values of around 0 mV in the A/Ag and−400 mV in the 2Ag horizon. Soil drainage resulted in a quick return to oxidising conditions. Both in the A and Ag horizons total aqueous As concentrations were low (up to 20 μg l − 1 ) and not related to E H. In contrast, aqueous As concentrations of the 2Ag horizon were between 5 and 140 μg l − 1 and increased as the E H decreased. However, the As species distribution showed no clear trend with E H since both As(III) and As(V) were detected under reducing conditions. High release of As in the 2Ag horizon is consistent with low contents in Fe (hydr)oxides. In the A and Ag horizons, pedogenetic enrichment of sesquioxides contributes to a comparably lower sensitivity of E H to water saturation and favours As retention. Thereby, As released under saturated conditions in the 2Ag horizon may be stabilised in the topsoil following capillary rise. Thus, surface horizons may act both as historic and as recent sinks for geogenic As at the site.

Importance of the fiddler crab Uca pugnax to salt marsh soil organic matter accumulation

The potential influence of fiddler crab burrow density on the processes controlling organic matter (OM) accumulation was examined in Virginia salt marshes. As burrows may affect important chemical and biological processes that influence belowground plant production and decomposition, experimental manipulations were designed to modify fiddler crab burrow density by either increasing them (artificially constructed holes) or decreasing them (exclosures) and comparing them to areas naturally with and without fiddler crab burrows. The only significant difference among treatments was associated with the presence of holes without regard for whether they were artificial or natural. Higher burrow density resulted in an increase in soil redox potential, which most likely caused higher decomposition, even though sulfate-reduction rates were not different among treat- ments. Belowground production decreased with increased burrow density resulting in less OM addi- tion to the soil. Higher decomposition and lower belowground production resulted in a net loss of 3 g C m -2 yr -1 in marshes with higher burrow densities, while areas with few to no burrows accumulated up to 245 g C m -2 yr -1 , equaling a surface accretion rate of 4 mm yr -1 . Examination of 6 other salt marshes in the region revealed a negative correlation between fiddler crab density and soil OM con- tent, lending strength to the argument that bioturbation is a potentially important explanatory factor of OM accumulation in salt marshes and may influence how sea level rise impacts coastal marshes.

Restoring Ecological Function to a Submerged Salt Marsh

Impacts of global climate change, such as sea level rise and severe drought, have altered the hydrology of coastal salt marshes resulting in submergence and subsequent degradation of ecosystem function. A potential method of rehabilitating these systems is the addition of sediment‐slurries to increase marsh surface elevation, thus ameliorating effects of excessive inundation. Although this technique is growing in popularity, the restoration of ecological function after sediment addition has received little attention. To determine if sediment subsidized salt marshes are functionally equivalent to natural marshes, we examined above‐ and belowground primary production in replicated restored marshes receiving four levels of sediment addition (29–42 cm North American Vertical Datum of 1988 [NAVD 88]) and in degraded and natural ambient marshes (4–22 cm NAVD 88). Moderate intensities of sediment‐slurry addition, resulting in elevations at the mid to high intertidal zone (29–36 cm NAVD 88), restored ecological function to degraded salt marshes. Sediment additions significantly decreased flood duration and frequency and increased bulk density, resulting in greater soil drainage and redox potential and significantly lower phytotoxic sulfide concentrations. However, ecological function in the restored salt marsh showed a sediment addition threshold that was characterized by a decline in primary productivity in areas of excessive sediment addition and high elevation (&gt;36 cm NAVD 88). Hence, the addition of intermediate levels of sediment to submerging salt marshes increased marsh surface elevation, ameliorated impacts of prolonged inundation, and increased primary productivity. However, too much sediment resulted in diminished ecological function that was equivalent to the submerged or degraded system.

Nitrate Immobilization in Anaerobic Forest Soils along a North–South Transect in East China

It has been reported that 15N‐NO3− added to soils cannot be completely recovered in the remaining NO3− and its measurable products under anaerobic conditions. Abiotic immobilization of NO3− into dissolved organic N (DON) has been hypothesized to explain this phenomenon. Six forest soil samples were collected from temperate, subtropical, and tropical regions in East China, amended with K15NO3, and incubated at 25°C under anaerobic conditions for 264 h. The results showed that only 36 to 56% of the added N was recovered in the temperate soils, while up to 90% of the added N was recovered in the subtropical and tropical soils 1.5 h after incubation commenced. Compared with the initial concentration, the DON concentration dramatically increased when measured in temperate soils at 1.5 h, but increased only slightly in the subtropical and tropical soils. In all the soils, there was a strong positive correlation between recovery of added N at 1.5 h and soil redox potential (Eh) (R2 = 0.87, P &lt; 0.01), and a strong negative correlation between added N recovery at 1.5 h and dissolved organic C (measured at the beginning of incubation) (R2 = 0.72, P &lt; 0.05). There was a significant decrease in recovery of added N along with an increase in DON as soil Eh fell in the subtropical and tropical soils during long‐term incubation, suggesting that a majority of unrecovered N is incorporated into dissolved organic matter. All these results indicate added N was rapidly converted to DON via abiotic immobilization and was an important component of the N cycle in temperate forest soils but not in subtropical and tropical soils with high soil Eh, which was the controlling factor affecting this process.