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Published online 29 June 2007
Published in Soil Sci Soc Am J 71:1406-1417 (2007)
DOI: 10.2136/sssaj2006.0155
© 2007 Soil Science Society of America
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WETLAND SOILS

Major Biogeochemical Processes in Soils-A Microcosm Incubation from Reducing to Oxidizing Conditions

Kewei Yua,*, Frank Böhmeb, Jörg Rinklebeb, Heinz-Ulrich Neueb and Ronald D. DeLaunea

a Wetland Biogeochemistry Institute, School of the Coast and Environment, Louisiana State Univ., Baton Rouge, LA 70803
b Helmholtz Centre for Environmental Research–UFZ, Dep. of Soil Chemistry, Theodor-Lieser-Str. 4, 06120 Halle/Saale, Germany

* Corresponding author (kyu1{at}lsu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Six soils used for rice (Oryza sativa L.) production were incubated using an automatic microcosm system. Production of trace gases (CO2, CH4, and N2O) and transformation of N, S, and metals (Fe and Mn) were studied in soil suspensions incubated from reducing to oxidizing conditions. Results show that soil pH variation was inversely correlated to soil redox potential (EH) change (P < 0.01). Soil CO2 production exponentially increased with soil EH increase. In contrast, soil CH4 production and DOC showed an exponential decrease with soil EH increase. Without the presence of soil oxidants, methanogenesis occurred across the entire EH range, with probable H2–supported methanogenesis at higher soil EH conditions constituting up to 200f total CH4 production. The CH4 compensation point, where CH4 concentration became constant due to equilibrium between CH4 production and consumption, exponentially decreased with soil EH increase. At pH 7, the critical EH above which soils consumed atmospheric CH4 varied among the soils, but was generally >400 mV. Significant N2O production was observed between 200 and 500 mV. Nitrification could also contribute to N2O production when EH is >500 mV, a possible critical EH for the initiation of nitrification. The critical EH for substantial immobilization of Fe and Mn was estimated to be around 50 and 250 mV, respectively. The intermediate EH range (approximately –150 to 180 mV) provided optimum conditions for minimizing cumulative global warming potential resulting from CO2, CH4, and N2O production in soils. Our results have implications in interpreting the overall benefits of soil C sequestration efforts.

Abbreviations: DOC, dissolved organic carbon • EH, redox potential • GC, gas chromatograph • OM, organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Oxidation and reduction reactions regulate many biogeochemical reactions in Earth surface environments. The intensity of soil reduction can be rapidly characterized by soil oxidation–reduction (redox) potential (EH), which allows the prediction of the stability and availability of various nutrients and metal elements in soils and sediments. Soils tend to undergo a series of sequential redox reactions in a homogenous environment when soil redox status changes from aerobic (high EH) to anaerobic (low EH) conditions. Major reactions include, in order of EH from high to low, nitrification, denitrification, Mn(IV) reduction, Fe(III) reduction, SO42– reduction, and methanogenesis (Patrick and DeLaune, 1972; Ponnamperuma, 1972; Smith and DeLaune, 1984; Reddy et al., 1989; Patrick and Jugsujinda, 1992). Meanwhile, soil respiration going from aerobic to anaerobic conditions results in CO2 production across the entire EH range. Although the reduction reactions proceed in a thermodynamic order (Ponnamperuma, 1972; Patrick and Reddy, 1978), the given oxidation–reduction system is only partially applicable to field conditions, because the mineral phases present in soils are mixed and often unknown. Changes in pH and activities of reactants and products can also alter the order of redox reactions. As a consequence, reduction potentials of a given redox reaction can span a wide range along the redox scale. Chemical reactions that are favored thermodynamically are not necessarily favored kinetically. The lack of effective coupling and the slowness of redox reactions mean that catalysis is required if equilibrium is to be attained. In soils, the catalysis of redox reactions is mediated by microorganisms. Equilibrium depends entirely on the growth and ecological behavior of the soil microbial population and the degree to which the reactants and products can diffuse and mix. Most of the information on soil redox processes has been obtained from flooded rice systems, but applies to natural wetland soils, and probably upland soils as well (Yu et al., 2001).

Wetland rice ecosystems are a unique aerobic and anaerobic environment. In wetland rice soils, two distinct aerobic–anaerobic interfaces have been identified: (i) the water–soil interface that receives sufficient O2 from the floodwater (Patrick and DeLaune, 1972)-the thickness of the layer may range from several millimeters to several centimeters depending on perturbation by soil fauna and the percolation rate of water; and (ii) the plant rhizosphere maintained by O2 diffusing through the aerenchyma of rice plants (Reddy et al., 1989). Redox processes play an important role in soil nutrient availability, biogeochemical cycling of elements, and ecological functions of rice ecosystems. Carbon dioxide, CH4, and N2O are the most important atmospheric trace gases that contribute to the global greenhouse effect. Biological N2O can be produced from nitrification under aerobic conditions, and denitrification under moderately reducing conditions where the reducing condition is not intense enough to completely reduce NO3– to N2 gas. Denitrification is the final step of the N cycle by which atmospheric N2 fixed in the biosphere returns to the N2 pool. Significant CH4 formation (methanogenesis) in soils generally occurs under strictly reducing conditions when soil redox potential decreases below a critical point. Rice fields have been the most studied methanogenic ecosystem because of their economical importance and high potential as an atmospheric CH4 source. Culturable microorganisms associated with CH4 and N2O dynamics were found to be strongly related with key edaphic soil properties (i.e., pH, C/N ratio) in soils (Kravchenko and Yu, 2006). Transformation of Mn, Fe, and S between their oxidized and reduced forms can significantly affect N2O and CH4 dynamics in soils (Yu and Patrick, 2004).

Almost all reported information on major redox processes has been obtained by incubating soils in a direction going from oxidizing to reducing conditions. This is analogous to flooding a thoroughly drained soil where all soil redox-active components are in their oxidized forms. During the incubation, however, soil microbial communities and enzyme activities for anaerobic processes may develop progressively, which ultimately influences the dynamics of various redox reactions. For example, N2O production is significantly affected by the development of denitrifying enzymes during the incubation, especially the N2O reduction enzyme that controls the N2O/N2 ratio (Rudaz et al., 1991; Dendooven and Anderson, 1995). In this study, the soil incubation was initiated under oxidizing conditions, continuing until reducing conditions (Phase I) developed, which allowed anaerobic microbial activities to fully function and soil redox-active components to be transformed to their reduced forms. In the subsequent reducing to oxidizing phase of the incubation (Phase II), major soil biogeochemical processes were analyzed at different soil redox conditions, including nitrification, denitrification, methanogenesis, methanotrophy, and transformation of Fe, Mn, and S. This was an analog to draining a long-flooded rice field, with the results complementing the previous information obtained by incubating soils from oxidizing to reducing conditions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Sampling
Six soils (surface 20 cm) were collected from four major rice-cultivating states in the USA (Arkansas, California, Louisiana, and Texas), and from two Asian regions: Hangzhou (China) and Java (Indonesia). The soils were air dried, sieved (1-mm sieves), thoroughly mixed, and stored at room temperature (20°C) before the experiment. Major soil characteristics were analyzed and are provided in Table 1.


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  		Table 1. Selected physical and chemical characteristics of soils used in a microcosm incubation study.

 
Description of the Soil Microcosm System
Soils were incubated using an advanced microcosm system, which allows continuous monitoring and control of soil EH, pH, and temperature in soil suspensions (Fig. 1). Soil EH was maintained within a specific range by adding N2 (to lower EH) and O2 (to raise EH) through an automatic-valve gas regulation system. All microcosm systems were connected to a gas chromatograph (GC) via a computer-operated valve–pipe system. Thus, gas concentration in the headspace of each microcosm was automatically quantified.


Figure 1
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  		Fig. 1. Components of the soil microcosm system used in an incubation study with six soils: (1) thermometer; (2) redox potential (EH) electrode; (3) pH electrode; (4) dispersion tube for N2; (5) dispersion tube for O2; (6) stirrer; (7) sampling tube; (8) microcosm vessel; (9) temperature control by a thermostat and water circulation; (10) data logger for EH, pH, and temperature; (11) automatic redox regulation by N2 and O2 valves; (12) control computer for data logger, pump, and valve system (gas sampling), and gas chromatograph (start signal); (13) gas chromatograph (GC) with flame ionization detector/electron capture detector for trace gas measurements (CO2, CH4, and N2O); and (14) computer for GC control and GC data storage.

 
Soil Incubation and Measurement
In total, 12 microcosm systems were used, allowing for two replicates of each soil except for the Louisiana and China soils (not replicated due to limited amounts of soil sample). Soil suspension was established by adding 200 g dry soil to a 2.88-L microcosm vessel with 1.6 L of deionized water. To each soil suspension, 5 g ground rice straw (388.0 g kg–1 C, 7.2 g kg–1 N, and C/N ratio 53.8) was added as an additional source of organic matter (OM). The original oxidized soils were incubated by continuously flushing the microcosm with N2 until stable reducing conditions with an EH below –200 mV (corresponding value at pH 7) were established. In this oxidizing to reducing phase (Phase I) of the incubation, only soil EH and pH were monitored and recorded every 15 min. A single Pt electrode with a Ag–AgCl reference electrode was used for the EH measurement. Original soil NO3– was completely denitrified during this earlier phase of the incubation. In the subsequent reducing to oxidizing phase (Phase II) of the incubation, soil EH was stepwise elevated to a specific EH value by providing O2 to the microcosms. Soil EH and pH were continuously monitored and recorded. To provide a N source for denitrification, 6 mL of 0.1 M NO3–N as KNO3 was added to each microcosm when soil EH reached moderately reducing conditions (EH approximately 0 mV at pH 7). Nitrate was added only once because it can significantly buffer soil redox conditions. Concentrations of CO2, CH4, and N2O in the microcosm headspace were quantified every 2 h. At selected EH levels, soil suspension was withdrawn (20 mL each time) from each microcosm, and was immediately filtered in an N2 atmosphere through a 0.45-µm Millipore membrane (Whatman Inc., Florham Park, NJ) into two 10-mL test tubes. One subsample was used immediately to monitor concentrations of dissolved organic carbon (DOC), NO3–, and NH4+. For the other subsample, three drops of 2 M HNO3 was added to preserve the solution for later analysis of Na+, K+, soluble Mn2+, Fe2+, and S (mainly in form of SO42–). The microcosm was flushed with N2 after a series of gas measurements and after sampling the soil suspensions. Changes in soil mass, water, and headspace volume in the microcosm were considered in calculations while soil/water ratio remained the same in the microcosms.

Sample Analysis
Initial soil pH was measured in a soil/water (1:1) slurry. Soil OM was measured colorimetrically after oxidizing with K2Cr2O7 and concentrated H2SO4. Soil total N was analyzed in dry combustion by a Leco N analyzer (Leco Corp., St. Joseph, MI). Initial soil Mn, Fe, S, K, and Na contents were analyzed by inductively coupled plasma (ICP) after extracting with diethylene triamine pentaacetic acid (DTPA) solution. All metal elements and S concentrations in the soil solution were analyzed directly on ICP–mass spectrometry using an ELAN 5000 (PerkinElmer, Wellesley, MA). Nitrate and NH4+ were colorimetrically measured using a FIAstar 5000 Analyzer (FOSS Analytical, Hillerød, Denmark). Dissolved organic C was analyzed after combustion of the finely sprayed solution with a micro N/C analyzer (Analytik Jena AG, Jena, Germany). Gas concentrations were analyzed using a Shimadzu GC-14BPFE (Shimadzu Corp., Kyoto, Japan) with an electron capture detector (for N2O), a flame ionization detector (FID, for CH4), and another FID detector (for CO2) coupled with a methanizer for transformation of CO2 into CH4. The GC columns were filled with HayeSep Q (80/100 mesh).

Calculation and Statistical Analysis
The experiment was conducted at room temperature (20 ± 1°C). Gas production rate was calculated by linear regression of three consecutive analyses with time after flushing the microcosm with N2. The amount of gas dissolved in the liquid phase was determined by using the mole fraction solubility of 5.07 x 10–4 for N2O, 2.81 x 10–5 for CH4, and 7.07 x 10–4 for CO2 (Lide, 1991). Soil EH was adjusted to the standard H2 electrode by adding 210 mV (correction factor for the Ag–AgCl electrode) to the recorded instrument reading. All EH data were reported as their corresponding values at pH 7 that were calculated according to the inverse relationship of EH and pH as described by the Nernst equation. Redox potential change per pH unit may vary from 59 to 177 mV, depending on redox couples and kinetics of the reaction (Bohn, 1971). Since EH values represent mixed potentials, a simple correction of 59 mV per pH unit (assuming equal numbers of protons and electrons involved in the reactions) was used.

Statistical analysis was conducted using SAS 9.1 (SAS Institute, Cary, NC). Simple linear regressions using PROC REG were conducted. Multiple regressions were conducted when more than one variable was considered in the model, with stepwise analysis to identify the most significant factor(s). Variables were considered statistically significant at P ≤ 0.05 ({alpha} = 0.05).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Relationship between Soil Redox Potential and pH
The measured EH in soils generally represented a composite value that reflects a weighted average contributed by all redox couples present. In aerobic soils where the O2–H2O redox couple functions, the EH range is between 300 and 700 mV. Soil EH and pH were recorded in both the earlier oxidizing to reducing (Phase I) and later reverse (Phase II) phases of the incubation. The soil EH generally ranged from –200 to 600 mV in this study, a typical EH range that occurs in wetland soils under natural conditions. At the extreme two ends of this EH range, nitrification occurs under oxidizing conditions (high EH) and methanogenesis under strictly reducing conditions (low EH). Soil pH fluctuated with changes in soil EH conditions. All major soil redox reactions (such as denitrification, and reduction of Mn, Fe, and SO42–) increase soil pH. The pH increase, however, is limited by the precipitation of Fe(II) and Mn(II) carbonates occurring at about pH 7, and production of CO2 and organic acids from decomposing OM. Under reducing conditions, all soils tend to reach near-neutral pH values for either originally acidic or alkaline soils (Ponnamperuma, 1972). Dynamics of the soil EH and pH measurement are summarized in Table 2. Statistical analysis showed a significant (P < 0.01) negative correlation between the soil EH and pH. The activities of many biogeochemical processes will be significantly altered due to such pH shifts with redox fluctuations.


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  		Table 2. Variations of soil pH and redox potential (EH), and their linear regression analysis in a microcosm incubation study with six soils{dagger}.

 
Carbon Dioxide Production
All major reduction reactions of soil oxidants generate CO2 (soil respiration), with OM as the electron donor. Soil oxidants [mainly Mn(IV), Fe(III), and SO42–] can be regenerated by introducing O2 into a reducing system, as was the case in Phase II in this study. Thus, aerobic respiration by soil microbes using O2 as an electron acceptor also probably contributed to CO2 production in this study.

Dissolved organic C, as an active soil electron donor, was measured by sampling soil suspensions at different stages of the Phase II incubation, because of its direct relationship with soil CO2 and CH4 production. Carbon dioxide production rates, for all studied soils, increased with soil EH. The same tendency has been reported in incubations from oxidizing to reducing conditions (Yu and Patrick, 2003, 2004). In this study, only CO2 production rates at times of DOC measurements are included in Fig. 2. Higher CO2 production rates were found at higher redox conditions, despite decreasing DOC content in the soil microcosms with increasing EH (Fig. 3-i). The DOC content decrease found at higher redox conditions indicates that formation of DOC from soil OM could not balance mineralization to CO2 during the incubation. Multiple regression analysis of all soils (n = 52) indicated that both the soil EH and DOC were positively correlated with the logarithm of CO2 production rates (r2 = 0.42, P < 0.01). Stepwise regression analysis indicated that the soil EH was more highly related to CO2 production (for EH analysis: r2 = 0.41, P < 0.01; for DOC analysis: r2 = 0.06, P = 0.08). For each microcosm, DOC-adjusted CO2 production rates were calculated by assuming the CO2 production rate is proportional with DOC content (DOC-adjusted CO2 production rate = measured CO2 production rate x DOC content). With this adjustment, the calculated CO2 production rates are independent of DOC content in each microcosm. The CO2 production rates and corresponding DOC measurements are summarized in Table 3. Without DOC interference of CO2 production rates (after adjustment), CO2 production rates tended to increase exponentially with the soil EH. For all soils, regression of the CO2 production rates (after adjustment) with the soil EH showed a significant relationship (r2 = 0.57, P < 0.01).


Figure 2
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  		Fig. 2. Carbon dioxide production rates under different redox potential (EH) conditions in a microcosm incubation study with six soils. Only results when soil dissolved organic carbon (DOC) measurements were conducted are included.

 

Figure 3
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  		Fig. 3. Soil CH4 production rates and dissolved organic carbon (DOC) concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils: (i) DOC measurement; (ii) original CH4 flux rate; and (iii) DOC adjusted CH4 flux rate. Only results when soil DOC measurements were conducted are included. Adjusted CH4 flux rates were calculated by: DOC adjusted CH4 flux rate = measured CH4 flux rate x DOC content.

 

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  		Table 3. Variations of CO2 production rates and associated dissolved organic carbon (DOC) contents, and exponential regression analysis of DOC-adjusted CO2 production rates with soil redox potential (EH) in a microcosm incubation study with six soils.

 
Soil CO2 production rates are generally low during anaerobic respiration, with less energy yield for soil microorganisms. Such inefficient respiration is a principle mechanism for soil C sequestration, as found in wetland ecosystems (Smith et al., 1983) and no-till agroecosystems (Kessavalou et al., 1998). Conversely, significant increases in respiration rates after flooding were reported in paddy soils (Bossio and Scow, 1995) and floodplain soils (Rinklebe and Langer, 2006), possibly due to an unspecific stress to aerobic microorganisms. The overall benefit of C sequestration, however, deserves careful evaluation. Some of the C sequestration benefit may be offset, in terms of contribution of total soil radiative forcing, by enhanced soil N2O production (Batjes, 1998), and especially by significant CH4 production (Roulet, 2000; Yu et al., 2006).

Methane Production
Dissolved organic C is a relatively mobile and labile form of soil C. In flooded soils, DOC may serve as a C source for CH4 production. Methane emission rates have been found to be positively correlated with the dynamics of DOC in the rice root zone (Lu et al., 2000). Significant CH4 production was found under relatively low EH conditions in this study (Fig. 3-ii). As a substrate for methanogenesis, soil DOC is directly responsible for the observed CH4 production. Following the same assumption as for CO2 production, CH4 production rates were adjusted with the corresponding DOC content in each microcosm. After adjusting the CH4 production rates with the measured DOC, the results showed similar CH4 production patterns under different EH conditions (Fig. 3-iii). Increasing soil EH by supplying O2, however, may exert a toxic effect on the methanogenic bacteria along with increasing the soil redox status. In fact, experiments with cultures of methanogenic bacteria showed that O2 had a greater adverse effect on methanogenic activity than high redox potentials, and that methanogens were able to initiate CH4 production at EH values up to 420 mV (Fetzer and Conrad, 1993). Other studies in which soils were not treated with O2 showed initiation of CH4 production at EH values around 0 to 100 mV (Peters and Conrad, 1996; Ratering and Conrad, 1998). Nevertheless, soil redox potential is generally a good indicator for the onset of soil methanogenesis, but should be combined with careful characterization of the soil and its CH4 production behavior.

Statistical analysis (n = 37) indicated that there was no significant correlation between the CH4 production rates (both before and after DOC adjustment) and DOC contents (P > 0.05) in the studied soils. Methane production tended to exponentially increase with decreasing soil EH (r2 = 0.27, P < 0.01). After adjusting the CH4 production rates by the soil DOC contents, the correlation between the CH4 production rates and soil EH remained significant (r2 = 0.15, P < 0.01).

The results of this incubation study from reducing to oxidizing conditions suggest that soil redox potential may not be a good indicator for the cessation of ongoing soil methanogenesis. The relative poor relationship (low r2 values) between the CH4 production rates and soil EH may be due to two mechanisms involved in CH4 production. Past studies have shown that, when soil incubations were initiated under oxidizing conditions, small amounts of CH4 production were observed at the beginning of the incubation with high EH (Fetzer and Conrad, 1993; Roy et al., 1997; Yao and Conrad, 1999; Yu and Patrick, 2003). Such initial methanogenesis was H2 dependent and was generally insignificant (2–60f total CH4 production) compared with the vigorous acetate-dependent methanogenesis under strictly anaerobic conditions (Yu and Patrick, 2003). Under conditions where soil goes from oxidizing to reducing, redox-active soil oxidants, such as NO3–, Mn(IV), Fe(III), and SO42–, can significantly reduce H2 production, limiting early CH4 production. Significant CH4 production can only take place when these oxidants are reduced into their reduced forms. At the same time, soil oxidants can contribute to oxidation of the existing CH4 in the system without using O2 (Iversen et al., 1987; Miura et al., 1992; Kumaraswamy et al., 2001). The critical EH for significant CH4 production has been determined to be about –150 mV or less in most studies (Neue et al., 1995; Yu et al., 2001; Yu and Patrick, 2003). In this study, measurement of CH4 production started under strictly reducing conditions when all soil redox-active oxidants had been transformed into their reducing forms. There was no limitation for methanogenesis in such a reducing environment. In transition from reducing to oxidizing conditions, inhibition mechanisms for methanogenesis (by redox-active soil oxidants, and even increase of O2 partial pressure) developed gradually. No clear EH boundary could be found for the two phases of methanogenesis in this study. With less competition of various soil oxidants for H2, CH4 production under higher EH conditions was significant (up to 200f total CH4 production), compared with the CH4 production under strictly reducing conditions. A reported field study concluded that H2–dependent methanogenesis contributed about 25 to 300f the CH4 produced in soils (Conrad and Klose, 1999a, 1999b). Methane production tended to terminate only when soil EH was >400 mV in this study. Measurement of CH4 production under different redox conditions provides valuable guidance for managing rice fields to mitigate CH4 emission and also helps to understand CH4 dynamics in natural wetlands under different hydrological conditions.

Methane Compensation Point under Different Redox Potential Conditions
Compensation occurs when consumption is balanced by simultaneous production. Trace gas consumption generally increases with ambient trace gas concentration. Trace gas production, however, is normally independent of the product concentration. Therefore, there exists a concentration level for a specific gas at which its production equals consumption, the so-called compensation point (Conrad, 1994). In this study, no compensation point could be determined for CO2 due to lack of consumption activity in the system, or for N2O due to the limited number of measurements.

Mechanisms of CH4 production have been discussed above. The major soil CH4 consumption mechanism is aerobic oxidation using O2. The significance of anaerobic CH4 oxidation has not been quantified, but the occurrence has been reported in marine sediments and in saline inland waters (Iversen and Joergensen, 1985; Iversen et al., 1987), and also in soils coinciding with reduction of Fe(III) (Kimura et al., 1992; Miura et al., 1992). Compensation points for CH4 have so far not been determined in either field or laboratory conditions. The automatic microcosm system used in this study made detailed monitoring of CH4 concentration in the microcosm possible. In such a closed system, two approaches for measuring CH4 compensation point were implemented. One was to monitor CH4 concentration increasing in the headspace of microcosms until it reached a steady state (Fig. 4-i), and the second was to monitor CH4 concentration decreasing to a steady state (Fig. 4-ii). Under low- EH conditions, a high CH4 compensation point was found due to strong CH4 production and weak CH4 oxidation capacity. The CH4 compensation point was low under high-EH conditions due to strong methanotrophic activity and weak methanogenesis activity. This analysis was applied to each soil across the entire EH range studied, and the results showed that the CH4 compensation point decreased exponentially with increasing soil EH (Table 4). For all studied soils, Fig. 4-iii clearly shows the reverse relationship between the CH4 compensation concentration and soil EH. The critical EH where the CH4 compensation point equals the ambient atmospheric CH4 concentration, above which soils start to consume atmospheric CH4, is important. Such critical EH values varied among the different soils studied. Regression analysis indicated that the critical EH value was 414 mV for all soils combined, above which the soils functioned as a sink of atmospheric CH4 (Table 4).


Figure 4
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  		Fig. 4. Determination of CH4 compensation point under different redox potential (EH) conditions, and exponential regression analysis of CH4 compensation point and soil EH in a microcosm incubation study with six soils: (i) CH4 concentration increases to a steady point; (ii) CH4 concentration decreases to a steady point; and (iii) relationship between CH4 compensation points and soil EH conditions from analysis of all six soils. Examples in determining CH4 compensation point are given from analysis of the Indonesian soil.

 

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  		Table 4. Exponential regression analysis of CH4 compensation point and soil redox potential (EH) in a microcosm incubation study with six soils.

 
The CH4 compensation point largely depends on the CH4 oxidation capacity. In rice fields, variations in CH4 emission have been primarily attributed to variations in methanotrophic activities (Sass et al., 1990; Schütz et al., 1989). Similar results have been reported in a Florida swamp where the CH4 emission increase associated with a decrease in environmental oxidation was not due to stimulation of methanogenesis but due to a decrease in the methanotrophic activity (King et al., 1990). Therefore, CH4 oxidation strength determines not only the potential of soils to act as a sink of atmospheric CH4, but also the CH4 emission strength of soils.

Nitrous Oxide Production and Associated Ammonium and Nitrate Content
With no addition of NO3– at the beginning of the incubations (Phase I), NO3– was essentially depleted when soils reached strongly reducing conditions (data not shown). During the reducing to oxidizing phase of the incubation, the soils occasionally reached higher EH conditions when excess O2 was introduced, and N2O formation was detected when the soil EH reached 500 mV or higher (Fig. 5). Without external NO3– addition, soil nutrient analysis showed an elevated level of NO3– in soil suspensions under such higher EH conditions. Nitrification was the probable cause of NO3– formation. Nitrous oxide production was attributed to nitrification itself or to the subsequent denitrification, the so-called coupled nitrification–denitrification process. No information on the critical EH for nitrification has been previously reported. Based on the results from this study, an EH value of approximate 500 mV is the likely critical EH threshold for nitrification activity to occur. Ammonium concentration in soil suspensions tended to decrease with increasing EH. Both immobilization by soil microorganisms and nitrification activity could contribute to the disappearance of soil NH4+. Considering that NH4+ was generated from soil organic matter mineralization, nitrification could play an important role in the consumption or depletion of the soil NH4+.


Figure 5
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  		Fig. 5. Soil N2O production rates and NH4+ and NO3– concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils. Additional NO3– was amended when EH > 0 mV. Soil EH occasionally fluctuated beyond the desired level, especially during the period before NO3– was added, where measurements were also conducted.

 
Nitrate was only applied when soil EH was >0 mV. Significant N2O production was observed in the EH range of 200 to 500 mV. Complete denitrification with N2 as the end product occurred when soil EH was <200 mV. The results agreed well with previous studies where soil incubations were conducted from oxidizing to reducing conditions (Yu and Patrick, 2003, 2004). When soil EH was <300 mV, strong denitrification activity rapidly consumed the NO3–, resulting in only trace amount of NO3– in the soil microcosms. Elevated levels of NO3– in the soil suspensions were found when the soil EH was in a higher range (>300 mV), especially when >500 mV. This could be attributed to a redox condition too high for denitrification but favorable for nitrification. Ammonium content remained low under such oxidizing conditions of the incubation. Initiation of nitrification at EH >500 mV was evidenced by NO3– concentration elevation during the Phase I incubation without adding NO3– (Fig. 5). The prolonged soil incubation under high- EH conditions in this study provides corroborating data for previous incubation studies where soil EH remained above 500 mV for just a few hours (Yu and Patrick, 2003, 2004).

Following a soil EH increase during the incubation, there was a parallel and substantial pH decrease in soil suspensions (Table 2). Microorganisms involve in soil N transformations generally function optimally under near-neutral pH conditions (Paul and Clark, 1996); however, nitrification can proceed rapidly at low pH. Significant net nitrification rates at pH 5 have been observed in a tropical forest soil (Neill et al., 1995). Nitrous oxide production depends mainly on denitrification intensity and the N2O/N2 ratio (higher under acidic conditions) in denitrification products. Small pH shifts may have little effect on total N2 + N2O produced, but the relative effect of pH on N2O reductase may regulate the N2O/N2 ratio (Burford and Bremner, 1975; Firestone et al., 1980; Klemedtsson et al., 1997).

Relation between Metals and Sulfur Transformation and Soil Redox Potential Conditions
When soils reached strongly reducing conditions (EH < –200 mV), all soil redox-active species were transformed into their reducing forms, Fe(III) to Fe(II), Mn(IV) to Mn(II), and SO42– to S2–, resulting in observed higher soluble Fe and Mn concentrations in soil solutions (Fig. 6-i and 6-ii). Elevating soil redox status from reducing to oxidizing conditions in this study generated a reverse order of Fe and Mn immobilization (Fe earlier than Mn), compared with the reported order of Fe and Mn mobilization (Mn earlier than Fe) when soil was incubated from oxidizing to reducing conditions (Patrick and Jugsujinda, 1992). In this study, the critical EH value for substantial immobilization of Fe and Mn was estimated to be about 50 and 250 mV, respectively. Previous studies showed that 100 mV at pH 7 was the critical value for Fe reduction and consequent dissolution (Gotoh and Patrick, 1974). Another study showed that, at pH between 6 and 8, most of the Mn conversion was found to take place at EH of 200 to 300 mV (Gotoh and Patrick, 1972). Variations of critical EH values for Fe and Mn transformations exist among different soils and different studies, due to the composite nature of redox reactions in the system. With drying in fields (soil pH will decrease), Fe and Mn carbonates will dissolve and oxidize to form oxides, amorphous oxides, and hydroxides that slowly recrystallize to stable Fe(III) oxides. Not much is known about this transformation, but pure Fe(OH)3 and MnO2 are not likely to form. At EH values between that typically found in flooded soils and that of well-aerated soils, the stable form of Fe is Fe3(OH)8 (Schwab and Lindsay, 1983). At higher EH, Fe3(OH)8 is not stable, although some Fe3(OH)8 may persist during short, dry fallows in rice fields. Under aerobic conditions, ferrihydrite may also be formed (Neue, 1991).


Figure 6
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  		Fig. 6. Soluble (i) Fe, (ii) Mn, and (iii) S concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils. Data represent the cumulative results from all six soils. The vertical lines represent the approximate EH conditions at which (i) Fe and (ii) Mn become immobilized when EH further increases. To use the same scale for the y axis, values of each analyte are standardized by multiplying by a standardized factor. For Fe concentration (soil x standardized factor): Arkansas x 10, California x 1.67, Louisiana x 10, Texas x 1, China x 1.5, Indonesia x 3. For Mn concentration (soil x standardized factor): Arkansas x 1, California x 2, Louisiana x 4, Texas x 4.8, China x 2.4, Indonesia x 1. For S concentration (soil x standardized factor): Arkansas x 3.56, California x 1.33, Louisiana x 3.56, Texas x 1.78, China x 1, Indonesia x 1.

 
The most common form of soluble S in soils is SO42–. Since most S in soils occurs in the organic state, reactions are closely associated with organic matter transformations and the activity of microorganisms. Upon flooding, sulfates are reduced to sulfides, and proteins are dissimilated after hydrolysis to H2S, mercaptans, S22–, NH3, and fatty acids (Neue and Mamaril, 1985). No clear relation between soluble S (mostly in the form of SO42–) content and soil EH (r2 = 0.004, P = 0.59, n = 84) could be found in this study (Fig. 6-iii). Precipitation of certain metal ions as sulfides in flooded soils or sediments is an important mechanism regulating the solution concentrations of toxic S2– and metal ions (Fe2+, Mn2+, Zn2+, Cu2+, and Hg2+). For the metal ions involved, the toxicity to plants is inversely related to the solubility of their sulfide salts, with Hg2+ being the most toxic and Fe2+ the least toxic (Engler and Patrick, 1975; Patrick and Reddy, 1978). When a flooded soil or sediment is drained and subsequently aerated, sulfides are transformed to more soluble SO42–, while the metal ions become insoluble oxidized forms. For acid soils, a further increase in soil acidity may dissolve these metal oxides by chemical reactions.

Ponnamperuma (1972) reported that the specific conductance of the soil solution first increases after flooding by 1 to 2 dS m–1 as a result of production of NH4+, HCO3–, RCOO–, Mn2+, and Fe2+, followed by displacement of Na+, K+, Ca2+, and Mg2+ from soil colloids. The subsequent decrease in conductance is the result of HCO3– removal, degradation and transformation of organic constituents, changes in the pH-dependent charges, and precipitation. In this study, the non-redox-active metal species, such as Na and K, showed no relation with soil EH. Concentrations of Na+ and K+ in soil microcosms tended to strongly correlate with the duration of incubation. For all soils, Na+ and K+ concentration significantly increased during the incubation, probably due to both dissolution and ion exchange mechanisms (Table 5). During the study period, Na+ and K+ concentrations apparently did not reach saturation status. Breaking down the soil mineral structure under fluctuating redox conditions due to ferrolysis (Brinkman, 1979) may enhance the dissolution processes. Exchange reactions may also be important in regulating the behavior of water-soluble Fe and Mn (Gotoh and Patrick, 1972).


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  		Table 5. Variations of Na and K concentrations in soil suspensions, and their liner regression analysis with incubation time in a microcosm incubation study with six soils.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The microcosm incubation and measurement of changes in soil biogeochemical processes under reducing to oxidizing conditions has several advantages: (i) it allows all possible microbial communities and enzymes to be fully developed during the preincubation from oxidizing to reducing conditions, (ii) all soil redox-active oxidants are converted into their reduced forms, creating an environment representing prolonged flooded soils and sediments, (iii) the information obtained on trace gas dynamics and nutrient transformation by elevating soil redox status is more similar to conditions occurring in draining flooded fields, and (iv) it provides an extended period of aerobic conditions by elevating soil EH with O2.

The EH range of –150 to 180 mV (corresponding value at pH 7) represents optimum soil conditions for minimizing the cumulative global warming potential from CO2, CH4, and N2O (Fig. 2, 3, and 5), which is in good agreement with previous studies (Yu and Patrick, 2003, 2004). The results have a significant implication in evaluating the overall benefits of soil C sequestration efforts, because part of the C captured in soils may be substantially offset by enhanced soil CH4 production at lower EH conditions (Roulet, 2000), and enhanced N2O emission under higher EH conditions (Li et al., 2005). Optimum EH conditions for minimizing soil global warming potential are mainly due to three factors: (i) the reduction potential in this EH range is favorable for complete denitrification, with N2 as the end product, but is still not strong enough to initiate significant CH4 production, (ii) a slightly acidic pH condition limits methanogenesis, and the N2O/N2 ratio of denitrification is relatively smaller than more acidic conditions (EH > 180 mV), and (iii) soil oxidants (mainly Fe and Mn) significantly lower the CH4 compensation point by competing with H2 produced from OM decomposition, and possibly by anaerobically oxidizing CH4.

The comprehensive analysis of soil nutrient and metal transformations and trace gas dynamics in this study integrates the main environmental factors regulating major biogeochemical processes in soils. Fluctuation in soil EH status represents changes occurring in aerobic and anaerobic environments on scales as small as soil aggregates (Tiedje et al., 1984) to as large as riparian zones and wetland ecosystems (Yu et al., 2006). The achieved quantification of the CH4 compensation point under different EH conditions provides a better understanding of the role of soils as a source or sink of atmospheric CH4. Biological CH4 oxidation is an important mechanism in controlling the CH4 emissions from anoxic soils and sediments, because up to 900f the produced CH4 is consumed before being released to the atmosphere (Frenzel et al., 1992). A prolonged period of incubation under high soil EH conditions provides some evidence of soil nitrification activity, which is normally studied in a soil slurry system. Our analysis indicates that 500 mV is probably a critical EH value for soil nitrification to take place, where NO3– (also possibly part of N2O) is formed from the oxidizing NH4+.


   ACKNOWLEDGMENTS
 
This paper is dedicated to Dr. William H. Patrick, Jr., (1925–2004) who participated in collecting soil samples and planning for this collaborative study.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
And: Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China

Current Address: KataLeuna GmbH Catalysts, Am Haupttor, 06237 Leuna, Germany

Current Address: Univ. of Wuppertal, Dep. D, Soil and Groundwater Management, Pauluskirchstraße 7, 42285 Wuppertal, Germany

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication April 12, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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