Paper Menu >>
Journal Menu >>
American Journal of Climate Change, 2012, 1, 205-209 http://dx.doi.org/10.4236/ajcc.2012.14017 Published Online December 2012 (http://www.SciRP.org/journal/ajcc) Wet Soil Redox Chemistry as Affected by Organic Matter and Nitrate Duane T. Gardiner, Stephanie James Texas A&M University-Kingsville, Kingsville, USA Email: duane.gardiner@tamuk.edu Received September 12, 2012; revised October 13, 2012; accepted October 23, 2012 ABSTRACT Wet soil microcosms were established to determine effects of organic matter and nitrate additions on microbial respira- tion and redox potentials. Organic matter (1%) and nitrate (100 ppm and 200 ppm) treatments were applied in factorial combination. Soil pH, redox potential, and CO2 emissions were measured. Data were analyzed by ANOVA for repeated measures and separately by sampling day. Addition of organic matter significantly (P < 0.05) and consistently increased CO2 emissions and decreased redox potentials. On Day 42 nitrate significantly (P < 0.05) increased redox values. This study indicates a tendency for organic matter to decrease soil redox potential both in absolute terms and relative to the suboxic-anoxic boundary. Our findings portend that additions of organic matter may quickly and markedly decrease soil redox potentials and increase CO2 emissions in wetlands, whereas additions of nitrate may have complex and sporadic effects on redox potentials. Keywords: Oxidation; Reduction; Wetlands; Carbon Dioxide Emission 1. Introduction Microbial respiration in the soil, accomplished by linking oxidation and reduction half-reactions, directly impacts the redox potential (Eh) of soil-water systems. Soil redox potential affects, and is affected by, many attributes of the ecosystem. In soil the primary electron donor for oxi- dation half-reactions is organic matter. In aerobic soil the prevailing electron acceptor for reduction half-reactions is molecular oxygen (O2). Unlike higher organisms, mi- crobial populations can use electron acceptors less sus- ceptible to reduction than O2, especially in environments where O2 is absent. Where microbes and organic matter are abundant, the absence of O2 (i.e., an anaerobic condi- tion) leads to the reduction of other substances, creating a low redox potential as expressed in units of millivolts or by the term pe implying the negative logarithm of con- centration of electrons in search of an electron acceptor. Electron acceptors are generally reduced in the fol- lowing order as redox potentials decline: O2, 3 NO , Mn4+, Fe3+, [1]. Oxic soils (also termed normal, oxidized, and aerobic) are those soils with high redox potentials indicating that O2 is present. Anoxic soils (also termed reduced, and water-logged) are soils in which O2 has been reduced and is absent, as are sulfate 2 4 SO 2 4 SO and other less preferable electron acceptors. Suboxic soils (also termed wet but not water-logged) are those having intermediate redox potentials in which O2 is ab- sent but sulfate has not been completely reduced. A con- cept used to delineate anoxic and suboxic soils is the pe-pH boundary between oxidized and reduced sulfur, as described by Baas Becking et al. [2] and modified by Sposito [1] and Essington [3]. This boundary between suboxic and anoxic conditions is described by the equa- tion: pe = 9 − pH. Nitrate 3 N O is a particularly effective electron accepter in the absence of O2, and can presumably miti- gate the reduction of sulfate and chemicals less prefer- able as electron acceptors, preventing very low redox potentials. Whitmire and Hamilton [4] demonstrate an increase in sulfate coinciding with denitrification, sug- gesting that the presence of nitrate for an electron accep- tor may alleviate sulfate reduction or encourage sulfur oxidation. A decrease in wet soil redox potential has been linked to the loss of nitrate [5]. The phenomenon of denitrification occurs when nitrate, functioning as an electron acceptor, is reduced to gaseous nitrogen. The extent to which denitrification occurs depends upon many environmental factors, including the quality of the carbon in the system [6], the relation between organic N mineralization and the presence of inorganic electron ac- ceptors [7], and competing fates of nitrate [8]. The ef- fects of nitrate on soil redox potential are intrinsically complex. The presence of three soil factors, organic mat- ter, O2, and nitrate, could strongly influence microbial respiration and soil redox potential. C opyright © 2012 SciRes. AJCC D. T. GARDINER, S. JAMES 206 Wetlands, either natural or constructed, are used for nitrogen removal from water catchments. The effective- ness of wetlands for nitrate attenuation has been assessed in lab [9], mesocosm [10], and field studies [11], with differing results. Also, wetlands are often constructed in locations where they did not formerly exist to compensate for the loss of wetlands elsewhere. As wetlands come into existence, one might well ask whether or not wetland functions have also come into existence, or has the process only created lands that are wet [12]. Answers to such ques- tions hinge on the chemistry, biology, and hydrology of a site. Moreover, legally recognized diagnostic characteris- tics for wetlands in the United States [13] include the presence of hydric soils, i.e., soils with anaerobic condi- tions. Yet some soils, natural or otherwise, can be inun- dated for long periods of time while maintaining high redox potentials, indicating a lack of anaerobic condi- tions. Soil redox potential influences many processes of practical importance, such as the degradation of pesti- cides [14]. Therefore understanding the nature of both constructed and natural wetlands requires an understand- ing of the factors associated with aerobic and anaerobic conditions as indicated by measurements of soil redox potential. The primary objective of this study was to determine if, and to what extent, organic matter addition would in- crease respiration in wet soils therefore consuming O2 and decreasing redox potentials. If redox potentials were reduced by organic matter, the secondary objective would be to determine whether or not nitrate would mi- tigate those effects, either in an absolute sense or in rela- tion to the wet-waterlogged (suboxic-anoxic) boundary. 2. Materials and Methods Experimental units were 18 wet soil microcosms con- sisting of 1-liter opaque bottles, each containing 1.0 kg of soil collected from a seasonally wet site of the Aransas National Wildlife Refuge in southern coastal Texas, USA. The soil was excavated from the upper 30 cm of a prob- able Mustang soil (Mixed, hyperthermic Typic Psamm- aquents) mapped as the Galveston-Mustang Association [15]. The soil contained 89.5% sand, 3.8% clay, 6.2% silt, 0.6% organic matter, 2.6 g kg–1 total nitrogen, and 31 mg kg–1 nitrate. Experimental treatments were applied in a 2 × 3 facto- rial combination of 0 and 1% organic matter additions (0 and 10 g kg–1); and 0, 100, and 200 ppm nitrate additions (0, 100, and 200 mg kg–1), replicated three times. The organic matter added was dried Burmudagrass containing 1.33 mg kg–1 nitrogen. Nitrate was added as NaNO3. The treatment combination that included neither organic mat- ter nor nitrate constituted the control. Each microcosm was randomly assigned to a treatment. Soils were main- tained in a wet condition with a few mm of water cover- ing the surface, and incubated in a growth chamber held a 30˚C with 11 hours of simulated sunlight. Electrode measurements of pH and redox potential were taken at 14-day intervals for a period of 112 days. Redox values were determined using a calomel electrode, then adjusted to standard Eh values relative to a hydro- gen electrode by adding 245 mV to the calomel-electrode values [16]. Redox values relative to the sloping line separating suboxic and anoxic soils were calculated by determining the difference between the measured redox value and the threshold redox value corresponding to the soil pH, given that pe = 9 − pH, and that Eh expressed in millivolts = 59.16 pe. Carbon dioxide emissions were determined by meas- uring CO2 concentrations in the headspace above each soil [17] on day 1, then again on days 8, 15, 22, 29, and 70. Tops of the containers were open during incubation, but sealed for 10 min for CO2 sampling, at which time 100 mL samples were collected from the 415 mL head- space using a Sensidyne gas detection pump (Sensidyne Inc., Clearwater, FL) connected in series to an RAE gas detection tube (RAE Systems Inc., Sunnyvale, CA). Effects of organic matter and nitrate on dependent va- riables CO2 and redox potential were analyzed by ANOVA for a repeated measures design. Also, separate data ana- lyses for each individual sampling day were conducted by ANOVA for a factorial design [18]. 3. Results Tables 1 and 2 show P values indicating significance of treatment effects. We considered any P value less than 0.05 to be significant. Results from the analysis of vari- ance for repeated measures of the dependent variables CO2, redox potential, and relative redox potential are presented in Table 1. For CO2, organic matter main ef- fects and the organic matter interaction with time were significant. Nitrate did not significantly affect CO2 levels. For redox values and relative redox values, interactions between time and organic matter, and between nitrate and time, were very highly significant (P < 0.001). Also for both redox and relative redox values, the main effects of organic matter were very highly significant (P < 0.001) but main effects of nitrate were not significant (P > 0.05). Because of the interactions with time (i.e., sampling day) in a temporally repeated measures design, data were also analyzed for each individual sampling day. Table 2 in- dicates results of analysis of variance on redox values and relative redox values performed for individual sam- pling days. Microbial respiration as measured by CO2 evolution (Figure 1) increased significantly in response to addi- tions of organic matter as measured on Days 8, 15, 22, and Copyright © 2012 SciRes. AJCC D. T. GARDINER, S. JAMES 207 Table 1. P values for temporally repeated measures of car- bon dioxide emissions (CO2), soil redox potential (redox), and soil redox potential relative to the suboxic-anoxic boun- dary (relative redox). Variables were nitrate (N), organic matter (O) and time (T). CO2 Redox Relative Redox Source dfP valuedfP valuedfP value N 20.281 20.143 2 0.094 O 1<0.0011<0.001 1 <0.001 N × O 20.374 20.384 2 0.355 T 50.011 8<0.001 8 <0.001 N × T 100.769 16<0.001 16<0.001 O × T 50.028 8<0.001 8 <0.001 N × O × T 100.839 160.103 160.120 Table 2. P values for effects of nitrate (N) and organic mat- ter (O) on soil redox potential (redox) and soil redox poten- tial relative to the suboxic-anoxic boundary (relative redox) as determined by analysis of variance for a factorial design. On no sampling day was the N × O interaction significant. Redox Relative Redox Sampling Day N O N O 1 0.940 0.063 0.543 0.046 14 0.098 <0.001 0.016 0.004 28 0.215 <0.001 0.393 <0.001 42 0.042 <0.001 0.010 0.001 56 0.093 <0.001 0.245 0.003 70 0.151 <0.001 0.226 <0.001 84 0.075 <0.001 0.076 <0.001 98 0.137 0.004 0.191 0.006 112 0.274 <0.001 0.216 <0.001 29. Because neither nitrate main effects nor nitrate × or- ganic matter interactions were significant on any sam- pling day, data from nitrate treatments were pooled such that organic matter effects presented in Table 1 include averages of all treatments with the organic matter addi- tion vs all treatments without it. Organic matter additions consistently produced a strong negative effect on soil redox values, with highly signifi- cant results observed on all days after Day 1 (Table 2; Figure 2). Nitrate did not significantly affect soil redox values except on Day 42, when redox values were higher for the 100 ppm nitrate treatment than for the control. No significant interactions between nitrate and organic mat- ter were observed. 4. Discussion The data strongly support the hypothesis that adding soil organic matter will decrease soil redox potentials. Furth- Figure 1. Carbon dioxide emitted from 1.0-kg wet soil mi- crocosms. Values are means of all treatments either with or without a 1% addition of organic matter. Vertical bars in- dicate standard errors of the means. Figure 2. Mean soil redox potentials as affected by organic matter and nitrate. Treatments levels were 100 ppm nitrate (N1), 200 ppm nitrate (N2), and 1% organic matter (OM). ermore, the control treatment having native levels of or- ganic matter and nitrate resulted in intermediate redox potentials through Day 42, after which it produced the highest redox values of all treatments. This further sup- ports the hypothesis that the negative impact on redox values was caused by the organic matter addition, not by the native organic matter in the soil. This study uniquely measured redox potential relative to the pH-dependent boundary line separating suboxic and anoxic soils (Figure 3). Effects of organic matter were significant on every sampling day, whereas effects of nitrate were significant on Days 14 and 42 only (Ta- ble 2). Interactions between nitrate and organic matter were not significant on any sampling day. Redox values relative to the suboxic-anoxic boundary can be more telling than absolute redox values because of their rela- tion to actual events or reactions. In suboxic soil sulfate and perhaps other species ranging between O2 and sulfate in redox susceptibility remain in their oxidized state; whereas in anoxic soil sulfate is absent, having been re- duced to sulfide or other forms of reduced sulfur. As one might expect, many data points in Figure 3 are in close proximity to the boundary line, suggesting that for these soils sulfate may be reducing but not completely reduced. With the disappearance of sulfate, soil redox potentials Copyright © 2012 SciRes. AJCC D. T. GARDINER, S. JAMES 208 Figure 3. Soil redox potential relative to the boundar y sepa- rating suboxic and anoxic soils. The boundary is a function of pH. Treatments levels were 100 ppm nitrate (N1), 200 ppm nitrate (N2), and 1% organic matter (OM). can fall appreciably below the boundary line, as was ob- served in most measurements here. The findings raise the question of why redox potentials relative to the suboxic-anoxic boundary were somewhat more strongly influenced by nitrate than were absolute redox values. Denitrification rates have long been known to be inhibited by acidic soil; therefore denitrification may have occurred to a lesser extent where pH values were lower. However, we postulate that a second expla- nation may also apply, i.e., that denitrification may have produced the mildly alkaline reaction product bicarbon- ate as suggested Equation (1) [19]. 23232 5CHO 4NO2N4HCOCO3HO. 2 (1) Figure 3 indicates that the combination of N plus or- ganic matter generally tended to increase pH; and illus- trates that as pH increases the boundary line slopes downward. With all other factors remaining constant, a treatment causing a slight elevation in pH would increase redox value relative to the boundary line, and would tend to shift equilibria between reduced and oxidized sulfur toward the oxidized state. Only for Days 14 and 42 did the results support the hypothesis that nitrate would mitigate the impact of or- ganic matter. Because denitrification is one of many pos- sible fates of nitrate, this result was not unexpected. Studies of N attenuation in wetlands reveals a wide range of results, including 100% nitrate and 69% total N at- tenuation [11], 13% to 73% nitrate attenuation, [9], and 51% total N attenuation [20]. Investigators [10,21] show generally positive correlations between nitrate reduction and soil organic matter. Matheson et al. [8] found that in unplanted wetland microcosms 49% of nitrate nitrogen was reduced to ammonium, 29% was denitrified, and 22% was immobilized. Adding to the complexities of quantifying denitrification in wetlands is its tendency to occur simultaneously or sequentially with nitrogen recy- cling [22]. Whitmire and Hamilton [4] observed nitrate depletion within 5 to 20 hours, leading to the speculation for the present study that added nitrate might have al- ready been reduced to another form when the first redox reading was taken, perhaps to cycle back as nitrate later in the study, such as on day 42 when nitrate treatment effects were surprisingly significant. In conclusion, this study of wet soil microcosms de- scribed the effects of organic matter and nitrate on respi- ration, redox potentials, and relative redox potentials (i.e., relative to the suboxic-anoxic boundary). Relative redox potentials are of particular relevance because they pro- vide an indication of the effect of a factor on electron acceptors having lesser susceptibility to reduction than O2 but greater than sulfate. We found that an organic matter addition of 1% markedly decreased the redox po- tential of wet soil both in the absolute sense and relative to the suboxic-anoxic boundary. On two sampling days nitrate raised redox potentials relative to the suboxic- anoxic boundary. Our findings suggest that adding orga- nic matter to a wetland may quickly and markedly de- crease soil redox potential and increase CO2 emission. Our findings offer little information from which a res- ponse to nitrate could be predicted, but rather serve to corroborate previous descriptions of the complex nature of nitrate in wetlands. 5. Acknowledgements We thank the USDA Natural Resources Conservation Service for funding this project. REFERENCES [1] G. Sposito, “The Chemistry of Soils,” Oxford University Press, New York, 1989. [2] L. G. M. Baas Becking, L. R. Kaplan and D. Moore, “Limits of the Natural Environment in Terms of pH and Oxidation-Reduction Potentials,” Journal of Geology, Vol. 68, No. 3, 1960, pp. 224-284. [3] M. E. Essington, “Soil and Water Chemistry: An Integra- tive Approach,” CRC Press, Boca Raton, 2004. [4] S. L. Whitmire and S. K. Hamilton, “Rapid Removal of Nitrate and Sulfate in Freshwater Wetland Sediments,” Journal of Environmental Quality, Vol. 34, No. 6, 2005, pp. 2062-2071. [5] T. Mansfeldt, “Redox Potential of Bulk Soil and Soil Solution Concentration of Nitrate, Manganese, Iron, and Sulfate in Two Gleysols,” Journal of Plant Nutrition and Soil Science, Vol. 167, No. 1, 2004, pp. 7-16. [6] N. P. Hume, M. S. Fleming and A. J. Horne, “Denitrifica- tion Potential and Carbon Quality of Four Aquatic Plants and Wetland Microcosms,” Soil Science Society of Ame- rica Journal, Vol. 66, No. 5, 2002, pp. 1706-1712. [7] J. R. White and K. R. Reddy, “Influence of Selected In- organic Electron Acceptors on Organic Nitrogen Minera- lization in Everglade Soils,” Soil Science Society of America Journal, Vol. 65, No. 3, 2001, pp. 941-948. Copyright © 2012 SciRes. AJCC D. T. GARDINER, S. JAMES Copyright © 2012 SciRes. AJCC 209 [8] F. E. Matheson, N. L. Nguyen, A. B. Cooper, T. P. Burt and D. C. Bull, “Fate of 15N-Nitrate in Unplanted, Planted and Harvested Riparian Wetland Soil Microcosms,” Eco- logical Engineering, Vol. 19, No. 4, 2002, pp. 249-264. [9] T. E. Davidsson and M. Ståhl, “The Influence of Organic Carbon on Nitrogen Transformations in Five Wetland Soils,” Soil Science Society of America Journal, Vol. 64, No. 3, 2000, pp. 1129-1136. [10] M. R. Burchell II, R. W. Skaggs, C. R. Lee, S. Broome, G. M. Chescheir and J. Osborne, “Substrate Organic Matter to Improve Nitrate Removal in Surface-Flow Constructed Wetlands,” Journal of Environmental Quality, Vol. 36, No. 1, 2007, pp. 194-207. [11] X. Zhang, S. E. Feagley, J. W. Day, W. H. Conner, I. D. Hesse, J. M. Rybczyk and W. H. Hudnall, “A Water Chemistry Assessment of Wastewater Remediation in a Natural Swamp,” Journal of Environmental Quality, Vol. 29, No. 6, 2000, pp. 1960-1968. [12] National Research Council, “Compensating for Wetland Losses under the Clean Water Act,” National Academy Press, Washington, 2001. [13] Wetland Training Institute, “Field Guide for Wetland De- lineation, 1987 Corps of Engineers Manual,” Wetland Training Institute, Glenwood, 2002. [14] R. D. DeLaune, I. Devai, C. Crozier and C. W. Lindau, “The Influence of Soil Redox Conditions on Atrazine Degradation in Wetlands,” Agriculture Ecosystems and Environment, Vol. 66, No. 1, 1997, pp. 41-46. [15] United States Department of Agriculture-Soil Conserva- tion Service, “Soil Survey of San Patricio and Aransas Counties, Texas,” US Government Printing Office, Wash- ington, 1979. [16] W. H. Patrick, R. P. Gambrel and S. P. Faulkner, “Redox Measurements of Soil,” In: D. L. Sparks, et al., Eds., Methods of Soil Analysis, Soil Science Society of Amer- ica, Madison Wisconsin, 1996, pp. 1155-1273. [17] United States Department of Agriculture, “Soil Quality Test Kit Guide,” US Government Printing Office, Wash- ington, 1998. [18] Analytical Software, “Statistix 8 Users Manual,” Analy- tical Software, Tallahassee, 2003. [19] L. J. Puckett and T. K. Cowdery, “Transport and Fate of Nitrate in a Glacial Outwash Aquifer in Relation to Ground Water Age, Land Use Practices, and Redox Processes,” Journal of Environmental Quality, Vol. 31, No. 3, 2002, pp. 782-796. [20] M. Healy and A. M. Cawley, “Nutrient Processing Ca- pacity of a Constructed Wetland in Western Ireland,” Journal of Environmental Quality, Vol. 31, No. 5, 2002, pp. 1739-1747. [21] P. J. Bohlen and S. M. Gathumbi, “Nitrogen Cycling in Seasonal Wetlands in Subtropical Cattle Pastures,” Soil Science Society of America Journal, Vol. 71, No. 3, 2007, pp. 1058-1065. [22] L. J. Lund, A. J. Horne and A. E. Williams, “Estimating Denitrification in a Large Constructed Wetland Using Stable Nitrogen Isotope Ratios,” Ecological Engineering, Vol. 14, No. 1-2, 2000, pp. 67-76. |