Published in Soil Sci. Soc. Am. J. 68:969-978 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS
Assessing the Reliability of Permanganate-Oxidizable Carbon as an Index of Soil Labile Carbon
A. Tirol-Padre and
J. K. Ladha*
Crop, Soil, and Water Sciences Div., IRRI, DAPO Box 7777, Metro Manila, Philippines
* Corresponding author (j.k.ladha{at}cgiar.org).
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ABSTRACT
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Soil C oxidized by neutral KMnO4, or permanganate-oxidizable C (POC), has been used as an index of labile C by several workers, although the nature of organic C (OC) oxidized has not been well elucidated. This study aimed to determine the reactivity of diverse organic compounds found in the soil with KMnO4 to judge the reliability of POC as an index of labile C. Sugars, amino acids, and other organic acids reacted slowly with 33 mM KMnO4 (2–45% C oxidized in 1 h), while compounds containing glycol groups (e.g., ascorbic acid and pyrogallol) were oxidized quickly by KMnO4 (25% C oxidized in 1 min). Permanganate did not oxidize cellulose, which is decomposed by soil microbial enzymes. The POC of organic manures and plant residues was positively correlated with lignin content. The rates of oxidation of SOM with KMnO4 varied among different rice (Oryza sativa L.) soils and were highly correlated with total soil C. The clay + silt/OC ratio negatively affected POC rendering physical protection for oxidizable C groups. In the soil, KMnO4 more rapidly oxidized less readily available organic compounds than the water-soluble carbohydrates, indicating that it did not discriminate the nonlabile from labile C. Soil POC was better correlated with total C (P < 0.01) than with water-soluble C (WSC) (P < 0.05) and was not correlated with microbial biomass C (MBC). Carbon oxidized by KMnO4 is not a reliable measure of labile C and should be referred to as POC when used as a parameter for characterizing soil C.
Abbreviations: FYM, farmyard manure • MBC, microbial biomass C • OC, organic C • POC, permanganate-oxidizable C • SOM, soil organic matter • TOC, total organic C • WSC, water-soluble C
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INTRODUCTION
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SOIL ORGANIC MATTER (SOM) can be divided into labile or rapidly decomposed, and stable or slowly decomposed fractions. It is generally accepted that labile constituents decompose within a few weeks or months, whereas their stable counterparts can persist in the soil for years or even decades. Included among the labile constituents are comminuted plant litter, macroorganic matter or light fraction, the living component or biomass, and nonhumic substances that are not bound to mineral constituents (Theng et al., 1989). The most labile components of SOM include cellular contents such as carbohydrates, amino acids, peptides, amino sugars, and lipids, followed by less readily metabolized structural materials such as waxes, fats, resins, lignin, cellulose, and hemi-cellulose. Labile pools also contain some recalcitrant plant residues (Woomer et al., 1994). Stable organic constituents in the soil include humic substances and other organic macromolecules that are intrinsically resistant against microbial attack, or that are physically protected by adsorption on mineral surfaces or entrapment within clay and mineral aggregates (Theng et al., 1989).
Soil organic matter can also be divided into functional pools based on their turnover rates. Typically, there is a small pool (1–5%) with a rapid turnover (weeks to years) and two larger pools with a slow (decades) and very slow (centuries) turnover rate (Scholes and Scholes, 1995).
One model structure, which has been used to represent multiple-pool SOM relationships divides soil OC into metabolic (0.5 yr), active (1.5 yr), structural (3 yr), slow C (25 yr), and passive soil C (1000 yr) (Parton et al., 1988). The first three fall into the active fraction, which consists mainly of living microbial biomass plus some readily metabolizable organic compounds leached out of litter, roots, and recently dead microbes.
Labile SOM fractions such as the light fraction, macroorganic matter or particulate C (Christensen, 1986; Hussain et al., 1999), MBC (Sparling, 1992; Yoshikawa and Inubushi, 1995), mineralizable C (Franzluebbers et al., 1994), carbohydrates, and enzymes (Deng and Tabatabai, 1996a; 1996b; 1997) are highly responsive to changes in C inputs to the soil and will provide a measurable change before any such change in total organic matter (Gregorich and Janzen, 1996). In contrast, the more stable (humified) pools are probably the more appropriate and representative fractions for C sequestration characterization (Cheng and Kimble, 2001).
Fractionation of soil OC, based on its susceptibility to oxidation with KMnO4 solutions of various concentrations (33–333 mM), was introduced by Loginow et al. (1987) on the premise that microbiological decomposition of organic matter in the soil is also largely associated with an oxidation process of enzymatic character. Blair et al. (1995) modified the procedure using a single concentration of KMnO4 (333 mM) as the oxidizing agent. Carbon, which is oxidized by KMnO4 in 1 h was considered as labile, and the remaining C that is not oxidized, as nonlabile. The method was used to develop a C management index (CMI) based on changes in the total C and labile and nonlabile C fractions as determined by KMnO4 oxidation. Subsequently, several publications dealing with the use of this method to determine short-term changes in the labile C fraction have appeared (Blair et al., 1997, 2001; Murage et al., 2000; Shrestha et al., 2002; Whitbread et al., 1998). However, the nature of soil C directly oxidized by neutral permanganate and the rates of oxidative reaction of organic compounds with KMnO4 have not been well elucidated. Alkaline or acidic KMnO4 has often been used for the oxidation of lignin and humic acids (Matsuda and Schnitzer, 1972; Maximov et al., 1972). The products of oxidation of organic compounds by alkaline or acidic KMnO4 have been reported by several workers (Maximov et al., 1977; Almendros and Leal, 1990).
The overall objective of this study was to assess the suitability of neutral POC as an index of soil labile C. The specific objectives were to determine (i) the degree of reactivity of diverse organic compounds found in the soil with neutral 33 mM KMnO4; (ii) the effect of soil texture and agronomic treatments on POC; and (iii) the relationships of POC with total C and other labile C parameters.
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MATERIALS AND METHODS
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Organic Materials and Soils Used
Laboratory-grade sugars, amino acids, and other organic acids (with purity 
98%) were obtained from Sigma-Aldrich (St. Louis, MO). Pyrogallol, ACS was obtained from Fisher Scientific (West Haven, CT) and cellulose, microcrystalline (Avicel) was from Merck (Whitehouse Station, NJ). Dried farmyard manure (FYM) (cow [Bus indicus] dung) was obtained from the Bhairahawa experiment station in Nepal, while the wheat (Triticum aestivum L.) straw came from an experimental field in Parwanipur, also in Nepal. Rice straw together with Azolla microphilla and Sesbania rostrata were taken from a long-term biofertilizer experiment at the IRRI. The FYM was air-dried while the plant materials were oven-dried at 70°C. All materials were finely ground to 0.5 mm.
Soils used for paddy rice production of varying texture and total C and N contents were from farmers' fields and a long-term experiment at the IRRI farm in the Philippines and rice–wheat soils from two long-term experiments in Ludhiana and Palampur in India, involving different organic matter treatments. Their physicochemical properties are given in Table 1. All soil samples were air-dried and ground to pass through a 2-mm sieve. For total C, N, and POC, soils were ground to pass through a 0.5-mm sieve.
The double-cropped rice long-term experiment at the experimental farm of IRRI, Philippines, began in 1985 with inorganic (urea) and organic (Azolla microphylla Lam. and Sesbania rostrata Bremek. Oberm.) fertilizer treatments. Details of this experiment were reported in Ladha et al. (2000). Soils sampled from the 0- to 20-cm layer in the 12th and 14th yr after the wet-season crop (25th and 29th crop) from the treated and control plots were analyzed for total and labile C.
The rice–wheat long-term experiment at the experimental farm of the Punjab Agricultural University, Ludhiana, India, included treatments with different combinations of inorganic and organic fertilizers for rice: 0 NPK (control), 100% of the recommended NPK, 50% recommended NPK + 6 Mg ha–1 FYM, 50% recommended NPK + 50% N in green manure (Sesbania cannabina Linu & Merrill) [GM], and 75% recommended N + 50% PK + 6 Mg ha–1 wheat cut straw. Details of this experiment were reported in Bhandari et al. (2002).
Determination of Permanganate-Oxidizable C [Modified from Blair et al. (1995)]
Standardization of KMnO4 Solutions
A 33 mM solution of KMnO4 was prepared by dissolving 5.2 g of KMnO4 crystals in 1000 mL of deionized-distilled water. Low heat was applied to completely dissolve the crystals. The KMnO4 solution was stored in an amber bottle and is stable for up to 1 mo. The exact concentration of KMnO4 solution was determined by titration against 0.0500 g As2O3, dissolved in 2 mL of 20% (w/v) NaOH. The solution was acidified with 2 mL of concentrated HCl and treated with four drops of 0.25 mM KI before titration with 33 mM KMnO4. A very small amount of KI is added as a catalyst. Without a suitable catalyst, Mn7+ is reduced only to an average state between Mn3+ and Mn4+ instead of Mn2+ (Willard et al., 1956). A digital buret (Digitrate, Jencons Scientific Ltd., UK) that could measure up to 0.01 mL was used to dispense the KMnO4. A faint pink color, which persisted for about 30 s, was considered as the end point. The reactions involved in the titration are as follows:
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Thus for every five moles of As2O3, four moles of MnO–4 is reduced. The exact concentration of KMnO4 was calculated using the following equation:
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where:
Aliquots of 0.6, 1.0, 1.4, 1.8, and 2.0 mL KMnO4 were measured using an accurate volume dispenser (Pipetman, Gilson Medical Electronics, France) in a 50-mL volumetric flask and diluted to volume. The concentrations of the diluted KMnO4 solutions were computed as follows:
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The absorbance of the diluted KMnO4 standards was then measured at 565 nm using a spectrophotometer (Beckman DU 650, Beckman Coulter Inc. Fullerton, CA), which was adjusted to zero absorbance using deionized-distilled water. The concentration of KMnO4 was plotted against absorbance at 565 nm to obtain a standard calibration curve. The calibration curve was found to be highly reproducible (Fig. 1)
. Thus the concentration of KMnO4 solutions can be quickly determined from the absorbance reading at 565 nm using the calibration curve.
Analysis of Permanganate-Oxidizable Carbon in Pure Organic Compounds
Triplicate samples of pure organic compounds (sugars, amino acids, organic acids, pyrogallol, and cellulose) containing 5 mg of C, and triplicate samples of 25 mg cellulose were placed in 50-mL centrifuge tubes provided with caps. These samples were arranged in a randomized complete block design (i.e., samples were grouped by rep and the organic materials randomized within each rep). Samples were analyzed replicate-wise (i.e., all Replication 1 first, followed by Replication 2 and Replication 3). Five sets of these samples were prepared to measure POC at five different time intervals (1 min, 1, 3, 6, and 24 h). Using a digital buret (Digitrate), 25 mL of 33 mM KMnO4 was dispensed into each sample in the centrifuge tubes. The same volume of KMnO4 was also dispensed into three empty centrifuge tubes to serve as blanks. The tubes containing the samples and KMnO4 were capped and covered with aluminum foil before shaking for 1 min and 1, 3, 6, and 24 h on a reciprocal shaker (with tube lying on its side) or an end-over-end tumbler. The 1-min samples were filtered through a Whatman No. 1 filter paper until enough filtrate was collected for dilution and absorbance measurement. A corresponding blank sample was prepared that also passed through a Whatman no. 1 filter paper. The 1-, 3-, 6-, and 24-h samples were centrifuged at 1030 x g for 5 min. Two-milliter aliquots of KMnO4 from each sample and blank were transferred using a Pipetman into 50-mL volumetric flasks and diluted to volume. The absorbance of the samples and blanks was then measured at 565 nm. The concentration of KMnO4 from the samples and blanks was determined using the standard calibration curve. The amount of POC in the sample was computed as follows:
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where mM Blank and mM Sample are the concentrations (mmol L–1) of KMnO4 in the blank and sample, respectively, determined from the standard regression curve: 50/2 = the dilution factor (mL mL–1); 25 = the volume (mL) of KMnO4 added to the soil sample; 9 = the amount of C oxidized for every mole of KMnO4 (g mol–1 or mg mmol–1) (When Mn+7O–4 is reduced to Mn+4O2 and C is oxidized from the neutral state (0) to C+4, three moles of C are oxidized for every four moles of Mn+7 reduced). To check whether inorganic substances in the soil have some catalyzing effect on the oxidation of sugars by KMnO4, 5 mg of C as glucose was mixed with air-dried soil samples containing 15 mg C, from Maahas, Bay and Pangil (Table 1) and was allowed to react with 33 mM KMnO4 for 1 min. The POC in spiked (with glucose) and control (without glucose) soils were analyzed in triplicate.
Analysis of Permanganate-Oxidizable Carbon in Organic Materials
Three replicate samples of dried FYM, rice, and wheat straw, sesbania and azolla, each containing 15 mg of C were used to measure POC following the above procedure. Permanganate-oxidizable C was measured after 1 and 6 h.
Analysis of Permanganate-Oxidizable Carbon in Soil Samples
Three replicate samples of eight air-dried rice-field soils from Pila, Bugallon, Maahas, Maligaya, IRRI, Gapan, Luisiana, and Urdaneta (Table 1), each containing 15 mg of C were used to measure POC following the above procedure. The amount of soil equivalent to 15 mg of C was calculated from known total C contents of these eight soils. Permanganate-oxidizable C was measured after 1, 3, 6, and 24 h. In some soils, the amount of dissolved OC in the KMnO4 extract was determined to obtain an estimate of the amount of C not completely oxidized to CO2. One-milliliter aliquot of the KMnO4 extract after centrifugation was transferred into a 10-mL test tube, then 5 mL of acidified 0.2 M FeSO4 was added to it to reduce the excess KMnO4. The amount of OC in solution was then directly measured using 1020A combustion TOC analyzer from Oceanography International (OI) Corp., College Station, TX.
Biochemical and Physical Analysis of Soils
Particle-size analysis was done by the pipette method and pH was measured in 1:1 soil/water suspension. Total C and N were determined by automated combustion using a PerkinElmer 2400 Elemental CHN analyzer from PerkinElmer Corp., Norwalk, CT (Jimenez and Ladha, 1993). Organic C was measured by adding one to two drops of 15% (w/v) HCl to 60 mg soil sample in a silver capsule to convert carbonates to CO2. The sample in the silver capsule was then dried in an oven at 80°C for about 2 h. The sample was sealed in the silver capsule and analyzed for C using the PerkinElmer 2400 CHN analyzer.
Water-soluble C was determined from 12 g of air-dried soil by shaking with 50 mL of deionized-distilled water for 1 h. The soil suspension was centrifuged for 30 min at 6953 x g and filtered through Whatman No. 42 filter paper. The total organic C (TOC) in solution was measured using the 1020A combustion TOC Analyzer.
Microbial biomass C was measured by fumigation-extraction following the modified procedure of Witt et al. (1998) but using 10 g each of air-dried soil for the fumigated and unfumigated samples. The soils were incubated for 1 mo under water-saturated conditions at 30°C before fumigation with chloroform. All analyses were done in triplicate.
Statistical Analysis
Analysis of variance and linear regression analyses were done using SAS systems (SAS Institute, 1995). The F test for homogeneity of regression coefficients was used to determine if slopes of regression lines of POC vs. time (in natural log) obtained from different rice soils differed from one another (Gomez and Gomez, 1984).
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RESULTS AND DISCUSSION
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Oxidative Reaction of Simple Organic Compounds with KMnO4
Reactivity with 33 mM KMnO4 varied among the organic compounds tested based on the change in KMnO4 concentration. Generally, the reaction followed a logarithmic trend, and started to plateau after 4 to 5 h (Fig. 2)
. For the sugars tested, only 2 to 41% C was oxidized to CO2 after 1 h in the order arabinose > xylose > fructose > mannose > glucose > maltose > sucrose. The disaccharides maltose and sucrose are considered to be nonreducing sugars because they lack a free aldehyde or keto group. However, they may be oxidized after undergoing hydrolysis to glucose and fructose (for sucrose). Thus, maltose and sucrose were found to be the least reactive with neutral KMnO4 with only 2 to 3% C oxidized to CO2 in 1 h and 23% in 24 h (Fig. 2a). Increasing the concentration of KMnO4 to 333 mM, at most, doubled the amount of C oxidized in monosaccharides (data not shown). For the amino acids tested, only 2 to 25% C was oxidized by KMnO4 after 1 h with threonine being the most reactive and valine the least (Fig. 2b). Among the organic acids, gluconic acid was oxidized by KMnO4 to the greatest extent (45% C in 1 h and 100% in 24 h), whereas benzoic acid was unreactive. For oxalic acid, 20% C was oxidized in 1 h but this did not increase further up to 24 h (Fig. 2c). Cellulose was unreactive with 33 and 333 mM KMnO4 (data not shown) as it has only one free aldehyde or keto group at the reducing end. In addition, cellulose greater than six glucose units are not soluble. Potassium permanganate was used to separate lignin and cellulose in acid detergent fiber (Van Soest and Wine, 1968) because of the difference in reactivity of lignin and cellulose to KMnO4 oxidation.
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Fig. 2. Kinetics of organic C oxidation by 33 mM KMnO4 (0–24 h) at room temperature in various organic compounds: (a) sugars, (b) amino acids, and (c) organic acids.
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Benzene carboxylic acids are considered to be permanganate-resistant molecules, whereas alkanoic acids and phenolic acids are progressively broken down into short dicarboxylic molecules, for example, oxalic acid (Almendros et al., 1989). Among the organic compounds tested, pyrogallol and ascorbic acid showed the quickest reaction with KMnO4 (25% C oxidized in 1 min) (Table 2). Both contain multiple glycol groups that are quickly oxidized by KMnO4. In contrast, no detectable amount of C was oxidized by KMnO4 after 1 min among the sugars. For the amino acids and the other organic acids, 4 to 7 and 0.6 to 18% C, respectively, were oxidized in 1 min by 33 mM KMnO4 (Table 2).
One minute shaking with KMnO4 was not long enough to oxidize glucose that was mixed with the soil judging from the observation that there was no difference in the amount of C oxidized from soils spiked with glucose and from those that were not spiked (Table 3). About 1 mg C in Maahas and Pangil and 3 mg C per g dry soil in Bay were oxidized by KMnO4. The estimated amount of soil C oxidized by KMnO4 in 1 min exceeded the WSC content in the three soils tested (Table 3), suggesting that KMnO4 more rapidly oxidized less available organic compounds. Since air-dried soils were used, it is unlikely that there were reduced ions in these soils that could have reacted with KMnO4.
The above results show that oxidation of sugars and amino acids by neutral KMnO4 does not proceed very rapidly and that the reactivity of water-soluble organic compounds with KMnO4 varies depending on their structure and functional groups. One hour of oxidation with 33 or 333 mM KMnO4 cannot discriminate labile from nonlabile C because of the inability of neutral KMnO4 to oxidize some simple sugars on the one hand and its ability to oxidize less readily metabolized organic compounds on the other hand. Data obtained suggested that neutral KMnO4 strongly oxidized a bigger pool of less readily metabolized organic compounds. Thus, POC is not a good measure of the active, labile, or available C fraction.
Maximov et al. (1977) showed the predominant degradation by KMnO4 of compounds containing glycol groups in humic acids. In agreement, we found that ascorbic acid and pyrogallol, which contained multiple glycol groups, were more rapidly oxidized by KMnO4 (in 1 min) than the other organic acids and sugars. Conteh et al. (1999) have also shown that KMnO4–oxidizable C contains polysaccharide and humic C and it is an order of magnitude greater than the K2SO4–extractable or MBC fractions.
Unprotected glycol groupings and double bonds are usually attacked by alkaline KMnO4, often resulting in C-C bond cleavage (Green, 1980). Under controlled conditions, KMnO4 effects cis-hydroxylation of double bonds. Primary and secondary alcohols are also rapidly oxidized by alkaline KMnO4; the reaction is slower in neutral and mildly acidic solutions. The rapid oxidation in alkaline solution has been attributed to the formation of a nucleophilic alkoxide anion. D-glucose may be oxidized completely to CO2 and water by hot, weakly alkaline solutions of KMnO4; as the alkalinity increases above 0.015 M, oxalic acid is produced, and, in 1.8 M KOH, yields of 42% oxalic acid are obtained (Green, 1980).
Oxidation of Complex Organic Compounds by KMnO4
We tested the susceptibility of various organic materials (crop residue [rice and wheat straw], FYM, and GM [azolla and sesbania]) to oxidation by KMnO4 (Table 4). Based on the amount of KMnO4 consumed, azolla was the most susceptible to oxidation, having the highest POC (65 and 99 mg g–1 in 1 and 6 h, respectively) and POC/TC (14.5% in 1 h and 22.3% in 6 h), whereas sesbania was the least susceptible, with a POC of 31.1 mg g–1 in 1 h and 65.6 mg g–1 in 6 h. The POC of the organic materials did not correlate with their total C content but correlated (P < 0.05) with their lignin content (Fig. 3)
. The C/N and lignin/N ratios have been shown to be negatively correlated with the mineralization rates of organic manures (Becker et al., 1994). Thus, rice and wheat straw, having high C/N (55 and 125, respectively) and lignin/N (12 and 32, respectively) ratios, have lower mineralization rates than sesbania, azolla, and FYM with low C/N (27, 13, and 20, respectively) and lignin/N (3, 5, and 5, respectively) ratios. On the contrary, rice and wheat straw had higher KMnO4 oxidation rates (36 and 48 mg POC g–1 h–1, respectively) than sesbania (31 mg POC g–1 h–1). Permanganate-oxidizable C was not correlated with the lignin/N nor with the C/N ratios of the organic manures tested, suggesting that POC is not predictive of mineralizable C but may indicate the lignified nature of the organic material.
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Fig. 3. Relationship of permanganate-oxidizable C (POC) in farmyard manure (FYM), rice and wheat straw, azolla and sesbania with their (left) lignin and (right) total C contents.
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Complex organic compounds in the soil are not completely oxidized to CO2 by KMnO4. Organic C was detected in the soil KMnO4 extracts in amounts greater than the WSC, suggesting that intermediate products of oxidation were formed (Table 3). Thus, for complex organic materials, it may be difficult to estimate the amount of C oxidized to CO2 from the amount of KMnO4 consumed since KMnO4 results in the formation of intermediate C compounds other than CO2. Loginow et al. (1987) have shown that the estimated amount of POC from various organic materials was generally higher when measured on the basis of KMnO4 consumption than the actual amount of CO2 evolved. They also found that lignin used more oxidizer than cellulose although the amounts of CO2 evolved from these organic materials were similar, which suggests a portion of KMnO4 consumed by lignin produces lower molecular weight organic compounds. In contrast, cellulose may be broken down into glucose units, which can be completely oxidized to CO2. The three monomers that make up almost all lignin found in nature are p-coumaryl, coniferyl, and sinapyl alchohol (Barker and Owen, 1999). Lignin contains glycol groups and double bonds that are rapidly oxidized by KMnO4. Thus, lignin is more susceptible to KMnO4 oxidation than cellulose although cellulose is more susceptible to microbial decomposition (Bohn et al., 1985).
Soil Permanganate-Oxidizable Carbon as Affected by Total Soil Carbon and Soil Texture
The POC measured after 1, 3, 6, and 24 h was compared in eight different rice-field soils. The amount of C oxidized by 33 mM KMnO4 increased with time following a logarithmic trend (Fig. 4) . The F test for homogeneity of regression coefficients gave a highly significant (P < 0.01) F value indicating that the slopes of the regression lines of POC vs. time in natural log varied among the different rice soils. The 95% confidence intervals of the slopes confirm significant differences in KMnO4 oxidation rates of these soils. The oxidation rate (slope) was highly correlated with the TOC content (R2 = 0.94, P < 0.01) (Fig. 5)
. The total cumulative POC as a fraction of TOC (POC/TC) ranged from 8 to 14% after 1 h, from 11 to 19% after 3 h, from 15 to 23% after 6 h, and from 23 to 33% after 24 h.
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Fig. 4. Kinetics of soil organic matter oxidation by 33 mM KMnO4 at room temperature in eight rice-field soils (** slopes are significantly different at the 1% level by the F test for homogeneity of regression coefficients).
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Fig. 5. Relationship between total C content and the oxidation rate of soil organic C by KMnO4 in eight rice field soils.
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Soil POC of 10 soils from the Philippines together with soil POC from plots with different organic matter treatments in two rice–wheat long-term experimental sites in India, were regressed with their total C contents and were found to be highly correlated (R2 = 0.96, P < 0.01)(Fig. 6)
. Regression analyses showed that the standard error of the estimate was higher across different soil types than within the same soil with different organic fertilizer treatments.
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Fig. 6. Relationship of soil permanganate-oxidizable C (POC) and total C in 10 different rice-field soils from the Philippines and rice-wheat field soils with different organic fertilizer treatments in two sites in India.
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The effect of soil texture on soil POC was examined by analyzing the relationships of clay/OC, silt/OC, and clay + silt/OC ratios with soil POC in 12 different rice-field soils from the Philippines and India. The scatter plots in Fig. 7
show two points with a large deviation from the regression line. These two soils contained extreme OC contents: Bay with a very high OC content of 49.6 g kg–1) and Ludhiana with a very low OC content of 4.35 g kg–1. For soils with very low or very high soil OC, soil texture may not have any significant effect on POC. The clay/OC ratio had no significant correlation with soil POC even if the soils with extreme OC contents were excluded from regression analysis. However, silt/OC and silt + clay/OC had a significant (P < 0.01) inverse relationship with soil POC if the two soils were excluded. Silt + clay/OC accounts for 82% of the variability in soil POC in 10 soils (Fig. 7).
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Fig. 7. Scatter plots and linear regression of soil permanganate oxidizable C (POC) with (a) clay/OC, (b) silt/OC, and (c) clay + silt/OC.
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The above results show that although soil POC is predominantly influenced by total soil C, it is also affected by soil texture as shown by its high correlation with the (clay + silt)/OC ratio. Clay and silt particles may render some degree of physical protection for oxidizable C groups in lignin.
Soil Permanganate-Oxidizable Carbon as Affected by Organic Fertilizer Treatments
Soil POC and total C were measured in a long-term rice experiment at IRRI after 27 and 29 rice crops. Azolla and sesbania fertilization increased soil POC over the control and urea treatments mainly due to the added POC from azolla and sesbania. The total POC added to the soil was 5.2 g kg–1 soil from azolla and 5.9 g kg–1 soil from sesbania (Table 5). However, the measured soil POC from the azolla and sesbania treatments was lower than that estimated from the added POC of the GM. About 48 and 43% of POC from azolla, and 48 and 51% of POC from sesbania in 1997 and 1999 respectively, was lost from the upper 0- to 20-cm soil depth [computed from data in Table 5: (C – A) x 100/C]. Some of the POC may have moved down to the lower soil depth as the azolla and sesbania treatments also had higher POC than the control and urea treatments in the 20- to 50-cm soil depths (2.6 mg g–1 in the control and urea treatments and 3.1 mg g–1 in the azolla and sesbania treatments). The unaccounted POC may have been lost or was stabilized in the soil clay particles. In terms of total amount of GM and total C added (azolla has a total C content of 42.5% while sesbania has a total C content of 44.2%), more was added from sesbania as compared with azolla but the soil C from the two treatments were similar showing greater C loss from sesbania (Table 5).
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Table 5. Soil permanganate-oxidizable C (POC) as affected by organic and inorganic fertilization after 27 and 29 crops in a long-term double rice cropping at IRRI.
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In a rice–wheat long-term experiment in Ludhiana, India, the effect of organic fertilizer treatments on total soil C and POC over 16 yr was determined. Analyses of variance over years showed significant differences among years and treatments in terms of total C, POC and the POC/TC measured after 6 h, without any treatment x year interaction (Table 6). All the organic fertilizer treatments (FYM, wheat straw, and GM) had significantly higher total soil C and POC than the unfertilized treatment. However, in terms of POC/TC, only the FYM treatment was significantly different from the control and the 100% NPK treatment. The increase in total soil C and POC in the fertilized over the control plots could be due to C sequestration from added organic fertilizers and increase in root residues in fertilized plots. Increase in POC/TC in the FYM treatment could be due to the higher POC/TC of pure FYM (25.7%) (Table 4) as compared with the control soil (17.9%). One-hour oxidation did not show significant differences among treatments and years in terms of POC/TC (data not shown).
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Table 6. Effect of agronomic treatment on total C and permanganate-oxidizable C (POC) in a long-term experiment in Ludhiana, India.
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These results show that soil POC is affected by the POC of the organic material added to the soil. In these experiments, the effect of organic fertilizers was evident not only in the change in soil POC but in the change in total soil C as well. Thus, POC cannot really be claimed as a more sensitive indicator of the changes in the labile C pool than total C.
Relationships of Soil Permanganate-Oxidizable Carbon with Water-Soluble Carbon and Microbial Biomass Carbon
The relationships of soil POC with total C, WSC, and MBC were determined using eight rice-field soils from the Philippines (Fig. 8)
. Permanganate-oxidizable C was significantly correlated with both total C and WSC. However, POC had a better correlation with total C (P < 0.01) than with WSC (P < 0.05). The significant correlation between POC and WSC is dictated by only one point (Pila soil, which has the highest POC and WSC). The correlation becomes insignificant at P < 0.05 if this point is removed. Moreover, POC was not correlated with MBC, whereas WSC was significantly correlated with MBC. The high correlation between MBC and WSC is also dictated by Pila soil. Removing this point lowers the correlation coefficient but it will still be significant at the 5% level.
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Fig. 8. Relationship of soil permanganate oxidizable C (POC) with (a) total C, (b) WSC, (c) MBC, and (d) MBC with WSC in eight rice-field soils.
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The lack of correlation of POC with WSC and MBC and its high correlation with total C further suggest that POC does not measure labile C but may represent a more stable fraction of total C. Permanganate-oxidizable C appears to be a better indicator of lignin content than labile C and thus it may also be used to monitor soil quality and sustainability by showing changes in the stored organic matter or the slow C pool resulting from various agronomic practices. Permanganate-oxidizable C may also indicate the extent of physical protection of clay particles for oxidizable C in lignin.
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ACKNOWLEDGMENTS
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The authors acknowledge the assistance of Ms. C. Bueno in analyzing the water-soluble C and Mr. M. Alumaga in laboratory work. We thank Dr. K. Inubushi for his constructive comments.
Received for publication March 24, 2003.
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