Permanganate Oxidizable C Research Articles

Long-term effects of wheat production management practices on some carbon fractions of a semiarid Plinthustalfs

This study evaluated wheat production effects on carbon (C) fractions and soil organic C (SOC) molecular composition of a semiarid Plinthustalf in a trial established near Bethlehem. Treatments applied for 20 consecutive years included two straw management (unburned and burned), three tillage (no-tillage, ploughing and stubble mulch) and two weed control (chemical and mechanical) methods. Samples collected from 0–50 mm depth of specific treatment combinations were analyzed for SOC, soil inorganic C (SIC), permanganate oxidizable C (POXC), cold (CWEC) and hot (HWEC) water extractable C, extractable humic substances (CEX), humic acids, fulvic acids (CFA) and SOC functional groups. Humification (HI) and polymerization (PI) indices and alkyl C/O-alkyl C ratios were calculated. No-tillage combinations demonstrated potential to reverse losses of soil C fractions in the 0–50 mm layer. Increased POXC, CWEC, CEX and CFA revealed the labile nature of accumulated SOC in no-tillage, suggesting that SOC therein could be rapidly lost if no-tilled soils were again cultivated. Although the HI and PI were not always significant, their decrease in the no-tillage combinations suggest minimal decomposition, which is a benefit in Plinthustalfs with low storage capacity. Positive correlations between SIC and SOC fractions implied that an increase in SOC fractions protected SIC, resulting in its accumulation in no-tillage combinations. Functional groups seemed to mimic fractionated SOC fractions because O-alkyl C decreased with concomitant increase in alkyl, aromatic and carbonyl C. These responses highlighted that no-tillage combinations could be ideal to restore SOC quality in drought-prone agro-ecosystems dominated by sandy soils.

Enzymes and C pools as indicators of C build up in short-term conservation agriculture in a savanna ecosystem in Cambodia

Soil organic carbon (SOC) pools, particularly labile pools, and soil enzymes are good indicators of short-term impacts of soil management. We hypothesized that labile SOC pools drives C accumulation and enzyme activity can be an efficient indicator for C build up in short-term conservation agriculture. The aims of this study were to quantify the impacts of tillage and crop rotations with diverse crop residue inputs on changes in SOC labile pools and enzymatic activities in rice-, soybean- and cassava-based cropping systems designated as RcCS, SbCS and CsCS, respectively. The four treatments in each cropping system consisted of: conventional tillage (CT), no-till in which main crops (rice, soybean and cassava) were grown in a one-year frequency pattern (NT1) and, no-till in which the main crops were grown in bi-annual rotations with maize (NT2 and NT3). After 5 years experiment period, greater hot-water extractable organic C (HWEO-C) stocks of 61%, 55% and 53%, and permanganate oxidizable C (POX-C) stocks of 23%, 21% and 32% were attributed to NT than those in CT soils under RcCS, SbCS and CsCS, respectively, at 0–5 cm soil layer. The pyrophosphate extractable organic C (PEO-C) and chemically stabilized organic C (CSO-C) stocks were almost constant in each depth among treatments, except 0–5 cm in CsCS. The β-glucosidase activity was 18%, 28% and 49% greater in NT than those in CT soils at 0–5 cm under RcCS, SbCS, CsCS, respectively. Arylsulfatase activity was 36% and 39% greater in NT than in CT under SbCS and CsCS, respectively but no significant differences in RcCS. A strong and positive correlation (P < 0.001) between β-glucosidase and arylsulfatase activity with POX-C, HWE-C, SOC and total N indicated how these variables were inter-related. Comparison among three NT treatments, bi-annual crop rotations showed a better increasing trend of HWEO-C, POX-C and enzymatic activities than those with one-year frequency pattern. In conclusion, short-term NT crop rotations with permanent soil cover significantly increased the storage of HWEO-C and POX-C and enhanced β-glucosidase and arylsulfatase activities at the surface soil layer as a result of higher biomass-C input and the absence of soil disturbance.

Biochemical proxies indicate differences in soil C cycling induced by long-term tillage and residue management in a tropical agroecosystem

A potential benefit of conservation agriculture (CA) is soil organic carbon (SOC) accrual, yet recent studies indicate limited or no impact of CA on total SOC in tropical agroecosystems. We evaluated biochemical indicators of soil C cycling after 9 years (18 seasons) of contrasting tillage with and without maize residue retention in western Kenya. Potential activities of C-cycling enzymes (β-glucosidase, GLU; β-galactosidase, GAL; glucosaminidase, GLM; cellobiohydrolase, CEL), permanganate-oxidizable C (POXC), and soil organic matter (SOM) composition (by infrared spectroscopy) were measured. POXC tended to be greater under reduced tillage and residue retention, but did not significantly differ among treatments (≤ 2% of SOC). Despite no significant differences in SOC concentrations or stocks, activities of all 4 C-cycling enzymes responded strongly to tillage, and to a lesser extent to residue management. Activities of GLU, GAL, and GLM were greatest under the combination of reduced tillage and residue retention relative to other treatments. Reduced tillage produced an enrichment in carboxyl C = O (+6%) and decreased polysaccharide C-O (−3.5%) relative to conventional tillage irrespective of residue management. Though enzyme activities and POXC are typically associated with SOC accrual, changes in soil C cycling at this site have not translated into significant differences in SOC after 9 years. Elevated enzyme activities may have offset potential SOC accumulation under CA. However, the ratio of C-cycling enzyme activities to SOC was higher under reduced tillage and residue retention relative to other treatments, indicating that stoichiometric scaling of SOC and enzyme activities does not explain absence of significant differences in SOC among tillage and residue managements. Potential factors that may explain the low SOC accrual rates in this tropical agroecosystem included the low, albeit realistic, levels of residue retention, nutrient limitations, and high temperatures favoring decomposition.

Quantification of Soil Permanganate Oxidizable C (POXC) Using Infrared Spectroscopy

Core Ideas Infrared spectroscopy and chemometrics can be used to quantify soil labile C. Soils amended with organic materials need to be taken into account during calibration development. Selection of spectral regions can give parsimonious predictions of soil total and labile C. Labile soil carbon is an important component of soil organic matter because it embodies the mineralizable material that is associated with short‐term fertility. Permanganate‐oxidizable C (POXC) is a widely used method for the study of labile C dynamics in soils. Rapid methods are needed to measure labile C, and better understand how this pool varies with soil C at regional scales. Infrared spectroscopy is an inexpensive way to quantify SOC and observe fluctuations in C functional groups. Using a sample set that encompassed several soil types and plant communities (seven different research projects, n = 496), soils were analyzed via diffuse reflectance Fourier transformed mid‐infrared (MidIR, 4000–400 cm–1) and near‐infrared (NIR, 10000–4000 cm–1) spectroscopy. Spectral data were used to develop calibrations for POXC, soil organic C (SOC), and total N (TN) using partial least squares (PLS) regression. The MidIR predicted POXC slightly better than the NIR, with calibration and/or validation R2 values ranging from 0.77 to 0.81 depending on spectral pretreatments. Predictions for POXC were better than SOC and TN, but site variability influenced the calibration quality for SOC and TN. Using a selected MidIR region, which included bands correlated to POXC (3225–2270 cm–1), reduced the calibration quality, but still gave acceptable R2 values of 0.76 to 0.77 for the calibration and validation sets. We show that POXC can be predicted using NIR and MidIR spectra. Selecting informative spectral bands offers an alternative to using full spectra for PLS regressions.

Comparison of Permanganate‐Oxidizable Carbon and Mineralizable Carbon for Assessment of Organic Matter Stabilization and Mineralization

Core Ideas POXC and mineralizable C were evaluated across diverse agroecosystems. The two are related but differentially influenced by management practices. POXC better reflected SOM stabilizing practices. Mineralizable C reflected SOM mineralizing practices. Both predicted agronomic performance better than other soil C fractions. Permanganate‐oxidizable C (POXC) and mineralizable C (as determined by short‐term aerobic incubation of rewetted soil) are measures of active organic matter that may provide early indication of soil C stabilization and mineralization processes. To date, the relationship between these two promising active organic matter tests has not been comprehensively evaluated, and little is known about their functional role in the soil ecosystem. Here, we examined the relationship between POXC and mineralizable C across a wide range of soil types, management histories, and geographic locations across the United States (13 studies, 76 total sites; n = 1071) and the ability of POXC and mineralizable C to predict crop yield and total aboveground biomass. Results from this comparative analysis showed that POXC and mineralizable C are related (r2 = 0.15–0.80) but that the relationship was differentially influenced by management practices. Overall, POXC better reflected practices that promote organic matter accumulation or stabilization and therefore can be a useful indicator of long‐term soil C sequestration. Conversely, mineralizable C better reflected practices that promote organic matter mineralization and therefore can be a useful indicator of short‐term soil nutrient availability. Our results also show that both mineralizable C and POXC were better predictors of corn (Zea mays L.) grain yield, aboveground biomass, and tomato (Solanum lycopersicum L.) fruit yield than other soil C fractions evaluated here. Thus, the integrated use of POXC and mineralizable C can provide a complementary framework to assess the relative dynamics of soil C stabilization and nutrient mineralization functions in agroecosystems.

Evaluating Measures to Assess Soil Health in Long‐Term Agroecosystem Trials

Monitoring soil health is an important component of any land management system that sustains soil resources. As metrics of soil health, we evaluated surface soil organic matter (SOM) properties from five field experiments ranging from 2 to 30 yr old and representing diverse agroecosystems across the inland Pacific Northwest (iPNW). The SOM properties measured included soil organic C (SOC), total N, acid nonhydrolyzable C (NHC), acid nonhydrolyzable N (NHN), acid-hydrolyzable C (HC), acid-hydrolyzable N (HN), microbial biomass C (MBC), microbial biomass N (MBN), carbon mineralization (Cmin), permanganate oxidizable C (POXC), ion exchange membrane N (IEM N), and potential N mineralization (PNM). We further evaluated these SOM metrics and the Haney soil health index (SHᵢₙdₑₓ) relative to the following seven criteria as a framework to judge the effectiveness of soil health tools: (i) evidence based, (ii) sensitive to change, (iii) logistically sensitive, (iv) cost effective, (v) accurate and precise, (vi) performed in situ, and (vii) valued for management decisions. Measures of active SOM were highly variable, particularly 1-d Cmin (CV, 3–53%), IEM N (CV, 9–55%), and SHᵢₙdₑₓ (CV, 4–37%) and subsequently not highly sensitive to management. Permanganate oxidizable C displayed sensitivity to more stabilized SOM, indicated by strong correlations to NHC (r = 0.84) and NHN (r = 0.80), and coupled with IEM N provided complimentary information important to soil health. Using these seven criteria to gauge soil health metrics, POXC scored the highest and should be considered as a component for soil health assessment within the iPNW.

Soil treatment-induced differential gene expression in tomato: Relationships between defense gene expression and soil microbial community composition

Soil management affects chemical and microbiological properties of soils in ways that impact plant growth. We examined effects of amendment treatments and contrasting management treatments on chemical and microbiological soil properties and on expression of selected defense genes in laboratory-grown tomatoes (Solanum lycopersicum). Treatments imposed on a conventionally managed Maury silt loam included: compost, manure, vetch (Vicia villosa), inorganic N, and non-treated control. Non-amended, organically managed Maury soil, collected from the same location was also studied. Compost, manure, and vetch increased total organic C (TOC) and permanganate oxidizable C (POXC) in soil. Organically managed soil also had greater TOC and POXC than the control treatment. Soil N was increased by compost, manure, vetch and inorganic N treatment. Microbial communities from compost, vetch and manure treatments differed from each other and from communities in the other treatments. Defense genes: ChiB (chitinase), Osm (osmotin), and GluA (β-1,3-glucanase) were generally expressed less in plants from manure treated soil and organically managed soil. The PR1b (pathogenesis-related protein PR1b) gene was expressed more in plants from compost, inorganic N, and vetch treatments. Defense gene expression was negatively related to Gram-negative bacterial biomarkers, which were greatest in manure treated soil and organically managed soil. These results suggest that increased relative abundance of Gram-negative members of soil microbial communities in manure treated or organically managed soils may indirectly reduce the steady state expression of defense genes in plants, allowing plants to shift resources from defense to other beneficial functions such as fruit or biomass production.

Soil respiration, labile carbon pools, and enzyme activities as affected by tillage practices in a tropical rice–maize–cowpea cropping system

In order to identify the viable option of tillage practices in rice-maize-cowpea cropping system that could cut down soil carbon dioxide (CO2) emission, sustain grain yield, and maintain better soil quality in tropical low land rice ecology soil respiration in terms of CO2 emission, labile carbon (C) pools, water-stable aggregate C fractions, and enzymatic activities were investigated in a sandy clay loam soil. Soil respiration is the major pathway of gaseous C efflux from terrestrial systems and acts as an important index of ecosystem functioning. The CO2-C emissions were quantified in between plants and rows throughout the year in rice-maize-cowpea cropping sequence both under conventional tillage (CT) and minimum tillage (MT) practices along with soil moisture and temperature. The CO2-C emissions, as a whole, were 24% higher in between plants than in rows, and were in the range of 23.4-78.1, 37.1-128.1, and 28.6-101.2mgm(-2)h(-1) under CT and 10.7-60.3, 17.3-99.1, and 17.2-79.1mgm(-2)h(-1) under MT in rice, maize, and cowpea, respectively. The CO2-C emission was found highest under maize (44%) followed by rice (33%) and cowpea (23%) irrespective of CT and MT practices. In CT system, the CO2-C emission increased significantly by 37.1% with respect to MT on cumulative annual basis including fallow. The CO2-C emission per unit yield was at par in rice and cowpea signifying the beneficial effect of MT in maintaining soil quality and reduction of CO2 emission. The microbial biomass C (MBC), readily mineralizable C (RMC), water-soluble C (WSC), and permanganate-oxidizable C (PMOC) were 19.4, 20.4, 39.5, and 15.1% higher under MT than CT. The C contents in soil aggregate fraction were significantly higher in MT than CT. Soil enzymatic activities like, dehydrogenase, fluorescein diacetate, and β-glucosidase were significantly higher by 13.8, 15.4, and 27.4% under MT compared to CT. The soil labile C pools, enzymatic activities, and heterotrophic microbial populations were in the order of maize > cowpea > rice, irrespective of the tillage treatments. Environmental sustainability point of view, minimum tillage practices in rice-maize-cowpea cropping system in tropical low land soil could be adopted to minimize CO2-C emission, sustain yield, and maintain soil health.

Mechanisms of soil carbon accrual and storage in bioenergy cropping systems

AbstractAnnual row cropping systems converted to perennial bioenergy crops tend to accrue soil C, likely a function of increased root production and decreased frequency of tillage; however, very little is known about the mechanisms governing the accrual and stability of this additional soil C. To address this uncertainty, we assessed the formation and stability of aggregates and soil organic C (SOC) pools under switchgrass, giantmiscanthus, a native perennial grass mix and continuous corn treatments in Michigan and Wisconsin soils differing in both texture and mineralogy. We isolated different aggregate size fractions, &gt;2 mm, 0.5–2 mm, and &lt;0.5 mm, using a procedure intended to minimize alterations to aggregate biological and chemical properties. We determinedSOC, permanganate oxidizable C (POXC), and microbial activities (i.e. enzyme activities and soil respiration rates) associated with these aggregates. Soil type strongly influenced the trajectory of aggregate formation and stabilization with differences between sites in mean aggregate size, stability,SOCand microbial activity under perennial vs. corn cropping systems. At the Michigan site, soil microbial activities were highest in the &gt;2 mm aggregates, and higher under the perennial grasses compared to corn. Contrastingly, in Wisconsin soils, microbial activities were highest in the &lt;0.5 mm aggregates and evidence for soil C accrual under perennial grasses was observed only in a fast turnover pool in the &lt;0.5 mm aggregate class. Our results help explain cross‐site variability in soil C accrual under perennial bioenergy crops by demonstrating how interactions between belowground productivity, soil type, aggregation processes and microbial communities influence the rates and extent ofSOCstabilization. Bioenergy cropping systems have the potential to be low‐C energy sources but first we must understand the complex interactions controlling the formation and stabilization ofSOCif we are to maximize soil C accrual.

Turfgrass Growth and Color Correlated to Spring Illinois Soil Nitrogen Test and Soil Permanganate‐Oxidizable Carbon Concentrations

ABSTRACTThe Illinois soil nitrogen test (ISNT) and soil permanganate‐oxidizable C (POXC) concentrations have been used to estimate mineralization potential of agricultural soils, assess soil quality, distinguish differences between crop management treatments, and predict crop response to N fertilization. However, it is not known if these measures are correlated to growth and color quality responses of cool‐season turfgrasses. This study was conducted across 5 yr (2008–2012) in Connecticut to determine if a single spring measurement of Illinois soil nitrogen test‐N (ISNT‐N) and POXC concentrations could be used to estimate color and growth responses of Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinacea Schreb.) lawns. Randomized complete block field experiments were set out on the two species with varying rates of an organic fertilizer. Soil samples were collected in early May of each year and analyzed for concentrations of ISNT‐N and POXC. Turfgrass color, clippings yield, clippings total N concentration, and clippings total N uptake were measured from May through October. Turfgrass responses showed consistent positive linear responses (p &lt; 0.05) as a function of ISNT‐N and POXC. Across species and years, ISNT‐N and POXC were generally greater under tall fescue than under Kentucky bluegrass. A single spring measurement of soil ISNT‐N and POXC shows promise in categorizing Kentucky bluegrass and tall fescues lawns as to their likelihood of N fertilization response. The data suggest a low probability of meaningful Kentucky bluegrass and tall fescues lawn responses to N fertilization when spring ISNT‐N and POXC concentrations exceed 250 mg kg−1 and 1300 mg kg−1, respectively.

Methane production potential and methanogenic archaeal community structure in tropical irrigated Indian paddy soils

Soil characteristics regulate various belowground microbial processes including methanogenesis and, consequently, affect the structure and function of methanogenic archaeal communities due to change in soil type which in turn influences the CH4 production potential of soils. Thus, five different soil orders (Alfisol, Entisol, Inceptisol, Podzol and Vertisol) were studied to assess their CH4 production potential and also the methanogenic archaeal community structure in dryland irrigated Indian paddy soils. Soil incubation experiments revealed CH4 production to range from 178.4 to 431.2 μg CH4 g-1 dws in all soil orders as: Vertisol<Inceptisol<Entisol<Podzol<Alfisol. The numbers of methanogens as quantified using real-time quantitative polymerase chain reaction (qPCR) targeting mcrA genes varied between 0.06 and 72.97 (×106 copies g-1 dws) and were the highest in Vertisol soil and the least in Alfisol soil. PCR-denaturing gradient gel electrophoresis (DGGE)-based approach targeting 16S rRNA genes revealed diverse methanogenic archaeal communities across all soils. A total of 43 DGGE bands sequenced showed the closely related groups to Methanomicrobiaceae, Methanobacteriaceae, Methanocellales, Methanosarcinaceae, Methanosaetaceae and Crenarchaeota. The composition of methanogenic groups differed among all soils and only the Methanocellales group was common and dominant in all types of soils. The highest diversity of methanogens was found in Inceptisol and Vertisol soils. Methane production potential varied significantly in different soil orders with a positive relationship (p < 0.05) with methanogens population size, permanganate oxidizable C (POXC) and CO2 production. The present study suggested that CH4 production potential of different soils depends on physicochemical properties, methanogenic archaeal community composition and the population size.

Labile soil organic carbon and microbial activity in three subtropical plantations

The effects of tree species on soil properties have been attracted much attention, but the specific responses of labile soil organic carbon (SOC) and microbial activity to changes in tree species of subtropical forest ecosystems remain unknown. In this study, we investigated the labile fractions of SOC from three different single species plantations, namely Pinus massoniana (PM), Cinnamomum camphora (CC) and Schima superba (SS) in subtropical China. Specifically, we analysed the soil microbial biomass C (MBC), dissolved organic C (DOC) and permanganate-oxidizable C (POC), soil respiration, and activities of six enzymes in surface mineral soil (0–20 cm). The MBC, POC and soil respiration, as well as the activities of urease, acid phosphatase and polyphenol oxidase significantly differed among the three plantations in the study. In contrast, changes in the DOC as well as the activities of invertase, catalase and cellulase were not significant. The CC soil exhibited the highest POC, DOC and urease activity. PM soil had the highest MBC, soil respiration and polyphenol oxidase activity, but also had the lowest POC, qCO2, urease activity and acid phosphatase activity. DOC and MBC were significantly correlated with the soil respiration. Urease activity was significantly related to the SOC fractions, except for MBC. Invertase and polyphenol oxidase activities were correlated with MBC. The results suggested that the tree species had different effects on the labile SOC and microbial activity and the observed differences seemed not to be explained by the differences in the litter quality.

Conversion of Wheat–Maize to Vegetable Cropping Systems Changes Soil Organic Matter Characteristics

We investigated the effects of land use conversion from cereal grain fields (GF) to greenhouse vegetable fields (VF) on total soil organic C (SOC) and total N (TN) and the labile soil organic matter pools dissolved organic C (DOC), permanganate‐oxidizable C (POC), dissolved organic N (DON), mineral N, and microbial biomass C (MBC) in the top 30 cm of the soil profile. The soil samples were collected in 20 sets of paired soils (greenhouse vegetable vs. wheat [Triticum aestivum L.]–maize [Zea mays L.] rotation fields) in northeast China. Compared with those in GF, the SOC and TN concentrations in VF increased by 28 and 58%, respectively. The accumulation rate of TN vs. SOC, however, led to a significant decrease in soil C/N ratio in VF compared with GF systems. The greatest difference in C/N ratios between GF and VF was observed in the large macroaggregates (&gt;2000 μm). Conversion to VF led to increased DOC, POC, and DON concentrations and an increase in the percentage of SOC as DOC and POC. The pH, MBC, and percentage of SOC as MBC, however, were lower in the VF than GF systems. Furthermore, a significant positive relationship was found between pH and MBC in the VF systems, while MBC was positively related to total SOC and POC in the GF systems. In conclusion, the results show tradeoffs between increasing SOC sequestration when agricultural land is converted from wheat–maize to vegetable systems and decreased C/N ratios and soil acidification. Improvements in soil and crop management for vegetable production systems in China, particularly the avoidance of excess N fertilizer and manure applications, are recommended.

Human‐Transported Material Soils of Urbanizing Estuarine Landscapes

Additions of human‐transported materials (HTM) have significantly altered many coastal shorelines and wetlands. The hydrology and the ability of these anthropogenic soils to support ecologically important functions is poorly understood. In this study, we documented soil hydrologic patterns along disturbed estuarine shorelines and wetlands. Our goal was to determine if the soils had properties relative to the potential to support denitrification (i.e., labile C, saturation, and reducing conditions). Eleven anthropogenic sites, located in Rhode Island and 30 to &gt;60 yr old, were studied. Auger transects were completed to characterize anthropogenic soils. Water table levels were monitored twice a month. Anthropogenic soils were described and sampled from pits at five representative locations. Soil organic C (SOC), permanganate‐oxidizable C (POC), bulk density, and pH were measured. Deposits of HTM, comprised of dredge and fill materials, ranged in thickness from 26 to &gt;285 cm, were predominantly sandy and often contained artifacts. In the thickest HTM deposits, water table levels rose as much as 2.5 m above the original buried soil surface. Redoximorphic features were identified within the range of water table activity in 16 of the 18 monitored anthropogenic soils, suggesting reducing conditions. Soil organic C ranged from 1.6 to 88.9 g kg−1, was highest in surface horizons, and had an irregular distribution with depth. Labile C, estimated from POC measurements, followed the SOC distribution. Evidence of labile C, saturation, and reducing conditions in the majority of these soils suggest that most of the disturbed estuarine soils we studied have the capacity for denitrification.

Enhancing Soil Quality through Residue Management in a Rice-Wheat System in Fukuoka, Japan

A long-term experiment (LTE) on a rice-wheat system was initiated in 1963 at the Kyushu National Agricultural Experiment Station, in Fukuoka, Japan, to determine the effects of continuous application of rye grass/wheat straw, rice straw and rice straw compost, alone or in combination with inorganic N on crop yields. Increase in rice yields and enhancement of total soil C and N contents with the application of organic residues in this LTE have been reported earlier. However, evaluation of the changes in the soil microbiological properties and the decomposable C fraction of soil organic matter that is needed for soil quality assessment is still lacking. Soil samples were collected after rice harvest in 2003 from the organic residue treatments and unfertilized control, air-dried and incubated for 1 month under aerobic [50% water-filled pore space (WFPS)] and flooded conditions prior to the analysis of the amount of microbial biomass C (MBC), soil respiration and the amount of potential mineralizable N (PMN). The contents of total C (TC), total N (TN), organic C (OC), hot water-extractable C (HWEC) and permanganate-oxidizable C (POC) were determined from air-dried soils. Organic residue incorporation brought about significant increases in the contents of TC, TN, OC, POC, HWEC and PMN. The largest accumulation of total C (23%) and N (72%) in the soil was from rice straw compost, compared with that from rice straw (C, 7% and N, 33%) and rye grass/wheat straw (C, 9% and N, 29%). Incorporation of rice straw compost also increased the amount of MBC under both aerobic and flooded conditions and basal soil respiration under aerobic conditions only. An efficient utilization of C by microorganisms was indicated by a significantly lower metabolic quotient (qCO2) in the composted and uncomposted rice straw treatments compared with the control in the “-” N treatment under aerobic conditions. Similarly, the flush of CO2 after rewetting of dry soil per unit of HWEC was lower in the organic matter treatments, indicating a more efficient C utilization and lower C losses per unit of available C. The content of HWEC was significantly correlated with the basal soil respiration (at 50% WFPS), the amounts of MBC, PMN and with the increase in the content of soil organic C in the residuetreated soils. In the treatments without inorganic N fertilizer, grain yield was significantly correlated with the amounts of total organic C, HWEC, MBC (at 50% WFPS), basal soil respiration (at 50% WFPS) and the amount of PMN.

Assessing the Reliability of Permanganate‐Oxidizable Carbon as an Index of Soil Labile Carbon

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 &lt; 0.01) than with water‐soluble C (WSC) (P &lt; 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.