Chloroform Fumigation-incubation Research Articles

Differential Impacts of Cropland Expansion on Soil Biological Indicators in Two Ecological Zones

Agricultural expansion in Sub-Saharan Africa is characterized by different farm ages in smallholder communities. This study investigated changes in microbial indices broadly (i) at the reconnaissance survey level in four agro-ecological zones and (ii) in different farms at the forest (Dompem) and forest–savanna transition (Adansam) zones, as influenced by the duration of cultivation. Soils from one-year (first cultivation of cleared forest/fallow), three-year, five-year, and ten-year farms were analyzed for basic soil properties, active or labile carbon (POXC), basal respiration (BR), microbial biomass (Cmic) using permanganate oxidizable C, alkali trap, and chloroform fumigation incubation. In both study levels, POXC content was <1% of soil organic carbon (SOC) in all zones, higher in the wet agro-ecological zones, and positively correlated with SOC (r = 0.70, 0.81; p < 0.01, p < 0.001). Dompem SOC and BR declined by 1–23% and 6–25% (p < 0.001), respectively, in the first three years; Cmic (p = 0.002) and %Cmic/SOC (p = 0.610) decreased from three-year farms onwards. Conversely, the Adansam SOC, BR, Cmic, and %Cmic/SOC rather had irregular trends. The microbial indices were influenced by exchangeable acidity, the sum of exchangeable bases, and effective cation exchangeable capacity negatively or positively, followed by SOC, pedogenic compounds, particularly dithionite-citrate iron (Fed), oxalate iron (Feox), and lastly, soil pH. Therefore, understanding the degree, direction, and changing aspects of these drivers of soil ecosystem services is necessary for sustainable soil management practices in different agro-ecological zones.

Lysis of soil microbial cells by CO2 or N2 high pressurization compared with chloroform fumigation

The classical chloroform fumigation-incubation (CFI) and fumigation-extraction (CFE) methods are nowadays among the most used for determining soil microbial biomass, although the chloroform lysing of microbial cells is not always complete. Here, we have tested a physical method, used for sterilizing foods but never in soil, based on N2 or CO2 high pressurization (N2HP or CO2HP, respectively) to cause microbial cell lysis. The N2HP and CO2HP were tested on two soils differing for their organic matter content, one agricultural (AGR) and one forest (FOR), and firstly were compared with the CFI. The CO2 extra-flush from both soils during 10-d incubation by N2HP was lower than that by CFI method, whereas that by CO2HP was greater. Then, the lysis by CO2HP was compared with that by the CFE method by varying CO2 pressure and duration. The CO2HP, at proper conditions, was more efficient than CFE method to cause the lysis of soil microbial cells. Moreover, both CO2 pressure value and duration were important in increasing the extractable organic C compared to the CFE. The most successful combination of high CO2 pressure and duration was 4.13 MPa and 32 h. However, we cannot exclude that CO2HP might have caused the release of soil organic C not ascribable to living organic matter. Further studies using 13C and/or 15N-labeled microbial cells should assess the release of abiotic organic C.

Bacterial community structure in fumigated soil

Soil microbial biomass has been determined since the mid 1970's by the chloroform fumigation incubation technique as proposed by Jenkinson and Powlson (1976). The microbial biomass C can be determined by subtracting the CO2 emitted from an unfumigated soil (mineralization of soil organic matter) from that emitted from a chloroform fumigated inoculated soil (mineralization of soil organic matter and killed soil microorganisms) and dividing the difference by a proportionality factor (kC = 0.45). The question remained which microorganisms recolonized a fumigated soil. An arable soil was fumigated for one day with ethanol-free chloroform or left unfumigated and incubated aerobically after removal of the chloroform for 10 days. The bacterial population structures were determined in the fumigated and unfumigated soil after 0, 1, 5 and 10 days by means of 454 pyrosequencing of the 16S rRNA gene. Fumigating the arable soil reduced significantly the relative abundance of phylotypes belonging to different groups, but increased the relative abundance of only four genera belonging to two phyla (Actinobacteria and Firmicutes) and two orders (Actinomycetales and Bacillales). The relative abundance of phylotypes belonging to the Micromonospora (Micromonosporaceae) increased significantly from 0.2% in the unfumigated soil to 6.7% in the fumigated soil and that of Bacillus (Bacillaceae) from 3.6% to 40.8%, Cohnella (Paenibacillaceae) from undetectable amounts to 0.6% and Paenibacillus (Paenibacillaceae) from 0.3% to 4.2%. The relative percentage of phylotypes belonging to the Acidobacteria, Bacteroidetes, Chloroflexi, Gemmatimonadetes and Proteobacteria (α- β-, δ- and γ-Proteobacteria) were significantly lower in the fumigated than in the unfumigated soil and in most of them the relative abundance of different bacterial orders (i.e. Gp3, Gp4, Gp6, Sphingobacteriales, Gemmatimonadales, Rhodospirillales, Burkholderiales, Xanthomonadales) was reduced strongly (P < 0.001). It was found that the relative abundance of a wide range of bacteria was reduced shortly after fumigating an arable soil, but only a limited group of bacteria increased in a fumigated arable soil indicating a capacity to metabolize the killed soil microorganisms or recolonize a fumigated soil.

DNA determinations during growth of soil microbial biomasses

We attempted to quantify microbial growth in soil by means of DNA determination after glucose amendment. An FDNA conversion factor of 5.0 was used to convert μg DNA g−1 soil to μg Cmic g−1 soil during the growth phase. The conversion factor acquired rested on a regression analysis between soil microbial biomass-C (Cmic) estimated by the substrate-induced respiration technique (SIR) and dsDNA using a modified, miniaturized dsDNA extraction procedure which included 44 field and forest soils with a coefficient of determination of r2 = 0.95. Verification of this conversion factor was tested on eight arable soils where Cmic was determined by substrate-induced respiration (SIR)-, chloroform fumigation-incubation (CFI)-, chloroform fumigation-extraction (CFE)-, and application of the FDNA conversion factor. The congruency between the Cmic values obtained through these different techniques was satisfactory since five of eight soils gave similar Cmic values which were not statistically significantly different. The soils were thereafter amended with glucose and microbial growth followed by Cmic determinations with CFI, CFE, and DNA conversion over a period of up to 264 h at 22 °C. Concomitant CO2 analyses gave clues to two kinds of growth processes with respect to speed. Based on DNA conversion the calculated traditional growth parameters such as the specific growth rate (μ) lay in the range between 0.0046 and 0.022 h−1 which is several fold slower than μ values based on CO2 conversion but are in accordance with data in the earlier literature on growth rates for bacteria and fungi in soil done with traditional plate counts. These results suggest that DNA determinations can be applied as an alternative index for growth studies in situ.

Biomassa e atividade microbiana de solo sob vegetação nativa e diferentes sistemas de manejos

The purpose of the present study is to evaluate the influence of different handling systems, the microbial population and its activity. To that end, soil samples were collected in October 2006 and March 2007, at Fazenda Sereno in the city of Jaciara, Mato Grosso State. The samples were removed at 0-10 cm deep with four replications in five areas with different handling systems: agriculture-livestock integration, corn/soy fields, pastures, native vegetation, and native vegetation under recovery. The method of Chloroform Fumigation-Incubation (CFI) was used to evaluate microbial biomass carbon (MBC) and microbial respiration (MR). The qCO2 was obtained through the relationship between MR and MBC. The averages were compared through the Scott-Knott test at 5%. During the analyses, it was observed that the different agricultural practices substantially affect the soil, causing disturbances in the microbial community, which may influence the biogeochemical processes occurring on the soil. The data obtained also makes it possible to conclude that: in the agriculture-livestock integration and pasture systems, the microbial population is influenced by collection times. In this work, in the field-livestock integration system, the microbial activity is constant at the different collection times and management systems influence the metabolic activity of microorganisms in the soil.

Using GC-MS for the quick analysis of carbon dioxide for soil microbial biomass determination

Microbial biomass carbon is a common soil parameter used to evaluate microbial abundance in soil environments, and microbial biomass can change in response to disturbances and/or changing environmental conditions. Microbial biomass carbon was determined on 192 soil samples using the chloroform fumigation-incubation method, and after 10 days, gas samples were collected. Gas samples were analyzed for carbon dioxide (CO2) concentration. Determination of CO2 concentration is usually conducted by infrared gas analysis or chemical analysis (an alkali trap and subsequent titration, or soda lime absorption), and these methods were either unavailable or too time consuming for the number of samples analyzed. As a result, we have developed an analytical method for the determination of CO2 in gases using the gas chromatograph/mass spectrometer (GC-MS). This method requires a 5 μL injection, 5 minute analysis time, and the resulting figures of merit are comparable to most procedures present in the literature. Additionally, the microbial biomass carbon values obtained were consistent with previously published values in similar ecosystems.

Influence of tillage, residue management, and crop rotation on soil microbial biomass and catabolic diversity

The densely populated, intensively cropped subtropical highlands of the world have agricultural sustainability problems from soil erosion and fertility decline. In 1991, the International Maize and Wheat Improvement Center (CIMMYT) initiated a long-term field experiment at its semi-arid highland experiment station in Mexico (2240 masl; 19.31°N, 98.50°W; Cumulic Phaeozem) to investigate the long-term effects of tillage/seeding practices, crop rotations, and crop residue management on maize and wheat grown under rainfed conditions. Soil ecology status contributes to agricultural system sustainability, and evaluations were made to determine the effect of different management practices on soil microbial biomass (SMB) (substrate-induced respiration (SIR) and chloroform fumigation incubation (CFI)) and micro-flora physiological and catabolic diversity (BIOLOG™ ecoplate well system). SMB-C (CFI, SIR) was significantly and respectively 1.2 and 1.3 times higher for residue retention (average 387 mg C kg −1 dry soil and 515 mg C kg −1 dry soil , respectively) compared to residue removal. SMB-C (CFI) was significantly higher for wheat (369 mg C kg −1 dry soil ) compared to maize (319 mg C kg −1 dry soil ). SMB-N (CFI) was significantly 1.3 times higher for residue retention (average 28 mg N kg −1 dry soil ) compared to residue removal. The average well color development (AWCD) obtained by the BIOLOG™ ecoplate essay indicated there were large differences in the catabolic capability of soil microbial communities after 15 years of contrasting management practices. While maize and wheat rotation under conventional tillage with residue retention showed a significantly higher overall AWCD value compared to the other treatments, AWCD of maize with zero tillage and residue removal was significantly lower than in the other treatments. AWCD was significantly higher for residue retention compared to residue removal and for wheat as compared to maize. For maize, the management practices were divided into two groups; zero tillage with residue removal was separate from all other treatments. For wheat, conventional tillage was separate from all zero tillage treatments. This study suggests that in the target area, a cropping system that includes zero tillage, crop rotation, and crop residue retention can increase overall biomass and micro-flora activity and diversity compared with common farming practices. In the long term, zero tillage combined with residue retention creates conditions favourable for the development of antagonists and predators, and fosters a new ecological stability. Zero tillage without residue retention is an unsustainable practice that leads to poor soil health in the long run.

Microbial biomass in a semi arid soil of the central highlands of Mexico cultivated with maize or under natural vegetation

Microbial biomass (MB) is the key factor in nutrient dynamics in soil, but no information exists how clearing of vegetation to cultivate maize in the central highlands of Mexico might affect it. Soil MB was measured with the chloroform fumigation incubation (CFI) and fumigation extraction (CFE) techniques and the substrate-induced respiration (SIR) method in soil sampled under or outside the canopy of mesquite ( Prosopis laevigata) and huisache ( Acacia tortuoso), N 2 fixing shrubs, and from fields cultivated with maize. Microbial biomass C as measured with the CFI technique ranged from 122 mg C kg −1 in agricultural soil to 373 mg C kg −1 in soil sampled under mesquite shrubs. Microbial biomass N as measured with the CFI technique ranged from 11 mg N kg −1 in agricultural soil to 116 mg N kg −1 in soil sampled under mesquite shrub. The ratio of microbial biomass C as measured with CFI related to the ninhydrin-positive compounds (NPC) was 12.23 after 1 day and 8.43 after 10 days while the relationship with extractable C was 3.15 and 2.96, respectively. The metabolic quotient ( qCO 2) decreased in the order OUTSIDE > MESQUITE > HUIZACHE > AGRICULTURE, and the microbial biomass:soil organic C ratio decreased in the order MESQUITE > HUIZACHE > OUTSIDE > AGRICULTURE using SIR to determine the microbial biomass. It was found that converting soil under natural vegetation to arable soil was not only detrimental for soil quality, but might be unsustainable as organic matter input is limited.

Microbial biomass C measurements in soil of the central highlands of Mexico

Microbial biomass is the key factor in nutrient dynamics in soil, but no information exist about it in soils of the central highlands of Mexico, a major agricultural area. We determined the microbial biomass in soils with a wide range of organic C and belonging to three soil texture classes. Twenty-four soils under different types of cultivation were sampled while microbial biomass C was measured with the chloroform fumigation incubation (CFI) and extraction technique (CFE). Microbial biomass C as measured with the CFI technique ranged from 138 to 2195 mg C kg −1. The ninhydrin-positive compounds (NPC) and extractable C released with CFE increased with increased time of exposure to chloroform and on average 53% of NPC and 83% of extractable C was released after 1 day compared to that released after 10 days. The ratio of microbial biomass C as measured with the CFI method related to the NPC was 31.8 after 1 day and 20.0 after 10 days while the relationship with extractable C was 3.18 and 2.69, respectively. The relationship between microbial biomass C as measured by the chloroform fumigation incubation technique and the soluble C and ninhydrin-N rendered extractable after 1 and 10 days of chloroform fumigation for soils of the central highlands of Mexico were comparable to values reported for soils in other regions of the world. The factors determined in this study can thus be used to determine microbial biomass.

Soil Microbial Activity Is Affected by Roundup WeatherMax and Pesticides Applied to Cotton (Gossypium hirsutum)

Adoption of glyphosate-based weed control systems has led to increased use of the herbicide with continued use of additional pesticides. Combinations of pesticides may affect soil microbial activity differently than pesticides applied alone. Research was conducted to evaluate the influence of glyphosate-based cotton pest management systems on soil microbial activity. Soil was treated with commercial formulations of trifluralin, aldicarb, and mefenoxam + pentachloronitrobenzene (PCNB) with or without glyphosate (applied as Roundup WeatherMax). The soil microbial activity was measured by quantifying C and N mineralization. Soil microbial biomass was determined using the chloroform fumigation-incubation method. Soils treated with glyphosate alone exhibited greater cumulative C mineralization 30 days after treatment than all other treatments, which were similar to the untreated control. The addition of Roundup WeatherMax reduced C mineralization in soils treated with fluometuron, aldicarb, or mefenoxam + PCNB formulations. These results indicate that glyphosate-based herbicides alter the soil microbial response to other pesticides.

C and N mineralization and microbial biomass in heavy-metal contaminated soil

Heavy metals such as arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills in Mexico. Interest in contamination by these heavy metals has increased recently as they can change the functioning of soil ecosystems qualitatively and quantitatively. They disturb the activities of soil fauna and contaminate drinking water in large parts of the world, which severely affects human health. Little, however, is known how heavy metals might affect the biological functioning of a soil. Soil was sampled from eight locations along a gradient of heavy-metal contamination with distance from a mine in San Luis Potosí (Mexico) active since about 1800 AD. Microbial biomass was determined with the original chloroform fumigation incubation (CFI) as well as extraction (CFE) techniques and the substrate induced respiration (SIR) technique while C and N mineralization were measured. Total concentrations of As in the top 0–10 cm soil layer ranged from 8 to 22992 mg kg –1, from 31 to 1845 mg kg –1 for Pb, from 27 to 1620 mg kg –1 for Cu and from 81 to 4218 mg kg –1 for Zn. There was a significant negative correlation ( P < 0.0001) between microbial biomass, soil organic carbon, total N and C mineralization and the heavy metal content of the soil. The microbial biomass C to organic C ratio, which varied from 0.4 to 1.9%, specific respiratory activity ( qCO 2), and oxidation of NO 2 – were not affected by heavy metals. It was found that long-term contamination of soil with heavy metals had an adverse effect on the amount of soil microorganisms as evidenced by a marked decrease in microbial biomass C, but not some of their characteristics. According to principal components analysis (PCA), the correlation matrix showed three distinct factors explaining 71% of the variance. A first factor including heavy metals (As, Pb, Cu and Zn) with a negative loading and total N, organic C, soil microbial biomass with a positive loading characterized the soil organic matter and contamination status. Loam and sand combined for the second factor characterizing the textural classification while the third factor was loaded by CEC and clay content.

Indicadores de Calidad de la Materia Orgánica del Suelo en un Andisol Cultivado

A B S T R A C T Agricultural practices influence the dynamics of soil organic matter (MOS) and its fractions. In this study, the following labile fractions of soil organic matter were determined: free light fraction (FLL), intraaggregate light fraction (FLI), microbial biomass and mineralized C-CO2, on volcanic soil with different rotations, and later their use was evaluated as biological indicators of the impact of agricultural practices on the soil. The study was carried out in an eight year field experiment, with different productive systems (rotations) in a randomized complete block design. The FLL was determined by density fractionation with NaI (1.8 g cm –3 ), and FLI was obtained by sonication (1,500 J s -1 ). Microbial biomass was quantified using the chloroform fumigation-incubation (FI) technique, and the basal soil respiration (C-CO2 evolution) was determined by incubation for a 10-day period. Increased soil use intensity decreased (P ≤ 0.05) C and N FLL contents, from 1.69 g C-FLL kg -1 soil (5-year rotation with alfalfa, Medicago sativa L.), to 0.49 g C-FLL kg -1 (annual crop rotation). However, these contents in FLI did not show a clear and consistent tendency (P ≤ 0.05). Soil biomass C and N decreased (P ≤ 0.05) with higher soil use intensity, from 551 to 264 ∝g C-CO2 g soil -1 and from 106 to 35 ∝g N-(NO3 - + NH4 + ) g soil -1 , respectively. The three studied indices were appropriate indicators to determine changes in soil organic matter quality as a result of agricultural practices.

Effects of Cover Crops on Soil Microbial Biomass in Vegetable Cropping Systems

The inclusion of cover crops into cropping systems may influence soil microbial activity which is crucial to sustained crop production. A study was conducted to measure short term effects of summer and winter cover crops on soil microbial biomass carbon (MBC) in a cucumber-tomato rotation system. The experiment was established in Summer 2002 as a factorial of summer cover crops (planted either as fallow or after harvest of cucumbers) and winter cover crops (planted in September). The design was a split-block with four replications. The main plot factor was summer cover crop and consisted of five treatments; sorghum sudangrass fallow (SGF), cowpea fallow (CPF), sorghum sudangrass after cucumber (SGC), cowpea after cucumber (CPC) and bareground fallow (BGF). The sub-plot factor was winter cover crop and consisted of three treatments including cereal rye (CR), hairy vetch (HV) and bareground (BG). In spring of 2003, soil samples were collected in each treatment at 30 days before (30 DBI), 2 days after (2 DAI) and 30 days after (30 DAI) cover crop incorporation. MBC was measured using the chloroform fumigation-incubation method. Both summer and winter cover crops affected soil microbial activity. MBC in the summer cover crop treatments at 30 DBI was 47.7, 51.4, 49.2, 43.7 and 42.5 μg·g-1 soil for SGF, CPF, SGC, CPC and BGF, respectively. At 30 DAI, 113.1, 88.9, 138.5, 105.6, and 109.3 μg·g-1 soil was obtained in SGF, CPF, SGC, CPC, and BGF plots, respectively. Soil MBC was similar at 2 DAI in the summer cover crop treatments. Among winter treatments MBC was similar at 30 DBI and 30 DAI, but significant at 2 DAI with values of 62.8, 53.3, 59.3 μg·g-1 soil for CR, BG, and HV, respectively.

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K 2SO 4-extractable organic C and 33.3 mM KMnO 4-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO 2 production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation–extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation–incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO 2-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO 2 production rate, relative C mineralization rate (CO 2-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.

Carbono da biomassa microbiana em solos de cerrado sob vegetação nativa e sob cultivo: avaliação dos métodos fumigação-incubação e fumigação-extração

A biomassa microbiana do solo é um componente essencial da matéria orgânica que, entre outras funções, regula a ciclagem de nutrientes no solo. Dentre os métodos mais utilizados para determinação do carbono da biomassa microbiana, destacam-se: os de clorofórmio-fumigação-incubação (CFI) e clorofórmio-fumigação-extração (CFE). Trabalhos na literatura têm comparado a eficiência desses métodos em diversos locais. No entanto, para a região do Cerrado, não existem informações a esse respeito. O objetivo deste trabalho foi avaliar a eficiência dos métodos CFE e CFI na determinação do carbono da biomassa microbiana do solo (CBMS) em áreas de Cerrado sob cultura anual (rotação soja-milho) e pastagem consorciada (Andropogon gayanus e Stylosanthes guianensis) e sob três fitofisionomias - mata de galeria, campo sujo e cerradão. Amostras de solo coletadas em duas profundidades, 0-5 cm e 5-20 cm, foram analisadas em quatro épocas: agosto de 1998, janeiro e agosto de 1999 e janeiro de 2000. Nas áreas cultivadas, os resultados obtidos com os métodos CFE e CFI foram semelhantes, independentemente dos tratamentos e das épocas amostradas, tendo as pastagens consorciadas apresentado maiores teores de CBMS do que as áreas sob culturas anuais. A interação profundidades x métodos foi significativa. Não houve diferenças entre as profundidades 0-5 e 5-20 cm, quando se utilizou o método CFI, mas as diferenças obtidas com o método CFE foram significativas. Os métodos CFI e CFE apresentaram as mesmas tendências nas áreas nativas, independentemente dos tratamentos, profundidades ou épocas analisados, tendo a mata de galeria apresentado níveis de CBMS superiores aos do cerradão e campo sujo. As interações profundidades x métodos e épocas x métodos foram significativas pelo fato de as diferenças nos teores do carbono da biomassa microbiana, nas profundidades e épocas amostradas, terem sido mais acentuadas com o método CFE. Os resultados indicaram que os métodos CFI e CFE foram apropriados para determinação da CBMS em solos de Cerrado sob cultivo e sob vegetação nativa.

Molar concentration of K 2SO 4 and soil pH affect estimation of extractable C with chloroform fumigation–extraction

Methods of determining soil microbial biomass need to be reliable and produce consistent results across soils with a wide range of properties. We investigated the effect of extractant molarity (distilled water and 0.001, 0.01, 0.1, and 0.5 M K 2SO 4) on the flush of C (i.e. the difference between fumigated and unfumigated subsamples) with the chloroform fumigation–extraction method in soils of different pH. Extraction efficiency of 0.5 M K 2SO 4 relative to water was dependent upon soil pH. The ratio of extractable C in water to that in 0.5 M K 2SO 4 for five acidic soils was 1.5±0.3 in unfumigated controls, 1.4±0.2 in fumigated samples, and 1.8±0.7 in fumigated minus control flushes, respectively. Ratios in six alkaline soils were 1.0±0.2, 0.9±0.2, and 0.8±0.2, respectively. Flocculation/dispersion of organic colloids and changes in the diffuse double layer surrounding clay particles are possible reasons for differences in extractable C with changes in extractant molarity and soil pH. Chloroform fumigation–extraction with any of the extractants was less related to soil organic C and potential C and N mineralization during 50 days of incubation ( r 2=0.51±0.11) than was chloroform fumigation–incubation without subtraction of a control ( r 2=0.74±0.08). Changes in microbial biomass estimates with changes in extractant molarity and soil pH suggest that chloroform fumigation–extraction may not be reliable in a wide range of soils.

Assessing biological soil quality with chloroform fumigation-incubation: Why subtract a control?

Microbial biomass, as part of the active pool of soil organic matter, is critical in decomposition of organic materials, nutrient cycling, and formation of soil structure. We evaluated chloroform fumigation-incubation with subtraction of a control (CFI/F–C) and without subtraction of a control (CFI/F) as methods to assess biological soil quality. Relationships between CFI/F and potential C mineralization, particulate organic C, and soil organic C were stronger (r2 = 0.86 ± 0.07, n = 232) than those between CFI/F–C and the same soil C pools (r2 = 0.25 ± 0.09) in soils from Georgia. From published data, relationships of CFI/F with potential C mineralization and soil organic C were stronger than those of chloroform fumigation-extraction and substrate-induced respiration with these soil C pools. Effects of land management on biological soil quality using CFI/F were consistent with those determined using other soil C pools as response variables. However, land management effects on biological soil quality using CFI/F–C were either contrary to those using other soil C pools or not detectable because of greater inherent variability in CFI/F–C. Chloroform fumigation-incubation without subtraction of a control is a robust and reliable method to assess biological soil quality under a wide range of soil conditions. Key words: Active soil carbon, chloroform fumigation-extraction, microbial biomass, soil organic matter, soil quality, substrate-induced respiration

Soil C extracted with water or K2SO4: pH effect on determination of microbial biomass

Routine determination of soil microbial biomass C has shifted during the past decade from chloroform fumigation-incubation to chloroform fumigation-extraction using 0.5 M K2SO4 as extractant. We compared extractable C with water and 0.5 M K2SO4 in eight soils ranging in pH from 5.4 to 8.3. In unfumigated soils with low pH, extractable C was 0.8- to 1.2-fold greater with water than with 0.5 M K2SO4. However, in unfumigated soils with pH &gt; 7.7, extractable C, although not statistically significant, was 11 to 19% less with water than with 0.5 M K2SO4. In fumigated soils, no difference in extractable C between water and 0.5 M K2SO4 was detected among soils with pH &lt; 7.7, but extractable C was 13 to 17% less with water than with 0.5 M K2SO4 with pH &gt; 7.7. Our results suggest that 0.5 M K2SO4 (1) may flocculate soil and cause adsorption of solubilized C onto colloids at pH &lt; 7.7, but (2) may disperse calcareous soils at pH &gt; 7.7, thereby differentially affecting the fate of solubilized C depending upon soil pH. Our results put into question the widespread adaptability of using chloroform fumigation-extraction to estimate microbial biomass C. Key words: Extractable carbon, chloroform fumigation-extraction, microbial biomass

Relationships of chloroform fumigation–incubation to soil organic matter pools

Microbial biomass is part of the active pool of soil organic matter that plays focal roles in decomposition of organic materials, nutrient cycling and biophysical manipulation of soil structure. We compared two commonly used variants of the chloroform fumigation–incubation method in their relationships with other active, passive and total soil C and N pools in soils from Texas, Georgia, Alberta and British Columbia. The relationship of potential C mineralization with chloroform fumigation–incubation without subtraction of a control was much stronger ( r 2=0.81±0.10 among five data sets with a total of 844 observations) than with subtraction of a control ( r 2=0.30±0.22). Similarly, the relationship of soil organic C with chloroform fumigation–incubation without subtraction of a control was better ( r 2=0.80±0.13) than with subtraction of a control ( r 2=0.38±0.32). Relationships of net N mineralization, flush of N following fumigation–incubation, flush of CO 2-C during the first day following rewetting of dried soil, particulate organic C and N, mean weight diameter of water-stable aggregation and total porosity with chloroform fumigation–incubation were also better without subtraction of a control than with subtraction of a control. In analyses of data taken from published reports, chloroform fumigation–incubation without subtraction of a control was better related with active soil C pools than with subtraction of a control. Chloroform fumigation–incubation without subtraction of a control, unlike that with subtraction of a control, should be considered a more robust method to determine microbial biomass under a wide range of environmental conditions.

Les variations au champ de la biomasse microbienne d'un sol cultivé: conséquences sur la réserve organique mobilisable (cas d'un sol ferrugineux tropical au Sénégal)

In an arenosol of the central part of northern Senegal, the microbial biomass expressed as mg C·kg −1 soil (BM-C) using the chloroform-fumigation incubation method was studied in two long term field trials respectively at Bambey and Thilmakha. BM-C expressed as percent of total soil organic C was higher than in temperate soils. The BM-C and the non-hydrolysable non-distillable nitrogen (Nhnd) were found to be correlated for both agrosystems. Agricultural activity (farmyard manuring and ploughing) did significantly increase these two parameters. The BM-C increased during the rainy season. This might be a key factor in the nitrogen flush at the onset of the rainy season in dry tropical areas, which is essential to the installation of the crop.