Journal of Environmental Quality 30:147-150 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
PLANT AND ENVIRONMENT INTERACTIONS
The Relationship between Microbial Carbon and the Resource Quality of Soil Carbon
E.A. Webstera,
D.W. Hopkinsa,
J.A. Chudekb,
S.F.I. Haslamc,
M. 
imekd and
T. Pîcekd
a Dep. of Environmental Science, University of Stirling, Stirling FK9 4LA, Scotland UK
b Dep. of Chemistry, University of Dundee, Dundee DD1 4HN, Scotland UK
c Dep. of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland UK
d Institute of Soil Biology of the Academy of Sciences of the Czech Republic and Biological Faculty of the University of South Bohemia, Na sàdkàch 7, 
eské Budéjovice CZ 370 05, Czech Republic
Corresponding author (e.a.webster{at}stir.ac.uk)
Received for publication March 16, 2000.
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ABSTRACT
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The biological health of soil is an important aspect of soil quality because of the many critical functions performed by organisms in soil. Various indicators of soil quality have been proposed, but measurements of microbial biomass are most commonly used. During decomposition of plant residues in soil the relative intensities of the O-alkyl-C signal decreases and the alkyl-C signal increases in nuclear magnetic resonance (NMR) spectra. This leads to the suggestion that the alkyl-C to O-alkyl-C ratio of a soil may indicate the degree of decomposition. Consequently, the overall resource quality of soil C as a substrate for heterotrophic microorganisms may be inversely related to the alkyl-C to O-alkyl-C ratio. Our hypothesis is that a relationship exists between the size of the soil microbial community (microbial biomass) and the quality of soil carbon as a resource for microorganisms. New data have been combined with previously published data to show that there was a significant, negative correlation between the biomass C to total C (Cmic to Corg) ratio and the alkyl-C to O-alkyl-C ratio (p < 0.01), which supports our hypothesis.
Abbreviations: Cmic, biomass carbon • Corg, total carbon • CP, cross polarization • MAS, magic angle spinning • NMR, nuclear magnetic resonance
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INTRODUCTION
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SOIL biological health, defined as the capacity of soil to function as a living system, is a critical aspect of soil quality because organisms perform many key functions in soils, such as release of nutrients, the maintenance of soil structure, and the breakdown of organic contaminants (Jenkinson and Ladd, 1981; Wardle, 1992; Elliot et al., 1996; Pankhurst et al., 1997). This means that biological indicators of soil quality have the capacity to integrate across a range of factors that might affect soil health. Various biological indicators of soil quality have been proposed (Elliot et al., 1996; Pankhurst et al., 1997; Capriel, 1997), but microbial biomass measurements are often used because they can be applied to a range of soils and determined relatively easily. Sparling (1992) showed that biomass C is more sensitive to change than total C and it has been proposed that the ratio of Cmic to Corg in soil may provide early warning of changes in organic matter dynamics (Powlson et al., 1987; Wardle, 1992). Moreover, the fact that biomass is depressed in contaminated, polluted, or poorly managed soils (Nannipieri et al., 1990; Elliot et al., 1996) is consistent with its use as an indicator of soil health.
If biomass C is to be used as an indicator of soil quality, it would be useful to be able to assess objectively how much microbial biomass C there should be in a given soil under "healthy" conditions. This could then provide an assessment of the degree of health impairment and a target for restoration purposes. The number of heterotrophic microorganisms that normally dominate the soil microbial community will be influenced by both the amount and resource quality of the soil organic matter as a substrate for growth. Our objective was to investigate the relationship between the size of the soil microbial community, as a proportion of total soil C, and the resource quality of soil organic matter. We have assessed the resource quality of soil organic matter using solid-state 13C NMR spectroscopy in which the alkyl-C to O-alkyl-C ratio provides an index of resource quality. Spectra for undecomposed plant litter give 13C NMR spectra with large O-alkyl-C signals, representing mainly plant polysaccharides, and comparatively small alkyl-C resonances (e.g., Knicker and Lüdemann, 1995; Hopkins and Chudek, 1997; Webster et al., 2000). As decomposition proceeds, the relative intensity of the alkyl-C signal, representing partly decomposed plant residues and microbial compounds, increases whilst that of the O-alkyl-C signal decreases (Baldock et al., 1997; Preston, 1996; Hopkins and Chudek, 1997; Webster et al., 2000). This leads to the possibility that the alkyl-C to O-alkyl-C ratio may indicate the degree of decomposition of soil C, with a larger value indicating soil C that is more decomposed and consequently more resistant to further rapid C loss. A smaller alkyl-C to O-alkyl-C ratio would indicate soil C with a greater potential for decomposition. This hypothesis is supported by the observations that the intensity of the alkyl-C signal increases whilst that of the O-alkyl-C decreases with decreasing particle size and increasing depth (Baldock et al., 1992; Preston et al., 1987, 1994). From these observations, we derived the hypothesis that the quality of the resource as a substrate for heterotrophic soil microorganisms declines as the ratio of alkyl-C to O-alkyl-C increases and that this ratio may provide a useful index of the quality of soil C.
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MATERIALS AND METHODS
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Twenty-nine soils representing a range of types and land uses (Table 1) were collected from the 0- to 15-cm depth. Samples 21 through 23 (Table 1) were collected from the litter layer, and consisted of mostly undecomposed plant material. Two chronosequences were included: Soils 1 through 5 were sampled from a transect across an area where sand is accumulating due to aeolian deposition, while Soils 21 through 26 represent the development of O horizons after severe, accidental fires (Haslam et al., 1998). All soils were sieved (<4 mm) in the field-moist state and stored for up to 8 wk at 4°C before use. Values of soil pH were determined from a 1:2.5 soil to water mixture. Prior to determining microbial biomass the soils were restored to 60% water holding capacity and maintained at 22°C for 7 d. Subsamples of soils were dried and the C and N contents determined using a Carlo Erba (Milan, Italy) CHN analyzer. Microbial biomass C was estimated in triplicate for each soil using the glucose induced respiration approach of Anderson and Domsch (1978) with minor modifications described by Hopkins and Ferguson (1994). Prior to 13C NMR analyses, the soils were freeze-dried and finely ground in a mortar and pestle. Cross polarization magic angle spinning (CP MAS) 13C NMR spectra were recorded using a Chemagnetics CMX LITE 300 MHz spectrometer (Varian/Chemagnetics, Fort Collins, CO) at 4 kHz MAS, 1 ms contact time, and 2 s relaxation delay with tetramethylsilane as the external reference (Golchin et al., 1996; Webster et al., 1997, 2000). The alkyl-C to O-alkyl-C ratios were calculated from the areas of the 13C NMR spectra within the shift ranges ascribed to alkyl-C (0–45 ppm) and O-alkyl-C (45–90 ppm) (Wilson, 1987). Spectral areas within these shift ranges were measured using an ADC AM100 area meter (Analytical Development Company Ltd, Hoddeston, Hertfordshire, England). In addition, the alkyl-C to O-alkyl-C ratios were estimated for the 13C NMR spectra published by Beyer (1995) and Beyer et al. (1995).
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Table 1. Characteristics of the soils. pH (H2O) and C and N values are the mean of three measurements with standard deviations shown in brackets
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RESULTS AND DISCUSSION
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The soils used covered a wide range of soil pH values (from pH 3.3 to 6.9), total C contents (from 5.6 to 479 g C kg-1), and total N contents (from 1.2 to 20.9 g N kg-1 soil) (Table 1). There was a significant correlation between total C and biomass C (p = 0.0001). The relationship between total C and the alkyl-C to O-alkyl-C ratio (p = 0.42) was not significant, and the relationship between biomass C and the alkyl-C to O-alkyl-C ratio was significant at p = 0.02.
Contrasting 13C CP NMR spectra are shown in Fig. 1
. In the spectrum for Soil 21 the alkyl-C resonance was relatively large indicating a relatively large amount of microbial metabolites and more stable plant components in this organic soil. By contrast, in the spectrum for Soil 19 the alkyl-C resonance was much smaller than the O-alkyl-C resonance consistent with the large amount of carbohydrates in undecomposed plant material. The spectrum for Soil 11 is typical of mineral soils with a greater signal to noise ratio due to the smaller C content.
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Fig. 1. Solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR) spectra for soils (a) 21, (b) 19, and (c) 11 from Table 1
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The relationship between Cmic to Corg and alkyl-C to O-alkyl-C for all soils is shown in Fig. 2
where the data for the 29 soils described in Table 1 are shown as closed symbols. Included in Fig. 2 are data from a further 18 soils from Beyer et al. (1995) and Beyer (1995) that included both microbial biomass measurements and 13C CP NMR spectra. The correlation analysis is presented in Table 2. The relationship between Cmic to Corg and alkyl-C to O-alkyl-C for all soils was significant (p < 0.001), whereas the relationship between Cmic and alkyl-C to O-alkyl-C (p > 0.05) or between Corg and alkyl-C to O-alkyl-C (p > 0.1) was not significant. There was, however a significant relationship between pH and Cmic to Corg (p < 0.001) and between pH and alkyl-C to O-alkyl-C (p < 0.001). The data from the different sources complement each other, and it is important to note that there were no soils with both large Cmic to Corg and large alkyl-C to O-alkyl-C ratios. The relationship between Cmic to Corg and alkyl-C to O-alkyl-C (Fig. 2) would be inadequate for the purpose of predicting biomass C normalized for total C because of the number of points with both small Cmic to Corg and small alkyl-C to O-alkyl-C values. Some of the soils may have been subject to conditions that limited the Cmic to Corg ratio to a value below that which the alkyl-C to O-alkyl-C value alone would predict. It is probable that such limitation may be the result of a natural property of the soil such as texture, pH, or physical protection of chemically labile O-alkyl-C, which may reduce the microbial activity of the soil to a value below that predicted from the chemical composition of the soil C. For other soils the Cmic to Corg value may have been restricted by a non-natural factor, such as poor management or pollution.
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Table 2. Correlation coefficients and p values for our data (Dataset 1; n = 29 df = 27) and for our data combined with published data (Dataset 2; n = 46 df = 44)
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There are some difficulties in using solid-state 13C NMR to estimate the resource quality of soil organic matter, including the fact that all C is not necessarily detected with the same efficiency in solid-state NMR spectra, and the problem of obtaining quantitative information from the spectra. These difficulties have been discussed by Kinchesh et al. (1995), and we have attempted to minimize some difficulties by using relative signal intensities. More specifically, the use of alkyl-C to O-alkyl-C ratios for making comparisons between soils may be limited because of differences in the chemical composition of the plant material from different species (Baldock et al., 1997) and because the alkyl-C to O-alkyl-C ratio will be influenced by the nature of the original C input to the soil. For example, the proportion of lignin is a determinant of plant litter quality (as a substrate for heterotrophic microorganisms), and the principal substituents of lignin contribute to both the O-alkyl-C regions and the aromatic regions of 13C NMR spectra (Hatcher, 1987). The presence of a large amount of lignin will protect some of the polysaccharide C in plant residues from microbial attack and decay (Huang et al., 1998). When significant amounts of lignin are present, the signal intensity in the O-alkyl-C region may overestimate the amount of readily decomposable C resulting in an alkyl-C to O-alkyl-C ratio that is greater than would be expected if only polysaccharide materials were present. The effect of protection of polysaccharides by lignin on the relationship between Cmic to Corg and alkyl-C to O-alkyl-C as a predictor of microbial biomass would be to overpredict biomass.
Despite the limitations to interpretation, our data suggest a relationship between the size of the microbial community, as a proportion of total C, and the quality of the soil C that supports our underlying hypothesis. If this relationship is to be used for predictive purposes, it will be necessary to increase the number and range of soils analyzed and take account of factors such as soil texture, pH, and the source of the original organic input.
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ACKNOWLEDGMENTS
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We are grateful to the UK Natural Environment Research Council (GR3/10019) and NATO (CRG 97/1331) for research grant support and to Linda Kubatovà for technical assistance. Part of this work was supported by the Grant Agency of the Czech Republic (Project no. 206/96/0169). We also thank Dr. J. Stana and Dr. P. Ruzec for providing soil samples.
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TOP
ABSTRACT
INTRODUCTION
) urban soils, (
) mineral soils, (
) organic soils (