Elsevier

Soil Biology and Biochemistry

Volume 30, Issue 14, December 1998, Pages 2009-2016
Soil Biology and Biochemistry

Mineralization of carbon from d- and l-amino acids and d-glucose in two contrasting soils

https://doi.org/10.1016/S0038-0717(98)00075-3Get rights and content

Abstract

Following addition of either the d- or the l-isomers of alanine, glutamine or glutamic acid or d-glucose, the CO2 production from an arable and a forest soil was measured until the pulses of CO2 production associated with substrate addition subsided. The maximum rate of additional CO2 production from the d-glucose amended soils occurred within the first 48 h for both soils. The greatest rates of additional CO2 production from l-amino acid amended soils occurred within 108 h for the forest soil and 60 h for the arable soil. Following addition of d-amino acids to the forest soil, the maximum rate of additional CO2 production was less than that following addition of the corresponding l-amino acid addition. However, for this soil the pulse of additional CO2 production following d-amino acid amendment lasted longer and by the time it had subsided (360 h), the total additional CO2 production did not differ between isomeric forms of the same amino acid. Following d-amino acid addition to the arable soil, there were delays of between about 24 and 48 h before the onset of rapid additional CO2 production and the CO2 pulse subsided relatively rapidly. The total additional CO2 produced from the arable soil was significantly less for the d-amino acid than for the corresponding l-amino acid treatments. Successive additions of d-glucose led to significant increases in the subsequent rates of additional CO2 production from the forest soil, but not from the arable soil. Each successive l-amino acid amendment led to increases in the rate of additional CO2 production from both soils, as did successive additions of the d-amino acids to the forest soil. However, successive additions of the d-amino acids to the arable soil did not lead to consistent responses in the additional rate of CO2 production.

Introduction

The presence of an asymmetric carbon atom, i.e. one to which four different functional groups are attached, that acts as a chiral centre in the molecule means that all amino acids except glycine may exist as enantiomers (or d- and l-forms). Both isomeric forms of amino acids occur naturally, but in living organisms and their metabolites the l-isomers are far more abundant. The biological occurrence of d-amino acids is restricted to a few amino acids with a few specific roles, such as the d-alanine and d-glutamic acid in the cross-linking of bacterial peptidoglycan (Weidel and Pelzer, 1964) and some antibiotics (Kuhn and Somerville, 1971).

Amino acids are collectively an important component of the soil organic N pool (Stevenson, 1982) and they are likely, therefore, to be important natural sources of C and N for soil microorganisms. Most studies of amino acid biochemistry in soils have, quite correctly although implicitly, assumed that the l-amino acids are the predominant form. There have been very few studies of d-amino acid metabolism in soils. Waksman (1932)commented on chirality as a factor influencing the rate of amino acid mineralization but, tantalisingly, did not say which isomeric form he thought was mineralized the more rapidly. It has also been suggested that bacterially-produced d-amino acids accumulate during the transformations of organic compounds in soil because of their persistence relative to that of the corresponding l-amino acids (Wagner and Mutatkar, 1968). This suggestion has been supported by little direct evidence, although d-alanine has been used as biomarker for bacteria (Gunnarson and Tunlid, 1986). Hopkins and O'Dowd (1997)have discussed chirality as a factor influencing substrate utilization by soil microbial communities.

Our previous studies have shown that short-term (usually 0–6 h) substrate induced respiratory responses following l-amino acid additions to a range of soils are greater than those following the addition of the corresponding d-amino acids (Hopkins and Ferguson, 1994; Hopkins et al., 1994, Hopkins et al., 1997; O'Dowd et al., 1997). Prior exposure of the soil microbial community to d-amino acids enhanced the subsequent rate of metabolism of the same d-amino acid to a proportionately greater extent than was observed for l-amino acids (Hopkins and Ferguson, 1994). Although the short-term l-amino acid induced respiration rate correlates well with microbial biomass C, as determined by glucose-induced respiration (Hopkins et al., 1994, Hopkins et al., 1997), d-amino acid metabolism does not appear to be related to the activity of the bacterial component of the soil microbial community (O'Dowd et al., 1997), as we had earlier hypothesized (Hopkins and Ferguson, 1994). Using soils with a range of biological and chemical properties, Hopkins et al. (1997)showed that the ratio of the maximum short-term rate of l-amino acid metabolism to that of the corresponding d-amino acid (the l-to-d ratio) declined markedly with both increasing activity per unit of biomass (CO2 production/biomass C; qCO2) and declining soil pH. This has led to the suggestion that increased l-to-d ratio is associated with increasing energetic demand by the soil community, so that the organisms are less fastidious with respect to C sources and will use relatively exotic C sources more readily in the presence of environmentally-imposed stress (Hopkins et al., 1997). This suggestion may be consistent with the recent observations of Staddon et al. (1997)that organic soils showed greater metabolic diversity than mineral soils, and has been supported indirectly by the observation that in soils with similar pH, the l-to-d ratio was much greater for soil with small available inorganic nutrient contents and organic C inputs compared to one with much larger available nutrient contents and C inputs (Hopkins and O'Dowd, 1997).

To date our studies comparing d- and l-amino acid metabolism in soils have been restricted to short-term investigations in which only the immediate physiological response of the soil microbial community has been determined. The objectives of the work presented here were two-fold. First, to compare the patterns with time of CO2 production from soil following addition of d- and l-amino acids and d-glucose over a sufficient period for the pulse of additional CO2 production to have subsided thereby allowing the substrate-induced CO2 production to be estimated. This will show whether the smaller amino acid induced respiration rates for d-amino acids compared to l-amino acids is due to a lag before the onset of C mineralization from d-amino acids. Second, to determine how mineralization of C from the different isomeric forms of amino acids added to soils was affected by repeated prior exposure of the soil microbial community to the amino acid. Given that we have already shown that prior exposure of the soil microbial community to an amino acid increases the subsequent substrate induced respiration rate to a greater extent for d-amino acids than for l-amino acids, this second objective will show whether the prior exposure affects any lag period.

Section snippets

Soils

Soils from the 0–15 cm depth at a forest and an arable site were collected, sieved (<4 mm) in the field-moist state and stored at 4°C for 2 weeks prior to experimentation. These soils were from the same sites as those used by O'Dowd et al. (1997)and they were selected because of their contrasting biological and physico–chemical characteristics (Table 1). Despite marked differences in soil pH and organic matter content, both soils had similar microbial biomass C contents (d-glucose induced

Results

The initial pulses of elevated CO2 evolution from the unamended soils subsided after 160 h in the forest soil and 48 h in the arable soil (Fig. 1). The individual rates of CO2 production following this time were used to estimate the basal respiration rates, which were 41 nmol CO2 g−1 soil h−1 (sd=3.9; n=5) and 21 nmol CO2 g−1 soil h−1 (sd=9.6; n=5) for the forest and the arable soils, respectively. Given that the biomass C contents of the two soils were similar (Table 1), the qCO2 for the forest soil

Discussion

Dissolution of CO2 in the soil solution and its loss in the CO2/HCO3 equilibrium is a possible source of error in the arable soil, because of its neutral pH, which could have caused loss of CO2 from the head-space thereby leading to an apparent lag period. It seems, however, unlikely that this is a major source of error because although the bulk pH(water) was approximately 7.0, much of the soil solution would have been below pH 7.0. In addition, the buffering capacity, which would also be

Acknowledgements

We are grateful to the Departmentof Agriculture for Northern Ireland and the U.K. Biotechnology and Biological Sciences Research Council (Soil-Plant-Microbe Interactions initiative) for their respective contributions to a postgraduate studentship for RWO.

References (17)

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