Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils
Introduction
A better understanding of how soil organic matter affects soil quality, ecosystem functioning, and atmospheric CO2 concentrations would be gained through knowledge of its dynamics. Soil organic matter comprises different groups of constituents that vary in mass and rate of turnover. Its dynamics reflect the biological activity, soil properties (e.g. texture), and the quantity and quality of plant residues returned to the soil. Humus is the largest, most stable pool of carbon in soil, comprising mostly resistant material (Jenkinson, 1990). The soil microbial biomass is a source and sink of biologically mediated nutrients and is responsible for transforming organic matter and nutrients within soil. Water soluble organic C (WSOC) is not only a C source for microorganisms, but its production is also believed to be microbially mediated (e.g. Christ and David, 1996). McGill et al. (1986) suggested that the flow of C through water soluble components supplies substrate for microbial biomass turnover.
Our objective was to analyze the organic C content and stable C isotope (13C) composition of whole soil and water soluble and microbial biomass fractions to determine the source of this C in some eastern Canadian agricultural soils. A simulation model was used to evaluate the turnover of these carbon pools to get a better understanding of the relationships among these soil organic constituents.
Section snippets
Materials and methods
We sampled five fields in eastern Canada in the spring of 1996. All had been under continuous maize cropping for 4–37 yr and had different textures and soil C contents. Relevant characteristics of the sites and soils are given in Table 1. The maize was grown using practices typical in the region. At each site, soils that had been maintained under grass adjacent to the maize plots were used as reference samples. Ten to fifteen subsamples of the surface layer (about 0–20 cm depth) were collected
Results and discussion
The flush of C induced by fumigation (i.e. fumigated minus nonfumigated samples) and extracted by either 125 mM K2SO4 or ultrapure water was similar (Fig. 1). The actual amounts of C extracted by the water and the salt solution were also about the same in the control soils and the fumigated soils (data not shown). The C flush, expressed in mg kg−1 soil, for the water extraction accounted for 75±10% of the fumigated value, and for the K2SO4 extraction this was 76±8%. These findings suggest that
Acknowledgements
Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada and the Program on Energy Research and Development/Climate Change Program of Natural Resources Canada. We thank Dr. Ron Beyaert for allowing us to sample his experimental plots.
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