Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils

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Abstract

Understanding the dynamics of soil C is key to managing soil organic matter to enhance soil quality and ecosystem functioning, and reduce trace gas emissions from soils. Our objective was to determine the source and turnover of C pools in some agricultural soils in eastern Canada. Soils from five field experiments under continuous maize cropping for 4–37 yr were sampled, and the organic C content and stable C isotope (13C) composition of whole soil and water soluble and microbial biomass fractions determined. The 13C results showed a clear distinction between the water soluble organic C and microbial biomass C, with the water soluble organic C more like the whole soil and the microbial biomass more like the maize residues. A simple linear model was used to explore the relationship among the soil organic constituents and evaluate the turnover of these carbon pools. Even though the water soluble organic C had a higher turnover rate than the microbial biomass C, the proportion of C4-derived C in the biomass was about 2.5 times greater than that in water soluble organic C. Apparently the large amount of native soil C, the small amount of water soluble organic C, and its equilibrium with the native soil C, cause humus to dominate the isotopic composition of water soluble organic C even though the water soluble C is very active. Our results suggest that the quantity, as well as the turnover rate, of soil organic matter constituents that are in equilibrium influence the isotopic composition of such constituents.

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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