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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Maize Residue Decomposition Measurement Using Soil Surface Carbon Dioxide Fluxes and Natural Abundance of Carbon-13

^a Dep. of Biological Sciences, Univ. of Lethbridge, 4401 University Dr., Lethbridge, AB, Canada, T1K 3M4

rochettep{at}em.agr.ca

The decomposition rate of crop residues in soils directly impacts organic matter content and nutrient cycling. We hypothesized that natural abundance ¹³C analyses could be used with soil CO₂ flux measurements to quantify the short-term decomposition rates of maize (Zea mays L.) residues under undisturbed field conditions. For this purpose, maize was grown in a sandy loam (Umbric Dystrochrept) that developed under C3 vegetation. Residues were returned to the field at the end of the growing season. During the following snowfree period (May to November), the maize residue decomposition rate was calculated for plots that were either under no-till or moldboard plowed, using the C isotope ratio (¹³C/¹²C) of the soil CO₂, the C isotope ratio of the plant and soil substrates, and the soil respiration rate. The incorporation of residue-derived C into the soil microbial biomass was also evaluated. Maize residue decomposition increased the C isotope ratio of the soil CO₂ by 2 to 7 relative to unamended control plots. Decomposition rates peaked in June (2–3 g C m^-2 d^-1) and were low at both the beginning and end of the growing season (<0.5 g C m^-2 d^-1). For a given soil temperature, the decomposition was more active early than late in the season because of decreased substrate availability as decomposition proceeded. The decomposition rate of maize-derived C correlated with the fraction of the microbial biomass derived from maize residues. This active pool represented 9% of microbial biomass and showed a high level of specific activity. The total maize residue-C losses during the study corresponded with 35% of the added residue C under no-till plots and 40% with moldboard plowing. Natural abundance ¹³C analyses may be successfully used with respiration measurements to quantify crop residue decomposition rates under undisturbed field conditions.

Abbreviations: DM, dry matter • MBC, microbial biomass C

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