Soil tillage effects on the contributions of soil and plant carbon pools to CO2 emissions using 13C natural abundance

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Ramnarine, Ravindra

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University of Guelph

Abstract

Soil management practices such as tillage may impact CO2 emissions in agricultural soils due to their effect on the decay of soil organic matter (SOM). This study identified the carbon substrates contributing to CO 2 emissions, by measuring the [delta]13C signatures of the constituents of soil organic carbon and of crop residue carbon inputs. Isotope labelling using the 13C natural abundance technique involved a crop rotation of C3 (soybean - 'Glycine max' (L.) Merr. and winter wheat - 'Triticum aestivum' L.), and C4 species (corn - 'Zea mays' L.) on conventional tillage (CT) and no-tillage (NT) plots. Soils on the research plots (Elora Research Station, Ontario, Canada) are derived from calcareous parent material and it was critical to remove carbonates that can confound measurements of [delta] 13C in soil organic matter. This study utilized both physical and chemical fractionation techniques to characterize soil and plant C pools. Organic carbon content and [delta] 13C of soil (0-10, 10-20, 20-30, and 30-50 cm depths) and plant residues were measured using high temperature combustion techniques coupled with isotope ratio mass spectrometry. Density separation followed by acid hydrolysis was used to separate the SOM pools into light, heavy, hydrolysable and non-hydrolysable fractions. The [delta]13C of the soil microbial biomass carbon (MBC) was determined on soil extracts following chloroform fumigation. After six years of no-tillage, total organic C and N contents for the 0-10 or 0-50 cm depths of CT and NT soils were not significantly different. The light fraction organic matter (LFOM) in the 0-10 cm depth of the NT soils was about 40% higher than in the CT soils. Differences in the isotopic signature of LFOM showed a preservation of newly-derived C in the NT soils compared to the CT soils. The MBC in the CT soils had higher [delta]13C values than the NT soils indicating that the microbes in the CT soils are assimilating a higher proportion of their substrate-C from corn residues. The no-tillage management system resulted in an increase in C content of soils, but only in the 0-10 cm layer. However, the net soil C accumulation was not significantly different for the 0-30 cm depth range for soils managed under NT after 6 years of inception from CT. The [delta] 13C values for the non-hydrolysable fraction C were not significantly different between tillage treatments for all soil depths (about -26.0). The respired CO2 from plots in the fall after coma harvest, showed that CT plots were more enriched (-16.7) than NT plots (-20.6), reflecting that a significant portion of the total CO2 flux was from the decay of corn stover (-12.2) and previous crop litter (-14.0). This study indicates that the carbon substrates contributing to seasonal CO2 fluxes are from the labile pools of SOM and not from the stabilised organic matter fractions. Therefore, no-tillage may be a viable strategy to sequester CO2 from the atmosphere and reduce CO2 emissions, based on its ability to protect and stabilize organic matter against decomposition.

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Keywords

soil tillage, CO2 emissions, soil carbon pools, plant carbon pools, |C13

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