Soil Organic Matter Characterization and CO2 Flux in Long-Term Experiments

Abstract

Increasing global concentration of atmospheric CO2 predating the beginning of industrial era mostly due to fossil fuel combustion, deforestation and changes in land management has necessitated studies that will quantify the relationship between soil organic matter fractions and CO2 efflux from soils. Our studies evaluated the different fractions of soil organic matter (SOM) and CO2 effluxes from known long-term experiments and chemically characterized functional groups of SOM from those plots. These long-term plots, the Old Rotation [c.1896] and the Cullars Rotation [c.1911] are located in Auburn University, Auburn, Alabama. A two-year seasonal study was conducted to characterize fractions of SOM and determined carbon (C) and nitrogen (N) contents in Total, Light and Heavy SOM components. Efflux of CO2 was also measured and SOM was chemically characterized. Total organic carbon (TOC) was determined by combustion. The Light component was determined by multiple extractions through a Millipore extraction set up with a 1.7 g cm-3 sodium iodide (NaI) solution and analyzed for TOC in the light fraction (LF). The passive and slow components that made up the heavy fraction (HF) were calculated by differences between the Total and the Light components. The CO2 efflux was measured using LiCOR 6200 placed on a ten centimeter polyvinyl chloride (PVC) pipes placed in between rows. The chemical characterization was performed through a sequential extraction of fulvic acid (FAs) component of SOM with 0.1 M sodium pyrophosphate. The extracted FAs were freeze-dried and then kept refrigerated until ready to be analyzed. The analysis was done by Fourier Transform Infrared Analyzer (FTIR) to elucidate the functional groups in the samples. Our study indicated that crop rotation and fertilizer application increased organic carbon accumulation in the soils. The Heavy fraction of carbon was ten times greater than the Light fraction. Plots with two to three year crop rotations in the Old Rotation had accumulated more organic matter fractions than the yearly and mono cropping systems. In the case of the chemical characterization, less transmittance intensities were seen for sodium pyrophosphate extractable organic functional groups in the 0-5 cm samples compared to 5-10 cm depth. In generally, treatments and crop rotations contributed to the presence of diverse organic functional groups. The CO2 effluxes for the summer seasons were greater than their corresponding fall seasons for both years of the study. The summer 2010 CO2 effluxes were greater than the summer 2008 effluxes for both locations. Soil CO2 was affected by crop rotation and fertilizer treatments in both locations. In general, CO2 effluxes were affected by factors that are interrelated and interdependent.