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Title: TILLAGE-INDUCED GAS FLUXES: COMPARISON OF METEOROLOGICAL AND LARGE CHAMBER TECHNIQUES

Author
item DENMEAD, O - CSIRO, AUSTRALIA
item Reicosky, Donald

Submitted to: International Soil Tillage Research Organization Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/18/2003
Publication Date: 7/18/2003
Citation: Denmead, O.T., Reicosky, D.C. 2003. Tillage-induced gas fluxes: Comparison of meteorological and large chamber techniques. In: Proceedings of the International Soil Tillage Research Organization, July 14-18, 2003, Brisbane, Australia. p. 357-363.

Interpretive Summary: Recent studies involving portable chambers for measuring gas loss after tillage methods indicate major gaseous loss of carbon immediately after intensive tillage. "Chamber effects" may occur due to soil variability, chamber size and placement, and the level of turbulence in the chamber. We compared chamber measurements of soil CO2 flux after tillage with those calculated simultaneously by novel, non-disturbing micrometeorological techniques suitable for small treated areas. There was good quantitative agreement between all three micrometeorological methods for both CO2 and water vapor fluxes, but the agreement between them and the chamber was variable, depending on the external wind speed. All methods agreed at a moderate wind speed, but the chambers gave higher fluxes when the wind speed was less than that value and lower fluxes above that value. Interestingly, the "wind" speed within the chamber was a constant yielding the inference that wind speed has a large effect on gas fluxes from the tilled soil, particularly in the early stages of the emission. The tillage-induced change in soil properties led to short-term CO2 losses that were higher than the undisturbed soil. These results are significant to farmers and policy makers in that intensive tillage results in substantial short-term gaseous losses of CO2. This information will assist scientists and engineers in developing improved tillage methods to minimize the gaseous loss and to improve soil carbon management. Farmers can develop and utilize new management techniques for enhancing soil carbon by increasing the quantity and quality of crop residues and by changing the type and intensity of tillage. This information will be of direct benefit to the farmers to enable them to maintain crop production with minimal impact on air quality and the environment.

Technical Abstract: CO2 fluxes from soils following tillage are usually large and decline rapidly with time. Establishing the time-course of the CO2 flux in the early stages of the emission is important, but conventional micrometeorological techniques require large treated areas that take time to create. Hence chambers are often used to measure the flux. "Chamber effects" may occur due to soil variability, chamber size and placement, and the level of turbulence in the chamber. We compared chamber measurements of soil CO2 flux after tillage with those calculated simultaneously by novel, non-disturbing micrometeorological techniques suitable for small treated areas. The experimental area was a wheat field with some crop residue; 12 or 16 furrows were plowed cross-wind in a strip 50m long and 5.6 or 7.3m wide. The chamber was a large (3.25 m**3), tractor-mounted, dynamic, closed chamber employing rapid mixing for fast response. Increases in both CO2 and water vapor concentrations were measured with a Licor 6262 infrared gas analyzer. Flux measurements required only one min. Three meteorological techniques were employed: one using a line-source solution, one using a solution for a semi-infinite strip, and one using a backward Lagrangian stochastic (bLs) model. Inputs were measurements of wind speed and gas concentrations on upwind and downwind edges of the treated area at 0.2m above the surface. Chamber and micrometeorological measurements were made for 1h before and 2h after plowing. There was good quantitative agreement between all three micrometeorological methods for both CO2 and water vapor fluxes, but the agreement between them and the chamber was variable, depending on the wind speed. All methods agreed at a wind speed (at 0.25m) of 2.27m s**-1, but the chambers gave higher fluxes when the wind speed was less than that value and higher fluxes above it. Interestingly, the "wind" speed within the chamber is a constant 2.2m s**-1. The clear inference is that wind speed has a large effect on gas fluxes from the tilled soil, particularly in the early stages of the emission. It is suggested that the micrometeorological techniques employed in this study provide attractive alternatives to chambers or could be used to calibrate "chamber effects."