CALIBRATION DESIGN TO QUANTIFY CO2 EMISSIONS (SOIL RESPIRATION) DURING DAYTIME INTERVALS

Abstract

Soil emissions account for the second largest flux of carbon dioxide (CO₂) from the Earth’s surface to the atmosphere. This flux depends on the soil organisms and the plants roots and is based on soil temperature, soil moisture and texture. The aim of the present study was to design a method to quantify CO₂ emissions and to validate it using a regression model that helps develop an improved experimental model for the use of a flux measurement chamber. Soil emissions were measured in an area covered with Cynodon dactylon (L.) Pers. (Bermuda) with a closed LICOR LI-8100A dynamic chamber equipped with an infrared gas analyzer. Soil temperature and moisture were also determined using built-in sensors, in four different daytime intervals between sample collections. The intervals were studied to find the best estimation of CO₂ emissions, under different environmental conditions and two observation times: the first one lasted 240 s every 5 min, for 8 h; the second one lasted 90 s with two repetitions every 30, 60 and 120 min for 12 h. The main results showed that there were significant differences in samples evaluated at 8:00 and 18:00; at other times, soil respiration was relatively homogenous. The ideal measurement interval that describes the spatial fluctuation of CO₂ throughout the day and between different periods of solar radiation was 120 min. The validation of the linear regression model determined 50% of the variation, and the rest could be explained with other variables such as grass development and soil physiochemical properties. The model presented an error of ±2 µmol m^-2 s^-1, and it is worth mentioning that it was performed under soil moisture conditions between 0.42 and 0.53 m³ m^-3 and a soil temperature between 17.6 and 30 °C.