DETERMINING CARBON DIOXIDE (CO2) EVOLVED DURING THE ECOMPOSITION OF ORGANIC MATTER IN THE SOIL BY FLOW INJECTION ANALYSIS (FIA)

Suwardi

Staf Pengajar Jurusan Tanah, Fakultas Pertanian, Institut Pertanian Bogor,

Jl. Meranti Kampus Darmaga, Bogor. Telp. 0251-627792, Fax. 629358,

Email: soilipb@indo.net.id

ABSTRACT

A laboratory method to determine the carbon dioxide (CO₂) evolved during the decomposition of organic matter in the soil by Flow Injection Analysis (FIA) was observed. The objective of this experiment was to evaluate the accurance of FIA to analize CO₂ evolved from organic matter. The CO₂ gas was trapped by NaOH solution in an original designed incubation flask and its concentration was measused by FIA at 590 nm. A calibration curve was established using standard solutions of 0, 40, 80, 120, 160, and 200 mg CO₂ L^-1 prepared from NaHCO₃. The quadratic form y= - 0.0007 x² + 0.983 x + 17.4 was obtained where y is the absorbance and x is the concentration of CO₂ with the correlation coefficient (r) of 0.9999. There was a rapid increase in CO₂ (more than 3.9 mg L^-1 min^-1) after the sample solutions made contact with the atmosphere which compelled the isolation of the samples and the atmosphere. Using FIA, the determination time of one sample was 30 seconds, faster than the other instruments nowadays available. The volume of solution needed for measuring the CO₂ was only 100 mL possible to measure a small volume of solution contain of CO₂.

For application of the above method, half gram of 3 kinds of chicken manure (CM-1, CM-2, and CM-3), which were subjected to the differences in their C/N ratio (11.5, 8.54, and 4.79, respectively), were applied to 50 g of soil. The rate of CO₂ evolved from the soil applied by chicken manure was greatest in the first three days, then decreased gradually until the end of the 30-day incubation period. The cumulative of CO₂ during 30-day incubation was 2510, 4010, 6080, and 1460 mg kg^-1 in the soil applied by CM-1, CM-2, CM-3, and control, respectively. The cumulative of CO₂ correlated with the C/N ratio of chicken manure where the cumulative of CO₂ decreased as the C/N ratio increased.

It was concluded that although there are some constraints in relation to the increasing CO₂ concentration, FIA is considerable as a convenient instrument for determining CO₂ evolved from the soil. The production of CO₂ is considerable for estimating the nitrogen mineralization since the relation between the amount of CO₂ and nitrogen mineralization during incubation has been given off.

Key words: carbon dioxide (CO₂), C/N ratio, decomposition of organic matter, Flow Injection Analysis (FIA), trapping reagent