Sediment organic matter

Source: Loh 2005 (SAMS)

Introduction

There are some data in the literature on LOI using this method, but generally little for fish farms. It might be expected to work very well in fish farms owing to the high flux of labile organic matter to sediments, and it should be possible to use this to evaluate recovery status. This indicator is less complex and time-consuming than total organic carbon (TOC) measurements, which are routinely carried out in EIA studies, yet correlate very poorly with impact, as TOC includes both labile and refractory material.

Method

Loss on ignition is carried out in a temperature-monitored muffle furnace. Approximately 0.5 g of dried, ground and sieved (200um) sediment sample is weighed precisely into a crucible. Crucibles with sediment are then ashed (250°C for 16 hours). When cooled, the crucibles are reweighed. Sediments are then heated to 500°C (Kristensen and Andersen, 1987) for 16 hours (Sutherland, 1998). When cool, they are weighed again.

According to Kristensen (1990), the weight loss in the lower temperature range (130‑280°C) is due to evaporation (i.e. dehydrogenation of hydroxylated aliphatic structures, decarboxylation of acid groups, and generation of low molecular weight volatile compounds); oxidative degradative degradation of aliphatic carbohydrates; random chain‑scission of weak bonds; cross-linking and peroxide formation; formation of compounds of less-ordered structure; cyclization; and formation of carbonaceous char. Plant materials rich in carbohydrates are combusted at the lower temperatures. The weight loss in the high temperature region (280‑520°C) is due to oxidation of aromatic groups (polyphenolic compounds such as lignin, humic substances and kerogens) and char. The percentage weight reduction after both 250°C and 500°C are measures of the sediment labile and refractory organic matter, respectively.

Mook and Hoskin (1982) found significant (p<0.05) weight losses between 200°C and 300°C; hence the mean 250°C is chosen to be used here. Kristensen and Andersen (1987) found that most calcite was combusted between 500‑800°C, hence the 500°C is used in this study, as most organic carbon would be completely separated with this temperature. Hirota and Szyper (1975) also separated organic and inorganic compounds by heating at 500°C.

Rp index was introduced by Kristensen (1990) to characterize the composition of various biogenic organic materials at different decomposition stages. It is defined as, Rp = PII/(PI+PII) where PI is the weight loss in the temperature range 0 °C and 250 °C and PII is the weight loss in the temperature range 250 °C and 500 °C. Calculations of percentage organic matter and Rp values are as follows:

Let Ws = original weight of sediment sample (approximately 0.5 g)

W₀ = weight of crucible and sediment before combustion

W₂₅₀= weight of crucible and sediment alter combustion at 250°C

W₅₀₀= weight of crucible and sediment after combustion at 500°C

PI = weight loss in temperature range 0°C and 250°C = W_O ‑ W₂₅₀

PII = weight loss in temperature range 250°C and 500°C = W₂₅₀ ‑ W₅₀₀

and, Rp= PII/(PI+PII)

% labile OM =  (PI/Ws) x 100%

% refractory OM =  (PII/Ws) x 100%

% total OM = X 100%

or, % total OM = % labile OM + % refractory OM

References

Hirota, J. & J. P. Szyper, 1975. Separation of total particulate carbon into inorganic and organic components. Limnol. Oceanogr. 20: 896–900.

Kristensen, E. 1990. Characterization of biogenic organic matter by stepwise thermogravimetry (STG). Biogeochemistry 9:135–159

Kristensen, E. and F.Ø. Andersen. 1987. Determination of organic carbon in marine sediments: a comparison of two CHN-analyzer methods. J. Exp. Mar. Biol. Ecol. 109: 15-23.

Loh, P.S., 2005. An assessment of the contribution of terrestrial organic matter to total organic matter in sediments in Scottish sea lochs. PhD thesis, UHI Millenium Institute, 350 pp.

Mook, DH and Hoskin CM. 1982. Organic determinations by ignition: caution advised. Est. Coast. Shelf Sci. 15: 697-699.

Sutherland, R. A., 1998. Loss-on-ignition estimates of organic matter and relationships to organic carbon in fluvial bed sediments. Hydrobiologia 389: 153–167.