JOURNAL OF THE WORLD AQUACULTURE SOCIETY

Vol. 25, No. 2 June, 1994

Comparison of Different Methods for Measuring Organic Carbon Concentrations in Pond Bottom Soils MOHAMMAD AYUB’ AND CLAUDE E. BOYD~

Department of Fisheries and Allied Aquacultures, Alabama Agricultural Experiment Station, Auburn University, Alabama 36849 USA

There is considerable interest in measur- ing organic matter concentrations in pond bottom soils. Recent work by Avnimelech and Zohar (1986) showed that the devel- opment of anaerobic micro-environments in pond bottom soils resulted in growth re- tardation in fish. Because the accumulation of organic matter in pond soils favors an- aerobic conditions, many commercial aquaculturists, and especially shrimp farm- ers, are beginning to monitor organic matter concentrations in pond soils and trying to manage ponds in a manner that minimizes organic matter accumulation (Peterson and Daniels 1992).

Contacts with researchers and commer- cial aquaculturists suggest that four meth- ods are being used to determine organic matter concentrations in pond soils: the Walkley-Black method of sulfuric acid-po- tassium dichromate oxidation without ex- ternal heating; the Mebius modification of the Walkley-Black method with heating; dry combustion in an induction furnace-type carbon analyzer; and dry ashing in a muffle furnace. According to Nelson and Sommers (1 982), the carbon analyzer technique em- ploys high temperature and converts all soil carbon to carbon dioxide for detection. The carbon analyzer does not distinguish inor- ganic C from organic C. The Walkley-Black method does not completely oxidize all of the organic matter, and the percentage ox- idation may vary considerably among sam- ples. The modified Mebius method pro-

’ Present address: 58 Umar Din Road, Wassanpura,

* Corresponding author. Lahore-54900, Pakistan.

vides more rigorous oxidation of organic matter than the Walkley-Black technique because external heat is applied. However, the modified Mebius procedure is more dif- ficult to perform. Some clays retain consid- erable water even after drying to constant weight at 105 C. This water will be driven off upon ignition for organic matter analysis and cause high results. Water loss during ignition can be minimized if ignition tem- perature does not exceed 350 C (Jackson 1958).

This study was conducted to provide comparison of the four methods of soil or- ganic matter analysis. Such information is needed by those deciding on an appropriate technique for analyzing the organic matter content of pond soils.

A series of “prepared” samples was made by drying and pulverizing peat moss, 32% protein channel catfish feed, cow manure, and oat straw hay and by mixing these ma- terials at 0, 5, 10, 20, 40, 60, 80, and 120 g/kg in dry bottom soil from a pond near Auburn, Alabama. Samples were main- tained for 4 wk in moist condition in a greenhouse. Moisture content of samples was not measured, but water was applied daily with a mist-sprayer until soil surfaces were moist. Soils were never saturated with water. Samples of pond soils were collected from Auburn, Alabama; Greensboro, Ala- bama; Dawson, Georgia; Butare, Rwanda; and Comayagua, Honduras. Soil pH values were measured in 1 : 1 dry soil : distilled wa- ter slurries with a glass electrode. Soil tex- ture was determined from particle size anal- ysis data with a soil triangle (Boyd and Tucker 1992). Samples from Alabama, Georgia, and Rwanda were acidic (pH 5.0-

0 Copyripht by the World Aquaculture Society 1994

322

MEASURING ORGANIC CARBON IN POND SOILS 323

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FIGURE 1. Relationships between organic C analyses by the modifid Mebius technique and three other methods of organic C determination. All samples were synthetic, prepared by mixing organic matter in a single soil (see text). The dashed lines represent perfect agreement between the modifid Mebius technique and the procedures.

5 . 3 , sandy clay and clay loam soils. Sam- ples from Greensboro, Alabama, were slightly basic (pH 7.2-7.4) clay soils, while soils from Honduras were slightly acidic (PH 6.5-6.8) clays. Brackishwater shrimp pond soil from Ecuador and Thailand were slight- ly basic (pH 7.4-7.7), heavy clay soils. All samples also were dried at 60 C and pul- verized with a soil crusher (Custom Labo- ratory Equipment, Orange City, Florida) to pass a 60 mesh screen (<0.24 mm).

Before analysis, samples were oven dried at 105 C and stored in a desiccator. For dry ashing, samples of ~5 g were weighed to five decimal places and placed in tared cru- cibles. Samples were heated at 350 C for 8 h in a muffle furnace, cooled in a desiccator, and reweighed. The weight loss was as- sumed to be organic matter with 58% C (Nelson and Sommers 1982). The modified Mebius and Walkley-Black methods were conducted as described by Nelson and Som- mers (1982). The induction furnace tech- nique employed a total carbon analyzer (Model EC-12; Leco Corporation, St. Jo-

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FIGURE 2. Relationships between organic C analyses by the modijied Mebius technique and three other methods of organic C determination. AN samples are from brackish waterponds. The dashed lines represent perfect agreement between the modifid Mebius tech- nique and the procedures.

seph, Michigan). This instrument provides dry combustion of organic matter at 1,500 C. The amount of carbon dioxide released by combustion of organic matter and car- bonate minerals is measured with an infra- red detector, and the percentage total car- bon in the sample is displayed digitally. Soil samples from Greensboro, Alabama, had calcium carbonate concentrations of 5.1- 30.4Yo (0.61-4.2Vo carbonate C) as mea- sured from the amount of carbon dioxide evolved following treatment of samples with hydrochloric acid (Jackson 1958). Total C concentrations measured with the carbon analyzer were corrected to organic C con- centrations by subtracting carbonate C.

Because the modified Mebius method is widely used for organic matter analysis (Nelson and Sommers 1982), it was selected as the independent variable for comparison with other organic C procedures. Simple, linear regression analyses of data for the “prepared” samples and samples from brackishwater shrimp ponds revealed coef- ficients of determination greater than 0.94 (P < 0.01) for all comparisons (Figs. 1, 2). Slopes of regression lines ranged from 0.84 1

3 24 A W B AND BOYD

to 1.018, and Y-intercepts were between -0.340 and 0.289. Thus, as expected from previous studies (Nelson and Sommers 1982), the modified Mebius method was closely correlated with other procedures. Percentages of organic C obtained by the modified Mebius method and by the other techniques also were of similar magnitude over the entire range of organic C concen- trations examined.

Agreement between modified Mebius method and other procedures were not as good for freshwater fish pond samples (Fig. 3) as for "prepared" samples and brackish- water pond samples. The highest R2 of 0.865 was for the dry ash. The Y-intercepts were between -0.031 and 0.199, but slopes ranged from 0.662 to 0.720. Therefore, there were relatively large differences in magni- tudes of organic C found by the modified Mebius method and the other three analyt- ical techniques.

The higher degree of agreement between the modified Mebius method and the other procedures for "prepared" and brackish- water samples than for the freshwater sam- ples was thought to be related to the nature of the soil organic matter. The "prepared" samples consisted of a single soil with low organic matter content to which had been added different quantities of four kinds of organic matter. The ponds were relatively new (3-5 yr old) and the organic matter con- sisted primarily of remains of uneaten feed, dead plankton, and shrimp excrement. Freshwater fish ponds were much more variable. Ponds were 5 to 40 years old, soils were of different texture classes, some soils contained appreciable amounts of carbon- ates, and several levels of pond manage- ment (unfertilized, manured, chemically fertilized, and supplemental feeding) were represented. Thus, the greater variation in the freshwater samples is probably respon- sible for the poorer agreement obtained be- tween the modified Mebius method and the other procedures.

The precision of each procedure was eval- uated by making seven replicate determi-

0 6 0 6 UOMFIED MEBlUS (WC) MODIFIED MEBIUS (*C)

FIGURE 3. Relationships between organic C analyses by the modijied Mebius technique and three other methods of organic C determination. AN samples are from freshwater ponds. The dashed lines represent perfect agreement between the modijied Mebius tech- nique and the procedures.

nations on three samples. Precision was good; averages for coefficients of variation were: modified Mebius, 7.87%; Walkley- Black, 9.33%; dry ashing, 10.73Oh; carbon analyzer, 4.80%. Three samples were spiked with potassium acid phthalate to increase organic C by 25% of its original value. Re- coveries of organic C spikes by the different methods were: modified Mebius, 108.1 %; Walkley-Black, 103.0%; dry ash, 97.0%; carbon analyzer, 94.2%. Thus, the Walkley- Black and dry ash methods actually gave better spike recovery (an estimate of accu- racy) than the two more rigorous tech- niques.

The carbon analyzer technique is not suit- able for measurements of organic C unless samples are known not to contain carbon- ates or unless carbonate C is determined and subtracted from the total C value ob- tained by the analyzer. The modified Me- bius method and Walkley-Black procedure require considerable analytical expertise and employ hazardous chemicals. The dry ash- ing method is useful provided water-free samples (dry at 105 C and desiccate) are

MEASURING ORGANIC CARBON IN POND SOILS 325

used and the ignition temperature does not exceed 350 C. The dry ashing method is simple, and it provides results which are comparable with those obtained by more complicated techniques. Dry ashing is a suitable method of soil organic matter anal- ysis for application in pond aquaculture.

Literature Cited Avnimelech, Y. and G. Zohar. 1986. The effect of

local anaerobic conditions on growth retardation in aquaculture systems. Aquaculture 58: 167-1 74.

Boyd, C. E. and C. S. Tucker. 1992. Water quality and pond soil analyses for aquaculture. Alabama

Agricultural Experiment Station, Auburn Univer- sity, Alabama, USA.

Jackson, M. L. 1958. Soil chemical analysis. Pren- tice-Hall, Englewood Cliffs, New Jersey, USA.

Nelson, D. W. and L. E. Sommers. 1982. Total car- bon, organic carbon, and organic matter. Pages 539-579 in A. L. Page, R. H. Miller, and D. R. Keeney, editors. Methods of soil analysis, part 2. Chemical and microbiological properties. Amer- ican Society of Agronomy, Madison, Wisconsin, USA.

Peterson, J. and H. Daniels. 1992. Shrimp industry perspectives on soil and sediment management. Pages 182-186 in J. Wyban, editor. Proceedings of the special session on shrimp farming. World Aquaculture Society, Baton Rouge, Louisiana, USA.