D'ACQUI Luigi P. (1), SANTI Carolina A. (1), SPARVOLI Enzo (1), CHURCHMAN, Jock G. (2), RISTORI Giuseppe G. (1)

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1 Scientific registration n : 2099 Symposium n : 4 Presentation: poster Controlled removal of organic matter from undisturbed aggregates by Low-Temperature Ashing for studies on soil structure stability Destruction contrôlée de la matière organique d'agrégats non perturbés par la technique de minéralisation basse température. Applications à l'étude de la stabilité de la structure D'ACQUI Luigi P. (1), SANTI Carolina A. (1), SPARVOLI Enzo (1), CHURCHMAN, Jock G. (2), RISTORI Giuseppe G. (1) (1) Istituto per la Genesi e l'ecologia del Suolo CNR - P.le delle Cascine n Firenze, Italia. (2) Land and Water, CSIRO, Private Bag N 2, Glen Osmond, S.A. 5064, Australia. Introduction Recent studies have proposed Low-Temperature Ashing (LTA) by oxygen plasma for the removal of organic matter (OM) from soil samples with minimal or no damage to mineral constituents (Marcoen, 1975; Sullivan et al., 1987) and soil microfabric (Price and Jenkins, 1980). Studies on the chemical mechanisms governing LTA of soil OM (De Kimpe and Schnitzer, 1990), have suggested that the oxidation of humic and fulvic acids is a homogeneous surface reaction by which the molecules of humic substances are stripped off, layer by layer, without altering or damaging the residual layer. However, little is known about the dynamics of organic matter removal by LTA from soil aggregates and the effect of this removal on soil structure stability (D Acqui et al., 1998). In this work the LTA procedure was applied to remove OM from undisturbed soil aggregates to investigate: i) the dynamics of the plasma oxidation processes; and ii) the effect of the partial removal of OM on the structural stability of aggregates. Materials and methods Samples The analyses were performed on air dried undisturbed aggregates (3-4 mm in size) from the surface horizon (5-10 cm) of different type of natural soils: an Inceptisol under forest (Veneto, Italy), an Alfisol under savannah (Mazowe, Zimbabwe) and a Vertisol under pasture (Sardinia, Italy). Some physico-chemical characteristics of the soil studied are reported in Table 1. 1

2 Samples clay silt sand ph (H2O) C N Fe DCB Al DCB g Kg -1 % % % % Vertisol Alfisol Inceptiso l Table 1. Some physico-chemical characteristics. The samples were submitted to LTA treatment with different time exposures. Total C content was measured at each step of the experiment on the bulk sample, at the surface and in the core of the aggregates. The sampling on individual aggregates for the surface and core parts have been conducted by accurate lancet peeling. The effect of the oxidation of OM on soil structure was studied using a test of water aggregates stability based on the application of an osmotic stress in order to avoid the mechanical disturbance. Low temperature ashing The low temperature ashing procedure was performed with an apparatus assembled in our laboratory according to Favia et al. (1996). The plasma reactor is shown schematically in figure 1. The aggregates were placed in the plasma reactor and evacuated to 0.3 kpa under an oxygen flow rate of 20mL/min. To generate the oxygen plasma the radiofrequency generator was adjusted to MHz with a power input of 100 W and a reflected power of 5 W. In these conditions the surface temperature is maintained below 100 C (Thomas and Hollahan, 1974). The aggregates (about 5 g), placed in separate boats, were removed after ashing for different times: a) for studying the dynamics of OM removal, the treatment was continued until a constant C content was attained; b) for the structural stability tests, the duration of treatment was continued until a removal of about 50% of the original C was attained, except for the Vertisol (about 30%), due to the very low rate of removal. The samples were periodically mixed during the ashing. 2

3 RF-generator and matching network pumping system and pressure gauge plasma RF external reactor electrode sample grounded internal oxygen electrode substrate inlet holder Figure 1. Schematics of the plasma reactor. (D Acqui et al., 1998) Chemical analyses The organic carbon content of aggregates was determined, at each step of the experiment, by dry combustion with high temperature carbon analyser Carlo Erba NA1500. The others physico-chemical analyses were carried out according to ASA, SSSA (1986, 1996). Structural stability of aggregates The structural stability of aggregates was evaluated by using a dispersibility soil test by dialysis to avoid any mechanical disturbance (Churchman and Weissmann, 1995) on samples treated by LTA for different time. A solution of 1M NaCl was added to 1 g of air dried soil aggregates placed in dialysis bags. The samples were then dialysed against deionised water in order to remove enough electrolyte from the samples. When the electrolyte concentration was reduced, inducing an osmotic stress, the samples started to disperse. The dispersion was allowed to occur for 20 days. This technique provides a visual demonstration of aggregates dispersion due to the diffusion of finest particles (see figure 2) and also enables the evaluation of the extent of these phenomena, by measuring i) the distance of diffusion above the original surface of the aggregates inside the dialysis bag or ii) by weighing the amount of dispersed particles. 3

4 Figure 2. An example of the stability test by dialysis on Vertisol aggregates after 0, 70, 140 hours of exposure time to oxygen plasma. Results and discussion Dynamics of C removal Figure 3 shows an example of the dynamics of C removal by plasma oxidation from bulk, core and surface of the Sardinia Vertisol aggregates. The others soil samples, exhibiting a similar tendency, are not reported. The removal of C increased progressively until a plateau was attained. The highest and the lowest C content have been found respectively in the core and on the surface of the aggregates, whereas intermediate values have been measured on the bulk samples. The residual C content of bulk aggregates reached a plateau after about 250 hours. The core aggregates needed 300 hours more to reach a similar C content. This behaviour suggests that the oxidation of OM proceeds with different intensity from the surface to inside of the aggregates until the resulting residual C content is almost uniform throughout the sample. Indeed, in the external layers of the aggregate free electrons, excited atoms and molecules are more abundant than in the internal layers because the OM oxidation process reduces the diffusion of these activated chemical species. This also explain why more effective oxidation occurs at the aggregates surface (Figure 3) where the OM is in direct contact with the oxygen plasma 4

5 5 Vertisol C % bulk surface core time (hours) Figure 3. Dynamics of C removal by LTA from bulk, core and surface of the aggregates (Sardinia, Vertisol) Therefore, the LTA oxidation of OM seems in general to be controlled by its physical accessibility. This is confirmed by the dynamics of the residual C measured after grinding of the bulk aggregates that have been submitted to treatment for 500 hours (see figure 4). The curve of C removal reached a minimum in a relatively short time (50 hours) and the C content is reduced by about 97%. The grinding has produced a larger surface of contact between the soil and the oxygen plasma favouring a further oxidation. The dynamics of OM removal is also related to the soil type. For both the Alfisol and the Inceptisol 48 hours of LTA treatment were sufficient to remove more than the 50 % of C (Table 2), whereas for the Vertisol, only 34 % of C was removed after 140 hours. This difference could be attributed to high microporosity of Vertisols that reduces the rate of diffusion of activated chemical species of the oxygen plasma. 5

6 5 Vertisol 4 aggregates after grinding C % time (hours) Figure 4. Dynamics of C removal by LTA from bulk aggregates and after grinding the aggregates after 500 hours of treatment (Sardinia, Vertisol) Aggregates structural stability test The effect of OM removal by LTA is shown by the dialysis test. Table 2 reports the amounts of the dispersed material resulting from the structural stability test of soils aggregates treated for different ashing times, and the corresponding residual C content. In general the dispersed material increased with the reduction of C content confirming the important role of OM on structural stability in natural soils. The data shows that the Inceptisol maintained a good stability, even though more than 50% of C had been removed, whereas the Vertisol, in which about 34 % of C was removed, showed a significant increase of dispersed material. In the Alfisol, extensive particles dispersion did not occur in spite of a relatively low residual C content. This suggests a contribution of inorganic cements in the stabilization of soil structure. The high content of DCB extractable Fe and Al (Table 1) of the Alfisol confirms this hypothesis. The behaviour observed in the Vertisol and the Inceptisol, which both have low contents of Fe and Al oxides, indicates that not only the amount, but also the quality of OM plays an important role in stabilization of the soil structure. 6

7 Samples Vertisol (LTA treat.) Dispersed material % C % 0 h hs hs Alfisol (LTA treat.) 0 h hs hs Inceptisol (LTA treat.) 0 h hs hs Table 2. Amount of dispersed material measured in the column of the stability tests of the aggregates and the respective residual C contents before and after different exposures to LTA treatment. Conclusions The dynamics of OM oxidation by LTA in undisturbed soil aggregates was controlled by the nature of particles and their arrangement, i.e. soil structure. Therefore, is possible to regulate the extent of the OM removal by changing the physical accessibility of particles within aggregates. The results of aggregate stability tests of the various samples with different extent of OM removal enables i) to distinguish between the specific contribution of organic and inorganic cements on maintenance of soil structure and ii) to better understand the relative importance of the quantity and quality of OM. References ASA, SSSA. 1986, Method of Soil Analysis part Second Edition. Eds. A.L., Page, R.H. Miller, and D.R. Keeney. N 9, series Agronomy. ASA, SSSA. Madison, Wisc. Churchman, G.J. and Weissmann, D.A Separation of sub-micron particles from soil and sediment without mechanical disturbance. Clay and Clay Minerals 43: D'Acqui, L.P., Churchman, G.J., Janik, L.J., Ristori, G.G., Weissmann, D.A., Effect of Organic Matter Removal by Low-Temperature Ashing on Dispersion of Undisturbed Aggregates from a Tropical Crusting Soil. Submitted to Geoderma. De Kimpe, C.R. and Schnitzer, M Low-temperature ashing of humic and fulvic acid. Soil Sci. Soc. Am. J. 54:

8 Favia, P., Stendardo, M. and d'agostini, R Selective grafting of amine on polyethylene by means of NH 3 -H 2 RF glow discharges. Plasmas and Polymers, Vol. 1 N 2. Marcoen, J.M. and F. Delecour Possibilities for using low temperature ashing in soil science. Pedologie 26: Price, F. R. and Jenkins D. A., Removal of resin from standard soil thin-sections by low temperature ashing as a means of following transmitted optical by scanning electron microscopy. Clay Minerals 15: Sullivan, L.A. and A.J. Koppi In situ soil organic matter studies using scanning electron microscopy and low temperature ashing. Geoderma 40: Thomas, R. S and Hollahan, J.R., Use of chemically-reactive gas plasmas in preparing specimens for scanning electron microscopy and electron probe microanalysis. Scanning Electron Microscopy. 1: Keywords : organic matter, low temperature ashing, aggregates, soil structure stability Mots clés : matière organique, minéralisation basse température, agrégats, stabilité de la structure 8