Clay-associated organic matter in kaolinitic and smectitic soils

The primary source of soil organic matter is plant debris of all kinds, such as dead roots, leaves and branches that enter into the soil and are then biologically decomposed at variable rates. Organic matter has many different important functions on a local and global scale. Soil organic matter is an important source of plant nutrients: when microbes mineralize organic matter, CO 2 and nutrients such as N, P, S, and Ca are released. SOM increases the capacity to adsorb water, and it increases the structural stability of a soil e.g. by forming aggregates with mineral components. Furthermore, forms a major source/sink for atmospheric carbon.The effect of clay mineralogy on organic matter dynamics has not been studied before. My objective was to study the long-term effect of the structurally different minerals kaolinite and smectite on the decomposition of SOM in natural ecosystems.Smectites are expandible 2:1 layer silicate minerals. The crystal structure of smectite is composed of two tetrahedral silicon-oxide sheets sandwiching one octahedral aluminum-hydroxide sheet. Smectites have a high permanent surface charge, a large surface area, and a high cation exchange capacity (CEC). Kaolinites are 1:1 layer structured alumino-silicates with a low surface area and a low CEC.I studied four aspects: the amount, extractability (as a measure of the binding mechanisms), chemical composition, and mean residence time of soil organic matter in kaolinite and smectite-dominated soils originating from savanna systems in various countries. We employed a C/N analyzer to measure the amount of carbon the clay-size fraction, a sequential extraction method with NaOH and Na 4 P 2 O 7 to test extractability, solid state CPMAS 13C NMR and Pyrolysis-GC/MS to characterize the chemical composition, and by measuring the natural 14C activity to determine the mean residence time.For this study, two sets of soil samples were used. The first set was selected from the collection of the International Soil Reference and Information Center (ISRIC, Wageningen, The Netherlands). It contained 12 soils from seven different countries. Half of the soils had clay-size fractions dominated by kaolinite, the other half were dominated by smectite. The second set of samples was collected in April 1998 by Peter Buurman and myself, west of Montepuez in northern Mozambique. It contained 10 soils, four of which had clay-size fractions dominated by smectite, and six of which were kaolinitic. All soils used were under native savanna vegetation.As most clay minerals are present in the clay-size fraction, I first separated the clay-size fraction of each soil. All organic matter present in the clay-size fraction was defined as clay-associated SOM. In the ISRIC-soils (chapters 2 and 3), only this clay-associated SOM was studied. A study by scanning electron microscopy indicated that this fraction also included 'free' plant remains. Therefore, in the experiments in chapters 4 and 5, in which I studied soils from Mozambique, sodium iodide was used before the extraction to separate the 'free' (light) organic matter in the clay-size fractions from the mineral-complexed (heavy) SOM, defined as clay-bound SOM.The research in chapter 2 aimed at determining the effect of clay mineralogy on the amount and composition of organic matter that is associated with the mineral surface. 13C NMR and Py-GC/MS were used to chemically characterize the SOM associated with kaolinite and smectite in clay-size fractions from the ISRIC-soils. The total content of carbon in the clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggested that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. The organic matter in the clay-size fraction was first extracted by NaOH and thereafter by Na 4 P 2 O 7 . About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicated that kaolinite-associated SOM is relatively rich in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. The results suggest that different clay minerals use different mechanisms to bind SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominant in the soil.In chapter 3, the 14C activity of clay-associated organic matter of the ISRIC soils was analyzed. The soils originated from natural savanna systems. Assuming that carbon inputs and outputs are in equilibrium in such soils, the 14C age was taken as mean residence time of the organic carbon. The 14C activity was corrected for the Suess effect, the Bomb effect and difference between date of sampling and date of 14C measurement. Kaolinite-associated soil organic matter had a fast turnover (360 years on average). Smectite-associated soil organic matter had a relatively slow turnover, with an average mean residence time for the whole clay-size fraction of 1100 years. Differences in turnover times between organic matter associated to kaolinite and smectite were significant. Multiple linear regression indicated that clay mineralogy, parameterized by specific surface area and effective cation exchange capacity of the mineral phase of the clay-size fraction (ECEC min ), are the main factors explaining differences in the mean residence time of the extracted soil organic matter.In chapter 4, two kaolinite and two smectite-dominated soils from a native savannah in Mozambique were studied in order to determine the difference in amount and molecular composition of kaolinite- and smectite-bound organic matter in one climatic area. The amount of soil organic matter (SOM) bound was independent of clay mineralogy. Furthermore, the amount of carbon in the light fraction was negligible. The extractability by NaOH and Na 4 P 2 O 7 (sequentially) of the clay-bound organic matter showed no significant difference between the clays: 50% of the clay-bound SOM was extracted by NaOH and thereafter about 15% by Na 4 P 2 O 7 . The extracted SOM of all four soils was dominated by polysaccharides. The smectitic soils seem to contain slightly more aliphatic components than the kaolinitic soils. Aromaticity varied among the four soils.The seemingly contrasting results regarding the extractability of smectite-associated SOM between chapters 2 and 4 may be related to the ECEC min of the clay-size fraction. The smectitic soils used from the ISRIC had an average ECEC min of 67±22 cmol/kg, while the smectitic soil from Mozambique have an average ECEC min of 29 cmol/kg. The relatively low ECEC of the Mozambican soils may have enhanced extractability by NaOH.In chapter 5, the 14C activity of organic matter was analyzed in the whole and heavy clay-size fraction of six kaolinite- and four smectite-dominated soils from N'Ropa, in northern Mozambique. For both kaolinite- and smectite-dominated soils, the organic matter in the whole and heavy clay-size fraction and extracts had a fast turnover (400 to 500 years on average). The mean residence time of kaolinite-bound organic matter did not differ significantly from that of smectite-bound organic matter. These results seem to disagree with those of chapter 3. Multiple linear regression of the data from Mozambique and the ISRIC samples, however, agree with the trends found in chapter 3 and indicate that the variation in 14C activity is not governed by mineralogy directly, but by the ECEC min , which is related to mineralogy. Relatively low ECEC min 's of the investigated smectitic soils from Mozambique bridge the gap in 14C activity between the two minerals found in chapter 2.In short, when combining all data, neither the amount of clay-associated organic matter, nor the amount of clay-bound organic matter differs between kaolinite and smectite (chapters 2 and 4). The extractability of the organic matter is different for the two clay minerals: smectite-associated SOM is less extractable by NaOH than kaolinite-associated SOM, but it is relatively well extractable by Na 4 P 2 O 7 . Multiple regressions of the combined data of the soils from the ISRIC and from Mozambique show that the surface charge of the clay minerals, parameterized by the ECEC min , explains 76% of the variation in fraction of carbon extracted from the clay-size fraction by Na 4 P 2 O 7 . This indicates that the charge of the mineral phase influences the extractability much more than the clay mineralogy as such. The chemical composition of the organic matter associated with the two clay minerals varies slightly: kaolinite-associated SOM seems to have more O-alkyl carbon (from polysaccharides) than smectite-associated SOM. A possible explanation for this difference in composition is that bonds formed via the hydroxyl groups of kaolinite are probably relatively weak. This limits the 'SOM retention capacity' of kaolinite and enables relative fast decomposition. This results in a relatively short mean residence time and thus the relatively large presence of 'fresh' organic matter containing relatively many components still in an early stage of decomposition, such as sugars. The 14C activity of the organic matter in the clay-size fraction of kaolinite-dominated soils does not differ significantly from the organic matter in the clay-size fraction of smectite-dominated soils. The factor best explaining the variance in 14C activity for the combined data was the ECEC min (42.6%). The optimal fit, explaining 69.5% of the variance, was reached when using ECEC min , alkyl % , temperature , and extract type as input variables. The coefficient of determination of this fit was 0.75. Of these four input variables, the ECEC min gave a negative correlation with the 14C activity. Clay minerals with a high ECEC min , have relatively slow organic matter turnover as the exchangeable cations enable the clays to bind organic matter, while clay minerals with a low ECEC min , like kaolinites, have relatively fast organic matter turnover, as the ability to bind SOM is much less. Surprisingly, the percentage of alkyl-C showed a positive correlation with 14C activity. Possibly the alkyl-C represents relatively easily decomposable lipids. Temperature also showed a positive correlation with 14C activity of the extracts: microbial activity and thus decomposition increases with temperature. The type of extract (1 for hydroxide extracts and 0 for pyrophosphate extracts) was also positively correlated with 14C activity, indicating that the NaOH extract contains young, easily decomposable SOM and the pyrophosphate extract contains old, recalcitrant SOM. This suggests that pyrophosphate-extracted SOM was relatively strongly bound (e.g. via exchangeable cations) and hydroxide-extracted SOM rather loosely bound to the mineral surface.The results of the regressions agree with the ISRIC experiment, where the same factors explained 66.9% of the variance with an R 2of 0.77. This indicates that the soils from Mozambique and from the wide spectrum of locations of the ISRIC set behave similarly in terms of effect of clay mineralogy on organic matter turnover.