Clay minerals as the key to the sequestration of carbon in soils

Results from earlier laboratory and field experiments were interrogated for the possibilities of sequestration, or longterm accumulation, of carbon from excess greenhouse gases in the atmosphere. In the laboratory study, samples of three (top) soils dominated by kaolinite and illite (together), smectite, and allophane were examined for the adsorption and desorption of dissolved organic carbon (DOC). Adsorption and desorption of DOC were carried out on clay fractions extracted physically and after first native organic matter and then iron oxides were removed chemically. Labeled organic material was added to the soils to assess the priming effect of organic carbon (OC). In the field, changes in OC were measured in sandy soils that had been amended by additions of clay for between 3 and 17 years, both through incorporation of exogenous clay and delving of in situ clay. The laboratory experiments demonstrated that a portion of DOC was held strongly in all soils. The amount of DOC adsorbed depended on clay mineral types, including Fe oxides. Much adsorbed DOC was lost by desorption in water and a substantial amount of native OC was lost on priming with new OC. Addition of clay to soils led to increased OC. Therefore, addition of clay to soil may enhance net sequestration of C. Organic carbon close to mineral surfaces or within microaggregates is held most strongly. Carbon sequestration may occur in subsoils with unsaturated mineral surfaces. However, incorporation of carbon into macroaggregates from enhanced plant growth might be most effective in removing excess carbon from the atmosphere, albeit over the short-term

Relationship between soil clay mineralogy and carbon protection capacity as influenced by temperature and moisture

Environmental conditions like temperature and moisture could affect the carbon protection capacity of various clay types in soils. Using dominantly kaolinitic-illitic, smectitic and allophanic soils, we conducted systematic incubation experiments over 42 days at different temperatures (4, 22 and 37 °C) and moisture contents (30, 60 and 90% of water holding capacity (available water)). The basal respiration was monitored to study the relative effect of moisture contents and temperature on the carbon protection capacities and mechanisms of the three clay types. The results indicated that carbon decomposition increased with increasing moisture and temperature. A two-component quadratic equation could explain the carbon mineralisation process. The highest C respiration was observed at 37 °C with a 60% moisture level in each of the soil types. Under these conditions, the smectitic soil recorded the highest carbon decomposition followed by the kaolinitic-illitic and allophanic soils. The study of the priming effect using 14C labelled malic acid confirmed the trend of the bulk respiration results. The allophanic soil showed the lowest amount of carbon mineralisation under all experimental conditions. A strong inverse correlation (R2 = 0.90 at p < 0.05) was observed between CO2 emission rate and total sesquioxides (Fe and Al oxides) content. As evidenced by the pore size distribution, micromorphologies and thermogravimetric analyses, the microporous structure and microaggregate formation in the allophanic soil enhanced carbon sequestration. This study indicated that soil carbon stabilisation was related more to the sesquioxides content than to the clay types or their relative specific surface areas

Decomposition of soil organic matter as affected by clay types, pedogenic oxides and plant residue addition rates

The interactive effects of the types and contents of soil clay fractions (SCFs) and plant-residue addition rates on soil organic carbon (SOC) stabilisation are largely unknown. We conducted incubation experiments by amending a sandy soil sample with kaolinitic-illitic, smectitic and allophanic SCFs and adding wheat residues to the mineral mixtures to compare their C stabilisation capacity. The rate of carbon (C) decomposition was higher in the kaolinitic-illitic SCF followed by smectitic and allophanic clay minerals. The supply of easily degradable C substrate from decomposing residues markedly influenced the SCFs’ abilities to stabilise SOC. The removal of sesquioxides from the SCFs significantly decreased their C stabilisation capacity, which coincided with a decrease in the dehydrogenase activity of the mineral-residue mixture. The allophanic SCF showed the least microbial activity and the greatest C stabilisation due to having a higher proportion of micropores (75%). The high C stabilisation capacity of allophanic SCF could also be explained by its high specific surface area (119 m2 g−1). The results of this study are helpful to understand the role of various SCFs in stabilising added C originating from external wheat residue addition but warrant further validation under field conditions