Incubations with homogenized soil samples 2016-2017

Incubations with homogenized soil samples 2016

In order to be able to more quickly evaluate the geochemical and microbiological effects of mixing of ultrafine-grained limestone powder (2.5 μm in diameter, Nordkalk C2), very fine-grained peat (4–8 μm in diameter, Vapo Oy) and a combination of these into acid sulfate soil, a laboratory-scale screening study was performed in both oxygen-rich and oxygen-free environments. The role of the limestone powder was to increase the pH value in the soil and the role of the peat was to act as an antioxidant and as an energy source for the bacteria in the soil.

The soil used in the experiment was taken in April 2016 from an untreated reference field on the Risöfladan experimental field by pressing 1.5 m long pipes into the ground with an excavator. In the experiment, the acid sulfate soil layer was used at 75–90 cm.

The different treatments of the homogenized acid sulfate soil were manual mixing of 1) 1 % ultrafine-grained limestone powder 2) 1 % fine-grained peat and 3) 1 % mixture of equal parts limestone powder and peat per dry weight soil. As a control, untreated homogenized soil was used. To investigate the oxygen influence of the treatments, all samples were prepared in both oxygen-free and oxygen-rich environments. All treatments were also performed in three replicates, i.e., three different soil samples taken several meters apart, and different sets of samples for the geochemical and microbiological analyzes were used. After treatment, the soil samples were stored at 10 °C in the dark for 10 weeks.

After incubation, the pH of the geochemical samples was measured and then frozen at -20 °C. From the thawed samples, sulfur and iron speciations were performed, as well as an analysis of selected metals. For the microbiological analyses, DNA was isolated from intact bacterial cells leached from the soil samples. From the isolated DNA, the 16S rRNA gene was PCR-amplified in order to identify the bacteria. After the PCR amplification, the samples were sent for sequencing, after which in-depth data analyses of the sequence material were performed to identify the bacterial populations in the untreated and treated soil samples. The compiled sequencing and geochemical data were published in 2018 in the journal Science of the Total Environment in the article Chemical and microbiological evaluation of novel chemical treatment methods for acid sulfate soils.

The overall conclusions of the study were that the oxygen-rich incubations mimicked a dry summer when the acidophiles in the acid sulfate soil are likely to be most active, and the low pH of the control soils allowed the acidophiles to catalyze the oxidation of iron disulfides. The oxygen-free incubations, on the other hand, mimicked autumn to spring conditions when the cracks in the soil are filled with water, the microorganisms have low activity, and no oxidation of iron disulfides occurs. In the soil treated with ultrafine-grained limestone powder and equal parts ultrafine-grained limestone powder and peat, acidophile populations were also present, but they were most likely inactive because metal and acid leaching was low. The treatment with peat alone had no effect on metal and acid leaching. Based on the results of this study, a treatment of acid sulfate soil must both raise the pH and increase the content of organic matter in the soil (e.g. with peat) to change the composition of the microbial population.