Guido Fellet: Effects of biochar application on contaminated soils

STSM at UK Biochar Research Centre, University of Edinburgh (UK), Sept- Dec 2013

The main purpose of the STSM was to learn new techniques and new methods of investigations over the effects of the biochar application on contaminated soil, with particular attention to the mobility and availability of inorganic pollutants, i.e. heavy metals. In particular, the research activity aimed at describing the balance between release and adsorption of inorganic pollutants by biochar particles, taking account of both pollutants in the soil and those indigenous to the biochar. The intention was to use rhizoboxes to examine the interactions in planted soil as well as unplanted.

Host: Saran Sohi, UK Biochar Research Centre, University of Edinburgh (UK)

Description of the work carried out and main results obtained during the STSM period

Previous rhizobox work at the host institution used an uncontaminated sandy Woburn soil from Rothamsted Research (UK). The soil has been unplanted for several decades. The resultant low organic matter content has previously made nutrient interactions more measurable. The principle behind our initial design in this study was analogous.
The experimental design, which considered at first heavy metal spiked soil versus un-spiked soil was eventually changed: it was decided to postpone the use of the un-spiked soil in order to introduce a new type of contaminated soil represented by coal tar spiked soil which then received the same treatments in terms of biochar type and dose of application. This part of the work was undertaken in collaboration with Aoife Brennan, University of Strathclyde (Glasgow, UK), who has participated in a previous STSM in Spain.
As the soil was initially uncontaminated, it was spiked with Zn, Cu and Pb salt solutions at the minimum amount necessary to reach the thresholds reported on the UK guidelines for declaration of contaminated soils. This was done after part-drying and sieving to <2mm.

Biochar was chosen amongst the wide variety of biochars produced by the UKBRC pilot- scale pyrolysis plant (research facility). The selection involved two types obtained from similar feedstock but with different heavy metal content: rural sewage sludge and urban sewage sludge. This would enable sorption and release of the three metals to be assessed from biochar to soil and from the soil to biochar. As for the plant species, indian mustard (Brassica juncea L.) was selected for its tolerance to heavy metals and adaptability with respect to different environmental conditions. Indian mustard and rapeseed (Brassica napus L.) are considered the most promising species for extracting heavy metals from contaminated soils (Gisbert et al., 2006).


Aliquots of biochar and spiked soil were mixed immediately after spiking. The substrates were left to equilibrate for one week. Then the rhizoboxes were packed for a total of 5 replicates for each treatment. All of the rhizoboxes received aqueous NPK-fertilizer to provide the plants with 83.9 mg kg-1 N, 19.3 mg kg-1 P and 33.0 mg kg-1 K, and simultaneously bring the soil moisture content up to 100% WHC. The rhizoboxes were scanned with a scanner to provide a background JPEG image to later analyse for spatial co- localization of the biochar and plant roots. The rhizoboxes were then placed into a growth chamber set up at the optimum growth conditions (16h 22°C / 8h 18°C);. The seeding was done providing each rhizobox with five seeds later thinned to leave only one plant per rhizobox. A single rhizobox was also prepared with un-spiked soil but the same level of water, fertilization and growth conditions. This would provide a pilot for the future planned (postponed) experiment with uncontaminated soil (which will now be performed at the home institution). The rhizoboxes were not watered until 60% WHC was reached, 2 weeks after germination. Thereafter watering was administered to maintain constant WHC until the end of the growth period (4 weeks after germination).
After prompt germination in 2 days the plants grew rather slowly. Growth almost stopped during the third week. The biomass was minimal in both heavy metal and coal tar spiked soil. At the end of the experiment, the biomass was collected from the rhizoboxes with heavy metal spiked soil. These samples will be analysed soon, at the home institution. The plant in the pilot rhizobox, despite a similar growth pattern, developed notable biomass as well as a good root development covering the whole depth of the rhizobox.
Representative samples of biochar were recovered from the soil in the rhizoboxes with heavy metal spiked soil. Adsorption and desorption experiments were undertaken. The adsorption tests were performed using a sorption solution containing the three elements used for spiking the soil (Cu, Pb and Zn), at several concentrations in the same ratio as in the spiked soil in order to relate the results from the test to the rootbox experiment, assuming that the changes of bioavailability were the same for the three metals. The desorption tests were done on biochar samples recovered from the rhizoboxes and on the same material after artificial ageing. This was to assess the possible effect of time on the release of the heavy metals adsorbed from the soil. The ageing process was performed following the UKBRC method reported of Cross & Sohi (2013).

Samples of soil from the same rhizoboxes were collected for bioavailability tests which were performed at the University of Strathclyde using the EDTA method (MAFF, 1986). At the moment, ICP analysis for the adsorprion and release tests are in progress.
During the three months STSM, I learned to use ImageJ software for the interpretation of the rhizobox images. Such analysis should produce a final image where soil particles are digitally removed while proximity of roots and biochar particles can be assessed. I was taught to use the Smartroot plugin for ImageJ to elaborate the root images to describe the root space-filling properties.


The setup of the experiment was possible thanks to the contributions of other scientists. The preparation of the rhizoboxes was done following the advice and suggestions of Miranda Prendergast-Miller (now CSIRO Land and Water, Australia) who previously designed and developed the rhizoboxes at the UKBRC. Ondrej Masek established the pyrolysis facility that was used to create the biochar used in the practical work. A great contribution was also given by Andrew Cross (UKBRC) who helped in providing the required material and taught the procedures for compliance with the regulations at the biochar laboratories. He also helped with the ageing of biochar for the adsorption and desorption tests. Aoife Brennan, from the University of Strathclyde, helped with the spiking of the soil and setting up the rhizoboxes as well as, with performing the bioavailability tests on the soil samples in Glasgow. Last but not least, Nikola Zotev, an undergraduate student from the University of Edinburgh who developed ImageJ use under a Nuffield bursary in summer 2013, gave a significant contribution in revealing the potentials of Images for the interpretations of the rhizobox images.

Future collaboration with host institution

The STSM has cemented collaboration between the University of Udine (Italy) and the University of Edinburgh, UK Biochar Research Centre. As mentioned in this final report, further experimental activities are already planned in connection with the current STSM. Both partners have proposed actions that will establish continuous and long-term collaboration.

Other comments

The period at the UKBRC was extremely motivating and positive. It resulted in new links between people with common interests and different expertise; a synergy to be developed and cared now and in the future for a long lasting cooperation in science.

References

Cross A. and S. P. Sohi. 2013. A method for screening the relative long-term stability of biochar. GCB Bioenergy 5:215–220
Gisbert C. et al. 2006. Tolerance and accumulation of heavy metals by Brassicaceae species grown in contaminated soils from Mediterranean regions of Spain. Environmental and Experimental Botany 56:19-27
MAFF 1986, The Analysis of Agricultural Material (2nd edition), RB427, HMSO, London, UK.
Prendergast-Miller, M. T. et al. 2013. Biochar–root interactions are mediated by biochar nutrient content and impacts on soil nutrient availability. European Journal of Soil Science. doi:10.1111/ejss.12079