Effect of biochar application at a trace-elements polluted area on soil carbon stability

Biochar, the solid carbonaceous material produced by pyrolysis of biomass, is a promising alternative for restoring degraded soils [1]. Specifically, biochar has been reported to increase agronomic productivity of acidic soils. Nevertheless, the theoretical high stability and recalcitrance of biochar is being questioned by recent studies [2]. In addition, the alterations on biochar C after its application into low C soil is still under debate. Thus, this study intends to evaluate the changes in carbon stability when biochars from different feedstock are applied into trace element polluted soils.

For this purpose, biochars were produced from rice husk-RHB, olive pit-OPB and almond shell-ASB using a steel batch reactor (temperature of 500 ºC; reaction time of 2 h under N₂ atmosphere with a heating rate of 20 ºC min^-1). A certified wood biochar (CWB) was also studied for comparative purposes. Two soils with a moderate and a high concentration of trace elements (called MPS and HPS respectively) were sampled for this study. Mixtures of each soil and 10 % (w/w) of the biochars were prepared in triplicates. Each pot was inoculated with 1 ml of a standard microbial suspension, the moisture was adjusted to 50 % of the water holding capacity and incubated in the automatic respirometer Respicond (Nordgren Innovations, Sweden) at 25 ºC for 60 days similarly to the procedure described by De la Rosa et al (2018) [2]. The CO₂ released was measured automatically every 6 h and the kinetics of the biological decomposition of the materials were fitted by a double exponential model. Results showed that the feedstock nature influenced the decomposition rates. Thus, the biochar stability of the tested materials followed the order ASB>RHB>OPB according to MTR₂.

Soil respiration showed a different C decomposition rate in both soils, having greater mean residence time in HPS (MTR₂=14.9 years) than in MPS (MTR₂=5.7 years). Our findings suggest that biochar addition increased the MTR₂ of the slow C pool in both soils.

References:

[1] Lehmann, J., Joseph, S., 2015. Biochar for environmental management: science and technology. 2nd ed. London & New York: Earthscan from Routledge.

[2] De la Rosa, J.M., Rosado, M., Paneque, M., Miller, A.Z., Knicker, H., 2018. Effects of aging under field conditions on biochar structure and composition: Implications for biochar stability in soils. Science of the Total Environment 613-614, 969-976.

Acknowledgements:

The former Spanish Ministry of Economy, Industry and Competitiveness (MINEICO) and AEI/FEDER are thanked for funding the projects CGL2016-76498-R and GL2015-64811-P. P. Campos thanks the “Fundación Tatiana Pérez de Guzmán el Bueno” for funding her PhD.