Microstructural and associated chemical changes during the composting of a high temperature biochar: Mechanisms for nitrate, phosphate and other nutrient retention and release

Authors: JOSEPH, STEPHEN - University Of New South Wales; KAMMANN, CLAUDIA - Hochschule Geisenheim University; SHEPARD, JESSICA - University Of Edinburgh; CONTE, PELLEGRINO - University Of Palermo Italy; SCHMIDT, HANS-PETER - Ithaka Institute For Carbon Strategies; HAGEMANN, NIKOLAS - Eberhard-Karls University; RICH, ANNE - University Of New South Wales; Spokas, Kurt; MARJO, CHRISTOPHER - University Of New South Wales; ALLAN, JESSICA - University Of New South Wales; MUNROE, PAUL - University Of New South Wales; MITCHELL, DAVID - University Of New South Wales; DONNE, SCOTT - University Of New South Wales; GRABER, ELLEN - Volcani Center (ARO)

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2017
Publication Date: 3/15/2018
Citation: Joseph, S., Kammann, C.I., Shepard, J.G., Conte, P., Schmidt, H., Hagemann, N., Rich, A.M., Spokas, K.A., Marjo, C.E., Allan, J., Munroe, P., Mitchell, D.R., Donne, S., Graber, E.R. 2018. Microstructural and associated chemical changes during the composting of a high temperature biochar: Mechanisms for nitrate, phosphate and other nutrient retention and release. Science of the Total Environment. 618:1210-1223. https://doi.org/10.1016/j.scitotenv.2017.09.200.
DOI: https://doi.org/10.1016/j.scitotenv.2017.09.200

Interpretive Summary: Biochar has been proposed as a positive amendment for agriculture systems. In this study, we evaluated the impact of field exposure and small scale alterations in the biochar that occur due to its exposure in soil. We pre-treated the biochar with nutrients (through composting). We used a range of analytical techniques to investigate the mechanisms of nutrient retention in a high temperature wood biochar. We compared the composted and non-composted forms of this biochar. From this comparison we concluded that the un-treated biochar competes with plants and soil microbes for nutrients, since fresh biochar attracts nutrients and dissolved species creating a nutrient-rich covering layer on the biochar. Without pre-treating the biochar it would compete with plant roots and soil microbes for these nutrients. Thereby, these results suggest that additional information and guidance is needed before applying biochar to soils. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved models for predicting the mechanisms behind biochar’s impact on the soil system. This data is critical for improving soil carbon management.

Technical Abstract: Recent studies have demonstrated the importance of the nutrient status of biochar and soils prior to its inclusion in particular agricultural systems. Pre-treatment of nutrient-reactive biochar, where nutrients are loaded into pores and onto surfaces, gives improved yield outcomes compared to untreated biochar. In this study we have used a wide selection of spectroscopic and microscopic techniques to investigate the mechanisms of nutrient retention in a high temperature wood biochar, which had negative effects on Chenopodium quinoa above ground biomass yield when applied to the system without prior nutrient loading, but positive effects when applied after composting. We have compared noncomposted biochar (BC) with composted biochar (BCC) to elucidate the differences which may have led to these results. The results of our investigation provide evidence for a complex series of reactions during composting, where dissolved nutrients are first taken up into biochar pores along a concentration gradient and through capillary action, followed by surface sorption and retention processes which block biochar pores and result in deposition of a nutrient-rich organomineral (plaque) layer. The lack of such pretreatment in the BC samples would render it reactive towards nutrients in a soil-fertiliser system, making it a competitor for, rather than provider of, nutrients for plant growth.