Towards a better understanding of the origin and fate of fine colloidal and dissolved black carbon in agro-ecosystems

• Applicant: Dr. Melanie Braun
• Subject Area: Soil Sciences
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 336943844

Black carbon (BC) comprises a variety of organic combustion residues, which enter the soil by atmospheric deposition, e.g. from vegetation fires and/or by direct application of BC in form of biochar. Black C is highly recalcitrant and hence accumulates in soil within several decades. However, the degradation and loss of BC in soil is hardly understood. In the last years several studies showed that BC occurs in water in the form of dissolved BC (DBC), which includes particles < 450 nm. Though, a direct evidence for loss of BC from soil in form of water extractable BC (WEBC) to this dissolved fraction is missing. I hypothesize that i) a large part of BC in arable soils exists in water-transportable form (WEBC), and that ii) WEBC consist mainly of colloidal to nano-sized particles, which can be characterized via analytical and preparative field flow fractionation (FFFprep). Besides, I assume that this WEBC is produced by microbial BC degradation. To test these hypotheses, I will first investigate arable soils from field experiments with different BC inputs on their WEBC content and composition (0, 4.5 and 9 t biochar/ha applied with compost to coppice plantations). WEBC will be extracted from soil and filtrated to < 450 nm prior to BC analyses (oxidation to benzene polycarboxylic acids (BPCA method). Secondly, for the determination of BC abundance in colloidal- to nano-size classes an FFFprep method will be established and later on optimized for preparative isolation of BC particles. These can then be used for subsequent analysis of BPCAs, which serve as marker for the degree of aromatic condensation. To elucidate the contribution of microbial BC degradation to WEBC formation, samples of a 5-year incubation experiment (including sterile and non-sterile variants) are characterized by FFFprep and subsequent BPCA analyses. These experiments will first time investigate the size distribution of WEBC in soil and to which degree it will change during degradation. This study will further reveal if biochar application rates correlate with WEBC in soil. I will then be able to provide first ideas if degradation of BC to fine-colloidal and nanoparticles might be relevant for BC losses from soil and the global carbon cycle at least in decadal time-scales.

• DFG Programme: Research Grants