Towards a better understanding of the origin and fate of fine colloidal and dissolved black carbon in agro-ecosystems
Final Report Abstract
Black carbon (BC) enters 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 accumulates in soil. However, in the last years several studies showed that BC occurs in water in the form of “dissolved BC” (DBC, < 450 nm) presumably originating from BC degradation in soil. Though, a direct evidence for loss of BC from soil in form of water extractable BC (WEBC) to this “dissolved fraction” was missing. I hypothesized that i) WEBC consist mainly of colloidal to nano-sized particles, which can be characterized via analytical and preparative field flow fractionation (FFFprep). I assumed that ii) a large part of BC in arable soils exists in form of WEBC, which is increased by application of biochar and further iii) produced by microbial BC degradation. To test these hypotheses, I first established a FFFprep method for preparative isolation of BC particles for the determination of BC abundance in colloidal- to nano-size classes. Then, I investigated arable soils from field experiments with different BC inputs (0, 4.5 and 9 t biochar/ha) on their WEBC content and composition. WEBC was extracted from soil and filtrated to < 450 nm prior to BC analyses (via oxidation to benzene polycarboxylic acids (BPCA method). To elucidate the contribution of microbial BC degradation to WEBC formation, samples of a 5-year incubation experiment (including sterile and non-sterile variants) were analyzed regarding BC and WEBC. I first developed a method for the determination of different carbon materials, including latex particles, soil and soil spiked with BC (charcoal), via field flow fractionation online with ICP-MS, yielding 76% to 105% recovery. To separate BC from other organic carbon and to enhance sample amounts of e.g. trace element analysis I tested a semi-preparative asymmetrical flow field flow fractionation channel (SP-AF4). The established method yielded a reproducibility between 95 and 99%; for preparative application 50% of total BC was recovered. Analysing WEBC in soils of the fertilizer trial I found, that 7.61±2.87 to 13.85±1.7 mg kg^-1, i.e. 0.4 to 1.7% of bulk BC in agriculturally soil exists in form of WEBC, which can be potentially leached from soil. Interestingly, I found no correlation of WEBC amount and BC input; accordingly, I have to refute my initial hypothesis that biochar application increases the amount of WEBC, at least for short timescales of few years. Found WEBC consisted of less condensed and aromatic BC structures than bulk BC, presumably because less condensed BC structures are less persistent and first transferred into the water-transportable from. During the 5-year incubation experiment I could not detect BC degradation, also no WEBC formation over time was detected, leading to the conclusion that such a short time is not sufficient for WEBC formation. Consequently, I cannot confirm that microbial degradation favors WEBC formation. However, detected structural changes of BC let assume that BC was altered within the 5 years of incubation. Applying the developed FFFprep method to the WEBC samples from the incubation experiment I could show, that at the beginning of the incubation in the variant topsoil+BC only 25% of the “dissolved phase” were truly dissolved, while 27% and 48% exists in form of small colloids and nanoparticles, respectively. Unfortunately, I could not determine temporal changes in the abundance of BC in different size classes during incubation since not enough sample material was left. Summarizing, I showed that WEBC exists in from of nanoparticles and small colloids in soils, which is potentially leachable and can therefore contribute to BC losses from soil and to DBC found in rivers. To elucidate the role of BC degradation for WEBC formation, however, more research is needed, focusing on longer timescales than 5 years.
Publications
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(2017): Fein-kolloidaler und gelöster schwarzer Kohlenstoff in Agrarökosystemen: Projektbeschreibung und erste Ergebnisse. Jahrestagung der Deutschen Bodenkundlichen Gesellschaft, Göttingen
Bläsing, M.; Gottselig, N.; Klumpp, E.; Amelung, W.; Lehndorff, E.
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(2018): Extending the capabilities of field flow fractionation online with ICP-MS for the determination of particulate carbon in latex and charcoal. J. Anal. At. Spectrom (8), 1363–1371
Nischwitz, V.; Gottselig, N.; Missong, A.; Klumpp, E. & Braun, M.
