Deciphering timing and rates of Central German Chernozem/Phaeozem formation through high resolution single-grain luminescence dating.

Chernozems/Phaeozems are important agricultural resources and have been intensively used for millennia. However, their origin and age are still controversial. In Europe, the westernmost widespread Chernozem/Phaeozem area is located in Central Germany. In contrast to other German regions with anthropogenic Chernozems/Phaeozems, their natural origin is suggested in connection with intensive bioturbation. Yet, radiocarbon is unsuitable for decoding Chernozem/Phaeozem formation so this hypothesis remains untested, whereas single-grain luminescence dating allows to discriminate between different soil sub-processes and formation phases. We applied single-grain feldspar luminescence to a Central German Chernozem that was buried during the Bronze Age and subsequently protected from pedogenic processes. For the first time, we could directly determine timing and rate of Chernozem/Phaeozem formation in Central Europe by dating bioturbation as the dominant soil forming process. Accordingly, Chernozem/Phaeozem formation started at the latest in the Early Holocene prior to Neolithic settlement indicating a natural origin of Central German Chernozems/Phaeozems, and Chernozem/Phaeozem formation ceased around 6-5 ka when the regional climate became more humid. Our effective soil reworking rates show that earthworm bioturbation in Chernozems/Phaeozems is more intense than ant-dominated bioturbation, but significantly less intense than bioturbation by lugworms or ploughing. The latter effect allows to identify prehistoric ploughing in paleosols.

Substitution of mineral fertilizers with biogas digestate plus biochar increases physically stabilized soil carbon but not crop biomass in a field trial.

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B0), with 1 Mg biochar ha-1season-1 for two growing seasons (B2), or with 40 Mg biochar ha-1 application (B40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 µm aggregate-occluded and mineral-associated OM. CO2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha-1), while biochar had no effect. B40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 µm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha-1 with digestate resulted in 20% more SOC in the <20 µm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B0 and B2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates.

Acceleration of Biochar Surface Oxidation during Composting?

Biochar composting experiments were performed to determine whether composting is a suitable method to accelerate biochar surface oxidation for increasing its reactivity. To assess the results, surface properties of Terra Preta (Brazil) and ancient charcoal pit (Northern Italy) biochars were additionally investigated. Calculation of O/C ratios by energy-dispersive X-ray spectroscopy demonstrated the anticipated increasing values from fresh biochars (0.13) to composted biochars (0.40), and finally charcoal pit biochars (0.54) and ancient Terra Preta biochars (0.64). By means of Fourier transformation infrared microscopy, formation of carboxylic and phenolic groups on biochars surface could be detected. Carboxylic acids of three composted biochars increased up to 14%, whereas one composted biochar showed a 21% lower proportion of carboxylic acids compared to the corresponding fresh biochar. Phenolic groups increased by 23% for the last mentioned biochar, and on all other biochars phenolic groups decreased up to 22%. Results showed that biochar surface oxidation can be accelerated through composting but still far away from ancient biochars.