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Summary Band 54

Borchard, Nils (2012): Interaction of biochar (black carbon) with the soil matrix and its influence on soil functions
Bonner Bodenkundl. Abh. 54 142 S., 10 Abb., 21 Tab. Bonn 2012



Biochar, i.e., the product of anthropogenic biomass carbonization, has been suggested to be used for both: sequestration of soil C and improvement of soil functions like plant productivity and element retention. Most of the available research results, however, related to tropical soils and a single biochar type, mainly charcoal. The aims of this study were to elucidate the fate and effects of three different kinds of biochar in temperate soil, and to test the role of hot steam activation, and composting on soil fertility, and biochar sorption properties.
The tested biochars were a gasification coke, a charcoal and a flash-pyrolysis char, all showing the isotopic signature of C3-plants. These biochars were mixed in varying amounts to sandy and silty textured soils to investigate effects on soil properties and yields of C4-crops. Two experiments were performed: a four-month pot-experiment with ryegrass (Lolium perenne L.) and a three-year field trial with maize (Zea mays L.). To monitor the fate of biochar-C in soil, I determined benzene polycarboxylic acids (BPCA) as molecular marker as well as stable 13C isotope signatures. Activated charcoal was used in pot-experiments next to charcoal treatments. A six-month composting experiment followed in order to study its impact on nutrient contents and physicochemical properties of biochars. Finally, sorption experiments of micro-nutrients (Cu and S) were performed with fresh and composted biochars to assess their potential to retain nutrients and heavy metals in temperate soils.
The added biochars elevated soil-C contents significantly; the biochar-C was sequestrated over at least the three-year experimental period. In the short-term, biochar associated ashes fertilized the soils and raised their pH values. Compared with charcoal, the gasification coke contained larger amounts of available K, Mg and P; and the flash-pyrolysis char showed lowest contents of available nutrients. However, no positive fertilizer effects were observed on plant growth. On the contrary, biochar applications led to nutrient imbalances, causing reduced N uptake by the plants. Apparently, with increasing amounts of added biochar, soil-N was immobilised. In the worst case (~100 g biochar kg-1 soil), biomass yields decreased instead of improving plant productivity! These yield risks would have to be balanced through fertilizer application (especially N). Consequently, additional CO2-equivalents are used for both fertilizer production and transport; moreover, the release of N2O could counteract potential C-sequestration by biochar applications.
As fresh biochars are characterized by low sorption capacities, biochars were physically activated and/or composted. The physical activation doubled cation exchange capacity (CEC) and multiplied specific surface area of the charcoal. Nevertheless, the use of activated charcoal did not affect yields of ryegrass, but decreased losses of N (not in the sand) and P. Composting increased the CEC of the biochars up to a factor of four; the BPCA-C contents remained unchanged. The specific surface areas (measured with N2 and CO2) decreased during composting due to the accumulations of soluble organic compounds. These compounds, however, contained nutrients (NPK), which therewith were reversibly sorbed. The sorption experiments with copper (Cu2+) as model cation revealed that Langmuir sorption coefficients increased after composting in the order of flash-pyrolysis char < charcoal < gasification coke, while the sorption capacity decreased in the order of gasification coke > flash-pyrolysis char > charcoal. Composting increased the Cu2+-sorption coefficient by a factor of 5 in the case of charcoal and by a factor of 3-4 in the case of gasification coke. Also retention of SO42-, used here as model anion, increased, though less strongly than that of Cu2+. Even at realistic biochar applications rates, composting may not improve the retention of Cu2+ and SO42- by biochars in soils of temperate climates.
In summary, my results confirm the potential of biochar made from wood for C-sequestration in soils. However, the results of this study provide no evidence of a beneficial effect of biochars on crop growth, but on the contrary they indicate nutrient imbalances and yield insecurity, therefore, a widespread application of biochar to fertile agricultural soils in temperate regions should be performed with caution. However, what remains to be examined is the potential use of activated or composted biochar to improve metal and nutrient retention of acidic, sandy soils with low organic matter contents.