Nina Eibisch successfully defended her doctoral thesis „Physico-chemical properties and effects of biochars from hydrothermal carbonization and pyrolysis of organic residues in agricultural soils” on March 30, 2015, at the Gottfried Wilhelm Leibniz Universität Hannover.
Nina Eibisch used chars that were produced by pyrolysis (pyrochars) and hydrothermal carbonization (HTC; hydrochars) from different feedstock materials (Miscanthus, woodchips, green cuttings, straw and digestate). Feedstock materials and chars were physico-chemically characterized and in agricultural soil underwent degradation, sorption and water retention experiments. The thesis aimed at evaluating if (a) different production conditions and (b) differences in physico-chemical properties of feedstock materials are reflected in the degradability of the chars, in the sorption capacity for pesticides (here: Isoproturon (IPU)) and in water retention dynamics after application to agricultural soils.
The results showed that pyrochars were hardly degraded during 56 days of incubation in soil (<2% of char-derived C), while hydrochars contained a relatively high proportion of easily available C: up to 14% of the hydrochar-derived C was degraded during incubation. Feedstock material had an influence especially on degradability of hydrochars: chars from lignin-containing feedstock material with high C/N and low O/C and H/C ratios showed lower degradation rates. Furthermore, the degree of degradation decreased with increasing carbonization temperature. The sorption experiments with the water-soluble pesticide IPU revealed that hydrochar application increased sorption to the 13-fold while application of pyrochar led to 2283-fold sorption compared to the soil without char. Mineralization of IPU was reduced by 56% (hydrochar) to 81% (pyrochar). Pyrochar application likely leads to long-term sorption of the pesticide within their micro structure while hydrochars sustain IPU bioavailability due to their more favorable decomposability. Hydrochars and pyrochars had similar effects in retaining plant available water in soil: they increased the plant available water capacity by 3 to 15% compared to pure soil. Possible counteracting water repellency characteristics of chars were over-compensated by other char characteristics, like particle size, inner porosity, form or plasticity. Summarizing, different char types may lead to different effects when applied to the soil, depending among others on production conditions, feedstock material. Therefore, no general recommendation for or against the application of hydrochars and pyrochars to agricultural soils can be given, but the suitability of a respective char type needs to be evaluated before application.
Nina Eibisch conducted her thesis in the frame of the BMEL project „Biochar as soil amendment and for carbon sequestration in agricultural soils“ which she successfully finished on March 30, 2015