Persistence in soil of Miscanthus biochar in laboratory and field conditions

Abstract

Evaluating biochars for their persistence in soil under field conditions is an important step towards their implementation for carbon sequestration. Current evaluations might be biased because the vast majority of studies are short-term laboratory incubations of biochars produced in laboratory-scale pyrolyzers. Here our objective was to investigate the stability of a biochar produced with a medium-scale pyrolyzer, first through laboratory characterization and stability tests and then through field experiment. We also aimed at relating properties of this medium-scale biochar to that of a laboratory-made biochar with the same feedstock. Biochars were made of Miscanthus biomass for isotopic C-tracing purposes and produced at temperatures between 600 and 700°C. The aromaticity and degree of condensation of aromatic rings of the medium-scale biochar was high, as was its resistance to chemical oxidation. In a 90-day laboratory incubation, cumulative mineralization was 0.1% for the medium-scale biochar vs. 45% for the Miscanthus feedstock, pointing to the absence of labile C pool in the biochar. These stability results were very close to those obtained for biochar produced at laboratory-scale, suggesting that upscaling from laboratory to medium-scale pyrolyzers had little effect on biochar stability. In the field, the medium-scale biochar applied at up to 25 t C ha-1 decomposed at an estimated 0.8% per year. In conclusion, our biochar scored high on stability indices in the laboratory and displayed a mean residence time > 100 years in the field, which is the threshold for permanent removal in C sequestration projects.

Miscanthus Biochar had Limited Effects on Soil Physical Properties, Microbial Biomass, and Grain Yield in a Four-Year Field Experiment in Norway

“…In Norway, biochar application is recognized as one of several methods with the potential to significantly reduce the carbon footprint of the agricultural sector [8]. Our previous investigations using the same biochar and soil used in this study confirm a low annual C mineralization rate of 0.8% and estimated mean residence time of >100 years for biochar in the soil [9].…”

“…The objectives were to assess the agronomic effects of miscanthus biochar under field conditions over four years and to give farmers and authorities in Norway insight for the suitability of biochar as a climate change mitigation method. Our results provide an agronomic context to Reference [9] (where we previously investigated biochar C stability from the same field site). In the present study, we hypothesized that biochar could improve soil water retention and alleviate short-term soil water deficits as are common in the early summer in Norway.…”

“…Similarly, labile C sources from crop residues have been reported to increase soil aggregation more than changes in soil properties induced by mulching (Rasse, Smucker, and Santos, 2000) [58]. By contrast, the miscanthus biochar used for the field experiment is highly stable [9], which probably explains the absence of effect on microbial biomass and, therefore, the absence of significant effect on soil aggregate stability.…”

“…Corresponding BC application rates and mass percent concentrations in the 23 cm Ap soil horizon were 11.4 t BC ha −1 or 0.38% (w/w) for BC8 and 35 t BC ha −1 or 1.16% (w/w) for BC25. Miscanthus was chosen as a feedstock because it is a C3 plant with contrasting δ 13 C to the C4 soil, which was relevant for the primary objective of the field experiment as reported in Rasse et al [9] (i.e., biochar-C stability over two years under field conditions). Biochar or Miscanthus straw were applied and raked out on the surface of the plots in September 2010 and all plots (including controls) were then mouldboard ploughed to a depth of 23 cm.…”

“…Miscanthus biochar was low in heavy metals and complied with the highest quality class (class 0) set for organic soil amendments under the Norwegian law (Table S3). Total C content was high (80%) and H:C and O:C ratios (Table 1), and Reference [9] were well below the respective thresholds (H:C 0.6, O:C 0.4) that characterize biochars thought to be suitable for long term soil carbon sequestration [46,47]. This biochar fulfills the minimum requirements for safety and utility as a material to increase carbon in agricultural soil.…”