1. Production of isotopically labelled sewage sludge and biochar for incubation and tracing experiments
The 13C-and 15N labelled sewage sludge has been prepared at a small pilot water treatment tank in collaboration with The Foundation Centre for New Water Technologies (CENTA), which manages a sewage sludge treatment Experimental Plant close to Seville. After adjusting the amount of substrate and duration of incubation to optimize the homogeneity of label incorporation into the biomass, the sludge was incubated and 13C-glucose was added (100mg/25L sludge/day) during 8 months to produce the 13C-labelled substrate. In addition, 15NO3 and 15NH4 were added to produce feedstock enriched in both 13C and 15N, which permits tracing the partitioning of 13C and 15N of the isotopically labelled biochars on the biochar amended soils.
Un-labelled biochars were produced from different feedstock (wood, sludge) have been produed by fast pyrolysis (600 ºC) and hydrothermal carbonization (HTC-at 200 and 260 ºC) at the Leibniz Institute for Agricultural Engineering in Postdam-Bornim e.V. (ATB) in 2013 and in August of 2014 during a short stay of José María de la Rosa (JMR).
Hydropyrolysis reactor: ATB-Potsdam
2. Characterization of biochars produced from sewage sludge and vegetation residues
A complete chemical characterization of the pre-product and biochar (end-product) have been carried out. This is as essential as a better understanding of the potential impact of the end-product on soil processes.
This characterization of biochars and feedstock comprised the elemental composition (C,H,N,S), several physical and chemical properties (pH, water holding capacity, ash content, cation exchange capacity, specific surface area), but also the use of advanced techniques such as nuclear magnetic resonance (NMR) spectroscopy , fourier transfor-Infrared (FT-IR) spectroscopy, analytical Pyrolysis (Py-GC/MS) and scanning electron microscopy (SEM). They permitted a significant advance in the chemical characterization of biochars (De la Rosa et al., 2014). This knowledge will improve the prediction of the real potential of biochars to act as a C sink within the global C cycle.
BRUKER AVANCE 400 MhZ NMR equipment at IRNAS-CSIC (Campus Olavide-Instituto de la Grasa)
3. C-sequestration potential and fate of biochar in typical Mediterranean agricultural soils
This goal has been carried out by combining two different proxies:
- Respicond apparatus monitored the potential CO2 released from the labelled and un-labelled biochars and of the respective amended soils.
The Respicond apparatus is a sophisticated computerized respirometer available at IRNAS-CSIC which allows monitoring the CO2 released, that is the respiration rate, from 96 vessels. The obtained CO2 accumulation curve was fitted using a two exponential decay model which allows the calculation of the mean residence times of the respective material. Preliminary results showed that pyrolysis confers greater persistence and therefore thus biochar mineralizes more slowly than the biomass they were produced from. Nevertheless, average mean residence times (MRT1/2) of C from biochars were much lower than expected (the traditionally theoretical centennial scale).
- C-sequestration of biochars in field studies was determined by studying the aging of biochar particles at biochar amended plots located at “La Hampa” experimental field.
This field study was performed at the experimental station “La Hampa” from IRNAS-CSIC, located in the Guadalquivir river valley during the spring and summer of 2013. Soil was amended with different doses five biochars, making a total of 12 different treatments. After 7 months of growth, sunflower plants were harvested. Once finished the growing season, field plots were maintained and monitored. Particles from each one of the biochars tested were picked up by hand from the soil at 0, 7, 12 and 24 months after planting and kept frozen until being analyzed by elemental, isotopic and spectroscopic techniques (FT-IR and 13C NMR spectroscopy). Results showed that biochar experienced changes in physical and chemical properties:
Physical changes included fragmentation to smaller particles, aggregation with other soil particles, and availability of surfaces. These changes permitted the translocation of biochar particles in the soil profile.
Chemical properties included weathering effects of poly-aromatic ring size, selective loss of certain components (alkyl-compounds) and increase of polar functional groups.
4. Assessment of Polycyclic aromatic hydrocarbons (PAHs) in biochar amended soils
Results of this project respecting the assessment of Polycyclic aromatic hydrocarbons (PAHs) in pot experiments of a Mediterranean agricultural soil amended with 4 different sorts of biochars were recently submitted to an international scientific journal. They found that i) the extraction in Soxhlet using toluene as solvent achieved the highest extraction rates, ii) biochar produced by the traditional kiln methodology contained much greater amounts of PAHs than those produced by fast pyrolysis under controlled conditions, iii) in general hydrochars resulted in more homogeneous amounts of PAHs than dry-biochars, but also greater abundance in average and iV) the abundance of PAHs on the amended soil was not linear with the applied dose of biochar.
Clean-up process by J.M. de la Rosa (during the assessment of PAHs in biochar amended soils)