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DEPARTMENT OF AGRICULTURE AND ECOLOGY FACULTY OF LIFE SCIENCES UNIVERSITY OF COPENHAGEN Stability and Physical Protection of Biochar in Soil Sander Bruun and Tarek El-Zehery Contact: Sander Bruun, [email protected], +4529406537 Biochar is mainly protected from microbial degradation in soil because of its recalcitrant chemical nature, which is making it much more stable than other forms of organic matter added to soils. This recalcitrance is dependant on the degree of thermal alteration. However, biochar is also stabilized by physical protection mechanisms. 0.6 Introduction The purpose of this study is to compare the degradation rate of char, thermally altered to different degrees and test if physical protection helps increasing its stability in soil. 0.5 0.4 Fraction respired A range of different mechanisms protect organic matter in soil from microbial decay. Organic matter may be protected by physical protection inside aggregates or otherwise in pores to small for microbes to enter, by complexation with or sorption to surfaces of mineral particles or it may by itself be chemically recalcitrant. Char is considered to be recalcitrant, because it is a highly complexity and stable chemical structure, whereas the particulate nature makes it less likely to be physically protected. 70 225 300 0.3 375 0.2 Materials and Methods Biochar was produced from homogeneously 14C labelled wheat at 225°C, 300°C and 375°C under limited oxygen supply. The resulting chars and litter dried at 70°C were used in a two-year incubation and the evolved 14C was trapped in a NaOH and counted on a scintillation counter. After two years, the soils were thoroughly mixed and incubated again to see whether this released any char from physical protection. 0.1 0 0 100 200 Results 300 400 500 600 700 Time (days) Fig 1: Evolution of carbon dioxide after incubation of litter and biochar produced at different temperature Table 1: Characteristics of litter and biochar Production Temp Fraction weight C N °C lost % % 70 225 300 375 0.00 0.21 0.52 0.60 42.7 41.2 37.8 - 2.31 2.62 5.45 - C/N 14C activity Fraction 14C lost DPM g-1 18.5 15.7 6.9 - 4.1×107 4.6×107 1.8×107 8.6×105 0.00 0.10 0.79 0.99 14C activity CO3-- Fraction 14C in DPM g-1 CO3-- 2.1×104 5.1×10-4 1.0×10-3 4.8×10-3 0.11 4.6×104 8.4×104 9.8×104 Two-year incubation Immediately after the experiment was started there was an initial flush of carbon evolution from all the chars included in the study with no apparent lag phase (Fig. 1). In contrast, there was a distinct lag phase for the litter. The small amount of carbonates in the chars produced at 225°C and 300°C excluded carbonates as the source of the initial flush. The initial flush is therefore more likely to be associated with oxidation of the char particle surfaces, which has been observed in other studies. After the initial flush, the carbon evolution quickly degreased to very low values for all the materials. However, the respiration rate was still dependant on the degree of thermal alteration. Effect of soil mixing The effect of mixing the soil after the respiration had levelled of after two years is illustrated in Fig 2. The tendency for organic matter to be liberated from physical protection by mixing the soils is evident for litter, but also for biochar although this tendency seems to be decreasing with the degree of thermal alteration. Again no lag-phase was apparent. 0.0012 Respiration rate (fraction of total repired per day) Characterics of biochar The characteristics of the biochars are shown in Table 1. For all materials carbonates seems to constitute a very small fraction of the total carbon content except for the biochar produced at 375°C. 0.001 70 0.0008 225 0.0006 300 375 0.0004 0.0002 0 0 10 20 30 Time (days) 40 50 60 Fig 2: Effect of soil mixing on the evolution of carbon dioxide from litter and biochar produced at different temperatures Conclusions Resistance to decay increases with the degree of thermal alteration, also after the initial flush of carbon evolution. Although the recalcitrant chemical nature of biochar is the primary reason for its slow degradation in soil, physical protection due to entrapment in small pores, may also play an important role. Whether other mechanisms of physical protection is also at play, is an open question. 2008 Conference of the International Biochar Initiative; Biochar, Sustainability and Security in a Changing Climate; Newcastle, United Kingdom, September 8 - 10, 2008