Download Stability and Physical Protection of Biochar in Soil

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