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Journal ArticleDOI

Partitioning of Microbially Respired CO2 Between Indigenous and Exogenous Carbon Sources During Biochar Degradation Using Radiocarbon and Stable Carbon Isotopes

01 Apr 2019-Radiocarbon (University of Arizona)-Vol. 61, Iss: 2, pp 573-586
TL;DR: In this article, a series of in-vitro incubations of the degraded biochars were used to determine CO2 efflux rates, 14C concentration and δ13C values in CO2 to quantify the contributions of biochar carbon and other sources of carbon to the CO 2 efflux.
Abstract: Pyrolized carbon in biochar can sequester atmospheric CO2 into soil to reduce impacts of anthropogenic CO2 emissions. When estimating the stability of biochar, degradation of biochar carbon, mobility of degradation products, and ingress of carbon from other sources must all be considered. In a previous study we tracked degradation in biochars produced from radiocarbon-free wood and subjected to different physico-chemical treatments over three years in a rainforest soil. Following completion of the field trial, we report here a series of in-vitro incubations of the degraded biochars to determine CO2 efflux rates, 14C concentration and δ13C values in CO2 to quantify the contributions of biochar carbon and other sources of carbon to the CO2 efflux. The 14C concentration in CO2 showed that microbial degradation led to respiration of CO2 sourced from indigenous biochar carbon (≈0.5–1.4 μmoles CO2/g biochar C/day) along with a component of carbon closely associated with the biochars but derived from the local environment. Correlations between 14C concentration, δ13C values and Ca abundance indicated that Ca2+ availability was an important determinant of the loss of biochar carbon.

Summary (2 min read)

Introduction

  • Biochar is pyrolyzed carbon (PyC) derived from the incomplete combustion of biomass.
  • The strong relationships between loss of indigenous carbon from the degraded biochars and amount and δ 13 C values of CO2 efflux in incubation trials led to two main hypotheses which remained unproven: 1. Biochar degradation was predominantly microbial and 2.

In vitro incubations and δ 13 CCO2 and 14 CCO2 measurements

  • In the present study the authors conducted two in-vitro experiments to measure the rate and isotopic composition of CO2 efflux from the field-exposed degraded biochar samples.
  • Duplicate samples of the pre-exposure 300 and 500 o C biochar and two vials with wet sand only were included in the incubation experiment.
  • A shorter-term, up-scaled incubation experiment was used to produce larger CO2 samples for 14 C analysis.
  • Flask were filled with CO2-free air immediately after sample loading.
  • Raw measurement results were corrected for possible contamination in graphitisation stage only (Hua et al. 2001) .

Calcium analysis of biochars

  • To assess the potential transport of calcium from soil, leaf litter and limestone into biochar samples during the 3-year field trial the authors undertook water and acid extractions of all biochar samples.
  • Upon completion, all extracts were mildly to moderately acidic (pH = 4.1-6.8).
  • Calcium concentrations were analysed by Inductively Coupled Plasma Mass Spectrometry .
  • Analytical quality control included analysis of certified reference waters, replicate samples and spiked samples.

Short-term incubation experiments

  • In contrast to CO2 efflux rate and 14 CCO2 concentration there was no significant (p=0.12) difference in  13 C -CO2 values derived from the 300˚C and 500˚C biochars in the short-term incubation experiment.
  • The variation in CO2 efflux rate and 14 CCO2 concentration in replicate field samples reflects unavoidable differences in the individual field placements including the thickness of covers, ingress of exogenous matter and water as well as rate of microbial colonisation.

Long-term incubation experiments

  • Compared to the 300˚C biochars, CO2 efflux rate and cumulative CO2 efflux were lower in the 500˚C biochars (initial rate ≈ 6-16 μmoles CO2/day/g C, final rate ≈ 2.5-3.8 μmoles CO2/day/g C) but the relative differences between the treatments were similar between the two biochar types (Fig. 1b ).
  • While  13 CCO2 values derived from the short-term and long-term incubations varied by 2-3 ‰ at equivalent incubation times the relative difference in values between limestone and no-limestone treatments were similar.
  • Furthermore, there was a higher proportion of et al. 2015) and is assumed to remain unchanged over the 3-year period.
  • The authors infer that most of the CO2 produced over the course of the incubations was due to microbial degradation and respiration.

BC

  • Limestone treated 300˚C biochars had a 5-6 fold higher percentage indigenous C loss respired as CO2 compared to treatments without limestone.
  • The results here suggest higher Ca 2+ availability led to the binding and immobilization in situ, of degradation products to the char surfaces, or minerals associated with the char surfaces (Oades, 1988; Wittinghall and Hobbie, 2012; Varcoe et al., 2010) .
  • The results presented here suggest that biochars from alkaline environments are not intrinsically more susceptible to degradation than biochars from non-alkaline environments, they simply retain degradation products in situ through Ca 2+ immobilization processes -products that have been lost by leaching and/or respiration from chars in non-alkaline environments.
  • As such the alkali-soluble component may potentially be able to provide a robust radiocarbon age determination if the solubilized indigenous component can be isolated from actual exogenous contamination.

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Figures (7)

Content maybe subject to copyright    Report

Munksgaard, N.C., McBeath, A.V., Ascough, P.L. , Levchenko,
V.A., Williams, A. and Bird, M.I. (2019) Partitioning of microbially
respired CO2 between indigenous and exogenous carbon sources during
biochar degradation using radiocarbon and stable carbon
isotopes. Radiocarbon, 61(2), pp. 573-586. (doi:10.1017/RDC.2018.128
)
There may be differences between this version and the published version.
You are advised to consult the publisher’s version if you wish to cite from
it.
http://eprints.gla.ac.uk/171024/
Deposited on: 10 October 2018
Enlighten – Research publications by members of the University of
Glasgow
http://eprints.gla.ac.uk

1
Partitioning of microbially respired CO
2
between indigenous and exogenous carbon sources during
1
biochar degradation using radiocarbon and stable carbon isotopes
2
3
Niels C. Munksgaard
a, b
, Anna V. McBeath
a, c
, Philippa L. Ascough
d
, Vladimir A. Levchenko
e
, Alan
4
Williams
e
, Michael I. Bird
a, f
5
a
College of Science and Engineering and Centre for Tropical Environmental and Sustainability
6
Science, James Cook University, Smithfield, QLD 4878, Australia
7
b
Research Institute for the Environment and Livelihoods, Charles Darwin University, Casuarina, NT
8
0810, Australia
9
c
Department of Agriculture and Fisheries, Queensland Government, South Johnstone, QLD 4859,
10
Australia
11
d
NERC Radiocarbon Facility, Scottish Universities Environmental Research Centre (SUERC), Scottish
12
Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, UK
13
e
Australian Nuclear Science and Technology Organisation (ANSTO), Kirrawee DC, NSW 2232,
14
Australia
15
f
ARC Centre for Excellence for Australian Biodiversity and Heritage, James Cook University,
16
Smithfield, QLD 4878, Australia
17
18
Abstract
19
Pyrolised carbon in biochar can sequester atmospheric CO
2
into soil to reduce impacts of
20
anthropogenic CO
2
emissions. When estimating the stability of biochar, degradation of biochar
21
carbon, mobility of degradation products and ingress of carbon from other sources must all be
22
considered. In a previous study we tracked degradation in biochars produced from radiocarbon-free
23
wood and subjected to different physico-chemical treatments over three years in a rainforest soil.
24
Following completion of the field trial, we report here a series of in-vitro incubations of the
25
degraded biochars to determine CO
2
efflux rates,
14
C concentration and δ
13
C values in CO
2
to
26
quantify the contributions of biochar carbon and other sources of carbon to the CO
2
efflux. The
14
C
27
concentration in CO
2
showed that microbial degradation led to respiration of CO
2
sourced from
28
biochar carbon (≈ 0.5 - 1.4 μmoles CO
2
/ g biochar C / day) along with a component of carbon closely
29
associated with the biochars but derived from the local environment. Correlations between
14
C
30

2
concentration, δ
13
C values and Ca abundance indicated that Ca
2+
availability was an important
31
determinant of the loss of biochar carbon.
32
33
Key Words
34
Biochar,
13
C,
14
C, Respiration, Degradation, Immobilisation
35
36
Introduction
37
Biochar is pyrolyzed carbon (PyC) derived from the incomplete combustion of biomass. When
38
incorporated into soil, biochar has the potential to provide long-term carbon sequestration that is
39
potentially able to offset a significant fraction of anthropogenic emissions (Woolf et al. 2010, Wang
40
et al. 2014). However, biochar includes a range of carbon compounds with variable degrees of
41
resistance to degradation (Bird et al. 1999; Kanaly and Harayama 2000; Hammes et al. 2008; Bird et
42
al. 2015).
43
The degree to which biochar is susceptible to degradation is controlled by the temperature of
44
pyrolysis, the nature of the material pyrolized and environmental conditions that influence the
45
activity of microbial communities and organo-mineral interactions during degradation (e.g. soil type,
46
temperature, moisture, pH and Ca
2+
availability (Pietikainen et al. 2000; Hockaday et al. 2007;
47
Whittinghill and Hobbie 2012; Bird et al. 2017). Recent research has mostly emphasised the role of
48
microbial degradation of biochar (e.g. Forbes et al. 2006; Fang et al. 2014; Kuzyakov et al. 2014;
49
Tilston et al. 2016) and these studies have directly demonstrated respiration of PyC using both
50
13
C/
12
C and radiocarbon (
14
C) as tracers of PyC conversion into CO
2
, microbial biomass, and soil
51
organic carbon. In contrast, a year-long in-vitro experiment by Zimmerman (2010) found abiotic CO
2
52
production rates equivalent to those of microbial oxidation in several types of biochars.
53
Recently Bird et al (2017) examined the controls on the degradation of biochars produced at
54
different temperatures from radiocarbon-free wood by subjecting them to different physico-
55
chemical treatments over three years in a humid tropical rainforest soil in NE Australia. Mass
56
balance calculations and measurements of
14
C concentration in the biochars demonstrated a strong
57
relationship between degradation and loss of indigenous (biochar) carbon, with carbon losses offset
58
to various degrees by the simultaneous addition of exogenous (leaf litter derived) carbon from the
59
local environment. High net carbon loss in biochars pyrolised at 300
o
C implied a relatively rapid total
60
degradation of the material to gaseous or solubilized forms over a few decades. Substantially lower
61

3
net losses of C in biochar pyrolised at 500
o
C showed these biochars to be comparatively resistant to
62
degradation. The strong relationships between loss of indigenous carbon from the degraded
63
biochars and amount and δ
13
C values of CO
2
efflux in incubation trials led to two main hypotheses
64
which remained unproven: 1. Biochar degradation was predominantly microbial and 2. High local
65
Ca
2+
concentrations immobilized degradation products in situ at high pH, rather than leaching and
66
loss of degradation products at low pH.
67
Here we present new evidence of the role of microbial activity in the degradation of the biochar
68
samples previously studied by Bird et al. (2017). We determined the efflux rate of CO
2
in in-vitro
69
incubation experiments and measured both
14
C concentration and δ
13
C values in the CO
2
efflux with
70
the aim of quantifying the contributions of indigenous radiocarbon-free PyC and exogeneous C
71
sources to CO
2
efflux from degrading biochar. We also tested the hypothesis that high local Ca
2+
72
concentrations lead to the immobilization of degradation products on the biochars.
73
74
Methods
75
Biochar samples
76
Detailed characteristics of the initial biochar material and the field trial was reported by Bird et al.
77
(2014, 2017). In brief, a c. 8 million year old wood log obtained from a brown coal seam was
78
pyrolyzed at 305, 414 or 512 ˚C using the system described by Bird et al. (2011). The radiocarbon
79
contents of the initial biochars were negligible and the TOC content and the proportion of stable
80
polycyclic aromatic carbon (SPAC) at high temperature increased with increasing temperature of
81
pyrolysis (McBeath et al. 2015). As temperature increases, the number of carbon rings increases,
82
leading to the development of recalcitrant microcrystalline graphitic sheets (Preston and Smith
83
2011). The biochar was used in a 3-year environmental degradation trial at the James Cook
84
University Daintree Rainforest Observatory, Cape Tribulation, Queensland (16.103
o
S; 145.447
o
E;
85
70m asl). This site is in a hot (mean monthly temperature ranging from 22 - 28 ˚C) and humid (3,500
86
mm annual rainfall) rainforest environment, where interactions between biochars and the
87
environment can be expected to be comparatively rapid.
88
In the field trial, aliquots of each biochar type contained in triplicate 125 μm aperture nylon mesh
89
bags, were pegged to the soil surface from June 2009 to August 2012 and subjected to one of the
90
following four treatments: (i) NL - all litter removed from the surface and aliquots laid directly on the
91
soil surface; (ii) L - as for NL but aliquots then covered with a ~5 cm thick layer of local leaf litter
92
replenished each six months; (iii) NL-LM - as for NL but aliquots then covered with a ~5 cm thick
93

4
layer of limestone chips (sieved at 2-10 mm); (iv) L-LM - as for NL but aliquots covered with a layer of
94
limestone chips (sieved at 2-10 mm) mixed with an equal volume of periodically replenished local
95
leaf litter each six months. The purpose of the limestone chips was to increase local pH, as alkaline
96
conditions have been shown to be a significant determinant of PyC degradation behaviour
97
(Braadbaart et al., 2009; Huisman et al., 2012).
98
Following three years of environmental exposure, Bird et al. (2017) identified correlated increases in
99
ash content (mineral matter after combustion at 550
o
C), mass of organic carbon, radiocarbon
100
concentration and decrease in
13
C values in the biochars. The changes were more substantial in
101
300˚C compared to 500˚C biochars and there were substantial changes in both biochar types
102
according to their physicochemical treatment. The changes were most pronounced in the no-litter
103
(NL) treatments, followed by the changes in the litter (L) treatments while both the no-litter
104
limestone (NL-LM) and litter limestone (L-LM) treatments were the least changed after three years.
105
In vitro incubations and δ
13
C
CO2
and
14
C
CO2
measurements
106
In the present study we conducted two in-vitro experiments to measure the rate and isotopic
107
composition of CO
2
efflux from the field-exposed degraded biochar samples. A small-volume
108
experiment was initially carried out over 66 days to investigate whether there were changes in the
109
rate of CO
2
production and whether changes in C sources may be revealed through changes in the
110
stable isotopic composition of the CO
2
efflux. Subsequently we conducted a second shorter-term
111
(14-18 days) in-vitro experiment to produce larger sample volumes necessary for the measurements
112
of CO
2
14
C concentration.
113
In the longer-term experiment, aliquots (80 mg) of dried 300
o
C and 500
o
C biochar (each treatment
114
in duplicate) were placed on a wet pre-combusted quartz sand bed (750 mg) in 12 mL capacity
115
Exetainer vials sealed with a septum cap for incubation in the dark at 25
o
C over 66 days, with no
116
applied nutrient source. Milli-Q
TM
grade water, filtered at 0.2 µm and UV-sterilized was added to the
117
surface level of the combusted sand. The wet sand base provided a stable source of moisture
118
available by capillary action without saturating the samples over the course of the experiment. Vials
119
were filled with CO
2
-free air immediately after sample loading. No new microbial material was added
120
as the purpose was to measure the response of a reinvigorated microbial population present on the
121
biochars in relation to the labile carbon supply inferred to exist based on the radiocarbon
122
measurements of Bird et al. (2017).
123
Duplicate samples of the pre-exposure 300 and 500
o
C biochar and two vials with wet sand only
124
(blanks) were included in the incubation experiment. The volumetric concentration and
13
C values
125

Citations
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01 Jan 2015
TL;DR: In this paper, the authors meta-analyzed the biochar decomposition in soil and estimated its mean residence time (MRT), and concluded that only a small part of biochar is bioavailable and that the remaining 97% contribute directly to long-term carbon sequestration in soil.
Abstract: The stability and decomposition of biochar are fundamental to understand its persistence in soil, its contribution to carbon (C) sequestration, and thus its role in the global C cycle. Our current knowledge about the degradability of biochar, however, is limited. Using 128 observations of biochar‐derived CO2 from 24 studies with stable (13C) and radioactive (14C) carbon isotopes, we meta‐analyzed the biochar decomposition in soil and estimated its mean residence time (MRT). The decomposed amount of biochar increased logarithmically with experimental duration, and the decomposition rate decreased with time. The biochar decomposition rate varied significantly with experimental duration, feedstock, pyrolysis temperature, and soil clay content. The MRTs of labile and recalcitrant biochar C pools were estimated to be about 108 days and 556 years with pool sizes of 3% and 97%, respectively. These results show that only a small part of biochar is bioavailable and that the remaining 97% contribute directly to long‐term C sequestration in soil. The second database (116 observations from 21 studies) was used to evaluate the priming effects after biochar addition. Biochar slightly retarded the mineralization of soil organic matter (SOM; overall mean: −3.8%, 95% CI = −8.1–0.8%) compared to the soil without biochar addition. Significant negative priming was common for studies with a duration shorter than half a year (−8.6%), crop‐derived biochar (−20.3%), fast pyrolysis (−18.9%), the lowest pyrolysis temperature (−18.5%), and small application amounts (−11.9%). In contrast, biochar addition to sandy soils strongly stimulated SOM mineralization by 20.8%. This indicates that biochar stimulates microbial activities especially in soils with low fertility. Furthermore, abiotic and biotic processes, as well as the characteristics of biochar and soils, affecting biochar decomposition are discussed. We conclude that biochar can persist in soils on a centennial scale and that it has a positive effect on SOM dynamics and thus on C sequestration.

418 citations

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Abstract: Pyrogenic Carbon (PyC) is ubiquitous in global environments, and is now known to form a significant, and dynamic component of the global carbon cycle, with at least some forms of PyC persisting in their depositional environment for many millennia. Despite this, the factors that determine the turnover of PyC remain poorly understood, as do the physical and chemical changes that this material undergoes when exposed to the environment over tens of thousands of years. Here, we present the results of an investigation to address these knowledge gaps through chemical and physical analysis of a suite of wood PyC samples exposed to the environment for varying time periods, to a maximum of >90,000 years. This includes an assessment of the quantity of resistant carbon, known as Stable Polyaromatic Carbon (SPAC) vs. more chemically labile carbon in the samples. We find that, although production temperature is likely to determine the initial “degradation potential” of PyC, an extended exposure to environmental conditions does not necessarily mean that remaining PyC always progresses to a “SPAC-dominant” state. Instead, some ancient PyC can be composed largely of chemical components typically thought of as environmentally labile, and it is likely that the depositional environment drives the trajectory of preservation vs. loss of PyC over time. This has important implications for the size of global PyC stocks, which may have been underestimated, and also for the potential loss of previously stored PyC, when its depositional environment alters through environmental or climatic changes.

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  • ..., associated with the remaining PyC structure); this process seems to be mediated by soil chemistry, particularly the presence of calcium ions (Bird et al., 2017; Munksgaard et al., 2018)....

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Abstract: Biochar is proposed as an option to sequester carbon (C) in soils and promote other soil-based ecosystem services. However, its impact on soil biota from micro to macroscale remains poorly understood. We investigated biochar effects on the soil biota across the soil food web, on plant community composition and on biomass production. We conducted a field experiment in a nature restoration grassland testing four treatments: two biochar types (herbaceous feedstock pyrolyzed at 400 °C or 600 °C – hereafter B400 and B600), and a positive (i.e. unpyrolysed biochar feedstock, hereafter Hay) and negative (no addition) control. Responses of plants and soil biota were evaluated one and three years after establishing the treatments. Soil pH and K concentrations increased significantly in the B600 treatment. Mite abundances were significantly higher in B400 whereas nematode abundances were highest in Hay (1st year) and lowest in B400 (3rd year). Other soil fauna groups (enchytraeids and earthworms) varied more between years than between treatments. Legume cover increased significantly in the biochar treatments but this effect was transient. Legumes, grasses and primary productivity also showed a statistically significant Treatment x Year interaction due to transitory effects that were no longer present by the 3rd year. Our results suggest that biochar produced from meadow cuttings and applied at the 10 t/ha rate cause transitory impacts on soil biota abundance and plant communities over the 3-year timeframe used for this experiment. Therefore, this type of biochar could potentially be used for soil carbon sequestration, with minimal impacts on soil biota abundance or diversity, within the groups studied here, or plant biodiversity and productivity. Further research is required to investigate the longer-term impacts of this potential soil C storage sink.

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References
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TL;DR: The maximum sustainable technical potential of biochar to mitigate climate change is estimated, which shows that it has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset.
Abstract: Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO 2 ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO 2 -C equivalent (CO 2 -C e ) per year (12 % of current anthropogenic CO 2 -C e emissions; 1 Pg = 1 Gt), and total net emissions over the course of a century by 130 Pg CO 2 -C e , without endangering food security, habitat or soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset.

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TL;DR: In this paper, the authors identified soil factors that retard mineralization of C in soils from correlations of C contents of soils with other properties such as clay content and base status, and showed that the rate and extent of C mineralization depends on the chemistry of the added organic matter and interaction with clays of the microbial biomass and metabolites.
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TL;DR: Because biochar lability was found to be strongly controlled by the relative amount of a more aliphatic and volatile component, measurements of volatile weight content may be a convenient predictor of biochar C longevity.
Abstract: Pyrogenic or “black” carbon is a soil and sediment component that may control pollutant migration. Biochar, black carbon made intentionally by biomass pyrolysis, is increasingly discussed as a possible soil amendment to increase fertility and sequester carbon. Though thought to be extremely refractory, it must degrade at some rate. Better understanding of the rates and factors controlling its remineralization in the environment is needed. Release of CO2 was measured over 1 year from microbial and sterile incubations of biochars made from a range of biomass types and combustion conditions. Carbon release from abiotic incubations was 50−90% that of microbially inoculated incubations, and both generally decreased with increasing charring temperature. All biochars displayed log−linearly decreasing mineralization rates that, when modeled, were used to calculate 100 year C losses of 3−26% and biochar C half-lives on orders ranging from 102 to 107 years. Because biochar lability was found to be strongly controll...

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TL;DR: It is suggested that charcoal from burning can support microbial communities, which are small in size but have a higher specific growth rate than those of the humus, and also ActC.
Abstract: Wildfires produce a charcoal layer, which has an adsorbing capacity resembling activated carbon. After the fire a new litter layer starts to accumulate on top of the charcoal layer, which liberates water-soluble compounds that percolate through the charcoal and the unburned humus layer. We first hypothesized that since charcoal has the capacity to adsorb organic compounds it may form a new habitat for microbes, which decompose the adsorbed compounds. Secondly, we hypothesized that the charcoal may cause depletion of decomposable organic carbon in the underlying humus and thus reduce the microbial biomass. To test our hypotheses we prepared microcosms, where we placed non-heated humus and on top one of the adsorbents: non-adsorptive pumice (Pum), charcoal from Empetrum nigrum (EmpCh), charcoal from humus (HuCh) or activated carbon (ActC). We watered them with birch leaf litter extract. The adsorbing capacity increased in the order Pum

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"Partitioning of Microbially Respire..." refers background in this paper

  • ...…and environmental conditions that influence the activity of microbial communities and organo-mineral interactions during degradation (e.g. soil type, temperature, moisture, pH and Ca2+ availability (Pietikäinen et al. 2000; Hockaday et al. 2007; Whittinghill and Hobbie 2012; Bird et al. 2017)....

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Frequently Asked Questions (1)
Q1. What are the contributions in this paper?

In a previous study the authors tracked degradation in biochars produced from radiocarbon-free 23 wood and subjected to different physico-chemical treatments over three years in a rainforest soil. 24 Following completion of the field trial, the authors report here a series of in-vitro incubations of the 25 degraded biochars to determine CO2 efflux rates, C concentration and δC values in CO2 to 26 quantify the contributions of biochar carbon and other sources of carbon to the CO2 efflux.