Addition of External Organic Carbon and Native Soil Organic Carbon Decomposition: A Meta-Analysis
Citation: Zhang W, Wang X, Wang S (
Addition of External Organic Carbon and Native Soil Organic Carbon Decomposition: A Meta-Analysis
Weidong Zhang 0
Xiaofeng Wang 0
Silong Wang 0
Caroline P. Slomp, Utrecht University, The Netherlands
0 1 Huitong Experimental Station of Forest Ecology, State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences , Shenyang , PR China , 2 Huitong National Research Station of Forest Ecosystem , Huitong , PR China , 3 Graduate School of Chinese Academy of Sciences , Beijing , China
Background: Extensive studies have been conducted to evaluate the effect of external organic Carbon on native soil organic carbon (SOC) decomposition. However, the direction and extent of this effect reported by different authors is inconsistent. Objective: The objective was to provide a synthesis of existing data that comprehensively and quantitatively evaluates how the soil chemical properties and incubation conditions interact with additional external organic C to affect the native SOC decomposition. Data Source: A meta-analysis was conducted on previously published empirical studies that examined the effect of the addition of external organic carbon on the native SOC decomposition through isotopic techniques. Results and Conclusions: The addition of external organic C, when averaged across all studies, enhanced the native SOC decomposition by 26.5%. The soil with higher SOC content and fine texture showed significantly higher priming effects, whereas the soil with higher total nitrogen content showed an opposite trend. The soils with higher C:N ratios had significantly stronger priming effects than those with low C:N ratios. The decomposition of native SOC was significantly enhanced more at early stage of incubation (,15d) than at the later stages (.15d). In addition, the incubation temperature and the addition rate of organic matter significantly influenced the native SOC decomposition in response to the addition of external organic C.
Soil organic carbon (SOC) decomposition is directly linked to
atmospheric CO2 emissions, and consequently, to global climate
change [1,2]. Thus, extensive studies have been conducted to
quantify, model, and predict the CO2 decomposition rate across a
range of ecosystem types. Microbial activity, environmental
variables (e.g., soil temperature, moisture) [3,4], the initial soil
chemical properties, and SOC inputs (e.g., plant litter, dead fine
roots, and root exudates) generally influence the decomposition
dynamics. When the inputs are not considered, SOC
decomposition depends on SOC quality , microbial activity, and
temperature. In all ecosystems, plant litter, dead fine roots, and
root exudates serve as SOC inputs and influence its output
through their priming effect (PE) . PEs are defined as short-term
changes in the turnover of SOC caused by the addition of external
organic C . A positive PE accelerates the decomposition of the
native SOC, whereas a negative PE retards it. Numerous studies
have been performed on the PE through the addition of 13C- or
14C-labeled plant materials  or easily degraded C sources
[11,12] to simulate the input of organic C that occurs in natural
ecosystems. However, the direction of the PEs reported by
different authors is inconsistent. Several studies have shown that
the native SOC decomposition is significantly enhanced by the
addition of external organic C . However, others have
reported either a suppression effect or no significant change
. The possible mechanisms of PEs have been suggested by
previous studies [7,17], but the relationship between the SOC
decomposition and external organic C is still unclear.
Consequently, the direction and extent of the PE is still unpredictable.
The different responses of native SOC decomposition to the
addition of external organic C across various studies can be
partially explained by differences in soil chemical properties. The
PE is driven by soil microorganisms ; thus, the PE of soil with
higher SOC, which often indicates a higher soil microbial biomass
, is activated more by the external organic C. As suggested by
Kuzyakov, the PEs in soil with higher SOC content are stronger
than those in poor soil . While the SOC content clearly
influences the extent of priming effect induced by external organic
C, other factors, including both nutrient status and incubation
conditions, may be important in explaining the large difference in
the extent of priming effects observed in the literature.
Meta-analysis quantitatively assesses the evidence for or against
a particular hypothesis, which is advantageous over narrative or
qualitative reviews that lack sampling rigor and robust statistical
methods . Meta-analysis methodology has been used widely by
ecologists to analyze the response of soil respiration and
aboveground plant growth to experimental ecosystem warming
, the effect of forest fires on soil microbial biomass and N
mineralization , and the response of crop productivity to
biochar application . Although several authors have reviewed the
studies in our analysis and the possible mechanisms of the PEs, to
our knowledge, no meta-analysis has been done on this topic.
The objective of the current meta-analysis is to provide a
comprehensive and quantitative synthesis of the effect of external
organic C on native SOC decomposition. This paper evaluates
how soil chemical properties (e.g., SOC content, total nitrogen
content, and C:N ratio) and the incubation conditions (i.e.,
substrate quality, incubation stage, incubation temperature, and
addition rate) influence the response of the native SOC
decomposition to the addition of external organic C.
More than 250 studies on SOC decomposition published since
1980 were reviewed and studies wherein the external organic C
was experimentally manipulated under laboratory conditions were
identified (22 studies and some unpublished data, Table S1). In
many of these studies, the effects of environmental factors such as
added nitrogen, temperature, and soil moisture on the soil organic
carbon decomposition were evaluated. This meta-analysis focused
on the influence of additional external organic C on native soil
organic carbon decomposition. The database for the meta-analysis
was created according to the following criteria: First, the
experimental studies reported direct manipulation of the soil
through the addition of external organic C in the form of plant
material and other small molecules, and had to assess the response
in terms of the CO2 release dynamics. Second, the CO2
production from organic C by the native soil system can be
separated by 13C- or 14C-labeled technique, and the native SOC
decomposition rate can be measured directly. Third, sufficient
information regarding environmental variables, such as native soil
chemical properties and experimental conditions, are provided in
the studies. For example, a large number of the studies that
examined the relationship between live roots and the soil reported
native soil respiration using isotopic techniques . However,
the rate of root exudates secretion is unquantifiable and
experimental conditions such as temperature were not provided.
Thus, studies those examined the effects of living plant on native
SOC decomposition were all excluded from the current analysis.
The native SOC decomposition rate is expressed as mg C kg21
soil h21. In some studies, the results were displayed as the CO2
production accumulation dynamics during the incubation stage.
The SOC decomposition rate and its variance were estimated
using the following equations:
where CO2 Ct+1 and CO2 Ct are the means of the accumulative
CO2 production derived from native soil at time t and t+1,
respectively; St and St+1 are the standard deviations of the
accumulative CO2 production derived from native soil at time t
and t+1, respectively; Nt and Nt+1 are the number of replicates for
the respective times; and t is the sampling time during incubation.
The Origin 7.0 software with the Digitize. OPK plug-in was
used to extract data from the figures.
In addition to the native SOC decomposition rate, the data on
the related environmental variables in these studies were also
collected, including the soil chemical properties (SOC content,
total nitrogen content, C:N ratio, and soil texture) and the
experimental conditions (substrate quality, incubation stage,
incubation temperature, and addition rate). The SOC content
ranged from 6 g kg21 to 340 g kg21. The total nitrogen content
ranged from 0.63 g kg21 to 26.8 g kg21. The C:N ratio ranged
from 0.63 to 38.1. The addition rate ranged from 0.1% to 33.3%
of SOC. The incubation temperature ranged from 6.5uC to
36.5uC. More than 15 different organic C types were represented
in the database. The incubation stage was monitored for periods
ranging from ,1 d to 98 d.
The strength of the effect for each paired observation was
calculated as the natural log of the response ratio R = xt/xc, where
xt is the mean of the added organic C for the treatment and xc is
the mean of the associated control without the addition of organic
C. The variance of ln R was estimated using the following
equation: vlnR = st2/(Nt 6 Xt2) + sc2/(Nc 6 xc2) . The standard
methods in the software MetaWin (version 2.0) were used to
perform the analyses. The procedure is analogous to the ANOVA,
wherein the total heterogeneity (Qt) for a group of comparisons is
partitioned into the within-group (Qw) and between-group (Qb)
heterogeneity. The Qw is tested against a chi-square distribution
with m 1 degrees of freedom, where m is the number of groups.
In addition to examining the overall effect of organic C addition
on native soil organic carbon decomposition, this study also
determined whether the chemistry of the native soil and the
experimental conditions elicited a significantly different response
to the addition of organic C. We hypothesized that the response of
the native soil organic carbon decomposition to the addition of
organic C is related to the properties of native soil. Therefore, the
combined data were grouped according to the chemistry of native
soil. The studies were categorized based on the soil organic carbon
content (,20 g kg21 or .20 g kg21), the total nitrogen content
(,2 g kg21 or .2 g kg21), the C:N ratio (,10 or .10) and soil
texture (Coarse, Medium and Fine texture). Furthermore, the
effects of the experimental conditions on the response of the native
SOC decomposition to organic C were also determined. Generally
speaking, small-molecule substrates such as fructose, glucose and
amino acids are easy to be utilized by microorganisms, and defined
as high-quality substrates in the current analysis. Otherwise, plant
materials, such as maize, slurry, and wheat straw with lower
microbial availability are defined as low-quality substrates. The
data were also grouped according to the substrate quality of the
added organic C (Low or High), the incubation stage (,15 or .15
d), the incubation temperature (#20, 2025, or .25uC), and the
addition rate in terms of the % SOC (,4% or .4%). That
information can be found in Table S2. Besides, matrix style table
was provided in Table S3 to show which different substrates were
used in which soil types (according to soil C and N content). The
heterogeneity among the categorical groups was partitioned to
determine the effect of each category (e.g., incubation
temperature) on the response of the native SOC decomposition to the
addition of organic C. The Qb for all categorical variables was
tested at a significance level of 0.05. Then, the mean effect strength
for each category was calculated, with the 95% confidence
intervals (CIs) generated using the bias-corrected bootstrap
procedure in the MetaWin software. The cumulative effect of all
studies or each categorical group was considered significant if its
95% CI did not reach zero, and the cumulative effect is
significantly different between two categorical groups if their
95% CIs were non-overlapping .
Results and Discussion
This meta-analysis is the first quantitative synthesis of published
literature describing the native SOC decomposition in response to
the addition of external organic C. A large number of studies have
examined the effects of external organic C addition on native SOC
decomposition, but little is known about the extent of the native
SOC response. Of the 520 observations in the meta-analysis, 69
showed significant inhibition effects, 181 showed significant
stimulation effects, and 270 showed no statistically significant
effects. Only 50.1% of the heterogeneity of the native SOC
decomposition rates after the addition of organic C can be
explained by the decomposition rates in the associated controls
(Fig. 1). The analysis showed that the addition of external organic
C enhances the native SOC decomposition by 26.5% (Fig. 2).
However, the amplitude of the native SOC decomposition
response was very large (ranged from 95.1% inhibition to
1207% stimulation). Previous studies have suggested mechanisms
for the so-called PEs. Generally speaking, positive priming effects
were induced due to microbial growth and the accompanying
increased enzyme production; possible negative priming
mechanisms include the toxicity of the substrates to microorganisms and
preference uptake of C-rich substrates by microorganisms .
However, the complicated interaction of the external organic C
and the native SOC has made predicting the effect of the addition
of external organic C difficult. The response of the native SOC
decomposition largely depends on the chemical properties of the
soil and the experimental conditions.
The results from the between-group heterogeneity analysis
showed that the response of native SOC decomposition to external
organic C addition depends on the chemical properties of the soil,
including the SOC content, TN content, C:N ratio, and soil
texture (Table 1). The SOC content in the current analysis ranged
from 6 g kg21 to 340 g kg21. Soil with a higher SOC content
generally has higher decomposition rates, which can be confirmed
by the observed SOC decomposition rate of the controls. The
CO2 release rate in soil with higher SOC content (.20 g kg21)
was 0.80 mg C kg21 soil h21, which is significantly higher than
that of soil with low SOC content (0.22 mg C kg21 soil h21).
Additionally, the SOC content also influenced the strength of the
PE. The analysis showed that the addition of organic C
significantly enhances the decomposition of native SOC with
low (,20 g kg21) and high (.20 g kg21) SOC content by 29.0%
and 43.2%, respectively. This result is consistent with that of
Kuzyakov, who suggested that the PEs in soils rich in C are more
intense than those in poor soils .
The PE strength also depends on the soil N content (Table 1),
which ranged from 0.63 g?kg21 to 26.8 g?kg21 in this analysis.
Soils with higher N content exhibited higher SOC decomposition
rates in the controls. However, the response of PE to N content
was contrary to that of SOC decomposition. The overall PEs in
soil with N content below 2 g?kg21 was 44.3%, significantly higher
than that in soils with higher N content (28.9%). A similar
response pattern was also observed for the C:N ratio. These
phenomena are likely related to the limitation in the soil microbial
nutrient. Microorganisms generally require C and other nutrients
at characteristic stoichiometric ratios. In soils with lower N content
and higher C:N ratios, the microorganism activity caused by
organic C addition involves the scavenging of N from native
SOM, resulting in more intense PEs.
The manner by which soil texture might influence the extent of
priming effects has not been paid considerable attention. In this
study, we observed significant between-group heterogeneity (Qb)
among soils with different soil textures (Table 1). Soils with fine
texture have significantly higher priming effect (56.7%) than soils
with medium and coarse textures (Fig. 2). This phenomenon could
be attributed to the low N content and high C:N ratio of these
soils. In this case, microorganism activity caused by organic C
addition decomposed the native SOM more intensely to obtain
available N for growth, resulting in the higher priming effects.
However, the difference in the priming effects between soils with
medium (9.1%) and coarse (8.5%) textures was not significant. The
data in the coarse texture category were derived from only three
studies, which caused large variation within groups (Fig. 2). Thus,
another factor other than soil texture may account for the
observed extent of priming effect.
The response of native SOC decomposition to organic
substrates addition also depended on experimental conditions
(Table 1; Fig. 3). PE strength changed because of the amount of
added C and energy, as suggested by Blagodatskaya and
Kuzyakov . A higher rate of organic C addition likely
activates more microorganisms, resulting in a stronger PE. SOC
content differed among various studies. Hence, the amount of
added substrate C was presented as percentage of SOC. In this
analysis, significant between-group heterogeneity was observed
among soils that received different levels of additional organic C
(Table 1). Corresponding to the percentage of additional organic
C to the SOC content, soils that received more substrate C showed
stronger PE (44.4%) than soils that received less substrate C
(29.6%). However, most studies on PE consisted of no more than
three addition rates. Strong evidence on the relationship between
the strength of PEs and addition rate of organic C is still lacking.
The minimum addition rate of organic C that induces a PE and
the maximum PE that occurs when excess external organic C is
added into soil remain unknown. Studies involving more addition
rates are still needed to clarify this relationship further.
PEs are defined as short-term changes in the turnover of soil
organic matter caused by comparatively moderate treatments of
the soil . The duration of this PEs remains unknown. Perelo
and Munch  observed significant positive PE that lasted for
more than 80 d after white mustard residues were added into the
soil. However, whether PEs has commonly long durations remain
undetermined. The present analysis suggests that PE strength is
influenced by the incubation stage (Fig. 3). The extent of PEs
(33.9%) during the early stage of incubation (,15 d) was
significantly higher than that (18.4%) during the later stage (.15
d). This result is reasonable considering that the organic substrates
in the soil decreased during incubation as a result of
decomposition. Previous studies have shown that the extent of PE decrease
with incubation , and this meta-analysis validated this finding.
The external organic C involved in the current analysis included
low-quality plant materials (e.g., maize, wheat, and slurry) and
high-quality small molecules (e.g., fructose, alanine, oxalic acid,
roots exudates, etc.). Given that high-quality substrates have more
available organic C for the microorganisms than low-quality
substrates, high-quality organic C is likely to induce higher PE
than the low-quality organic C. However, between-group
heterogeneity was not significant in terms of organic C quality (Table 1).
We attribute this discrepancy to two conditions. First, although
plant materials are categorized as low quality, they also contain
large amounts of small molecules that are highly available for
Figure 1. Relationship between soil organic C decomposition rates in organic material treatment and in control. Each point represents
a single comparison between the external organic C addition and control treatment. Values falling on the 1:1 line indicate a similar decay response for
organic C addition vs control treatments, whereas points above or below the line indicate a stimulation or inhibition in decomposition, respectively.
microorganisms. Second, the addition rate of organic C in the
lowquality group was much higher than that in the high-quality group
(13.66% of SOC vs. 2.85% of SOC), and the amount of added C
is important in determining the extent of PE as discussed above.
The response of priming effect to temperature has seldom been
studied, because most studies on priming effect used only one
incubation temperature. In this study, we observed significant
Soil organic C (g/kg)a
Material quality (Low or High)b
Incubation time (days)b
Incubation temperature (uC)b
Material added (as % soil C)b
between-group heterogeneity among different incubation
temperature ranges. As suggested by Kuzyakov , the PE below 20uC
(38.4%) was significantly more intense than above 20uC (20.17%).
In addition, based on short-term incubation (24 h), we found that
the extent of the PEs induced by both glucose and Chinese fir litter
in two soils (natural forest and Chinese fir plantation soils)
consistently decreased along the temperature gradient
(unpublished data), consistent with Kuzyakovs prediction . One
possible explanation for this phenomenon is the higher native
SOC decomposition level in the controls under higher
temperatures. In this case, native SOC decomposition level hardly
increased further through substrate addition, which resulted in
lower PE. However, the extent of PE above 25uC (88.2%) was
much higher than that below 25uC. We attribute this inconsistent
trend to other factors in the current analysis. For example, this
group only contained 68 observations and applied considerably
more organic substrate (11.1% of SOC). The relationship between
temperature and PE is evidently important in predicting the
response of SOC decomposition in future warming, and related
research is fairly insufficient. Further studies are needed to
examine directly temperature sensitivity of PE with other factors.
In summary, the results indicate that addition of external
organic C alters the native SOC decomposition, but the direction
and extent of response are mediated by interaction between native
soil chemical properties and incubation conditions. Native SOC
decomposition was enhanced under the following conditions: SOC
content higher than 20 g?kg21; total N content below 2 g?kg21;
C:N ratio higher than 10 and soils with fine texture. In addition,
PE was significantly stronger during the early incubation stage
(,15 d) and at higher organic C addition rates. This meta-analysis
indicated the importance of organic C input on soil carbon cycling
in ecosystems. The strength of this effect is closely related to the
native soil chemical properties and environmental conditions.
Moreover, given that microbes drive the priming effect, different
soils with diverse microbial communities may respond to substrate
addition differently. However, information on the influence of
microbial community in native soil on priming effect is very
limited, and this phenomenon requires further studies.
Substrates and soils used in the published
Conceived and designed the experiments: WZ SW. Performed the
experiments: XW WZ. Analyzed the data: SW. Contributed reagents/
materials/analysis tools: SW. Wrote the paper: WZ.
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