Pyrogenic organic matter (PyOM) is considered an important soil carbon (C) sink. However, there are evidences that its addition to soil may induce a priming effect (PE) thus influencing its C abatement potential. The direction, the size and the mechanisms responsible for PyOM induced PE are far from being understood. We collected approximately 650 data points from 18 studies to analyse the characteristics of the PE induced by PyOM. The database was divided between the PE induced on the native soil organic matter and on fresh organic matter. Most of the studies were short-term incubation therefore the projections of findings on the long term may be critical. Our findings indicate that over 1 year PyOM induces an average positive PE of 0.3 mg C g−1 soil on native soil organic matter and a PE of approximately the same size but opposite direction on fresh organic matter. We studied the correlation of PE with several properties of soil, of the added PyOM, and time after PyOM addition. We found that PyOM primes positively the native soil organic matter in the first 20 days while negative PE appears in a later stage. Negative PE was correlated with the soil C content. PyOM characterized by a low C content induced a higher positive PE on native soil organic carbon. No correlation was found between the factors record in our database and the PE induced on the fresh organic matter. We reviewed the mechanisms proposed in literature to explain PE and discussed them based on findings from our meta-analysis. We believe that the presence of a labile fraction in PyOM may trigger the activity of soil microorganisms on the short term and therefore induce a positive PE, while on the long term PyOM may induce a negative PE by promoting physical protection mechanisms.
Pyrolysis process consists in the heating of organic matter under anoxic conditions. Pyrolysis can occur during wildfires, where local and temporary limitation of oxygen can occur, or it can be a controlled process to produce heat and pyrogenic organic matter (PyOM), also termed biochar, from agricultural residues (Lehmann & Joseph, 2009). We define here PyOM as the residues of pyrolysis of biomass. PyOM can represent an important fraction of soil carbon (C) both in soils amended with biochar and in natural soils. In fact PyOM is virtually present in every soil (Preston & Schmidt, 2006), and massive inputs of PyOM to the soil occur every year by forest burning. The estimations of PyOM global production vary between 40 and 600 Tg C yr−1 (Crutzen & Andreae, 1990; Kuhlbusch & Crutzen, 1995). Such a high input rate together with its centennial mean residence time (Singh et al., 2012a) makes PyOM a fundamental component of soil organic matter. Moreover, PyOM content in soil is likely to increase in future due to the increasing fire frequency (Moritz et al., 2012; Flannigan et al., 2013) and to the growing interest in the use of PyOM – biochar – as a tool to increase soil fertility and store C at the same time (Lehmann, 2007; Verheijen et al., 2010).
Many recent studies indicated that PyOM beside from being a stable C pool can also affect the stability of non-PyOM C pools, i.e. it may induce a priming effect (PE). Here we adopt the terminology from Bingeman et al. (1953) who use the term PE to indicate the change in the mineralization rate of the soil organic matter in a soil receiving an input of exogenous organic matter. We specifically define PE as positive when the addition of PyOM increases the mineralization rate of non-PyOM C and negative, when PyOM decreases the mineralization rate of non-PyOM C. PyOM was previously found to induce a positive PE (Wardle et al., 2008; Luo et al., 2011) or a negative PE (Kuzyakov et al., 2009; Cross & Sohi, 2011). Also, no effect has been reported (Abiven & Andreoli, 2010; Santos et al., 2012).
In the literature, PE was often reported as a response of soil to the addition of easily decomposable substrates like sugar (Wu et al., 1993; Conde et al., 2005; Blagodatskaya et al., 2007, 2011; Blagodatsky et al., 2010; Kuzyakov, 2010), cellulose (Fontaine & Barot, 2005) or plant material, e.g. rye grass (Wu et al., 1993). Surprisingly it was found that also PyOM induced a PE, although it has a low microbial availability (Kuzyakov et al., 2009; Singh et al., 2013). Several mechanisms have been proposed to explain the PE observed in individual studies; however a comprehensive analysis of results and mechanisms is still lacking.
Predicting the importance of PE induced by PyOM on the mineralization of non-PyOM C is crucial to assessment of PyOM C abatement potential. Woolf & Lehmann (2012), predicted the importance of PyOM induced PE on C stabilization in agroecosystem by using a process based model, however very little is known on the mechanisms responsible for PyOM induced PE. For example, it is still not known whether the PE is related to the type of soil and to the characteristics of the primed soil organic matter, or to the characteristics of the PyOM. It is also not known whether the PE is important on the long term, or if it is only a short-term phenomenon. The aim of this study is to assess whether PyOM induces PE, its impact on soil C budget, and which factors are driving its size and direction. To reach these goals, we carried out a meta-analysis of the data reported in literature on PyOM induced PE separately on native soil organic matter and on fresh organic matter. We define here fresh organic matter, as an organic input added to the soil, e.g. plant material or glucose.
Moreover, we reviewed the existing mechanisms presented in literature and discussed them based also on results from our meta-analysis.
The PE induced by PyOM on the native organic matter and on the fresh organic matter was considered separately. We extracted data from individual sampling dates, to take into account the effect of time on PE. We expressed PE as the increase in CO2 efflux derived from non-PyOM C pool compared to CO2 efflux in the control treatment, i.e. in the treatment without PyOM addition. Where CO2 efflux was not expressed as rate but as cumulative CO2 efflux, we transformed the cumulative values into fluxes and attributed it to the last day of the cumulative period. To give the same importance to each study, we attributed to each data point a weight, inversely related to the number of data extracted from the study, using Eqn (1):
where W is the weight attributed to individual data and Ndata is the number of data extracted from the study.
To estimate how soil respiration changed over time in treatments where no PyOM was added (i.e. in control treatments), we used a linear regression of soil respiration on log-transformed time (Fig. S1). For PE on native soil organic matter, we restricted our data collection to studies employing isotopic techniques to discriminate among the sources of CO2. In the case of studies using both labelled and unlabelled techniques (Zimmerman et al., 2011) we considered only the treatments with 13C-CO2 estimations. We consider here that only the 13C-based studies can provide reliable information and direct calculation to the PE. For the PE on native organic matter, we collected data from 12 publications (Table 1). Our data set was composed in total of 464 data points, each of them reporting PyOM C content, soil C content, time, soil pH, PyOM-C added per gram soil C added, PyOM parent biomass (wood or grass), soil texture (classes).
Table 1. Observations of priming on the native soil organic matter, for the study without isotopes the priming effect (PE) column reflect whether pyrogenic organic matter (PyOM) addition induced changes in soil respiration rather than in native soil organic matter mineralization. Only studies employing isotopes were then included in the meta-analysis
Positive PE, relatively higher in the low pH soil, higher from PyOM of 350 °C
Cometabolism, due to the high concentration of available (i.e. dissolved) C present in biochar. Moreover, the 350 induced more priming than the 700. Relative higher PE in low pH than in high pH (liming induced PE)
where PE is the priming effect, NSOMPyOM is the mineralization rate of native soil organic matter in soil amended with PyOM, and NSOMcon is the mineralization rate of native soil organic matter in control (soil without any amendment). All the terms are expressed as mg C-CO2 g−1 soil day−1.
For the PE induced by the interaction between PyOM and fresh organic matter, we compiled a data set from six studies (Table 2), consisting in a total of 198 data. We estimated PE using Eqn (3):
where MINwithPyOM is the mineralization of the fresh organic matter in the PyOM treatment and MINwithoutPyOM the mineralization in the control treatment.
Table 2. Studies investigating the priming effect (PE) on the fresh organic matter. The column entitled ‘Formula for PE’ indicates whether Eqns (4) or (3)was used to measure PE. Equation (4) was used when it was not possible to discriminate between the CO2 derived from pyrogenic organic matter (PyOM) and the CO2 derived from fresh organic matter
In the studies where PyOM induced PE on fresh organic was measured in a soil matrix (i.e. not in quartz sand like in Hamer et al. (2004), it is necessary to partition the CO2 between three sources: PyOM, native soil organic matter and fresh organic matter. This would imply the use of two of different isotopic labels, i.e. 14C and 13C, as in the study from Blagodatskaya et al. (2011). Since such set up has not been used so far in studies on fluxes in PyOM amended soils, we included also studies where isotopic techniques were not applied. Therefore when it was not possible to separate the flux of fresh organic matter from the other flux, we used Eqn (4):
where SRobserved was the SR in SR(soil+fresh_organic_matter+PyOM) − SRsoil, and SRtheoretical was the sum of (SRfresh_organic_,matter+soil − SRsoil) + (SRPyOM+soil − SRsoil), therefore attributing the whole change in SR to the a change in the mineralization of the fresh substrate. We reported the PE as percentage of fresh organic matter decomposition rate. We also calculated the PE and expressed it as mg C g C-fresh organic matter added, using Eqns (5) and (6):
To compare PE on fresh organic matter with the PE on native soil organic matter, we then expressed it as mg C-CO2 g−1 soil day−1. We recognize, that this value is affected by the quantity of fresh organic matter added to the soil in the different experiments.
We tested the following factors as explanatory variables for PE induced on fresh organic matter: time, fresh organic matter C : N ratio, PyOM-C, PyOM-N, PyOM-C : N ratio, fresh organic matter N content, fresh organic matter C, fresh organic matter addition rate and soil C.
To evaluate which factors are influencing PE, we used the model simplification approach described in Crawley (2007) consisting in seeking for the minimal adequate model. To estimate if the relation between the response variable (PE) and the explanatory variable was significant without presuming their distribution, we used the bootstrapping method with 1000 resampling iterations (Crawley, 2007) to establish whether the slope of the regression was significantly different from 0 (P < 0.05). The regression was weighted by giving to each point the weight in Eqn (1). All the statistical analyses were performed using the statistical software ‘R’. The software ‘g3Data’ (http://www.frantz.fi/software/g3data.php) was used to extract data from the figures.
Results and discussion of the meta-analysis
The maximum positive PE on native soil organic matter mineralization was observed by Cross & Sohi (2011) in an incubation experiment 15 days after PyOM addition and was equal to 0.04 mg C-CO2 g−1 soil day−1, while the minimum PE (i.e. the maximum negative PE) observed was equal to −0.02 mg C-CO2 g−1 soil day−1 after 90 days (Zimmerman et al., 2011). The maximum observation period was the incubation experiment by (Singh et al., 2012b), lasting for 5 years. Overall, most of the data points collected in the data base were short-term measurements. In fact 50% of the data points we collected were measured within <3 months after PyOM addition to the soil (Fig. S2). Overall, the average weighted PE observed in literature was 0.0020 ± 0.0003 mg C-CO2 g soil day−1 (P < 0.001, weighted t-test). Among the explanatory variables, time (logarithm transformed) and the interaction between time and PyOM-C content were significant (P < 0.001, regression slope different from 0, boot strapping). We observed that PE decreased with time, and that PyOM having a low C content induced more positive PE on the short term (Fig. S3). Most of the highest positive effect occurred within the first 20 days and with a low PyOM-C content, while most of the negative priming occurred on a longer time scale (Fig. 1) . Modelling PE over time, on the basis of data collected in the database, we observed that the PE was positive until 200 days, and then negative (Fig. 2). When PE was cumulated over time, it was reaching a neutral PE, i.e. the positive PE induced in the beginning was counterbalanced by the negative observed afterwards, approximately 600 days (Fig. 2). We did not observe a correlation between PE and the other variables recorded in our database: PyOM C content, soil C content, time, soil pH, PyOM-C added g−1 C added, PyOM parent biomass (wood or grass), soil texture (classes).
We calculated the integral of the curve relating PE and time, and we found that PyOM after 1 year of addition induced a cumulative positive PE equivalent to 0.3 mg C g−1 soil. This loss represented 15% (0.32 mg C-CO2 g−1 soil−1, Fig. 2) of the average soil respiration in control treatment, i.e. where no PyOM was added (2.1 mg C-CO2 g−1 soil−1, Fig. S1). The theoretical influence of PE on the abatement potential of PyOM-C is reported in Fig. S4. Given that we did not find a correlation between PE and the amount of PyOM-C added to the soil, we considered a fixed PE of 0.3 mg C g−1soil yr−1. The impact of PE on C abatement potential is inversely proportional to the amount of PyOM-C added per gram of soil. Therefore when low amounts of PyOM are added to the soil, the impact of PE can be relevant. In the studies collected in our database, the mode of the addition rate was about 10 mg PyOM-C g−1 soil (Fig. S4), in this case losses by PE would represent 3% of PyOM-C added. Such addition rate would correspond in an ideal soil having 1 g cm−3 and tilled down to 20 cm to an addition rate of 20 t PyOM-C ha−1. Considering a pyrolysis C yield of 37% (Woolf & Lehmann, 2012), approximately 54 t of agricultural residues-C would be necessary to produce such amount of PyOM. This would correspond to the summation of the agricultural residues produced over several years. Therefore, we believe that the quantity of PyOM added to the field would generally be lower than the mode of PyOM generally added in laboratory experiment and therefore PE may significantly reduce the abatement potential of PyOM.
In the data set on PE induced on fresh organic matter, the longest observation period was 1.5 years in the incubation experiment by Liang et al. (2010). The maximum observed PE was 53% and was observed in Novak et al. (2010) after 25 days, while the minimum was observed in Zavalloni et al. (2011) where the decomposition of fresh organic matter plus PyOM did not significantly differ from the decomposition of fresh organic matter alone. We found that the mean of PyOM induced PE on the fresh organic matter was negative and was −10 ± 2% (weighted t-test, P < 0.001) of the theoretical mineralization rate, this is equivalent to −3.9 ± 1.5 mg C-CO2 g C-fresh organic matter day−1 and to −0.005 ± 0.002 mg C-CO2 g−1 soil day−1 when expressed on a gram of soil basis. We could not find a relation between recorded factors and PE. Nevertheless, this could be attributed to the setups that differed considerably among the experiments, and this may have increased the variability in the response.
Woolf & Lehmann (2012) predicted using a process based model, that the cumulative PE induced by PyOM, at the end of the year, was negative. They assumed that the C stabilization effect of PyOM was linearly related to amount of PyOM content in soil, while we observed a correlation between PyOM-C and PE and no correlation with the application rate. If negative PE is considered to be positively correlated with PyOM content in soil, assuming a yearly input of PyOM to the soil, and given the low decomposition rate of PyOM, the C stabilization will increase year after year. Given our results, we believe that assuming the C stabilization to be proportional to PyOM contained in soil, may lead to an overestimation of C stabilized particularly when predicting dynamics over decades. Moreover Woolf & Lehmann (2012) considered only the effect of PyOM on the fresh organic matter, for which however very few experiment exist (Table 2), and did not take into account the effect that PyOM can have on native soil organic matter, which may counterbalance the impact on the fresh organic matter, as we observe from our results that the two effects have an opposite direction.
When looking at the PE induced by other types of organic matter, Sayer et al. (2011) found that litter input increased native soil organic mineralization by 13% over 1 year, while Crow et al. (2009) found that litter addition induced a priming of 15–21% over 1 year. This indicates that the PE induced by PyOM is equivalent to other types of organic inputs, despite its low decomposability.
Based on this meta-analysis, we conclude that both negative and positive PEs can coexist. Looking at the native soil organic matter, the positive PE occurs on the short term and more intensively, while negative PE is acting on the longer term with less intensity. The characteristics of the PyOM (C content) also seem to play a role, but this is directly related to the time, i.e. PyOM characterized by a low C content may induce a positive PE on the short term. While for the PE on the fresh organic matter, it was not possible to establish a relationship between the observed PE and the variables recorded in our database. Also, it has to be noticed that most of the studies included in the present meta-analysis are short term (Fig. S2) and therefore projections of the impact of PE on the long term, may suffer from a lack of data. Moreover it has to be considered that on the long term, the factors that may influence PE size and direction may vary, e.g. repeated addition of PyOM, seasonal variations of soil temperature and moisture. As these factors were constant in most of the studies considered (incubation studies) it was not possible to consider the effect of their variation over time.
In the literature, several mechanisms have been proposed to explain the PE induced by PyOM. In the following sections, we will review these mechanisms and evaluate how influential they can be considering also the meta-analysis results. We believe that the mechanisms hereafter described do not exclude each other and they may all contribute simultaneously to the resulting net PE.
The labile fraction mechanism
Our findings on the positive PE induced on native soil organic matter agree with the theory of Fontaine et al. (2003) and suggest that the addition of fresh organic carbon to the soil represents an energy source that increases the microbial biomass. According to the theory from Fontaine et al. (2003), if the added substrate is sufficiently complex this will favour the growth of k-strategist decomposers responsible for the decomposition of the native organic matter. In fact several studies observed an increase in soil microbial biomass after PyOM addition (Bruun et al., 2008; Steiner et al., 2008a; Kolb et al., 2009; Lehmann et al., 2011; Maestrini et al., 2014). Moreover PyOM, being a complex blend of molecules (Schmidt & Noack, 2000; Keiluweit et al., 2010; Spokas, 2010), is likely to represent the kind of substrate that can trigger the growth of k-strategists microbes.
In addition, PyOM may contain a small easily decomposable fraction (Hamer et al., 2004; Keith et al., 2011; Santos et al., 2012; Singh et al., 2012b; Maestrini et al., 2014) that would constitute an energy source for microbial community on the short term. This agrees with our findings that low-C content PyOM induce positive priming on the short term (Fig. S1). Several studies show that PyOM characterized by a low C content contains a larger labile fraction. Singh et al. (2012b), using 13C cross-polarization NMR found that a low C content in PyOM was associated with a larger fraction of non aromatic C. Fabbri et al. (2012) found that a higher PyOM content in sugar, estimated by pyrolysis coupled to gas-chromatography, was often associated to a low C content. Singh et al. (2012b) also found a positive correlation between soil respiration and the presence of an easily decomposable fraction and Fabbri et al. (2012) found that higher CO2 fluxes were associated to PyOM containing a higher fraction of sugars and a low level of aromaticity.
Even though we did not find a correlation between PyOM C content and pyrolysis temperature there is a general agreement in the literature on the positive correlation between the two (Keiluweit et al., 2010; Zimmerman et al., 2011). Also, the pyrolysis temperature is likely to affect PyOM chemistry and thus its availability to soil micro-organisms. Keiluweit et al. (2010) proposed a multi-phase model that correlates PyOM structure and pyrolysis temperature. Their model proposes that the PyOM content of volatile matter (which is thought to be more labile) decreases with higher pyrolysis temperature while the nonvolatile fraction (more resistant to decomposition) shows a relative increase. In this model, a distinction is made between amorphous PyOM and composite PyOM. The amorphous PyOM is composed mainly of pyranones, phenols, pyrroles and is characterized by a relative increase in stable aromatic lignin compared to raw material, since lignin is more heat resistant than cellulose. The turbostratic PyOM is formed at higher temperature and is characterized by a higher degree of condensation. Turbostratic PyOM is composed of turbostratic crystallites embedded in a matrix of amorphous PyOM. It is therefore possible that less condensed and thus more available PyOM produced at low temperatures stimulates soil microorganisms inducing a higher positive PE.
Also the feedstock may affect PyOM chemical properties like PyOM-C content and forms. It is known that grass-derived PyOM are characterized by a lower C content and thus are less condensed, probably due to the lower thermal stability of cellulose compared to lignin (Hammes et al., 2006; Keiluweit et al., 2010; Knicker, 2010). For example, Hilscher et al. (2009) using 13C NMR technique, observed that a low C content of the ryegrass-derived PyOM corresponded to a higher content of alkyl C compared to the wood-derived PyOM. Overall, we can therefore expect that grass-derived and low temperature PyOM induce a higher positive PE on the short term.
This mechanism is related to the capacity of PyOM to adsorb other organic compounds. This property is already described extensively in the literature (Lehmann et al., 2005; Lehmann & Joseph, 2009; Joseph et al., 2010). PyOM capacity to adsorb organic compounds is related to its high porosity and cation exchange (Lehmann, 2007). It has been shown that PyOM has a higher sorption capacity than non-PyOM for dissolved organic matter (Cornelissen et al., 2005a) and organic xenobiotics (Cornelissen et al., 2005b).
Carbon stabilization by sorption is generally considered one of the most likely mechanisms for PyOM induced negative PE (Kimetu et al., 2009; Kuzyakov et al., 2009; Cross & Sohi, 2011), even though there is very little evidence for sorption of dissolved organic carbon to depress organic matter decomposition (Kaiser & Guggenberger, 2000). The PyOM sorption properties have been proposed as an explanation for negative PE from different perspectives. First, the PyOM has been hypothesized to adsorb part of the native soil organic matter, leading to a lower availability of substrate for decomposers (Kimetu et al., 2009; Kuzyakov et al., 2009; Cross & Sohi, 2011). Pietikäinen et al. (2000) observed a high capacity of PyOM to adsorb dissolved organic carbon compared to pumice. Second, it has been proposed that PyOM sorption potential may influence enzyme activity. Zimmerman et al. (2011) suggested that PyOM stabilization of extracellular enzymes alters their activity by: (i) blocking the active sites of the enzyme or (ii) by inducing a deformation on the tertiary structure of the enzyme. However, these hypotheses were not confirmed by Jin (2010) and Bailey et al. (2011) who found an inconsistent effect of PyOM on a wide range of enzyme activities: the addition of PyOM to the soil in some cases increased enzyme activity, while decreased it in others. They attributed the increase in the activity to the PyOM stimulation of soil microorganisms and the decrease to the sorption of the assay on PyOM surface. Moreover, it has to be considered that among the two forms of enzymes active in the soil, the extracellular stabilized enzymes and the intracellular enzymes, only the latter contribute directly to microbial activity (Nannipieri & Gianfreda, 1998; Nannipieri et al., 2012). The ecological role of extracellular stabilized enzymes is only indirectly connected to microbial activity as they may serve as a reservoir of potential enzymatic activity in case of changes in substrate availability and as originator of signalling molecules by cleaving off small fragments of larger polymer. The signalling molecules act as an inducer for microbes to release the enzyme for the target molecules (Gianfreda & Rao, 2011; Wallenstein & Burns, 2011). Therefore, we believe that the sorption of extracellular enzymes on clays and organic matter (including PyOM) cannot decrease the microbial activity, although it can decrease the enzyme activity measured using the classical approach which cannot separate between extracellular enzymes and enzymes associated to microbes (Nannipieri et al., 2012).
In our meta-analysis, the size of the negative PE was higher when the soil had a higher content in C when data on negative PE are pooled together (Fig. 3). The sorption of dissolved organic carbon agrees with these findings: in fact the dissolved organic carbon production is not directly related to the time, and the physical protection mechanism is likely to become relatively more influential in a second stage when the positive PE has ceased. Also, the dissolved organic carbon production is proportional to the amount of C in the soil (Kalbitz et al., 2000). This confirms findings from Zimmerman et al. (2011), and Stewart et al. (2013) who observed a higher C stabilization in soil richer in PyOM over a 2 years incubation. Zimmerman et al. (2011) suggests that soil more rich in C will produce more C stabilization due to the higher potential C to be sorbed. However, Kuzyakov et al. (2009) observed C stabilization in a loess sediment (very poor in C) but not in a C richer loess derived-soil, which although it was generally poor in C was still ten times richer in C than the loess sediment. Also, Chen et al. (2008) showed that while non-PyOM sorption capacity is linearly dependent on the concentration of the solute, the PyOM sorption capacity is nonlinearly dependent and shows greater affinity at low concentration rates, challenging the theory that negative PE is correlated with DOC content of soil. Nevertheless, the range of concentrations where PyOM sorption capacity is not linear is lower than the typical concentration of dissolved organic carbon in soil (mg C L−1), therefore in the soil, the sorption capacity of PyOM is likely to be linearly correlated in the concentration range of dissolved organic carbon.
Chun et al. (2004) and Chen et al. (2008) found that the pyrolysis temperature significantly altered the sorptive capacity of PyOM. They found that pyrolysis temperature altered the ratio of carbonized/noncarbonized material that are characterized by different adsorption properties. Also James et al. (2005), Bornemann et al. (2007) and Harvey et al. (2011) found similar results with increasing sorption affinity positively correlated with pyrolysis temperature. It is therefore possible that charcoal produced at higher temperature has higher sorption capacity. This would strengthen the importance of PyOM-C content which may be correlated on the one side to the size of the PyOM labile C content and on the other for PyOM sorption capacity. Therefore high temperature PyOM may contribute to stabilize C in the soil, while low temperature C may contribute to stimulate microbial activity.
Overall, the sorption of dissolved organic matter on PyOM surface is a likely explanation for the negative PE, particularly it is possible that in concurrence with the labile fraction it determines the temporal pattern of PE of initial positive PE followed by a phase of negative PE, as observed in several studies (Singh et al., 2012b; Maestrini et al., 2014).
Mechanisms related to the changes of soil properties
Addition of PyOM to the soil may change the soil chemical and physical properties. There is an increasing body of evidence that PyOM addition will increase soil pH, mostly by adding ash to the soil (Van Zwieten et al., 2009; Liu & Zhang, 2012; Maestrini et al., 2014). Luo et al. (2011) observed a relatively higher initial PE in a soil with a lower pH, however we did not find a relation between priming and soil or PyOM pH (data not reported). Nevertheless the alteration of pH may affect organic matter mineralization in several ways. When the pH of soil solution is higher than five, this may alter the equilibrium between CO2 and carbonates (HCO3−, and CO3−) promoting the formation of the latter. This results in an artificial lowering of the CO2 efflux until the saturation of the soil solution is reached, this generally occurs within a few days (Blagodatskaya & Kuzyakov, 2008; Lehmann et al., 2011). Theoretically the relevance of this process decreases with time, as PyOM amendment effect on pH has been shown to decrease (Cheng et al., 2006; Maestrini et al., 2014). However we observed that most of the negative PE occurred in later stages, therefore we believe that this is not a main driver for observed negative PE.
A shift in soil pH by PyOM addition may affect enzymatic activity (Jones et al., 2011). Even though we cannot rule out this hypothesis, each enzyme has its optimum and it is therefore likely that a shift in pH does not have a unidirectional effect on enzyme activity.
Kimetu & Lehmann (2010) suggested that PyOM may increase soil aggregation which can in turn increase C stabilization, particularly by incorporating C in microaggregates (Six et al., 2006). However, it is often not possible to distinguish the effect of sorption from the effect of aggregation, in fact these two effects are often classified together as physical protection mechanisms (Sollins et al., 1996). Vasilyeva et al. (2011), Brodowski et al. (2006) and Skjemstad et al. (1993), found relatively large quantities of PyOM in microaggregates, and propose that PyOM plays an important role in microaggregates formation acting as a binding agent. A similar conclusion was drawn by Fellet et al. (2011), who observed a higher water retention capacity in PyOM amended soil and attributes this to an increase in aggregation. Piccolo et al. (1997) found that coal derived humic substances can improve aggregate stability by creating a water repellent coating to the humic-mineral associations. Liang et al. (2010) found that PyOM increased the incorporation of fresh added organic matter in the aggregates. However in their study, this was not accompanied by a decrease in the PE induced by PyOM on the fresh organic matter. We would expect that the promotion of aggregates formation would have pronounced effects in soil with little structure, like sandy soil, however we did not find an effect of soil texture on PE (data not reported), and therefore although we believe that this mechanism can occur, we cannot confirm this with our meta-analysis.
An alternative explanation for the phase of negative PE observed in several studies is the shortage of available organic matter caused by the higher decomposition rates of the positive PE phase (Bingeman et al., 1953). However, Hamer & Marschner (2005) have shown by repeated substrate addition that there was no limitation in availability of soil organic carbon to PE. Moreover when this mechanism occurs, we expect that a soil richer in C would be less affected by C depletion effect, but results on negative PE did show that soils richer in C are affected by more negative PE.
Mechanisms related to the change in microbial community and activity
The shift in the microbial structure is an explanation that has been used to explain both positive and negative PE. Zimmerman et al. (2011) proposes that compounds toxic to microbes can be released from the PyOM and reduce microbial activity and organic matter mineralization. However, they reject this hypothesis since the toxicity should show its effect on microbial biomass on the short term, due to the volatile fraction they contain. Our results on the PE occurring mostly on the short term, confirm that even if such mechanism exists it is not prevalent at least on the short term. Moreover there is also an increasing body of evidence reporting an increase in microbial biomass following PyOM addition that indirectly do not confirm the presence of toxic compounds (Bruun et al., 2008; Steiner et al., 2008b; Kolb et al., 2009; Schneider et al., 2011; Maestrini et al., 2014). Nevertheless, it is also possible that the observed increase in microbial biomass derives from an increased turnover following from the death of microbial groups sensitive to the toxic compounds. In this case, the dead microbial cells could represent a growing substrate for microbes less sensitive to the toxic elements of PyOM, generating an increased turnover of microbial biomass (Blagodatsky et al., 2010).
Lehmann et al. (2011) proposed based on the observation that PyOM induces a negative PE and an increase in microbial biomass that the PyOM capacity of adsorbing organic matter increases microbial efficiency (i.e. microbes produce less CO2 for the same amount of C incorporated). They suggested that microbial biomass increase may be due to colocation of micro-organisms and soil organic matter; this would reduce the energy invested by microbes in producing extracellular enzymes. The basis for this hypothesis is the autoinducer theory (Redfield, 2002) that postulates that microbes release the enzymes only when the risk of losing them by diffusion is minimal. According to such theory a concentration of decomposable substrate higher than the quorum sensing, i.e. the substrate concentration threshold for bacteria to produce extracellular enzymes, will induce the production of extracellular enzymes, and therefore increase substrate decomposition and minimize the expenses for the production of enzymes.
Our database does not allow us to verify the hypotheses on the efficiency of microbial biomass in PyOM amended soils, however our results on positive PE induced by PyOM on the short term are challenging the initial assumption from Lehmann et al. (2011). Also Jones et al. (2012), who estimated microbial efficiency based on initial consumption rate of added glucose found that microbial efficiency in PyOM amended soil decreased, therefore not confirming this hypotheses.
Wardle et al. (2008) observed in a litter bag experiment an increase in C mineralization from a mix of charcoal and humus compared to the expected decomposition from the two substrates separately. They suggested that PyOM can act as a ‘foci’ (Zackrisson et al., 1996), i.e. a spot, where microbes can grow and decompose the phenols sorbed on PyOM surface being protected from microarthropod predation. In fact according to Warnock et al. (2007) and Zackrisson et al. (1996), the size of many charcoal pores (often below 16 μm in diameters) would allow the entrance of bacteria, fungi and microbe-feeding nematodes but not of predators like protists (8–100 μm) nor microarthropods (100 μm–2 mm). However, as Lehmann et al. (2011) recently reviewed there is no quantitative evidence that microbes inside the PyOM porosity are protected from predators. Moreover, the theory that microbial activity is enhanced by protection against predation is challenged by the many evidences that predation of microbes increase CO2 efflux for example by increasing the activity of soil micro-organisms by transporting them to unexploited substrates and by providing excretion and defecations that are readily usable (Ingham et al., 1985).
Mechanisms for PE on fresh organic matter
The mechanisms proposed in literature for PE on fresh organic matter do not differ substantially from the one proposed for the native soil organic matter. Wardle et al. (2008) suggests the foci hypotheses, similarly Hamer et al. (2004) proposed that PyOM offers a large surface for the growth of micro-organisms. Zavalloni et al. (2011) observed a stabilization effect mixing PyOM and wheat straw, and suggested that the addition of PyOM and decomposable organic matter produces an increase in C immobilization in microbial biomass. However incubations are generally short term, and therefore the possible immobilization of C in microbial biomass would be released on the long term by microbial turnover. Jones et al. (2012) suggests that PyOM induces a shift in the microbial community promoting less C-efficient micro-organisms, so that more C is respired and less is stored. Keith et al. (2011) report that PyOM may stabilize fresh organic matter by trapping it into organo-mineral fractions. This was confirmed by Liang et al. (2010) who found that more fresh organic matter was incorporated in the aggregate fraction in a soil containing high quantities of aged PyOM.
Looking at all the different reported explanation, it appears clear that the labile theory does not apply for the fresh organic matter, in agreement with the principle that fresh organic matter is generally a source of energy for microbes, while old soil organic matter is rather a source of nutrients (Fontaine et al., 2003). On the other hand, soil organic matter is rich in soluble compounds that once in soil can be mobilized and trapped in the aggregates whose formation is promoted by the presence of PyOM. Although the limited number of studies and the heterogeneity of their setup does not allow us to conclude on the definitive mechanisms responsible for the often reported PE on the fresh organic matter, we believe that it is likely that PyOM may stabilize fresh organic matter by physical protection mechanisms.
Although much work has been done on the mechanisms responsible for the PE, a big uncertainty still resides in the processes occurring at landscape levels that may influence PE induced by PyOM. We believe that particularly two aspects have to be elucidated: (i) impact of continuous input of organic matter, (ii) impact of different land uses. Very little is known on the impact of continuous input of fresh organic matter, their impact can be partially inferred from studies where fresh organic was repeatedly added to the soil and biochar mix (Hamer et al., 2004; Kuzyakov et al., 2009). Nonetheless, repeated input of the same substance or mix of substances can hardly reproduce of the diversity and continuity of rhyzodeposition.
Land use is a relevant driver for organic matter dynamics in soils, for example compaction, or litter production seasonality strongly influence decomposition. Nonetheless so far studies investigating PyOM induced PE only looked at the influence of different land uses on soil organic matter quality and quantity (Cross & Sohi 2011). We believe that understanding the impact of tillage on PE induced by PyOM is specially important in the context of biochar application. However this aspect was never investigated in the field, although its impact was simulated in incubation experiments by mixing of soil (Kuzyakov et al., 2009).
We believe that these two aspects are crucial to understand the impact of PyOM induced PE on soil C budget.
We found that on average PyOM induces a PE of similar magnitude but opposite direction on native soil organic matter (positive PE) and fresh organic matter (negative PE). The PE on native soil organic matter was found to be related with time and PyOM-C content, with the positive PE occurring mostly on the short term and induced by PyOM characterized by a low C content, and negative PE appearing at a second stage.
We discussed the different mechanisms that can be involved in the PyOM-induced PE on the native soil organic matter. We believe that the presence of a labile fraction in PyOM may induce a positive PE on the short term by triggering the activity of soil micro-organisms. Simultaneously, PyOM may promote the physical protection of organic matter by sorption on PyOM surfaces or into microaggregates, however the effect of this mechanism appears only in a second stage when positive PE has ceased.
We conclude that, although many uncertainties still exist, particularly on the parameters driving the amplitude and the direction of PE, adding PyOM to the soil induces a cumulative positive PE on a yearly time scale, on the native soil organic matter, which may be counterbalanced by the negative PE observed on fresh organic matter. However, further investigations on the factors influencing the PE induced on fresh organic matter are required; particularly studies employing double isotopic labelling would allow the determination of the PE both on the fresh and on the native soil organic matter.
We are grateful to three anonymous reviewers who carefully read the manuscript and provided insightful comments.