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Article

Effects of Wood-Derived Biochar on Soil Respiration of a European Beech Forest Under Current Climate and Simulated Climate Change

by
Andrea Vannini
,
Debora Tarasconi
,
Federico Pietropoli
,
T’ai Gladys Whittingham Forte
*,
Filippo Grillo
,
Michele Carbognani
† and
Alessandro Petraglia
†
Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11/A, 43124 Parma, Italy
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2025, 16(3), 474; https://doi.org/10.3390/f16030474
Submission received: 20 January 2025 / Revised: 21 February 2025 / Accepted: 26 February 2025 / Published: 8 March 2025
(This article belongs to the Special Issue Functions of Biochar in Carbon Emission and Carbon Sequestration in Forest Ecosystems)
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Abstract

:
Biochar (BCH) amendments represent a valuable strategy for increasing forest carbon stock, but their effects on soil respiration of beech forests under climate change are largely unknown. We conducted a short-term mesocosm experiment investigating the impact of BCH applications (0%, 10%, 20%, and 50%, v/v) on respiration of a European beech forest soil in N-Italy. The experiment, carried out in Parma, was conducted under both ambient and modified climatic conditions, involving higher soil temperatures (c. +1 K) and reduced precipitation (−50%). The experiment was performed during autumn 2022 and repeated in spring 2023, periods representing late and early summer, respectively. Soil respiration significantly increased with BCH applications when compared to controls, irrespective of the percentage applied. The highest values were recorded in the 20% amendment, while values were significantly lower in BCH 50%, similar to those recorded in BCH 10%. Although soil respiration and soil temperature were positively correlated, no effect of simulated warming was observed. No effects of precipitation reduction were also found, despite respiration being significantly influenced by soil moisture. These results provide an initial insight into the potentially negligible impact of BCH applications on soil respiration in European beech forests under both current and future climate scenarios.

Graphical Abstract

1. Introduction

Soil plays a fundamental role in carbon (C) storage, with three times the amount of C stored in this compartment in comparison with that stored in the atmosphere [1]. A dynamic process of C exchange occurs continuously between soil and atmosphere, consisting of C accumulation in the soil (biota, litter, and wood) and C release into the atmosphere through microbial and root respiration [2]. Since C accumulation is greater than release in many ecosystems, soil organic carbon (SOC) represents the major terrestrial C reservoir [3].
This delicate balance is currently threatened by climate change (CC). As atmospheric temperature rises, soil respiration increases, i.e., CO2 is released from the soil through microbial activity, which leads to increased C loss from the soil and increased atmospheric CO2 concentrations [4,5]. The strong correlation between temperature and soil respiration [5] means that each 1 K increase in soil temperature potentially leads to a global reduction of about 170 Pg of soil C [6], an amount which is roughly equivalent to the C stored in the subsoil of the United States [7] or equivalent to the release of 7480 billion tons of CO2, which is approximately 211 times the 2022 global CO2 emissions [8].
The use of specific soil C sequestration strategies can prevent the depletion of, or even enhance, the soil C stocks of terrestrial ecosystems [9]. One of these strategies consists in the application of C-rich amendments resulting from the anoxic thermochemical conversion of woody biomass (pyrolysis). In this process, where wood is used for energy purposes [10], not only is the release of CO2 into the atmosphere limited, in line with environmental needs, but it also leads to an increase soil C stock. Pyrolysis converts about 50% of C in woody biomass into an aromatic C-rich material (>60%), commonly known as black carbon or biochar (BCH) [11,12]. As this substrate is characterized by remarkable environmental stability [13], the application of BCH is recognized as a biological tool used to enhance atmospheric C sequestration [14] and increase soil C content [15,16,17,18,19,20]. However, this strategy seems to be feasible only if it is applied within a circular economy process, as summarized by Sohi and Kuppens [21]. According to estimates by Chagas et al., applying BCH > 5% to the soil can increase the total soil C content by up to 140% [22].
With regard to forest ecosystems, BCH amendments have the potential to enhance their C storage capacity by increasing wood biomass [23] as a result of BCH fertilizing and soil-ameliorating properties [24]. In addition, in some cases, BCH applications appear to reduce CO2 emissions from the soil [25]. However, it should be noted that studies on this topic have produced contrasting results. In fact, it would appear that BCH applications to forest soils have an overall positive effect on soil respiration, with an increase of approx. 20% when the amount of BCH applied ranges from 10 to 30 t/ha (as reviewed by [26]). This could be due to a process known as “the priming effect”, in which SOC is lost, in all likelihood, as a result of increased soil microbial biomass [27] commonly thought to be the result of enhanced soil microbial activity following BCH application [28]. Since “priming” is significantly influenced by temperature and soil moisture levels [29,30], studying the effects of the alteration of these variables on forest soils is of crucial importance—especially so when considering future CC scenarios where temperature increases and/or soil moisture decreases are projected [31].
The impact of climate change on forest soil stability is receiving increased attention [32,33] since the importance of forest soils as global C sinks is universally recognized [34,35]. In this context, particular attention should be paid to boreal and mountain forests ecosystems [36] where colder temperatures and high levels of soil moisture determine higher levels of soil carbon. In fact, together with temperature, soil moisture significantly affects soil respiration: as soil moisture levels approach 100%, gas exchange with the atmosphere is reduced, thus reducing heterotrophic soil respiration rates [37].
European beech (Fagus sylvatica L.) forests represent the most widespread ecosystems in Europe [38]. In order to assess the impact of wood-derived BCH amendments on these forests, for which no information is yet available, a mesocosm experiment was carried out investigating the effects of four different BCH applications (0%, 10%, 20%, and 50%, v/v) on respiration rates of beech forest soils under both current climate and forced warmer and/or drier conditions. This experiment was conducted in the University of Parma garden site and performed in two periods (autumn and spring) in order to evaluate BCH effects in different seasons and cover a broad range of climate variability. Two main questions were addressed: (1) Is beech forest soil respiration affected in the short term by different percentages of BCH applications? and (2) Will warmer temperatures and reduced precipitation, as projected in recent global models [31], influence the effects of BCH applications on soil respiration? Based on previous outcomes (see meta-analysis by [26]), we would initially expect soil respiration to be enhanced by BCH amendments. However, based on findings by Vannini et al., it would also be possible for the effects of BCH amendments to be either weak or negligible, or even slightly reduce soil respiration [39]. In fact, Vannini et al., who investigated BCH effects on litter decomposition in a local beech forest, found that although BCH amendments did not alter the decomposition of high-quality litter, they significantly reduced the decomposition of low-quality litter when this was buried in soils amended with 10%, 20% and 100% BCH (equal to applications of 20, 40 and 200 t/ha, respectively) [39]. In addition, we hypothesized that other factors, such as temperature increase and lower precipitation regimes, could influence soil respiration and potentially interact with BCH applications: warmer soil temperatures (c. +1 K) could stimulate soil respiration, whereas drier soils may tend to suppress it; on the other hand, improved soil water retention following BCH application could counteract the suppressive effects of reduced precipitation on soil respiration.

2. Materials and Methods

2.1. Collection and Preparation of Soil Samples

The soil used was collected from a European beech forest (Fagus sylvatica L.) located within the Tuscan-Emilian Apennines National Park (Emilia-Romagna region, Parma Province; coordinates: 44.419092 N, 10.027582 E) at an elevation of approx. 1200 m a.s.l. The collection procedure involved the removal of the litter layer, i.e., decomposing leaves, small branches, and seeds/fruits. The top 0–10 cm layer of soil (known as the A horizon) was then excavated using a pickaxe and shovel, placed in unsealed plastic bags and transported to the laboratory, where coarse fragments were removed by sieving to 4 mm, then 2 mm. Finally, the soil was homogenized manually inside an 80 L plastic tank. The analysis of the chemical and physical characteristics of the used soil is to be found in previous studies [39,40].

2.2. Preparation of Experimental Treatments

The BCH used in our experiment was produced through the pyrolysis of woody biomass collected from deciduous broadleaf forests located within the Tuscan-Emilian Apennines. Pyrolysis was conducted in an industrial gasifier with a capacity of 125 kW (Holz Energie, Massa-Carrara, Italy) with temperatures ranging between 500 and 650 °C. Before the BCH was incorporated into the soil, it was partially sterilized in a controlled-temperature oven at 120 °C for four days. Sterilization was carried out to eliminate the majority of microorganisms living on the BCH surface, to prevent them influencing soil respiration rates.
Three distinct BCH amendments were prepared: 10% (B10), 20% (B20) and 50% (B50) (v/v), equivalent to approximately 20, 40 and 100 tons per hectare (t/ha), respectively. Amendments were blended in 40 L plastic containers, ensuring homogenization of the soil and BCH at the required v/v ratios. The modified soil from each container was subsequently used to fill 20 plastic pots (Euro3 Plast SpA, Ponte di Barbarano, Italy), each with a capacity of 1.2 L and a surface area of 132.7 cm2. The 10% and 20% BCH applications were chosen on the basis of previous testing, specifically assessing the influence of this substrate on beech forest regeneration [40], whereas the 50% BCH amendment was chosen as a positive control (or worst-case scenario) in the event no discernible outcomes would emerge from lower amendment rates. To assess the impact of BCH applications, an additional 20 control (unamended) pots (BC) were included in the experiment, which were filled exclusively with 2 mm sieved soil. Six soil-only pots and four BCH-only pots were also prepared. The experiment was conducted twice, first in autumn 2022 (Fall experiment: 12 October–17 November 2022) and then in spring 2023 (Spring experiment: 30 March–11 May 2023). These periods were chosen to monitor CO2 emissions at the beginning and at the end of the summer season in the soil sampling area. For both experiments, soil and soil amendments were prepared from scratch. Soil characteristics in control and amended soils are reported in Appendix A: granulometric composition and total N in Table A1; soil organic carbon (%) in Table A2.

2.3. Experimental Design

The experiment took place in an enclosed garden area to the north-west of the University of Parma Biosciences building. A total of 10 blocks were selected within a 5 m × 1.5 m area, spaced 50 cm apart and each measuring 50 cm × 50 cm. Each block was dug to a depth of 10 cm (=pot height) to accommodate 9 pots, i.e., one per treatment (Figure 1). The blocks were isolated from the underlying earth by nylon sheets and the soil removed during excavation was used to fill the spaces between pots.
To simulate a temperature increase which reflects the predicted rise in average temperatures due to CC [41], half the number of blocks (five) were equipped with Open Top Chambers (OTCs) made of transparent PMMA (Bonomi srl; Parma) with a 0.81 m2 basal area and a 0.25 m2 open-top area. OTCs effectively raised soil temperature inside pots by c. +1 K. To enable the precise control of the water supply (carried out manually) and shield the experimental area from natural precipitation, a nylon sheet was placed over the area, supported by an aluminum structure (Figure 2).
In each of the 10 blocks, four pots received full irrigation (F), whereas the remaining four received less water (D) (Figure 1). The amount of water received differed between the two seasons, depending on the mean cumulative rainfall measured in the field during a climatically equivalent period. In detail, in the Fall experiment, F pots were artificially irrigated with 800 mL of water, based on the average cumulative rainfall (520 mm) in the soil sampling area during September–October, from 2001 to 2021 [42], a period exhibiting mean temperatures comparable to those recorded in Parma during the months in which the experiment was conducted (October–November 2023). On the other hand, D pots were irrigated with an amount which was 50% of that allocated to F pots (400 mL) simulating reduced precipitation due to CC. In the Spring experiment, F pots were artificially irrigated with 240 mL of water, based on the average cumulative rainfall (160 mm) in the soil sampling area during June–July, from 2001 to 2021 [42], a period exhibiting mean temperatures comparable to those recorded in Parma during the months in which the experiment was conducted (March–April 2023). On the other hand, D pots during the Spring experiment were irrigated with 120 mL.

2.4. Measurements of Soil Respiration

Soil respiration measurements were conducted weekly before each irrigation session. Prior to measurements, pots were removed from the ground and placed on a table. A CO2 meter EGM-4 (PP Systems, Amesbury, MA, USA) equipped with a 1 L dark cylindrical chamber was then used to record CO2 concentrations. In particular, each measurement lasted 120 s, with the recording of CO2 flux from the soil made every c. 5 s. The calculation of emitted CO2 concentration rates (g m−2 h−1) was made according to the ideal gas law. For both experiments, soil respiration measurements were conducted during the central hours of the day.

2.5. Measurements of Soil pH, Soil Temperature, and Soil Moisture

Soil pH measurements were only carried out on pots receiving full irrigation and not exposed to temperature manipulation; the methodology outlined in the Gazzetta Ufficiale della Repubblica Italiana (1999, see Serie Generale n° 248 21/10/1999) was followed, with slight modifications [43]. Approximately 10 g of air-dried (40 °C) and sieved (2 mm) soil samples were mixed with 25 mL of distilled water and vortexed for two hours. Samples were then centrifuged at 3800 rpm for five minutes, after which the supernatant was filtered using a 185 mm paper filter with a pore size of 12–15 μm. The resulting solution was measured using a benchtop pH meter (XS Instruments benchtop pH meter mod. pH 510). Soil samples were then analyzed in triplicate and results were expressed as the mean value.
To assess the impact of BCH amendments on soil temperature, pots from four distinct blocks (two with OTCs and two without) were outfitted with HOBO Pendant® temperature sensors (Temperature/Light 64 K Data Logger, code UA002-64; Onset Computer Corporation, Bourne, MA, USA), positioned at approx. −5 cm depth and programmed to record soil temperature at hourly intervals. Soil moisture content, measured in grams (g H2O per pot), was determined by weighing the pots before the start of each watering cycle. Water content was derived by calculating the difference in pot weight after and just before watering.

2.6. Data Analysis

Soil respiration measured in amended and unamended pots was analyzed using linear mixed-effects models, implemented with the ‘lme’ function from the ‘nlme’ package [44] in the R program, version 4.1.3 [45]. For all models, collinearity among predictors was assessed using the variance inflation factor, considering a value of “three” as the maximum threshold. The best random effect structure was selected through the Akaike information criterion by comparing alternative models with different random components. Assumptions of linear models were evaluated through residual graphical inspection, and appropriate variance structures were introduced to avoid heteroscedasticity. For all models, components related to random effects and variance structures were assessed using estimates based on restricted maximum likelihood. The selection of fixed effects, carried out in order to obtain the minimal adequate model including only significant terms, was based on estimates using the maximum likelihood method.
To address the first two questions regarding the effects of the three experimental treatments (BCH, warming, and precipitation) on soil respiration, a model was employed with respiration as the response variable. Fixed effects included the BCH treatment (four-level categorical variable: BC, B10, B20 and B50), the temperature treatment (two-level categorical variable: control and OTC), the precipitation treatment (two-level categorical variable: control (F) and drought (D)) and all their interactions. In this model, the random effect structure involved estimating an intercept for the measurement day (11-level categorical variable) nested within the measurement period (two-level categorical variable: fall and spring). A variance structure allowing for differentiated variation for BCH treatment levels and a structure modeling residuals as an exponential function of the fitted values were included to avoid heteroscedasticity.
To address the second question based on data collected for the subgroup of pots equipped with temperature sensors, a linear mixed-effect model was used with respiration as the response variable. Fixed factors included the BCH treatment (four-level categorical variable: BC, B10, B20 and B50), the period (two-level categorical variable: fall and spring), soil temperature (numerical variable, in °C), soil moisture (numerical variable expressed in g H2O per pot) and their two-way interactions. In this model, the structure of random effects involved estimating random intercepts for each pot (69-level categorical variable) crossed with the measurement day (11-level categorical variable). A variance structure allowing for differentiated variation for BCH treatment levels was included to avoid heteroscedasticity.

3. Results

3.1. Effects of Biochar, Simulated Warming and Drought

The applications of BCH significantly increased soil respiration compared to the unamended control soil, irrespective of the application percentage (p < 0.001; Figure 3). The post hoc test revealed that all treatments significantly differed between each other, with only B50 and B10 exhibiting statistically similar values (p = 0.678). In detail, soil respiration increased with increasing percentages of BCH application, reaching maximum values under the 20% BCH amendment and returning to lower rates in the 50% BCH application, overlapping values measured in the 10% BCH treatment.
Soil respiration was not significantly influenced by either simulated warming or precipitation reduction imposed through artificial water supply (p = 0.989 and p = 0.577, respectively).

3.2. Effects of Season, Soil Temperature, Soil Moisture and Their Interaction

The season during which the experiment was carried out significantly affected soil respiration, with higher values recorded in spring compared to fall (p = 0.024; see Figure A1A). Soil respiration was also positively influenced by soil temperature recorded during measurements (p = 0.003; see Figure A1B) and differed across treatments (p = 0.007) but was not affected by soil moisture (p > 0.05; see Figure A1C). Significant interactions between season and soil temperature (p = 0.017; Figure 4), season and soil moisture (p = 0.003; Figure 5), and soil temperature and BCH amendments (p < 0.001; Figure 6) were found. In detail, the regression slope between temperature and soil respiration was steeper in spring compared to the one displayed in fall (Figure 4). Soil respiration during the autumn showed a negative trend with increasing soil moisture, whereas an opposite trend was found in spring (Figure 5). When BCH amendments were considered, soil respiration response to soil temperature differed among treatments, with slopes becoming steeper starting from unamended control soils and moving toward the 20% BCH-amended soils. The correlation slope decreased under 50% BCH, reaching similar levels to those found for control soils (Figure 6).

4. Discussion

4.1. Effects on Soil Respiration of Biochar, Simulated Warming and Drought

The first aim of study was to assess whether soil respiration in beech forests is influenced in the short term by different percentages of BCH application. Results revealed an increase in the respiration of European beech soils irrespective of the percentage when compared to unamended control soils.
Soil respiration increase following BCH application is commonly ascribed to the increased decomposition of soil humus (i.e., the major component of soil organic matter, SOM, in forest soils; [2]), a process known as the ‘priming effect’. Several alternative explanations have been put forward for this process: (1) the increase in soil microbial biomass following BCH amendments; (2) the immediate consumption of labile fraction of organic matter in BCH by microorganisms; (3) the role of BCH as a refuge for soil microbial growth; (4) the transfer of acidified water from forest soil into BCH; and (5) soil disturbance during BCH applications.
The “priming effect” could be due to the increase in soil microbial biomass [27] resulting from the ability of BCH to increase the activity and/or survival of soil microorganisms [28]. In fact, BCH provides not only essential nutrients (K, Mg, Ca, P and N) to soil microbial populations, but also shelter and water [46], thus increasing soil microbial biomass which, in turn, promotes humus decomposition and, ultimately, CO2 release. This hypothesized mechanism still needs to be confirmed, as some observations suggest either the absence of this effect or negligible/stimulating effects of BCH applications on soil humic acid content [47,48]. However, a second explanation for the occurrence of a “priming effect” on soil respiration is provided by Sagrilo et al. who suggest that this could be triggered by the immediate consumption by soil microorganisms of the labile fraction of organic matter contained in BCH [49], which is then used as an energy source [50]. This labile fraction, which increases in direct proportion to the percentage of BCH applied [51], has the ability to enhance microbial activity and also change soil microbial communities [52,53], parameters which are known to increase soil CO2 emissions. In fact, a positive correlation has been observed between the amount of labile organic matter and the rate of soil CO2 release [54]. As a result of this mechanism, the “priming effect” would come to an end once this pool of organic matter is completely depleted.
In the present study, further data are still needed to fully support this second explanation. So far, studies focusing on the persistence of the “priming effect” on soil respiration have only been carried out for a period not more than 200 days [49,55], making it difficult to clearly assess the potential duration of this process. Moreover, the BCH used in this study was thermally treated at 120 °C prior to use, a treatment which may have potentially extinguished the fraction of organic matter which is usually labile at temperatures > 30 °C [54]. Despite this, we cannot completely exclude the possibility that some of this labile organic fraction may have resisted the thermal treatment, since measurements using the potassium permanganate method [56] showed that BCH retained c. 2.5% of the labile fraction even after heat treatment. The increase in CO2 release observed during the experiment could, therefore, be either entirely or partially due to the consumption of this fraction. In line with this, results for organic C (OC) content in the post-treatment soils indicate a mean percentage approx. 5% higher than that measured in control soils (Table A2).
A further explanation for the occurrence of a “priming effect” of BCH on soil respiration can be attributed to the protective role that BCH plays in the growth of soil microbial populations [57], which is facilitated by the numerous substrate micro-cavities promoting undisturbed microorganism development. This process seems to be stimulated by the presence of fine particles [58] and, in support of this hypothesis, 24% of the BCH used in this study consisted of particularly small-sized particles (<0.5 mm; [40]).
An additional, fourth explanation could be found in the transfer of acidified water from the forest soil into BCH, resulting in the dissolution of calcium salts (CaCO3) contained in BCH and the subsequent release of CO2 from the soil [55,59]. In line with this hypothesis, Wu et al. suggested that the release of CO2 from BCH-amended soils is greater when soil pH is acidic rather than basic [60]. In this study, soil pH was 4.7, whereas the pH of the BCH employed was 8.5 (see also [40]).
The last factor to be considered is the manipulation of the soil when amending it with BCH. In this case, the priming effect would be due to the oxygenation of the soil [39], a phenomenon that tends to diminish as the soil becomes more compact. In fact, as reported by Novara et al., bulk density was found to negatively affect soil CO2 emission [61].
Despite all these explanatory factors, however, the priming effect on SOC is inevitably triggered as soon as the C content of the soil is increased by the addition of BCH [49], with effects that may or may not be ecologically relevant. Regarding the results of this study, it should be noted that the observed increases in soil respiration following amendments are to be considered within the normal range of variability for beech forest soils during the summer period (June–October 2023; max = 1.5 g (CO2) m−2 h−1, min = 0.2 g (CO2) m−2 h−1; data collection in the soil sampling area in summer 2023), a period where maximum respiration takes place [2].
Although the increase in soil respiration was evident in all BCH amendments, results from this study also revealed that the extent of the response differed between BCH application percentages. In particular, the highest rate was recorded in the 20% amendment, while soil respiration was significantly lower in the 50% application. The higher soil respiration rates following 10% and 20% BCH amendments are likely to be the result of improved soil chemical and physical conditions, which favor microbial development; on the other hand, the decrease at 50% BCH could be the result of an ‘optimal’ threshold (unknown) being exceeded, leading to a reduction in soil microbial activity and/or biomass. In fact, it is noteworthy that BCH can also induce negative priming effects on SOM decomposition [50] with effects increasing in proportion with the application percentage [62]. Several mechanisms may drive this negative priming: (1) The sequestration of part of the native SOM which then reduces the substrate available for microbial metabolism [48]; (2) The immobilization of enzymes, such as laccase, involved in lignin decomposition [63]; and (3) The increase in soil pH values, which may negatively affect the activity of soil fungi [64], i.e., the main lignin decomposers. Our data support this last hypothesis, since BCH amendments did effectively increase pH compared to unamended soils (4.7): 10% BCH = 6.1; 20% BCH = 7.3; and 50% BCH = 7.7.
In addition to the above-mentioned hypotheses, an excessive increase in polycyclic aromatic hydrocarbons (PAHs) concentrations may also have negatively affected soil microbial biomass, thus reducing CO2 emission rates in the 50% BCH amendment when compared to the 20% BCH rates. In fact, PAHs are contaminants known to be present in BCH structure. These may sometimes reach high concentrations (9 ± 29 mg/kg; [65]) with values exceeding the maximum concentrations found in background (i.e., unpolluted) soils (7.8 mg/kg [66]). The application of BCH inevitably leads to an increase in the levels of these contaminants in the soil [67] which could, at a certain level, negatively affect both the diversity and the biomass of soil microbial populations [68,69]. However, at present, this hypothesis remains speculative as there is no clear evidence that the PAHs in BCH have a detrimental effect on soil respiration.
Finally, a 50% reduction in soil volume may, to some degree, have reduced soil respiration, since this manipulation inevitably reduces the microbial biomass by half. Despite this, our data suggest that negative effects of 50% volume reduction on soil respiration remained weak compared to the positive priming effect of BCH (Figure A2). In fact, average CO2 emission rates of pots half-filled exclusively with either biochar or forest soil were lower than rates measured in the 50% BCH amendment. Overall, it appears that even if all the above inhibitory factors of BCH on soil respiration were to occur concurrently, the resulting inhibitory effect would still be less than the priming effect of BCH on soil respiration.
With regard to the second aim of this study, i.e., an assessment of whether the potential effects (if measurable) of BCH application on soil respiration are modulated by predicted warmer temperatures and reduced precipitation [31], results showed no effect of climate manipulations on soil respiration rates. In detail, pots in OTCs exhibited similar respiration values to those experiencing ambient temperatures, suggesting that the c. +1 K thermal difference between soils inside and outside OTCs is not sufficient to significantly influence soil respiration. This is a crucial result given potential soil C loss due to CC predicted for the future [6]. This is supported by the limited difference in soil OC between control soils and 20% BCH amendments exposed (W) or not exposed (nW) to warming within the OTC (Table A2). Similarly, soil respiration was unaffected by precipitation reduction in the short term, suggesting that the predicted intensification of drought events like those simulated in this study is unlikely to affect CO2 emissions of beech forest soils.

4.2. Effects on Soil Respiration of Season, Soil Temperature, Soil Moisture and Their Interaction

Soil respiration was found to be higher in spring than in autumn. The positive effect of temperature on soil respiration in beech forests is well documented [70,71], as is the effect of soil moisture [72]. In particular, these studies suggest that heterotrophic soil respiration tends to increase with increasing soil temperature, whereas it tends to decrease as soil water content approaches 100%. In our research, higher respiration rates in spring are, in all likelihood, due to the higher temperatures recorded in this season, together with reduced water content in soil micro- and macro-pores caused by decreased artificial soil hydration.
Our findings indicate that the amount of soil respiration increases along with an increase in soil temperature, with the slope being steeper in spring compared to autumn. Soil temperatures measured in our measurements typically ranged from 15 to 25 °C in both seasons, suggesting that while soil temperature plays a significant role in driving soil respiration [73], it may not be the only contributing factor. Such observed differences are probably due to the different rainfall regimes experienced by the blocks during the two seasons, with higher rainfall recorded in autumn. The consequent increase in soil water content probably led to a significant oxygen reduction in soil macro-pores [74] resulting in increased soil resistance to transfer gas to the atmosphere [75]. Not only would this explain the negative correlation between soil respiration and soil water content during the autumn, but would also account for the steeper slope obtained during spring—a hypothesis supported by soil water content measured during the autumn (350–600 g) vs. spring (200–400 g).
Despite the well-established relationship between soil water content and soil respiration [76], results showed no correlation between these two variables. On the other hand, our study confirms the role of temperature in increasing soil respiration, albeit with variations between treatments. The steepness of the correlation slope increased starting from control soils to the 20% BCH-amended soil, but decreased in the 50% BCH amendment, a pattern which is in line with the previously discussed result showing greater soil respiration rate in the 20% BCH treatment and lower values in the 50% BCH amendment. This result highlights the importance of soil temperature in enhancing soil respiration as long as there are no other factors hampering soil microbial activity.

5. Conclusions

Our study investigated the short-term effects of biochar (BCH) applications on respiration of European beech forest soils. A mesocosm experiment was carried out to assess the effects under both current and simulated future climate. Results showed a significant increase in soil respiration with BCH amendments, peaking at an application of 20%. Despite data variability, soil respiration rates following BCH amendments fell within the natural respiration levels recorded for European beech forests. Nevertheless, to confirm this result, the effects of different BCH concentrations will have to be directly tested in situ, in field experiments on beech forest soils, in order to obtain a clearer picture of the real impact of this substrate. Although a positive correlation was observed, not only between temperature increase and soil respiration alone, but also in interaction with treatments, climate change was not found to modulate BCH effects on soil respiration. In particular, no direct effect of warming on respiration was found in this study, highlighting the fact that small temperature changes will not significantly affect soil respiration. A similar conclusion can be drawn when the effect of reduced precipitation is considered. This study, therefore, improves our understanding of the nuanced effects of BCH amendments on the respiration of European beech forest soils, while emphasizing the need for further research, especially rigorous field experiments, on the effects on soil CO2 emissions of this promising forest management strategy.

Author Contributions

Conceptualization, A.V., M.C. and A.P.; Data curation, A.V., D.T. and F.P.; Formal analysis, F.G. and M.C.; Funding acquisition, A.V. and A.P.; Investigation, A.V.; Methodology, T.G.W.F., M.C. and A.P.; Project administration, A.V.; Resources, A.P.; Supervision, M.C. and A.P.; Validation, M.C. and A.P.; Visualization, M.C.; Writing—original draft, A.V., T.G.W.F., M.C. and A.P.; Writing—review and editing, A.V., D.T., F.P., T.G.W.F., F.G., M.C. and A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Parma, Bando di Ateneo 2021 per la Ricerca (Action: B-Progetti di ricerca per giovani ricercatori) co-founded by the MIUR—the Italian Ministry for University and Research—D.M. 737/2021-PNR-PNRR—NextGenerationEU. This work has also benefited from the equipment and framework of the COMP-HUB and COMP-R Initiatives, funded by the ‘Departments of Excellence’ program of the MIUR (MIUR, 2018–2022 and MIUR, 2023–2027). The publication was made by a researcher with a research contract co-funded by the European Union-PON Research and Innovation 2014–2020 in accordance with Article 24, paragraph 3, letter a), of Law no. 240 of 30 December 2010, as amended and Ministerial Decree no. 10 August 2021 no. 1062.

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

We thank Alessio Malcevschi for providing the biochar employed in this study. We thank Rebecca Whittingham for the English revision of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BCHBiochar
CCarbon
CCClimate change
OTCOpen Top Chamber
PAHPolycyclic Aromatic Hydrocarbon
SOCSoil Organic Carbon
SOMSoil Organic Matter

Appendix A

Appendix A.1

Table A1. Granulometric composition and total nitrogen (N) in control (BC) and amended soils with biochar at 10% (B10), 20% (B20), and 50% (B50). Methodology is described in the Gazzetta Ufficiale della Repubblica Italiana [43].
Table A1. Granulometric composition and total nitrogen (N) in control (BC) and amended soils with biochar at 10% (B10), 20% (B20), and 50% (B50). Methodology is described in the Gazzetta Ufficiale della Repubblica Italiana [43].
Soil CharacteristicsBCB10B20B50
Sand (%)45.237.528.94.6
Silt (%)42.549.859.384.1
Clay (%)12.312.611.610.8
Total N (g/kg)4.26.626.710.6

Appendix A.2

Table A2. Organic carbon (%, mean ± standard deviation; mean of four replicates) in control (BC) and amended soils amended with biochar 10% (B10), 20% (B20), and 50% (B50). The abbreviation “W” indicates samples exposed to warming conditions, “nW” not exposed. Methodology is described by Miano and Mondelli [77].
Table A2. Organic carbon (%, mean ± standard deviation; mean of four replicates) in control (BC) and amended soils amended with biochar 10% (B10), 20% (B20), and 50% (B50). The abbreviation “W” indicates samples exposed to warming conditions, “nW” not exposed. Methodology is described by Miano and Mondelli [77].
SeasonWarmingBCB10B20B50
FallW19.2 ± 2.4-19.7 ± 6.2-
nW18.3 ± 0.521.1 ± 7.421.8 ± 6.623.3 ± 2.7
SpringW23.8 ± 0.4-27.4 ± 1.2-
nW23.3 ± 0.829.0 ± 1.128.8 ± 0.832.7 ± 2.1

Appendix B

Appendix B.1

Forests 16 00474 g0a1
Figure A1. Respiration rates (g (CO2) m−2 h−1) of European beech forest soils amended with different biochar application rates (A) in each of the two investigated seasons (fall and spring) and as a function of (B) soil temperature (°C) and (C) soil moisture (g (H2O) per pot).
Figure A1. Respiration rates (g (CO2) m−2 h−1) of European beech forest soils amended with different biochar application rates (A) in each of the two investigated seasons (fall and spring) and as a function of (B) soil temperature (°C) and (C) soil moisture (g (H2O) per pot).
Forests 16 00474 g0a1

Appendix B.2

Forests 16 00474 g0a2
Figure A2. Average respiration rates (g (CO2) m−2 h−1) of pots entirely filled with European beech forest soil (BC) and soil amended with 50% biochar (B50) and pots only half-filled only with soil (half-BC) or only with biochar (half-BCH).
Figure A2. Average respiration rates (g (CO2) m−2 h−1) of pots entirely filled with European beech forest soil (BC) and soil amended with 50% biochar (B50) and pots only half-filled only with soil (half-BC) or only with biochar (half-BCH).
Forests 16 00474 g0a2

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Figure 1. Schematic representation of pot arrangement in each of the 10 experimental blocks. The white circle indicates a half-filled pot, filled only with soil (halfBC) or only with biochar (halfBCH). Colored circles indicate different treatments: BC—unamended control soil; B10—soil amended with 10% BCH (v/v); B20—soil amended with 20% BCH (v/v); B50—soil amended with 50% BCH (v/v). In each block, the eight BCH-amended pots are arranged as follows: four pots undergoing full irrigation (F) and the remaining four pots subjected to “drought” irrigation (D).
Figure 1. Schematic representation of pot arrangement in each of the 10 experimental blocks. The white circle indicates a half-filled pot, filled only with soil (halfBC) or only with biochar (halfBCH). Colored circles indicate different treatments: BC—unamended control soil; B10—soil amended with 10% BCH (v/v); B20—soil amended with 20% BCH (v/v); B50—soil amended with 50% BCH (v/v). In each block, the eight BCH-amended pots are arranged as follows: four pots undergoing full irrigation (F) and the remaining four pots subjected to “drought” irrigation (D).
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Figure 2. Schematic representation of the experimental set-up, illustrating the aluminum structure supporting the nylon sheet cover. On the left, the area beneath the shelter is shown with the 10 blocks where pots were located: nW—blocks without open top chambers (OTCs); W—blocks with OTCs.
Figure 2. Schematic representation of the experimental set-up, illustrating the aluminum structure supporting the nylon sheet cover. On the left, the area beneath the shelter is shown with the 10 blocks where pots were located: nW—blocks without open top chambers (OTCs); W—blocks with OTCs.
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Figure 3. Respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different BCH application percentages: 0% (BC), 10% (B10), 20% (B20), and 50% (B50). Black lines indicate mean values, whereas gray bands indicate 95% confidence intervals. Different letters indicate significant differences between treatments (p < 0.05). Estimates are based on a linear mixed-effect models including biochar amendment as fixed effect, measurement date nested within period as random effects, a variance structure allowing different variation among fixed effect levels, and a variance structure modeling residuals as exponential function of the fitted values.
Figure 3. Respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different BCH application percentages: 0% (BC), 10% (B10), 20% (B20), and 50% (B50). Black lines indicate mean values, whereas gray bands indicate 95% confidence intervals. Different letters indicate significant differences between treatments (p < 0.05). Estimates are based on a linear mixed-effect models including biochar amendment as fixed effect, measurement date nested within period as random effects, a variance structure allowing different variation among fixed effect levels, and a variance structure modeling residuals as exponential function of the fitted values.
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Figure 4. Effects of soil temperature (°C) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages in each of the two investigated seasons (fall and spring). Slope estimates for controls are reported, with p-values of the comparison with null slope (for fall) and with fall slope (for spring). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
Figure 4. Effects of soil temperature (°C) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages in each of the two investigated seasons (fall and spring). Slope estimates for controls are reported, with p-values of the comparison with null slope (for fall) and with fall slope (for spring). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
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Figure 5. Effects of soil moisture (g (H2O) per pot) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages in each of the two investigated seasons (fall and spring). Slope estimates for controls are reported, with p-values of the comparison with null slope (for fall) and with fall slope (for spring). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
Figure 5. Effects of soil moisture (g (H2O) per pot) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages in each of the two investigated seasons (fall and spring). Slope estimates for controls are reported, with p-values of the comparison with null slope (for fall) and with fall slope (for spring). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
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Figure 6. Effects of soil temperature (°C) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages, i.e., 0% (BC), 10% (B10), 20% (B20), and 50% (B50). Slope estimates for fall are reported, with p-values of the comparison with null slope (for BC) and with BC slope (for biochar amendments). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
Figure 6. Effects of soil temperature (°C) on respiration rates (g (CO2) m−2 h−1) of beech forest soils amended with different biochar application percentages, i.e., 0% (BC), 10% (B10), 20% (B20), and 50% (B50). Slope estimates for fall are reported, with p-values of the comparison with null slope (for BC) and with BC slope (for biochar amendments). Black, dashed lines indicate estimated relationships, whereas gray bands indicate 95% confidence intervals. Estimates are based on a linear mixed-effect model including biochar amendment, soil temperature, soil moisture, and the interaction between biochar amendment and soil temperature as fixed effects, measurement date crossed with pot identity as random effects, and a variance structure allowing different variation among biochar amendment levels.
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MDPI and ACS Style

Vannini, A.; Tarasconi, D.; Pietropoli, F.; Forte, T.G.W.; Grillo, F.; Carbognani, M.; Petraglia, A. Effects of Wood-Derived Biochar on Soil Respiration of a European Beech Forest Under Current Climate and Simulated Climate Change. Forests 2025, 16, 474. https://doi.org/10.3390/f16030474

AMA Style

Vannini A, Tarasconi D, Pietropoli F, Forte TGW, Grillo F, Carbognani M, Petraglia A. Effects of Wood-Derived Biochar on Soil Respiration of a European Beech Forest Under Current Climate and Simulated Climate Change. Forests. 2025; 16(3):474. https://doi.org/10.3390/f16030474

Chicago/Turabian Style

Vannini, Andrea, Debora Tarasconi, Federico Pietropoli, T’ai Gladys Whittingham Forte, Filippo Grillo, Michele Carbognani, and Alessandro Petraglia. 2025. "Effects of Wood-Derived Biochar on Soil Respiration of a European Beech Forest Under Current Climate and Simulated Climate Change" Forests 16, no. 3: 474. https://doi.org/10.3390/f16030474

APA Style

Vannini, A., Tarasconi, D., Pietropoli, F., Forte, T. G. W., Grillo, F., Carbognani, M., & Petraglia, A. (2025). Effects of Wood-Derived Biochar on Soil Respiration of a European Beech Forest Under Current Climate and Simulated Climate Change. Forests, 16(3), 474. https://doi.org/10.3390/f16030474

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