Pyrolysis temperature determines the amendment effects of poplar residue‐derived biochars on reducing CO2 emission

Poplars and their hybrids are widely planted in both plantation forestry and agroforestry systems of the world. Along with the utilization and plantation management processes, a large amount of biomass residues are produced, but the relationship between biochar properties and soil CO2 emissions is largely unknown. Here, a laboratory incubation study was conducted to assess the effects of different biochars and their corresponding biomass residues on soil CO2 emissions during the 180 days of incubation. Poplar residue‐derived biochars were larger in the surface area and total pore volume but lower in nutrients and pH values than the rice straw‐derived biochar. Increasing pyrolysis temperature led to a decrease in the total nitrogen (TN) content of poplar leaf‐ and rice straw‐derived biochars, but enhanced the TN in the poplar twig‐ and poplar bark‐derived biochars. After 180‐day incubation, the total cumulative CO2 emission decreased by 33.1%–73.8% in the biochar amendments compared to their corresponding biomass residue addition, whereas the biochars derived from poplar twig and bark residues had more positive effects on reducing soil CO2 emissions, but depended on the pyrolysis temperature. Correlation analysis showed a significant and positive correlation between the CO2 emissions and TN content of bio‐based materials but the negative relationships to total carbon content and C/N ratio. Meanwhile the positive correlations of CO2 emissions to the surface area, t‐plot micropore area, and volume of the biochars were detected. Our results suggest that application of poplar twig‐ and poplar bark‐derived biochars has a great potential for mitigating global warming.


| INTRODUCTION
It was reported that agriculture contributes about 20% of total greenhouse gas (GHG) emissions directly or indirectly in the world (Ahsaan et al., 2021). Biochar, as a costeffective and environmentally friendly sorbent, is a carbon (C)-rich material produced during pyrolysis process, and has a wide range of applications such as soil amendment, soil decontamination, water treatment, and food additive for animal feed (Jamaludin et al., 2019). Recently, biochar has also been applied for reducing GHG emission as a feasible and potential option from farm waste and residues of agriculture and forestry, which can potentially mitigate soil GHG emissions as well as improve soil health (Pipíška et al., 2022;Subedi et al., 2016;Wang et al., 2023). As a sustainable strategy for climate change mitigation, biochar production and incorporation into soil are promoted by increasing reports of significant C sequestration in soil and reduction in GHG emissions from the amended soils (Giagnoni & Renella, 2022;Stewart et al., 2013). Although biochar has been recognized as stable C pools and known to sequester C via several mechanisms (Ahsaan et al., 2021;Giagnoni & Renella, 2022;Lorenz & Lal, 2014), the effects vary considerably according to the specific properties of the biochar (Gwenzi et al., 2015;Jeffery et al., 2015;Subedi et al., 2016), for example, there exist the possible tradeoffs between the expected biochar benefits, potential risks, and associated uncertainties. For example, few studies reported negative effect of biochar on GHG emission from soils (Troy et al., 2013), while a combined meta-analysis of 46 reports showed remarkable increase of 28% in CO 2 emission from biochar added soils and revealed strong correlation among biochar and native soil organic C content (Ahsaan et al., 2021).
Many studies indicated that biochar stability in soils and its interaction with the soil micro-organisms are influenced by several factors, such as the nature of the biochar feedstock, the type of process used for the biochar production, and the operating conditions (Ameloot et al., 2014;Giagnoni & Renella, 2022;Lehmann et al., 2011;Subedi et al., 2016). However, soil CO 2 emissions are largely dependent on biochar production conditions, especially pyrolysis temperature plays a key role in biomass thermochemical conversion in the process of biochar production (Das et al., 2021). It was found that biochar produced at higher pyrolysis temperature has larger surface area, higher pH, and porosity than biochar pyrolyzed at lower temperature, whereas the volatile matter and product yield show an opposite pattern (Das et al., 2021;Deng et al., 2022;Tomczyk et al., 2020). Meanwhile, it was also observed that low pyrolysis process mitigates more CO 2 than generated from fast pyrolysis (Ahsaan et al., 2021;Subedi et al., 2016). Moreover, it was noted that the optimum pyrolysis temperature is related to the biomass type and its application purpose. For instance, some studies indicated that the crop straw biochar pyrolyzed at 300°C had greater capacity to improve soil acidity than that produced at 500°C (Yuan et al., 2011), whereas the application of birch (Betula spp.) biochar pyrolyzed at an intermediate temperature (375°C) obtained the highest ryegrass yield (Hanger et al., 2016). Deng et al. (2022) reported that the highest biomass production and leaf secondary metabolite accumulation in Cyclocarya paliurus were obtained in the addition of woody biochar pyrolyzed at 500°C. The microbial degradation of biochar C in soil is often associated with labile organic compounds, such as alkanoic and benzoic acids, and phenols, whose concentration falls with increasing pyrolysis temperature (Graber et al., 2010;Novak et al., 2009;Troy et al., 2013), while biochars produced from plant residues contain stable aromatic structures and are more resistant to microbial attack (Case et al., 2014;Ippolito et al., 2012;Subedi et al., 2016). Therefore, it is very important to identify biochar properties produced from different plant biomass at various pyrolysis temperatures in order to select an appropriate biochar for clear-cut applications.
Poplars (Populus spp., over 100 species) and their hybrids are widely planted in subtropical and temperate regions and in both plantation forestry and agroforestry systems of the world (Fang et al., 2007;IPC, 2016;Zhang et al., 2022). As one of the most widely cultivated timber and ecological tree species in the middle latitude plain of the world, poplars have received growing interests and become a preferred tree species. For example, the total area of poplar plantations has reached 8.54 million ha in China (Fang et al., 2021;IPC, 2016;Tun et al., 2018), approaching one third of the world's total poplar plantations. Poplar wood is traditionally used as raw materials for various panels, paper, and fiber products (IPC, 2016). Along with these industrial application processes, a large amount of processing residues (such as twigs, wood shaving, and sawdust) are generated, accounting for 10%-15% of the poplar volume (Wu et al., 2023). Ferretti (2021) reported that biomass residues deriving from production account for more than 54% of the raw material in the poplar plywood sector. Meanwhile, lots of pruning and thinning residues are also produced during the plantation management (Sarauer et al., 2019). However, if the majority of the residues are improperly managed (e.g., burned or discarded as waste), it may lead to bioresource waste and environmental pollution (He et al., 2013). To achieve full resource utilization, researches on poplar residues for biofuel production and biorefinery have been extensively conducted (Olba-Zięty et al., 2022;Wu et al., 2023). However, application of biochars derived from the available woody residues is recently considered as a strategically important option for soil fertility improvement and mitigation of GHG emissions (Ahsaan et al., 2021;Jamaludin et al., 2019).
The property of biochars derived from some woody biomass, crop residue, and animal manures and their effects on soil fertility improvement and mitigation of GHG emissions have been reported (Ahsaan et al., 2021;Pipíška et al., 2022;Subedi et al., 2016;Wang et al., 2023), whereas there have been few reports on the use of biochars derived from various poplar residues, and especially the amendment effect on mitigation of CO 2 emissions is not compared between the biochars and their corresponding residue biomass. Therefore, the biochars derived from four plant residues (poplar biomass residues and rice straw residue [SR]) were prepared at three pyrolysis temperatures (300, 500, and 700°C) in this study. After measuring the surface characteristics and physicochemical properties of the biochars, a laboratory incubation experiment was conducted and the dynamic and cumulation of CO 2 emissions were determined after amending the soil with biomass residues and their corresponding biochars. We test the following hypotheses: (1) amendment of poplar residuederived biochars could significantly reduce CO 2 emissions compared with direct addition of its residue biomass; (2) the reduction magnitude in the CO 2 emissions would be closely related to the surface characteristics and physicochemical properties of the biochar; and (3) an optimal pyrolysis temperature would vary for each residue type in term of the reduction effect on CO 2 emissions. Results from this study would provide some references for assessing the practical feasibility of C off-setting strategy during the management of poplar plantations.

| Residue sample collection and biochar preparation
All four types of residue biomass were collected in October 2018 from Sihong county (33°16′ N, 118°21′ E), Jiangsu Province, China. Of them, three types of poplar residues were sampled from a Nanlin-895 (Populus × euramericana cv. 'Nanlin-895') plantation established in March 2007, named as twig residue (TR, the largest diameter of twigs <3 cm), bark residue (BR), and leaf residue (LR), respectively. Meanwhile, the SR was collected from the farmland near the poplar plantation. Once transported to the laboratory, all collected samples were cut, washed with deionized water, and dried at 70°C. Then, all dried samples were sieved to 1 mm particles prior to pyrolysis and further application. The basic properties of different types of residue biomass are shown in Table 1. Biochars were prepared from the residue powders of poplar twig, bark and leaf and rice straw using slow pyrolysis method (heating rate 10°C min −1 ) at 300, 500, and 700°C and residence time of 4 h in a condition of hypoxia. These biochars are named in this study as follows: T1 (biochar pyrolyzed at 300°C), T2 (biochar pyrolyzed at 500°C), and T3 (biochar pyrolyzed at 700°C) for TR, respectively. Similarly, the biochars prepared for other residues are named as B1, B2, and B3 for BR, L1, L2, and L3 for LR, as well as S1, S2, and S3 for SR, respectively.

| Soil collection and characterization
The soil used in this experiment was collected from the same poplar plantation to sample residue biomass in March 2019. The soil was collected from the top 20 cm soil using S-shaped sampling method. After transporting to the laboratory, the collected soil samples with a medium-clay texture were subsequently air-dried, grounded by manual operation, and then sieved to below 2 mm for measurement of soil physical and chemical property and incubation experiment. The soil had bulk density of 1.42 ± 0.02 g · cm −3 , pH of 7.18 ± 0.05, total carbon (TC) of 9.77 ± 0.01 g · kg −1 , total nitrogen (TN) of 1.13 ± 0.01 g · kg −1 , total phosphorus (TP) of 0.35 ± 0.00 g · kg −1 , total potassium (TK) of 9.52 ± 0.75 g · kg −1 , and cation exchange capacity (CEC) of 31.02 ± 0.66 cmol · kg −1 , respectively.

| Incubation experiment
The incubation experiments of different residue biomass (dehydrated) and biochar amendments were conducted in the constant temperature incubators (25°C, 65% relative humidity) for dark culture, and set-up as a randomized complete block design, with three replicates. Each replicate unit consisted of three cylindrical polyethylene jars (diameter 8 cm, height 12 cm), and totally nine jars for each treatment. In this study, five treatments were designed for residue biomass amendment, for example, TR, BR, LR, SR, and no addition (CK, only soil), while 13 treatments were set-up for the amendments of biochars prepared with various residue types and pyrolysis temperatures, named as T1, T2, T3, B1, B2, B3, L1, L2, L3, S1, S2, S3, and CK, respectively.
About 100 g air-dried and sieved soil for each jar was manually mixed with each type of dry residue biomass and biochar at 2% w/w (oven-dried weight), corresponding to an amendment rate of 20 g · kg −1 . All the soils were then moistened with deionized water in order to reach 60% of field moisture capacity, then the jars were sealed and incubated in the constant temperature incubators. During the incubation period, the soil water content was adjusted every 2 days to maintain about 60% of field moisture capacity, while the jars were ventilated for 20 min every week to maintain good aeration condition of the soil.

| Gas sampling and measurement
Gas samples were taken from the jar headspace at 1, 3,5,7,10,13,16,19,22,25,30,35,40,47,54,61,70,79,88,99,110,121,134,149,165, and 180 days after the treatments using the syringe sampling method (Subedi et al., 2016). During gas sampling, the jars were hermetically sealed with a lid, and gas samples were taken at 0 and 60 min after closing the jars, respectively. Approximately 15 mL of gas was extracted from the jars using a polyethylene syringe (30 mL) each time, and was stored in the preevacuated tinfoil gas collecting bag. During each sampling event, the jars after sampling were put back into the incubator culture for the next sampling.
The gas samples were analyzed within the 2 days after sampling, using a gas chromatograph (Model 7890A; Agilent Tech). For the determination of CO 2 concentration, argon-methane (5%) was used as the carrier gas at a flow rate of 40 mL min −1 , while the temperatures for the column and electron capture detector (ECD) were maintained at 40 and 300°C, and the oven and flame ionization detector (FID) were operated at 40 and 200°C, respectively. The CO 2 fluxes (F) and cumulative emissions (s) were estimated by the following equations, respectively: where F represents the gas flux (mg · kg −1 h −1 ); ρ is the gas density in standard state (g · L −1 ); dc/dt is the rate of change in gas concentration (ppm · h −1 ); V is the effective space volume of gas in culture jar (L); W is the sample mass (kg); and T is the absolute temperature (°C).
where s represents the cumulative CO 2 emissions (mg · kg −1 ); F i is the gas flux (mg · kg −1 h −1 ); and t i is the incubation time at sampling (day).
3.2 | Determination of biochar yield and ash content The biochar yield (C) and ash content (A) for different residue types and pyrolysis temperatures were measured according to the National Standard of P.R. China "Wood charcoal and test method of wood charcoal (GB/T 17664-1999)" and were calculated as C(%) = (biochar mass∕ material mass) × 100, and A(%) = residue mass after burning at 800 • C for 2 h∕biochar mass × 100 , respectively.

| Measurement of biochar Fourier
transform infrared spectra, specific surface area, and pore structure Fourier transform infrared (FTIR) spectra of all the biochar samples were recorded in the 4000-400 cm −1 region by a Fourier Transform Infrared Spectrometer (VERTEX 80/80V; Bruker) with a resolution of 1.0 cm −1 , using the KBr tablet method. The pore structure and specific surface area of the biochar samples were documented using a physisorption analyzer (ASAP2020 HD88; Micromeritics), and calculated by the methods of BJH (Barrett-Joiner-Halenda) and BET (Brunauer-Emmett-Teller), respectively.

| Physicochemical property determination
The TC and TN in the soil, residue biomass, and biochars were analyzed by means of elemental analysis (Vario Max CN analyser; Elementar), while the TP and TK were determined using vanadium molybdate blue colorimetric method (Bao, 2000) and flame photometric method (Niu et al., 2018), respectively. The soil pH was measured with a pH meter at a soil to water ratio of 1:2.5 (w/v), whereas the pH values of residue biomass and biochar were determined in deionized water at a 1:20 residue mass-water ratio and at a 1:10 biochar-water ratio using a pH meter, respectively. The CEC was measured via the ammonium acetate compulsory displacement method (Gaskin et al., 2008).

| Statistical analysis
The reported results are the means of the three replicates and presented as mean ± standard error. The data of basic properties in different types of residue biomass were analyzed by one-way ANOVA at p < 0.05. However, two-way ANOVA (at a p < 0.05 significance level) was performed to assess significant differences in other measured indexes, and followed by multiple comparisons of LSD test at a probability level of 0.05. Moreover, a Pearson bivariate correlation analysis was performed on the different measured variables, and a hierarchical cluster analysis was also conducted to classify the total cumulative CO 2 emission by Euclidean distance. All statistical analyses were performed using SPSS 20.0 software (SPSS, Inc.).

| Variations in basic properties of residue biomass and biochar yields
One-way ANOVA showed that there were significant differences in basic properties of residue biomass among the four residue types (Table 1, p < 0.05). The greatest pH was observed in the SR, followed by TR, but no significant difference was detected between the BR and LR. The highest TC occurred in TR (Table 1), which is 3.5%, 16.1%, and 15.3% greater than in the BR, LR, and SR, respectively. The TN ranged from 3.70 to 14.07 g · kg −1 with the highest in LR and the lowest in the BR, while the opposite trend was detected for C/N ratio. However, the greatest TP and TK were both observed in the rice straw, and the lowest values appeared in BR. For example, the TK in SR was 326.5%, 539.7%, and 214.3% higher than in the TR, BR, and LR, respectively. Residue type and pyrolysis temperature significantly affected the biochar yield and ash content (Figure 1, p < 0.05). For all the residue types, the biochar yield decreased but ash content increased as the pyrolysis temperature enhanced. The biochar yield ranged from 29.79% (TR pyrolyzed at 700°C) to 67.09% (LR pyrolyzed at 300°C) among the treatments (Figure 1a), while ash content was 7.55 (TR pyrolyzed at 300°C)-29.94% (LR pyrolyzed at 700°C) ( Figure 1b). It is noted that the biochar yield dramatically decreased by 32.7% (averaged across the four residue types) when the pyrolysis temperature increased from 300 to 500°C, but only decreased by 7.6% with enhancing from 500 to 700°C. However, an opposite variation tendency was observed for the ash content ( Figure 1b). As a general trend, the biochar yield among the four biomass residues was ranked as poplar leaf > poplar bark > rice straw > poplar twig at three pyrolysis temperatures, and ash content was in the order of poplar leaf > rice straw > poplar bark > poplar twig. Figure 2 shows that the different biochars produced similar FTIR graphs except for a slight change in peak intensity sharpness as increasing pyrolysis temperature. Some strong and broad peaks were found around 3432, 2920, 2851, and 1622 cm −1 for all biochars pyrolyzed at 300, 500, and 700°C ( Figure 2), but the intensity of absorption peaks at 2920, 2851, and 1622 cm −1 all decreased with enhancing pyrolysis temperature, indicating that alkyl in the biochars gradually burns off due to the increase in pyrolysis temperature. Meanwhile, the absorption peaks at 1430 cm −1 were not detected in the biochars pyrolyzed at 300°C, but all observed in the biochars pyrolyzed at 500 and 700°C, suggesting that nonpolar aliphatic functional groups in the residue biomass may be transformed into aromatic structure.

| Variations in biochar FTIR spectra and surface characteristics
The specific surface area, total pore volume, pore diameter, t-plot micropore area, and t-plot volume of various residue-based biochars pyrolyzed at the three temperatures are presented in Table 2. The results showed that both the surface area and total pore volume enhanced for the four residue-based biochars as the pyrolysis temperature increased. For instance, the surface area of poplar bark-derived biochar at pyrolysis temperature of 700°C was 53.8 times of that at the 300°C and 31.3 times of that at the 500°C, respectively, while the total pore volume of the biochar at the 700°C was 19.2 times of that at the 300°C and 2.7 times of that at the 500°C. However, as a general tendency, both the surface area and total pore volume of biochar derived for four biomass residues (averaged across the three pyrolysis temperatures) were ranked as poplar bark > poplar leaf > poplar twig > rice straw (Table 2).
It was observed that the no micropores were formed in poplar leaf-derived biochars at pyrolysis temperatures of 300 and 500°C, as well as in poplar twig-derived biochar at pyrolysis temperature of 300°C (Table 2). However, a dramatic increase in t-plot micropore area and t-plot micropore volume was detected in the poplar residue-derived biochars with the pyrolysis temperature enhancing from 500 to 700°C, whereas the highest values of t-plot micropore area and t-plot micropore volume in rice strawderived biochar appeared at pyrolysis temperature of 500°C (Table 2). For example, the t-plot micropore area in poplar bark-derived biochar pyrolyzed at 700°C reached 90.29 m 2 · g −1 , which are 9.1, 105.3, and 95.1 times of the F I G U R E 1 Variations in the yield (a) and ash content (b) of biochars produced from different biomass residues at the pyrolysis temperatures of 300, 500, and 700°C. Different lowercase letters indicate a significant difference among different pyrolysis temperatures for the same biomass residues, while different uppercase letters show a significant difference among various biomass residues at the same pyrolysis temperatures (p < 0.05).

F I G U R E 2
The Fourier transform infrared spectra of biochars produced from different biomass residues at the pyrolysis temperatures of 300, 500, and 700°C.
T A B L E 2 Specific surface area and pore structure of the biochars produced with different residue types and pyrolysis temperatures.

| Variations in biochar physicochemical properties
Two-way ANOVA results indicated that residue type and pyrolysis temperature significantly affected biochar physicochemical properties (Table 3, p < 0.05). The pH values in the biochars showed a wide range (from 8.44 to 11.9), and all increased as pyrolysis temperature enhanced. The pH in the different residue-derived biochars, where the values were averaged across three pyrolysis temperatures, was ranked as rice straw (11.14) > poplar leaf (10.20) > poplar twig (9.85) > poplar bark (9.65), whereas the pH in three pyrolysis temperatures (averaged across four residue types) was in the order of 700°C (11.09) > 500°C (10.26) > 300°C (9.28). The TC in the biochars ranged from 515.57 to 775.70 g · kg −1 , and also increased as pyrolysis temperature enhanced. The TC in the residue-derived biochars (averaged across three pyrolysis temperatures) was ranked as poplar twig (728.33 g · kg −1 ) > poplar bark (713.41 g · kg −1 ) > rice straw (601.86 g · kg −1 ) > poplar leaf (549.07 g · kg −1 ), whereas was in the order of 700°C (687.94 g · kg −1 ) > 500°C (660.42 g · kg −1 ) > 300°C (596.14 g · kg −1 ) for three pyrolysis temperatures (averaged across four residue types). The TN in the biochars ranged from 6.97 to 20.63 g·kg −1 , but the effects of pyrolysis temperatures on the TN varied among the residue types (Table 3). The highest TN achieved in the biochars pyrolyzed at 300°C for the LR and SR, whereas the greatest TN was observed at the pyrolysis temperature of 700°C for the TR and BR. The C/N ratio of the biochars widely ranged from 27.25 (poplar leafderived biochar pyrolyzed at 300°C, L1) to 95.58 (poplar bark-derived biochar pyrolyzed at 300°C, B1), while the C/N ratio in the residue-derived biochars (averaged across three pyrolysis temperatures) was ranked as poplar bark (92.26) > poplar twig (64.14) > rice straw (33.59) > poplar leaf (31.21).
The TP in the biochars also showed a wide range of variation, and an increasing tendency in TP was detected with enhancing pyrolysis temperatures (Table 3). The highest TP (4.53 g · kg −1 ) was observed in the rice strawderived biochar pyrolyzed at 700°C (S3), while the lowest (0.61 g · kg −1 ) occurred in the poplar bark-derived biochar pyrolyzed at 300°C (B1). On average across three pyrolysis temperatures, the TP in the residue-derived biochars followed the order of rice straw (3.68 g · kg −1 ) > poplar twig (3.13 g · kg −1 ) > poplar leaf (2.43 g · kg −1 ) > poplar bark (0.81 g · kg −1 ). Similar to the TN, the effects of pyrolysis temperatures on the TK varied among the residue types (Table 3). The highest TK was observed in the biochars pyrolyzed at 700°C for the TR (T3) and SR (S3), whereas the greatest TK was detected at the pyrolysis temperature of 500°C for the residues of poplar twig (T2) and bark (B2). The TK in the residue-derived biochars, averaged across three pyrolysis temperatures, was ranked as rice straw (44.73 g · kg −1 ) > poplar leaf (14.88 g · kg −1 ) > poplar twig (12.14 g · kg −1 ) > poplar bark (7.14 g · kg −1 ). Moreover, a significant difference in CEC was found among the biochars (Table 3, p < 0.05), and the CEC in the biochars ranged from 12.61 cmol · kg −1 in B3 to 43.90 cmol · kg −1 in B2. However, the response magnitude of CEC to pyrolysis temperatures varied from the residue types. For instance, the CECs in poplar bark-derived biochars in B1 and B3 decreased by 56.7% and 71.2%, respectively, when compared to the B2, whereas the CEC in rice straw-derived biochars decreased by 9.7% in S1 but increased by 5.3% in S3, compared with the CEC in S2 (Table 3).

| Effects of residue direct application on soil CO 2 emission
A detailed dynamic and cumulative emission of CO 2 from the soil is presented in Figure 3 after the direct additions of different biomass residues. After adding biomass residues into the soil, the daily release rate of CO 2 from the soil all showed a tendency with being relatively fast in the initial period, gradually slowing down in the middle stage, and tending to be stable in the later stage of the incubation, whereas a slight difference in release dynamic of CO 2 was observed among the different biomass residues. For example, the daily release rate of CO 2 from the soil reached a relative stability (<3.0 mg · kg −1 · day −1 ) after the addition of 60 days in TR, 80 days in CK, and 100 days in BR and SR, respectively (Figure 3a), however, more than daily release rate of 5.0 mg · kg −1 · day −1 was still detected in LR after 100 days of the amendment.
After 180 days of incubation, the total cumulative CO 2 emission from the soil was significantly affected F I G U R E 3 Effects of various biomass residue additions on the CO 2 daily emission rate (a) and the cumulative emission (b) from the soil during the incubation period of 180 days. TR, BR, LR, SR, and CK represent poplar twig residue, poplar bark residue, poplar leaf residue, rice straw residue, and no bio-based material addition (only soil), respectively. Different lowercase letters in (b) indicate a significant difference among the treatments at the level of p < 0.05. by the residue biomass types (Figure 3b, p < 0.05). The highest cumulative emission was observed in the SR treatment, reaching about 1610 mg · kg −1 , followed by the LR and the lowest emission was detected in CK. Compared to the CK, the total cumulative CO 2 emission increased by 291.4% in SR,209.9% in LR,38.9% in TR,and 36.4% in BR, respectively. However, it is noted that the cumulative emission accounted for more than 50.0% of the total cumulative emission during the first 30-day incubation for all the treatments, even if the proportion varied from the residue biomass types (e.g., the cumulative emission reached about 84.1% in SR,79.3% in LR,62.9% in TR,58.1% in BR,and 50.8% in CK during the first 30-day incubation respectively, compared to the total cumulative emission of 180-day incubation period).

| Effects of biochar application on soil CO 2 emission
After amendment of various biochars into the soil, the daily release rate of CO 2 showed a similar dynamic tendency to the biomass residue addition during the incubation period (Figure 4). However, there was an obvious difference in the peaks of daily CO 2 emission among the biochar addition treatments, and the peak values ranged from 7.04 to 106.40 mg · kg −1 · day −1 . The peak value (9.92 mg · kg −1 · day −1 ) in CK was detected on the day of 30 after treatment, while the peaks in biochar amendment treatments all appeared before 20 days after the treatments ( Figure 4A-D). For instance, the peaks (15.82-26.04 mg · kg −1 · day −1 ) of S1, S2, and S3 biochars were observed on the 13 days after the treatments, whereas the F I G U R E 4 Effects of the biochar amendments derived from various biomass residues at the three pyrolysis temperatures on the CO 2 daily emission rate (A-D) and the cumulative emission (a-d) from the soil during the incubation period of 180 days. T1, T2, and T3 represent the poplar twig-derived biochars pyrolyzed at 300, 500, and 700°C, respectively. Similarly, B1, B2, and B3 represent the biochars derived from poplar bark residue pyrolyzed at 300, 500, and 700°C, L1, L2, and L3 for poplar leaf residue, as well as S1, S2, and S3 for rice straw residue, respectively. CK means no any bio-based material addition to the soil. Different lowercase letters in (b) indicate a significant difference among the treatments at the level of p < 0.05. peaks in the B1, B2, and B3 showed up on the day of 16, 13, and 7 after treatments, respectively.
The total cumulative CO 2 emission after 180 days of incubation was significantly affected by pyrolysis temperatures for each residue type (Figure 4a-d, p < 0.05), the response magnitude of the total CO 2 emission to the amendment of biochars pyrolyzed at different temperatures varied from the residue types. For instance, compared to the CK (no addition), the total cumulative CO 2 emission from poplar twig-derived biochars decreased by 26.0% in T1 amendment, 21.4% in the T2, and 5.3% in the T3 respectively, while the total emission from rice strawderived biochars increased by 3.6% in S1 amendment and 23.3% in the S2 but reduced by 20.1% in the S3. However, the total cumulative CO 2 emission after 180 days of incubation (averaged across the three pyrolysis temperatures) reduced by 17.6% and 8.8% in amendments of poplar twigderived and poplar bark-derived biochars respectively, whereas increased by 33.6% in poplar leaf-derived biochar and 2.3% in rice straw-derived biochar, when compared to the CK. Furthermore, the total cumulative CO 2 emission decreased by 40.7%, 33.1%, 56.9%, and 73.8% in the treatments with addition of poplar twig-derived, poplar bark-derived, poplar leaf-derived, and rice straw-derived biochars (averaged across three pyrolysis temperatures) respectively, when compared with the values in the addition of their corresponding biomass residue (TR, BR, LR, and SR, Figure 3b), indicating that biochar amendment is a better option than the direct application of biomass residue in term of CO 2 emission.
Hierarchical cluster analysis indicated that the data from the 17 treatments (including CK, 12 biochar amendments, and four direct application of biomass residues) can be classified into four distinct groups based on the total cumulative CO 2 emission during the experimental period ( Figure 5). Cluster 1 had a medium total cumulative CO 2 emission, ranging from 390 to 508 mg · kg −1 and included seven treatments (T3, B1, L2, S1, CK, B2, and S2); Cluster 2 consisted of four treatments (T2, S3, T1, and B3) and gave a relatively low range of the total cumulative emission, ranging from 284 to 329 mg · kg −1 ; Cluster 3 showed a higher cumulative emission with the range from 561 to 620 mg · kg −1 and comprised of four treatments (L1, L3, TR, and BR); and Cluster 4 (including LR and SR treatments) gave the greatest total cumulative emission ranging from 1274 to 1610 mg · kg −1 .

| Influences of residue type and pyrolysis temperature on biochar properties
The ability of biochar in stabilizing organic and mineral compounds is attributed to its specific properties such as porous structure, expanded specific surface area, high organic C content, active functional groups, and high CEC (Jamaludin et al., 2019), whereas the biochar properties are mainly determined by the type of biomass used and the parameters of pyrolysis process (Deng et al., 2022;Hanger et al., 2016;Subedi et al., 2016;Taherymoosavi et al., 2018; F I G U R E 5 A cluster analysis dendrogram of the treatments with different bio-based material amendments based on the total cumulative CO 2 emission during the incubation period of 180 days. Tomczyk et al., 2020). Our results showed that the surface and physicochemical properties of biochars were considerably influenced by both residue type and pyrolysis temperature (Tables 2 and 3). It seems that poplar residuederived biochars were larger in the surface area and total pore volume, while the surface area and total pore volume in all biochars increased with enhancing pyrolysis temperature (Table 2). A smaller surface area and lesser porosity of rice straw-derived biochars may be explained by the presence of the organic molecules on the biochar surface, which obscures the biochar pores at the relatively low temperatures (300-500°C) (Chen et al., 2008;Fuertes et al., 2010;Gaskin et al., 2008) or could escape from the biochar pores via volatilization at greater temperatures (700°C) (Fuertes et al., 2010;Subedi et al., 2016). However, poplar residue-derived biochars were lower in nutrients (TN, TP, and TK) and pH values than rice straw-derived biochar (Table 3). Increasing pyrolysis temperature led to a decrease in the TN for both poplar leaf-derived and rice straw-derived biochars, but enhanced the TN in the poplar twig-and poplar bark-derived biochars and ash content in all the biochars (Figure 1). Moreover, the responses of C/N ratio and CEC to the pyrolysis temperature varied among the different residue types (Table 3), conforming that the characteristics of biochar are mainly determined by the biomass type used and the parameters of pyrolysis process (Das et al., 2021;Gwenzi et al., 2015). As reported by Angın (2013), temperature is the only pyrolysis parameter that can control the elemental compositions of biochar and their atomic ratios, thus the biomass type chosen to undergo pyrolysis process plays an important role in determining the biochar properties. Our results suggest that biochar production with a desired performance (such as pollutant removal capacity, soil fertility improvement, and mitigation of GHG emissions) can be achieved only by understanding the effect of each parameter on the biochar properties.

| Differential effects of residue biomass and its biochar on CO 2 emission
The application of crop residues into soils has been widely used in agricultural systems in the past several decades (Duan et al., 2020;Huang et al., 2018;Liu et al., 2014;Zhang et al., 2021). However, the biomass residues from the forestry systems are either retained at sites or burned (Garrett et al., 2021;Mendham et al., 2002;Sarauer et al., 2019;Špulák & Kacálek, 2020). Biochar, as an alternative use of crop and plant residues, has received much attention in recent years (Duan et al., 2020;El-Naggar et al., 2019;Jiang et al., 2022;Lehmann et al., 2011). In this study, the applications of biomass residue and its biochar on soil GHG emissions were compared in an incubation experiment. Our results indicated that the total cumulative CO 2 emission after 180-day incubation decreased by 33.1%-73.8% in the biochar treatments when compared with the values in the amendment of their corresponding biomass residues (Figures 3 and 4), in agreement with previous studies (Duan et al., 2020;Wang et al., 2014;Yang et al., 2017). Biochar application decreased the CO 2 emission from the soil, which confirms our first hypothesis that the amendments of biochars derived from the biomass residues into the soil would significantly decrease the CO 2 emission as compared with the direct incorporation of its corresponding residues, and can be used as an effective practice to mitigate soil GHG emission and develop technologies with a low C footprint in agriculture and forestry.
Many studies demonstrated that suppression of soil GHG (including CO 2 , N 2 O, and CH 4 ) emissions following the biochar amendment varied among the site conditions, addition quantities, and biochar types (Abagandura et al., 2019;Case et al., 2014;Fang, Lee, et al., 2016;Fang, Lin, et al., 2016;Koga et al., 2017;Subedi et al., 2016;Wang et al., 2014). For instance, in the field, biochar amendment reduced soil CO 2 emissions by 33% and annual net soil CO 2 equivalent (eq.) emissions (CO 2 , N 2 O, and methane, CH 4 ) by 37% over 2 years (Case et al., 2014), while the application of biochars (produced from corn [Zea mays] stover, pinewood [Pinus ponderosa], and switchgrass [Panicum virgatum]) can mitigate CO 2 and N 2 O emissions from the sandy loam soil, but not from the clay loam soil (Abagandura et al., 2019). Moreover, our results showed that biochars derived from TR and BR had more positive effects on suppression of soil CO 2 emissions than ones derived from LR and SR (Figure 4), further confirming the conclusion from Case et al. (2014) that hardwood biochar has the potential to reduce the GHG emissions.
Indeed, the positive or negative effects of biochar amendments on CO 2 emission from soils were reported (Ahsaan et al., 2021;Giagnoni & Renella, 2022;Martin et al., 2015;Sheng et al., 2016). Our results showed that additions with different biochars into soil had diversified impacts on the CO 2 emission (Figure 4). The total cumulative CO 2 emission after 180-day incubation decreased by 5.3%-26.0% in poplar twig-derived biochars when compared with the values in the CK (Figure 4a), showing a positive effect on CO 2 emission. However, a negative effect was observed after amendments of poplar leafderived biochars (Figure 4c), where the cumulative CO 2 emission increased by 2.0%-48.4% compared to the CK. Furthermore, both the positive or negative effects were detected in the biochars derived from BR and SR pyrolyzed at various temperatures (Figure 4b,d). For example, a negative effect on CO 2 emission occurred after additions of rice straw-derived biochars pyrolyzed at the relatively low temperatures (e.g., 300 and 500°C), while a positive effect on CO 2 emission was found from rice straw-derived biochars pyrolyzed at 700°C. The present results suggest that an optimal pyrolysis temperature would vary for each residue type in term of the reduction effect on CO 2 emissions, which confirms our third hypothesis.

| Relationship between the properties of bio-based materials and CO 2 emission
Applications of organic residue and its biochar have been reported to markedly affect soil CO 2 emission (Abagandura et al., 2019;Duan et al., 2020;Sarauer et al., 2019;Wang et al., 2014Wang et al., , 2023, but the positive or negative effects were both observed. Some studies have well discussed the correlations of soil environment, soil chemical property, and soil biological processes to soil CO 2 efflux following the amendment of biomass residues or its biochars (Duan et al., 2020;Fang, Lee, et al., 2016;Fang, Lin, et al., 2016;Subedi et al., 2016), whereas the relationship between the properties of bio-based materials and CO 2 emission is less demonstrated. As shown in Tables 2 and 3, pyrogenic conversion of poplar residues and SR to biochars generally resulted in increase in pH, TC, and nutrient contents (TN, TP, and TK), but lowering of C/N ratios, which leads to much lesser CO 2 emissions from the soil after amendment (Figures 3 and 4), indicating the CO 2 emissions from the soil is greatly affected by the properties of added bio-based materials.
Pearson bivariate correlation analysis showed a significant and positive correlation between the CO 2 emissions and TN content of bio-based materials (including biomass residues and its biochars) but the negative relationships to TC content and C/N ratio (Table 4, p < 0.01), suggesting that soil microbes could utilize N from biochars as an important food source to decompose organic C (Benbi & Brar, 2021;Subedi et al., 2016). Meanwhile, the CO 2 emissions were positively correlated with the surface area, t-plot micropore area, and volume of the biochars (Table 4, p < 0.05 except for the surface area), confirming our second hypothesis that the CO 2 emissions are not only mediated by the biochar physicochemical properties but also the porosity and surface area of the biochar. As mentioned before, both biomass residue and pyrolysis temperature significantly affected the physicochemical and surface properties of the biochars (Tables 2 and 3), and consequently modified the CO 2 emissions from the soil amended with the biochars (Figure 4). For instance, the best effect on reducing CO 2 emissions was achieved at the soil amended with the biochars pyrolyzed at 300°C for poplar twig, 700°C for poplar bark, 500°C for poplar leaf, and 700°C for rice straw, respectively. Therefore, results from the present study suggest that residue type and pyrolysis temperature can be used as a means to predict the effects of plant residue-derived biochars on CO 2 emissions, supporting our third hypothesis, and highlight that biochar application has a great potential for mitigating global warming through enhanced soil C sequestration. Some studies indicated that a possible use of woody biomass residues is conversion to biochar and returning it to the forest soil for C mitigation and to potentially improve soil properties (Benbi & Brar, 2021;Hanger et al., 2016;Sarauer et al., 2019). However, it is not feasible to mix the biochars into the soil profile like agricultural soils, whereas biochar is amended to the soil surface in forest management, where biochars initially influence the forest soil and not the mineral soil (Sarauer et al., 2019), leading to variation in GHG flux by impacting different soil processes. Our incubation experiment found that applications of poplar twig-and poplar bark-derived biochars had a better effect on mediating CO 2 emissions than the poplar leaf-and rice straw-derived biochars (Figure 4), however, application (such as biochar application rates at different sites) and long-term monitoring are further required to assess the effect of these biochars on soil GHG emissions in the field trial.

| CONCLUSIONS
Our results indicated that the surface and physicochemical properties of biochars were obviously affected by the residue type and pyrolysis temperature. Poplar residuederived biochars were larger in the surface area and total pore volume but lower in nutrients and pH values than the rice straw-derived biochar. Increasing pyrolysis temperature led to a decrease in the TN for both poplar leafderived and rice straw-derived biochars, but enhanced the TN in the poplar twig-and poplar bark-derived biochars. However, the ash content, surface area, and total pore volume in all biochars increased with enhancing the pyrolysis temperature. Both the daily release rate of CO 2 and the total cumulative CO 2 emission were significantly affected by the amendment of bio-based materials. Hierarchical cluster analysis indicated that 17 treatments can be classified into four distinct groups based on the total cumulative CO 2 emission. However, total cumulative CO 2 emission from the soil with biochar amendments is significantly lower than that with their corresponding biomass residue addition. Pearson correlation analysis showed that there was a significant and positive correlation between the CO 2 emissions and TN content of bio-based materials but the negative relationships to TC content and C/N ratio. The biochars derived from TR and BR showed more positive effects on suppression of soil CO 2 emissions, but the optimal pyrolysis temperature varied among the residues. We can recommend poplar twig-and poplar bark-derived biochars for future application as eco-friendly residue management practices, however, application and longterm monitoring are further required to assess the effect of these biochars on soil GHG emissions in the field trial. Our results also suggest that the biochar with a desired performance can be achieved by optimizing production conditions, and residue type and pyrolysis temperature can be used as a means to predict the effects of plant residue-derived biochars on CO 2 emissions.

AUTHOR CONTRIBUTIONS
Sihui Ding was involved in investigation, methodology, data curation, data analysis, and writing-original draft. Ziyu Lan was involved in software, data analysis, and data curation. Shengzuo Fang was involved in conceptualization, methodology, supervision, funding acquisition, and writing-reviewing and editing.