Soil Physical, Chemical, and Biological Properties
The observed soil type differences are primarily due to their differing parent materials, with the Black soil of high clay content developed from a weakly calcareous glaciolacustrine deposit having lower pH and EC levels compared with the Brown loam textured soil developed on moderately calcareous shale-modified glacial till. The contrasting SOC levels of the two soils result from the historically drier Brown soil zone supporting lower plant production and associated above- and below-ground residue inputs to soil, compared with the wetter Black soil zone region of the province. The lower Black soil Db is related to its higher SOC content, while the Black soil greater WHC and CEC are a function of its higher clay and SOC amounts (Table 1). Accordingly, the higher WFPS in the Brown soil is associated with its higher Db, coupled with lower porosity and volumetric moisture content (data not shown). The relative lack of treatment effects on the Black soil WEOC and WEON, compared with the Brown soil, is consistent with the much higher SOC and total N levels within the Black soil (Table 1), which would tend to mask treatment effects more.
The tendency of SCM to reduce Db and increase CEC compared with LHM, especially within the Brown soil, is due to the greater organic matter content of SCM per unit weight. Likewise, the improved WHC of both soils following either manure addition is primarily related to the reduced Db (and associated increased porosity) from the added organic matter. The higher EC following LHM addition compared with SCM is caused by the high EC of the LHM (1,480 dS m-1). The elevated pH following SCM addition compared with LHM is attributed to a liming effect, given the high concentration of base cations within SCM, such as calcium (33,800 mg kg-1), magnesium (12,200 mg kg-1), potassium (9,500 mg kg-1), and sodium (1,400 mg kg-1), along with carboxyl and phenolic hydroxyl functional groups neutralizing H+ [76-78]. The increased Brown soil CEC following SCM addition is due to the humic acid substances within SCM . The greater WEON (and corresponding decrease in WEOC:WEON) within the LHM treatments is attributed to its greater water soluble organic N content compared with SCM. The reduced Brown soil MB-C concentration with SCM alone is consistent with its enhanced MeQ compared with the unamended control, along with its smaller MB-C:MB-N than LHM alone (Figs. 1k,l). An increasing MeQ indicates decreasing heterotrophic respiration efficiency, where microbes tend to mineralize C instead of assimilating (i.e., immobilizing) it in their biomass, and is consistent with the observed inverse relationship between MeQ and both MB-C and MB-N (Fig. 3). The increased MeQ could be due to moisture stress associated with the surface hydrophobicity of composted SCM when initially applied to soil  or changing microbial community structure. The hydrophobicity of SCM was not an issue with the Black soil, given its greater WHC (Fig. 1b). Additionally, the wider MB-C:MB-N with LHM alone compared with SCM alone, likely reflects the proliferation of autotrophic nitrifiers, specifically ammonia-oxidizing Archea (AOA) and bacteria (AOB), following LHM addition containing predominantly NH4-N.
The reduction in Brown soil EC with Slow biochar alone is presumably due to electrolyte sorption by the biochar, while the increased Brown soil pH following the addition of alkaline biochar is often reported . The increased WHC of both soils with the Slow biochar only is consistent with Szmigielski et al. , who reported the Slow biochar increased WHC more than the Fast biochar regardless of application rate. Furthermore, the greater porosity and specific surface area of the Slow biochar, relative to the Fast biochar (Table 2), are recognized to collectively enhance soil aeration, WHC, sorption capacity, and microorganism refugia [82,19,20]. It is important to note the agronomic impact of this increased WHC, albeit minor, can have on crop productivity; particularly, within semi-arid Saskatchewan, where soil moisture deficits often limit annual and perennial plant species growth, including short-rotation coppice (SRC) willow production [83-85]. Specifically, in the companion four-year field study to this incubation work , isotopic fractionation δ13C composition data served as a proxy for historic water-use efficiency and, therefore, an index of moisture stress. The biochar-amended Brown and Black soils reduced the moisture stress of annual crops over a four-year period, compared with the crops growing in plots without biochar. Although both biochars reduced crop moisture stress, the positive effect was more pronounced with the Slow biochar (R. Hangs, unpublished data). The reduction in soil WEOC with the Slow biochar alone is likely due to its large sorptive capacity (Table 2) and biochar’s affinity to sorb low-molecular weight organic acids and organic pollutants [86-90]. Szmigielski et al.  found the Slow and Fast biochars reduced herbicide activity in soil through sorption. Additionally, in the third year of the companion field study , the added biochars mitigated wild oat (Avena fatua) antagonism against wheat (Triticum aestivum L.) growth; presumably by mollifying allelochemical activity via sorption (R. Hangs, unpublished data). The contrasting effects of the Slow biochar alone on the Brown soil MB-C and MB-N (decreased) compared with the Black soil (increased) could be related to a greater sensitivity of the heterotrophic community to reduced WEOC availability in the Brown soil containing less SOC (Table 1).
Compared with SCM alone, the decreased Db and increased WHC of SCM+Slow for both soils, along with a reduction in the Brown soil WFPS with SCM+Slow and SCM+Fast, may indicate a synergistic interaction between SCM and the biochars. Considering both SCM and biochar amendments are known to improve Db, WHC, and WFPS [77,50,91], co-applying these low-density materials may contribute to the improved soil structure, aeration, and hydraulic properties, especially within the higher Db Brown soil. The decreased Brown soil CEC and MeQ observed with SCM+biochars, compared with SCM alone, may be due to biochar sorption of SCM-related humic substances and SCM-WEOC, which reduced the soil CEC and labile-C available for heterotrophic metabolism, considering the recalcitrant nature of WEOC associated with composted SCM .
The Brown loam soil properties generally were more responsive to the imposed treatments than the Black clay soil in this study, which is consistent with the companion four-year field study , along with most reports indicating poorer quality soils are more responsive to biochar additions than higher quality soils. Overall, the added manures and biochars, alone or in combination, had a minor impact on the measured soil physical, chemical, and biological properties of both soils and likely reflects the good quality of the young temperate prairie soils used. Moreover, the limited treatment effects may be explained by only a single manure application, along with the relatively low rate of biochar used in this study. Recent investigative and meta-analyses indicate >10 Mg biochar-C ha-1 is generally required to influence young fertile temperate soil properties [93,35,94].
Soil Nitrogen Availability
The cumulative six-week NH4-N (13.5-24.2 mg 10 cm2 six weeks-1) and NO3-N (337.4-3456.9 mg 10 cm2 six weeks-1) supply rates are consistent with values reported elsewhere for Canadian prairie soils amended with LHM, SCM, or biochar [95,96,50,42]. The NO3-N supply represented 97.9% (averaged across all treatments and soils) of the total inorganic N available and was two orders of magnitude larger than the NH4-N supply; reflecting the rapid nitrification typically occurring within these temperate prairie agricultural soils . For example, although 93% of the total LHM-N added was in the form of NH4-N (7% organic N; data not shown), the LHM, with or without biochar, did not increase the NH4-N supply within either soil (Fig., 1m). The decreased NH4-N availability within both soils following manure addition, with and without biochar, may be attributable to a combination of enhanced NH4-N oxidation to NO3-N by AOA and AOB, along with net NH4-N immobilization from autotrophic metabolism, along with heterotrophic mineralization of the added labile-C [98,99]. The greater NH4-N levels observed with the Fast biochar alone compared with Slow biochar alone, within both soils, could be the result of its greater CEC protecting sorbed NH4-N from autotrophic and heterotrophic activity [100,101,32]. The enhanced net mineralization (i.e., increased NH4-N supply rate over and above immobilization) with the SCM+Slow and SCM+Fast compared with SCM alone, within both soils, is likely a combination of the biochars augmenting heterotrophic mineralization of the added SCM-organic N, while protecting a portion of the mineralized NH4-N from microbial metabolism through sorption mechanisms. Additionally, the increased WHC of SCM+Slow (Fig. 1b) would also increase the supply of diffusion-limited nutrients like NH4-N.
The greater Black soil NO3-N availability compared with the Brown soil is indicative of the larger mineralizable N content within the Black soil (Table 1). The higher NO3-N supply associated with the LHM compared with SCM, with or without biochar, is consistent with previous work comparing these manures  and represents their contrasting predominant N forms and associated temporal availability: LHM (narrow C:N, inorganic, and immediate) vs SCM (wide C:N, organic, and slow-release). Additionally, the organic N within the composted SCM used is more recalcitrant and mineralizes slower than fresh SCM [102-105]. The SCM alone increased the Brown soil NO3-N only; presumably due to the inherently higher Black soil NO3-N level masking the relatively small amounts of mineralized SCM-organic N. The lack of fertilizer N effect from the added biochar-N (105 kg N ha-1; on average) on the NO3-N availability within either soil, is not surprising given the inherently good quality of these young temperate soils (particularly, the fine-textured Black soil), the low N content of willow stem feedstock, and the recalcitrant nature of lignocellulosic biochar-N [58,106,19]. However, Hangs et al.  reported that 20 Mg C ha-1 of the Slow biochar immobilized soil N within the Brown soil and reduced NO3-N levels (≈ 50%), with and without added urea (100 kg N ha-1). In the same study, the Slow biochar did not affect the NO3-N level within a loam Orthic Black Chernozem (Typic Haplocryoll), with or without urea. In this study, only 8 Mg C ha-1 of the Slow biochar decreased the Black clay soil NO3-N supply (Fig. 1n); primarily caused by biotic fixation (i.e., autotrophic and heterotrophic immobilization; Fig. 1j), considering its wider C:N ratio than the Fast biochar (Table 2). Such seemingly contradictory results reaffirm the importance of assessing the many soil×biochar interactions that may exist before generalizing biochar amendment effects (and application rates) across broad soil types.
The higher NO3-N supply with the LHM+Slow than LHM alone and LHM+Fast within both soils (Fig. 1n), may be due to three biochar-related factors [107,108,93]: i) the Slow biochar provided a more favourable ecological habitat; resulting in enhanced autotrophic nitrification by AOA and AOB. ii) the Slow biochar contributes to faster movement of NO3-N to the PRS™-probe ion-exchange resin membrane surface, given its greater porosity (Table 2); and iii) the Fast biochar presumably has a stronger anion-exchange capacity to fix adsorbed NO3-N, which is common among inactivated lignocellulosic biochars produced at lower temperatures. Considering the majority of cereal crop plant N uptake from soil occurs within several weeks after plant emergence , the observed difference in plant available NO3-N between the Slow and Fast biochars co-applied with LHM observed during this six-week study is consistent with the following first-year results from the companion field study : i) applying the Fast biochar with LHM reduced the production of barley (Hordeum vulgare L.) grain (25%) and total biomass (29%) at both field sites, along with straw (29%) at the Brown site compared with LHM alone; ii) both LHM alone (72%) and LHM+Slow (57%) increased barley N uptake, whereas LHM+Fast had no effect; and iii) less fertilizer 15N was recovered by barley growing within the Fast biochar plots compared with Slow biochar plots. The lower Brown soil NO3-N level within the SCM+Slow compared with SCM alone, may be caused by reduced NH4-N availability for nitrification, via biochar sorption, which evidently rendered a portion of the NH4-N available for PRS™-probe measurement, but less accessible for microbial metabolism (Figs. 1k,m,n). Within the Black soil, the muted effects of either manure, alone or in combination with biochar, reflect the Black soil having more soil organic matter and inherently stronger buffer capacity to maintain soil solution N concentration (Table 1). It is important to note, however, after the first year of the companion field study, the SCM+Slow increased the residual NO3-N content within both the Brown (55%) and Black (49%) soils compared with SCM alone . Additionally, compared with either the SCM or Slow biochar applied alone, the SCM+Slow increased canola (Brassica napus L.) production (i.e., a high N-demanding crop) at the Brown (year 2) and Black (year four) sites . Consequently, the co-application of the Slow biochar with SCM appears to reduce SCM-N availability within these two soils in the short-term (i.e., weeks; this study), but enhance SCM-N availability for plant uptake over the long-term (i.e., years; companion field study). The precise mechanism(s) is unclear, but the Slow biochar could be inhibiting SCM-N mineralization and/or immobilizing SCM-N in the short-term, and subsequently augmenting SCM-N mineralization and/or preserving liberated SCM-N (NH4-N, NO3-N, and dissolved organic N) over the long-term via sorption [110,108,93,29].
The estimated six-week cumulative N2O emissions ranged from 37.9-1956.8 mg m-2 (Fig. 2a) and are consistent with Canadian temperate agricultural soils, with and without, LHM, SCM, or biochar addition [111,10,53,29,50]. The principal edaphic properties governing soil N2O emissions are abundant inorganic N, labile SOC for denitrifying heterotrophs, and anoxic conditions . These conditions were most prevalent within the Black soil (Tables 1 and 4; Figs. 1b,f,n), which helps to explain the larger N2O losses from the unamended Black soil compared with the untreated Brown soil. Despite less WFPS within the Black soil (61.3%) than the Brown soil (72.4%), anaerobic microsites occur more frequently within clay soil [112,113]. Moreover, both soils surpassed the critical threshold of 60% WFPS for denitrification [114,111,115].
The relationship between LHM addition, with or without biochar, and elevated N2O emissions from both soils may be the result of the following [116-119]: i) N2O losses associated with autotrophic nitrification (Fig. 1n); ii) denitrifying community responding to increased NO3-N availability (Fig. 3); and iii) prolific heterotrophic activity in response to the added LHM dissolved organic-C and -N creating anaerobiosis within soil aggregate microsites. Accordingly, the greater N2O emissions attendant with the LHM explain the lack of LHM alone effect on the Black soil NO3-N supply (Figs. 1n and 2a). Likewise, the relatively larger amounts of dissolved organic-C and -N within LHM supported higher rates of nitrification and denitrification compared with SCM-amended soils [Figs. 1n and 2a; 10].
Reduced N2O emissions following biochar addition are generally considered to be associated with the following : i) improved soil aeration/gas exchange due to reduced Db (and correspondingly greater porosity); ii) reduced WFPS; iii) biotic N immobilization and/or biochar sorption lowering soil NO3-N supply; and iv) elevated pH. All these factors likely contributed to lower N2O emissions in this study, but the influence of biochar on soil pH (indirect) and NO3-N availability (direct) appeared to be the most relevant factors (Fig. 3). Interestingly, however, the reduced N2O emissions from both soils with the LHM+Slow compared with LHM alone corresponded with enhanced NO3-N supply (Figs. 1n and 2a); thus, indicating enhanced LHM-N conservation by the Slow biochar. Likewise, the greater Brown soil N2O emissions from the LHM+Fast than LHM+Slow, help account for the lower NO3-N level within the LHM+Fast and could be due to the more labile-C within the Fast biochar supporting greater N2O emissions. Additionally, recent work involving western Canadian Chernozemic soils of similar textures, with and without the addition of a lignocellulosic slow pyrolysis biochar, suggests a narrowing WEOC:WEON is indicative of greater microbial metabolism , which agrees with the observed inverse relationship between WEOC:WEON and the microbially mediated pathways (i.e., autotrophic and heterotrophic) controlling the NO3-N supply and N2O fluxes in our study (Fig. 3).
The greater Brown soil N2O emissions with the LHM+Fast than LHM+Slow, may be due to reduced gas permeability and diffusivity, resulting from soil pore clogging associated with the much smaller Fast biochar particles (Table 2), causing less aeration and increased denitrification . The Brown soil was more susceptible to reduced gas exchange, given its greater Db (i.e., lower pore space volume), less WHC, and WFPS compared with the Black soil (Figs. 1a-c). The reduced NO3-N availability following Slow+SCM (Brown soil) and Slow biochar alone (Black soil), without attendant increased N2O emissions, could be due to the following [110,108,98,121,93]: formation of organo-biochar-mineral complexes that stabilized organic N and, therefore, slowed its mineralization, especially within clay soils; ii) preservation of SCM-N (NH4-N, NO3-N, and dissolved organic N) via Slow biochar sorption; and iii) biotic N immobilization. Presumably, this N will become available eventually and these slow-release N mechanisms would explain the apparent synergism between SCM+Slow, to increase canola growth at both the Brown (year 2) and Black (year 4) field sites following a single application of SCM and Slow biochar . As such, the results from this six-week incubation study are consistent with the meta-analysis of Gao et al. , who reported co-applications of biochar with organic fertilizers can improve inorganic N availability for crop uptake.
Although the capacity of lignocellulosic biochar to reduce soil N2O emissions is well known [e.g., 122,123,124], the inability of either biochar alone to influence the N2O emissions in our study is attributed to the relatively low rate of biochar applied and the inherently good N retention capacity of these two soils. Cayuela et al.  suggest lignocellulosic high-temperature slow pyrolysis biochars facilitate electron transport during denitrification; thus, coupled with its higher pH and greater surface area for N2O sorption [33,126] compared with Fast biochar (Table 2), may have promoted more complete denitrification (i.e., N2O reduction to N2), thereby decreasing the N2O/(N2O + N2) ratio. Increased pH under aerobic conditions favours NH4-N sorption by biochar and reduced nitrification, while under anerobic conditions supports complete reduction to N2 [Fig. 3; 127]. In saturated alkaline soils, N2 is the primary source of denitrification loss , which although not a greenhouse gas, results in decreased manure-N use efficiency.
The estimated six-week cumulative CO2 fluxes ranged from 665.2-1233.2 g m-2 (Fig. 2b), and are consistent with Canadian temperate agricultural soils, with and without, LHM, SCM, or biochar addition [128,129,10,53,50,29]. The observed CO2 fluxes were considered to be primarily governed by heterotrophic respiration, given the direct relationship between the CO2 flux and WEON, WEOC, MB-C:MB-N, MeQ, and WHC, along with its indirect relationship with Db (i.e., porosity) and WFPS (Fig. 3). For example, the Slow biochar addition to the Black soil enhanced heterotrophic mineralization-immobilization as evidenced by the increased CO2 flux and MB-N, with a corresponding reduction in NH4-N and NO3-N supply (Figs. 1j,n and 2b). Likewise, the enhanced Brown soil CO2 flux with the Fast biochar alone, particularly compared with Slow biochar alone, may be caused by the greater amount of labile-C added with the Fast biochar stimulating heterotrophic activity. Larger CO2 fluxes associated with lower temperature biochars, compared with higher temperature biochars, are often reported [e.g., 130,124,131] and help to explain the greater Black soil CO2 flux (mineralization) and lower NO3-N supply (immobilization) in the LHM+Fast than LHM+Slow (Figs. 1n and 2b). As such, the C:N values of our Slow (116) and Fast (51) biochars were not a reliable indicator of biochar-C mineralization potential compared with their volatile matter and fixed C contents (Table 2). Another useful metric for assessing the biochar-C mineralization potential is its hydrogen (H):C, which is indirectly related to its recalcitrance [132-134] and was relevant in this study.
Considering manure and biochar amendments are well known to promote microbial abundance, activity, and diversity by improving soil physical and chemical properties [e.g., 135,136,137], the inverse relationship between CO2 flux and both MB-C and MB-N were surprising (Fig. 3). For example, the C and N substrates added with the LHM and SCM treatments would be expected to increase heterotrophic respiration, especially within the lower SOC-containing Brown soil. Biochar is capable of immobilizing CO2 through surface sorption and precipitation reactions, to form CaCO3 (due to increased pH and calcium availability associated with the biochar), but these are relatively minor CO2 sinks . Likewise, CO2 sorption reactions with soil minerals and organic matter, along with pedogenic (i.e., secondary) CaCO3 recrystallization do occur in calcareous soils, but likewise, are minor CO2 sinks [138,139]. Instead, however, these apparently contradictory results likely reflect the concurrent opposing influence of autotrophic (i.e., CO2 sink) and heterotrophic (i.e., CO2 source) communities on the measured CO2 fluxes [i.e., net CO2 emissions; 50]. Specifically, treatments adding labile-C and -N were simultaneously supporting both autotrophic respiration (i.e., CO2 consumption) and heterotrophic respiration (i.e., CO2 production). The inherent complexity of the treatments varying influence on autotroph and heterotroph activity collectively impacted the measured CO2 fluxes and were manifest by the significant (P <0.01) interaction among all main treatment effects (Table 4) and minor treatment effects on CO2 flux (Fig. 2b). These concurrent CO2 sink/source pools also help to explain the highly variable MeQ data in this study (Fig. 1l), along with numerous inconsistent reports of MeQ for more than a decade following biochar additions that varied markedly depending on soil N availability [86,140]. Heterotrophic respiration has long been considered the largest regulator of soil CO2 flux and sole consideration when calculating MeQ . However, unless the proportional influence of autotrophic and heterotrophic respiration on measured CO2 flux can be quantified, then caution needs to be exercised when basing inferences regarding soil ecosystem health on MeQ data, which does not account for autotrophs (i.e., their biomass or CO2 consumption) during the measurement period.
Well-drained temperate agricultural soils typically are sinks for atmospheric CH4 and our study was no exception; with average CH4 consumption after six weeks among treatments (28.3-90.0 mg m-2; Fig 2c) consistent with Canadian temperate agricultural soils, with and without, LHM, SCM, or biochar addition [129,10,50,29]. Like the CO2 fluxes, the measured CH4 fluxes are a net balance of CH4 methanogenesis and methanotrophy. Several edaphic factors favour methanotrophy [142-144]: (i) enhanced aggregation, structure, and porosity causing more microaerophilic niches and, therefore, greater CH4 diffusivity to aerobic microsites; (ii) alkaline pH; (iii) low EC; (iv) moisture content at/or below FC (and/or less than 60% WFPS); and (v) large C:N accentuated by wide C:N amendments. The imposed treatments positively affected these physical and chemical properties to varying degrees, albeit slightly, for both soils (Figs. 1a-e). Consequently, the observed methanotrophy in this study was primarily influenced by NH4-N availability (Fig. 3). For example, the much greater CH4 consumption with the SCM alone than LHM alone was principally due to the comparatively large amount of NH4-N added with the LHM manure causing a shift in autotroph (primarily AOA) methanotrophic activity, which preferentially oxidize NH4-N instead of CH4, following the addition of NH4-N or NH4-N forming inorganic or organic fertilizers [145-147]. In the absence of plants, this shift in autotrophic methanotrophy to nitrification is evidenced by increased NO3-N availability concurrent with decreased CH4 consumption  and was most apparent when comparing the greater Brown soil NO3-N supply, coupled with less CH4 oxidation, associated with LHM alone than SCM alone (Figs. 1n and 2c). Furthermore, rapid nitrification often favours N2O production, along with reduced CH4 consumption (and vice-versa), which agrees with the enhanced Brown soil N2O emissions and reduced CH4 sink strength associated with the LHM alone and LHM+Fast compared with the unamended control and SCM alone (Figs. 2a,c). It is desirable to observe the Slow biochar co-applied with LHM mitigated the elevated N2O emissions (both soils) and decreased CH4 consumption (Brown soil) typically associated with LHM amendments (Figs. 2a,c). Presumably, this biochar-induced effect involved enhanced abiotic and biotic N immobilization, in addition to promoting greater methanotrophic activity. Interestingly, despite greater Black soil N2O emissions with the LHM alone than SCM alone, the LHM alone promoted more CH4 consumption than SCM alone (Figs. 2a,c) and could be related to the more favourable soil aeration and porosity following LHM addition (i.e., Db and WHC; Figs. 1a,b). The increased Black soil methanotrophy with the SCM+Fast than SCM alone is probably due to the preservation of mineralized SCM-N from nitrification through biochar sorption; indicated by its greater NH4-N supply (Fig. 1m). Enhanced CH4 consumption following biochar amendment is often reported [e.g., 143,50,148] and is attributed to not only its improvement of the edaphic factors favouring CH4 oxidation, but also, CH4 retention via biochar sorption further facilitating CH4 oxidation [149-151]. However, the inability of either biochar alone to affect the CH4 consumption of either soil, along with no treatment effects on the Black soil methanotrophy is not uncommon [151,31,34] and likely reflects the relatively low rate of biochar added and inherent good quality of these young temperate prairie soils.
Implications for Lifecycle Analyses of SRC Willow Bioenergy Production
Widespread biofuel adoption will be aided by these fuels being accepted as CO2e-negative in their influence on atmospheric GHG concentrations . Over the past two decades, the life cycle analysis (LCA) of net GHG emissions associated with SRC willow bioenergy production has progressed from initially being considered a weak C-source to a currently being deemed a large sink for atmospheric C, as increasing above- and below-ground C-sink crediting data have become available . Likewise, if accounted for, the reduced GHG emissions associated with the co-firing of willow biomass with coal for electricity generation within large jurisdictions [e.g., Saskatchewan; 153,154] would further support the CO2e-negative narrative of SRC willow bioenergy. This evolving perspective is an excellent example of how expanding LCA boundary conditions can profoundly influence subsequent conclusions. Consequently, the GHG mitigation benefits of bioenergy fuel chain co-products, such as biochar, warrant consideration within LCAs [155,30,156]. Currently, there is a general lack of interest in commercial scale slow pyrolysis biochar production, due to the perceived absence of any market value . However, if the agroecosystem benefits of slow pyrolysis biochar can be valued, then a further economic justification of SRC willow exists for growers . In our study, the Slow biochar effectively reduced the LHM-related N2O emissions, while increasing the soil CH4 sink strength, compared with LHM alone. Moreover, neither biochar co-applied with manure affected the CO2 fluxes in either soil compared with the manures alone.
As such, co-applying the willow biochar with manure appears to not only support present agronomic initiatives seeking novel approaches to maximize manure-N use efficiency within temperate regions , but also, represents an innovative practice for mitigating GHG fluxes associated with land-applied manure, which would help demonstrate Canada’s compliance to its Paris Agreement commitment [14,15]. Considering our study examined short-term GHG fluxes after a single application of manure and biochar under controlled conditions, we recommend assessing the long-term GHG fluxes in situ over several field seasons, after a single biochar application (at a variety of rates) and following repeated annual manure additions (representative of conventional practices), to validate our results and/or augment our understanding of the mechanisms governing our observed GHG fluxes. If consistent results are found under field conditions, then new LCAs encompassing an even broader perspective (i.e., “womb to tomb”) of SRC willow bioenergy production could incorporate the GHG mitigating benefits of co-applying biochar with manure, to further improve the estimated net GHG emissions associated with this renewable energy alternative.