Characteristics of growing medium components

Peat was sourced from Sungro Horticulture (Canadian Sphagnum Peat Moss Grower, Grade White, OMRI Listed) and coir from CANNA COCO Natural PLANT MEDIUM (CANNA Canada Corp.). Composite samples of peat and coir were analyzed at a private laboratory (A&L Canada Laboratory Inc., London, Canada) using recommended methods (Messiga et al. 2020). The particle size distribution for peat and coir was 3% and 10% for >2000 μm, 60% and 70% for 2000–1000 μm, 31% and 18% for 1000–250 μm, and 6% and 2% for <250 μm (Table 1). The pH equated 4.03 for peat and 6.08 for coir. Total N content was 0.61% for peat and 0.29% for coir, whereas the concentration of ammonium N (NH₄⁺-N) was 223 mg·kg⁻¹ for peat and 2.0 mg·kg⁻¹ for coir.

Table 1.

Physico-chemical properties of the growing media and amendments used in the study.

Biochar was sourced from Canadian AgriChar (OMRI Listed, Maple Ridge, Canada) and vermicompost from Nurturing Nature Organics Inc. (OMRI Listed, Lake Country, Canada). The particle size distribution of biochar and vermicompost was 10% and 17% for >2000 μm, 58% and 81% for 2000–1000 μm, 24% and 2% for 1000–250 μm, and 8% and 0% for <250 μm (Table 1). The pH was 9.56 for biochar and 4.22 for vermicompost. The dry matter content was 51.30% for biochar and 25.15% for vermicompost. Total N was 0.57% for biochar and 0.74% for vermicompost. The concentration of NH₄⁺-N was 11.50 mg·kg⁻¹ for biochar and 19.00 mg·kg⁻¹ for vermicompost (Table 1).

Incubation experiment

Six combination of growing media mixtures were assessed during a 60 d incubation experiment in three growth chambers (Conviron Adaptis A1000, Winnipeg, Canada) at Agassiz Research and Development Centre (BC, Canada). Eight destructive samplings for the growing media mixtures were retained, and the treatments ×sampling dates were arranged in a randomized complete block design with three replicates for a total of 144 experimental units (six growing media mixtures × eight sampling dates of 0, 3, 7, 12, 18, 32, 46, and 60 d × three replicates). The six growing media mixtures were prepared by mixing peat and coir with biochar or vermicompost amendments on a plastic tarp using the same proportions as described by Messiga et al. (2020) in v/v: (1) 100% peat (peat), (2) 90% peat + 10% vermicompost (peat + vermicompost), (3) 90% peat + 10% biochar (peat + biochar), (4) 100% coir (coir), (5) 90% coir +10% biochar (coir + biochar), and (6) 90% coir + 10% vermicompost (coir + vermicompost). We weighed 30 g of each growing media mixture into 250 mL Mason jars. As such, the total amount of N associated with the growing medium was 183 mg N·jar⁻¹ (6.11 g N·kg⁻¹) for peat, 181 mg N·jar⁻¹ (6.04 g N·kg⁻¹) for peat + biochar, 194 mg N·jar⁻¹ (6.48 g N·kg⁻¹) for peat + vermicompost, 87 mg N·jar⁻¹ (2.9 g N·kg⁻¹) for coir, 105 mg N·jar⁻¹ (3.49 g N·kg⁻¹) for coir + biochar, and 196 mg N·jar⁻¹ (4.79 g N·kg⁻¹) for coir + vermicompost. A bottle top dispenser was used to add distilled water (approximately 7 mL) into the Mason jars to reach approximately 60% water field pore space (WFPS), and then the moist growing media was mixed with a spatula. Maximum microbial activity is achieved at 60% WFPS (Franzluebbers 1999). The Mason jars were then closed with a parafilm. A needle was used to puncture 10 holes on the parafilm sheet to allow gas exchange. The 144 Mason jars were then arranged in the three growth chambers with each growth chamber representing one replicate. The growth chambers were set at day-length of 16 h (0600–2200), day temperature of 22 ±2 °C, night temperature of 20 ±2 °C, and relative humidity of 70%. These parameters were selected to mimic the environmental conditions we used in a previous pot experiment in the greenhouse with the same growing media (Messiga et al. 2020). One extra Mason jar per growing media was placed in each growth chamber for a total of 18 additional Mason jars. These 18 Mason jars were weighed every 3 d to check moisture content. Humidity was adjusted when needed by adding distilled water every time the moisture content decreased by more than 10%. All Mason jars (162 jars) were pre-incubated in the growth chambers for 7 d.

Gas sampling and analyses

At the end of the pre-incubation period of 7 d, gas samples were collected throughout the duration of the incubation experiment on the same set of sampling jars corresponding to 0, 3, 7, 12, 18, 32, 46, and 60 d after incubation (DAI). For all gas sampling periods, two set of samples were collected: the first corresponded to time t0 (T = 0 h) and was collected the day before; the second corresponded to time t1 (T = 24 h) and was collected after 24 h. Briefly, during the preparation phase, the punctured parafilm was removed from the sampled jars, and air inside the jar was mixed using a handheld fan. The sampled jar was then tightly closed with a lid equipped with a rubber septum. The headspace of the closed sampled jar was sampled using a 50 mL syringe, and the gas was injected into a 12 mL pre-evacuated Exetainer vial (Labco, High Wycombe, UK). After 24 h, the headspace of the closed sampled jar was sampled again as described above, and the collected gas was injected into a second pre-evacuated Exetainer vial. At the end of the sampling phase, the lid equipped with the rubber septum was removed, and the air inside the sampling jar was mixed with a handheld fane, and the jar was closed with a punctured parafilm until the next sampling date. All gas samples collected were analyzed for CO₂ and N₂O concentrations using a gas chromatograph (model 3800, Varian Inc., Walnut Creek, CA, USA) equipped with an electron capture detector (N₂O) and a flame ionization detector (CO₂). Gas emissions were calculated according to the equations of CO₂-C and N₂O-N fluxes (Rochette and Bertrand 2007), and cumulative gas emissions were calculated by linear interpolations between sampling dates.

Growing media sampling and analyses

At each of the eight sampling dates, the corresponding Mason jars were removed from the growth chambers, and four sets of 5.0 g moist growing media were taken for subsequent analyses. The first and second sets of subsamples were used for microbial biomass carbon (MBC) and nitrogen (MBN) extractions using the chloroform fumigation–extraction method (Voroney et al. 2008). Briefly, the first set (non-fumigated) was placed into a 50 mL centrifuge tube containing 30 mL of 0.25 mol·L⁻¹ K₂SO₄ and shaken end-over-end for 1 h; the second set (fumigated) was placed into 50 mL glass beakers which were arranged in a desiccator and fumigated with 50 mL ethanol-free chloroform for 24 h in the dark. The growing media were then transferred into 50 mL centrifuge tubes containing 30 mL of 0.25 mol·L⁻¹ K₂SO₄ and shaken end-over-end for 1 h. All suspensions (non-fumigated and fumigated) were centrifuged at 4500 r·min⁻¹ for 10 min, and the supernatant filtered using Whatman filter paper. The filtrates were stored at 4 °C and analyzed within 48 h using a TOC analyzer (Shimadzu Total Organic Carbon-Visionary Series; TOC-VCPH+TNM-1, Maryland, USA). The concentrations of MBC and MBN were then calculated by using extraction coefficients of 0.35 and 0.50 for MBC (K_EC) and MBN (K_EN), respectively (Voroney et al. 2008). The third set of 5.0 g moist growing media was used for mineral N (NO₃⁻ and NH₄⁺) analysis using KCl extraction. Briefly, 5.0 g moist growing media was extracted with 2 mol·L⁻¹ KCl using a 1:10 (w:v) growing media:extractant ratio. All extracts were analyzed colorimetrically for NO₃⁻ and NH₄⁺ using a flow injection analyzer (Tecator FIAStar 2010) as described by Maynard et al. (2007). The fourth set was used to determine the moisture content and was placed into an aluminum cup and oven-dried at 105 °C for 48 h.

Statistical analysis

An analysis of variance based on the randomized complete block design was performed for data of NH₄⁺-N, NO₃⁻-N, CO₂-C, and N₂O-N emissions in the growing media, using SAS PROC MIXED, version 9.3 (SAS Institute 2010), considering block as a random factor, sampling dates as repeated effects, and growing media mixtures as fixed effects. Data of cumulative CO₂-C and N₂O-N emissions and cumulative MBC and MBN in the growing media at the end of the 60 d incubation were analyzed using SAS PROC MIXED considering block as a random factor and growing media mixtures as fixed effects. Differences among least square means (LSMEANS) for all treatment pairs were tested at a significance level of P = 0.05.

Evolution of mineral nitrogen concentrations

For peat-based growing media mixtures, the concentration of NH₄⁺-N decreased from 430 mg·kg⁻¹ at 0 DAI to 37 mg·kg⁻¹ at 60 DAI (Fig. 1). On the other hand, the concentration of NH₄⁺-N of coir-based growing media slightly varied from 58.2 mg·kg⁻¹ at 0 DAI to 39.5 mg·kg⁻¹ at 60 DAI. During the first days of incubation, peat amended with biochar had lower NH₄⁺-N concentrations compared with peat alone and peat amended with vermicompost; NH₄⁺-N concentration in peat amended with biochar was lower than peat alone by 12.8% at 0 DAI, 19.7% at 3 DAI, and 8.0% at 7 DAI. Little changes were observed for coir amended with biochar compared with coir alone. However, coir amended with vermicompost had higher NH₄⁺-N concentrations (23.1% at 0 DAI, 152.3% at 3 DAI, 89.5% at 7 DAI, 78.4% at 12 DAI for coir alone; 47.4% at 0 DAI, 101.8% at 3 DAI, 161.6% at 7 DAI, 68.5% at 12 DAI for coir amended with biochar) compared with the other coir-based growing media. Vermicompost had no significant effect when mixed with peat, except at 46 DAI, which was 63.12% lower.

Fig. 1.

Evolution of ammonium (NH₄⁺-N) nitrogen concentrations of six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

In contrast to NH₄⁺-N, NO₃⁻-N concentrations increased steadily up to a peak around 32 and 46 DAI for peat and coir amended with vermicompost, respectively (Fig. 2). Specifically, for peat amended with vermicompost, NO₃⁻-N concentration peaked at 32 DAI with 275 mg·kg⁻¹ followed by a decreasing trend down to 44.7 mg·kg⁻¹ at 60 DAI (Fig. 2). For coir amended with vermicompost, NO₃⁻-N concentration peaked at 552 mg·kg⁻¹ at 46 DAI followed by a decreasing trend down to 251 mg·kg⁻¹ at 60 DAI (Fig. 2). For coir and peat alone and coir and peat amended with biochar, NO₃⁻-N concentration during the incubation period remained close to 0 mg·kg⁻¹ between 0 DAI and 18 DAI, then increased steadily up to 50 mg·kg⁻¹ (coir) and 33 mg·kg⁻¹ (peat) at 60 DAI (Fig. 2).

Fig. 2.

Evolution of nitrate (NO₃⁻-N) nitrogen concentrations of six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

Evolution of CO₂-C and N₂O-N during the incubation period

During the respective sampling dates, peat amended with biochar had the highest CO₂-C emission rate, whereas peat amended with vermicompost had in average the lowest CO₂-C emission rate; the peat alone having an intermediate emission rate (Fig. 3a). After 60 DAI, the cumulative CO₂-C emission was 180 μg CO₂-C·g⁻¹ dry weight (DW) for peat amended with vermicompost, 638 μg CO₂-C·g⁻¹ DW for peat alone, and 2070 μg CO₂-C·g⁻¹ DW with peat amended with biochar (Fig. 3b). Thus, the partial substitution of peat with biochar increased the cumulative CO₂-C emissions by 3.2 times (636–2070 μg CO₂-C·g⁻¹ DW), whereas the substitution of peat with vermicompost decreased the cumulative CO₂-C emissions by 3.5 times (636–179 μg CO₂-C·g⁻¹ DW). In contrast to peat, the partial substitution of coir with biochar did not modify the cumulative CO₂-C emissions, whereas the substitution of coir with vermicompost increased the cumulative CO₂-C emissions by 0.9 time (306–582 μg CO₂-C·g⁻¹ DW).

Fig. 3.

Evolution of (a) hourly CO₂-C and (b) cumulative CO₂-C emissions by six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

During the first week of incubation, coir amended with vermicompost had the highest N₂O-N emission rate among all growing media mixtures (Fig. 4a). However, peat amended with vermicompost followed by peat amended with biochar had higher N₂O-N emissions between 18 and 46 DAI. The cumulative N₂O-N emission after 60 DAI was affected by the growing media mixtures. The cumulative N₂O-N emissions varied from 16.77 μg N₂O-N·g⁻¹ DW with peat alone to 111.82 μg N₂O-N·g⁻¹ DW with peat amended with vermicompost (Fig. 4b). The partial substitution of peat and coir with vermicompost increased N₂O-N emissions compared with their substitution with biochar. Indeed, peat with biochar increased the cumulative N₂O-N emissions by 3.7 times compared with peat alone (16.78–62.78 μg N₂O-N·g⁻¹ DW), whereas the substitution of peat with vermicompost increased the cumulative N₂O-N emissions by 6.7 times (16.78–111.82 μg N₂O-N·g⁻¹ DW). Similar trend, but at a much lower level, was observed for coir amended with vermicompost where the cumulative N₂O-N emission increased by 1.3 times (19.57–47.53 μg N₂O-N·g⁻¹ DW). On the other hand, peat and coir alone had a similar low-cumulative N₂O-N emissions.

Fig. 4.

Evolution of (a) hourly N₂O-N and (b) cumulative N₂O-N emissions by six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

Evolution of MBC and MBN during the incubation period

The cumulative MBC was significantly affected by the growing media at 18, 32 46, and 60 DAI. The cumulative MBC varied from 18.2 μg·g⁻¹ DW with coir amended with biochar to 26.5 μg·g⁻¹ DW with peat amended with vermicompost (Fig. 5). The partial substitution of peat or coir with biochar decreased the cumulative MBC compared with peat and coir alone or amended with vermicompost. Thus, at 60 DAI, the substitution of peat with biochar decreased the cumulative MBC by 16.0% (24.9–20.9 μg·g⁻¹ DW), whereas the substitution of peat with vermicompost increased the cumulative MBC by 5.7% (24.9–26.4 μg·g⁻¹ DW). Similarly, the partial substitution of coir with biochar decreased the cumulative MBC by 16% (21.7–18.2 μg·g⁻¹ DW), whereas its amendment with vermicompost increased the cumulative MBC by 3.2% (21.7–22.4 μg·g⁻¹ DW).

Fig. 5.

Evolution of microbial biomass carbon of six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

The cumulative MBN was significantly affected by growing media and sampling dates. It varied from 0.47 μg·g⁻¹ with coir amended with vermicompost to 0.80 μg·g⁻¹ with peat amended with vermicompost (Fig. 6). The partial substitution of peat with biochar decreased MBN by 10% (0.57–0.51 μg·g⁻¹ DW), whereas the substitution of peat with vermicompost increased MBN by 41% (0.57–0.80 μg·g⁻¹ DW) compared with peat alone. On the other hand, the partial substitution of coir with biochar had no significant effect on MBN, whereas its substitution with vermicompost decreased MBN by 13% (0.54–0.47 μg·g⁻¹ DW). It is interesting to observe that there was a spike in MBN accumulation between 12 and 18 DAI, and then a plateau was reached, except for peat amended with vermicompost that continued to increase. For example, for coir amended with biochar MBN increased by 25% between 0 and 12 DAI, and by 623% between 0 and 18 DAI. Similar patterns were obtained with the other treatment combinations.

Fig. 6.

Evolution of microbial biomass nitrogen of six growing media mixtures during a 60 d incubation period. Means followed by asterisks are significantly different at a given day after incubation (*, p < 0.05; **, p < 0.01; ***, p < 0.001).

Dynamics of mineral nitrogen (NH₄⁺-N and NO₃⁻-N) during the incubation

High initial NH₄⁺-N concentrations in peat-based growing media could result from ammoniation, a process through which ammonia is sorbed onto peat at high temperatures (Nommik 1967). A study conducted in Quebec showed that at ammoniation rates above 30 g NH₃ kg·dry peat⁻¹, exchangeable NH₄⁺-N is the dominant N fraction in ammoniated peat (Abbes et al. 1995). Ammoniated peat has several benefits for plant growth including enhanced N uptake, root length, and root growth rate (Abbes et al. 1995). However, a recent greenhouse study showed that the use of this material as growing media can result in high concentrations of NH₄⁺-N in the leachates at early growth stages when the plant root system is not yet developed to take up or scavenge the inorganic N (Messiga et al. 2020). In greenhouse production systems with nutrient recirculation, the synchrony between plant N needs and N supply can be achieved through the recycling of the leached inorganic N back to the growing media. This source of N could be particularly beneficial for microgreen productions with short growth cycle (7–21 d) and relatively low N requirement compared with longer growing crops where this source of N will be negligible.

Although all peat growing media had high initial values, it is interesting to observe that they all had a similar and low NH₄⁺-N concentrations at 60 DAI compared with the coir-based growing media that had a constant and low initial value (Fig. 1). The lower initial NH₄⁺-N concentration of peat amended with biochar compared with peat alone or amended with vermicompost is in agreement with a previous greenhouse study using cabbage and lettuce as test crops (Messiga et al. 2020). These results are also in line with previous studies showing that biochar owing to its large surface area, surface charge, and cation exchange capacity can fix NH₄⁺-N, thus reducing potential risk of inorganic N leaching. For example, Teutscherova et al. (2018) showed that holm oak biochar produced at 600 °C increased NH₄⁺-N sorption in a sandy Acrisol. Lehmann et al. (2003) reported that reduced N leaching through soils amended with biochar was partly explained by NH₄⁺-N sorption due to the higher CEC and increased water-holding capacity. The substitution of peat with softwood biochar for soilless marigold growth also resulted into reduced extractable NH₄⁺-N as well as for organic tomato greenhouse crops (Dorais et al. 2017; Margenot et al. 2018). It is also interesting to observe that lower NH₄⁺-N concentrations in peat amended with biochar compared with the other peat-based growing media mixtures occurred only at the beginning of the incubation experiment indicating a rapid NH₄⁺-N sorption process. This could be explained by the fine particle size of biochar allowing better mixing conditions with peat. It is also possible that biochar altered the hydrophobicity or water repellency of peat by enhancing water infiltration and retention (Lehmann et al. 2003), thus removing the NH₄⁺-N from the ammoniated peat and transferring it onto biochar.

Coir-based growing media mixtures had low NH₄⁺-N concentrations throughout the incubation period showing that neither the carbon-rich material nor the two amendments, biochar and vermicompost, were sources of NH₄⁺-N (Fig. 1). On the other hand, coir and peat amended with vermicompost had high initial NO₃⁻-N concentrations compared with the other growing media mixtures indicating that vermicompost was the source of NO₃⁻-N (Fig. 2). The sharp increase in NO₃⁻-N concentrations obtained between 18 and 32 DAI for peat amended with vermicompost, and 18 and 46 DAI for coir amended with vermicompost (Fig. 2), suggests favorable conditions for the nitrification process. The amount of vermicompost was 12.9 g in coir + vermicompost but 8.61 g in peat + vermicompost (Messiga et al. 2020). The difference was due to the bulk densities of coir (258 kg·m⁻³) and peat (228 kg·m⁻³) (Table 1). The additional 4.29 g of vermicompost (0.082 mg NH₄⁺-N per pot) could have extended the nitrification process in coir + vermicompost mixture up to 46 DAI. Studies showed that nitrification is a dominant process associated with the decomposition of organic N during vermicomposting (Lv et al. 2019). Burrowing and shuttling activities of earthworms during vermicomposting improve aerobic conditions and resulting in high nitrification rates (Cáceres et al. 2018). The initial NO₃⁻-N concentration observed prior to 18 DAI could be explained by the immobilization of NO₃⁻-N during vermicomposting in the earthworm’s tissues and cells and microbial biomass or sorption onto vermicompost materials. The NO₃⁻-N or its precursors such as nitrites immobilized during the vermicomposting process could represent a transient phase that leads to nitrification once favorable conditions prevail. Among the favorable conditions that trigger the reactivation of microbial activity are the cycles of drying and rewetting (Chen et al. 2014) as well as mixing of coir or peat with vermicompost before and during the incubation. The nitrification proceeded over a long period (18–46 DAI) under coir amended with vermicompost owing to the physical properties of the growing media mixture (Fig. 2). Coir is a loose and fluffy material compared with peat, and therefore, the mixture between coir and vermicompost had more pores available for water and gas exchanges, conditions favorable for the nitrification process. The prolonged nitrification process favored N supply and supported higher leafy vegetable yields obtained under coir amended with vermicompost compared with peat amended with vermicompost growing media during the greenhouse experiment (Messiga et al. 2020).

CO₂-C and N₂O-N emissions and microbial biomass C and N

The large cumulative CO₂-C and N₂O-N emissions obtained under peat amended with biochar is in line with several studies (Liu et al. 2016; Wang et al. 2020), but in contrast with others (Dorais et al. 2017). The increase in CO₂ emission with biochar is generally associated with the labile C contained in biochar itself (Lévesque et al. 2020b). High NH₄⁺-N content associated with peat may have enhanced the mineralization of labile C of biochar. Thus, the association between NH₄⁺-N from peat and labile C from biochar was favorable for CO₂ emission in peat amended with biochar. Studies conducted on soils with varying properties showed that fluxes of CO₂ and N₂O with biochar addition increased with N fertilizer rates (Fang et al. 2016; Wang et al. 2020). In the other hand, Lévesque et al. (2020b) found that addition of compost decreased CO₂ emissions probably due to the low priming effect on native soil C decomposition. In stark contrast with peat, coir amended with biochar did not impact CO₂ and N₂O emissions (Figs. 3 and 4). This could be explained by differences in coir and peat characteristics that may have inhibited the mineralization of labile C contained in biochar resulting in sharper plateau after 18 DAI and lower MBC at 60 DAI (Table 1). The total N and NH₄⁺-N concentrations were lower in coir compare with peat which may have limited the mineralization of labile C associated with biochar when mixed with coir, resulting in a lower MBC after 60 DAI. The pH of coir was greater than peat (6.08 vs. 4.03), and studies have shown that biochar may increase net emissions in acidic environments (Sheng et al. 2016).

High fluxes of CO₂-C and N₂O-N under peat amended with biochar compared with peat alone did not translate into high concentrations of cumulative MBC and MBN (Figs. 5 and 6), whereas the opposite was observed for peat amended with vermicompost. Microbial biomass under coir amended with biochar was lower (MBC) or similar (MBN) compared with coir alone, which is in line with the trends of CO₂ and N₂O emissions that were not different. Except for peat amended with vermicompost, the limited MBN increase beyond 20 DAI suggests that the pool of labile C, associated with biochar or with the other growing medium materials, that could feed microbial growth was limited (Xu et al. 2016). In a previous experiment, we observed that the growth of young cabbage plants was delayed under peat and coir amended with biochar in the absence of N fertilizer until 2–3 wk after transplanting. We also observed re-growth following this period indicating that N was supplied to the young cabbage plants which is consistent with the increase of MBN (Messiga et al. 2020). It is, therefore, possible that the spike in MBN was re-mineralized into mineral N, which would support the increase in NO₃⁻-N observed for peat and coir amended with vermicompost after 32 and 46 DAI, respectively (Figs. 1 and 2).

The large cumulative N₂O-N emission obtained with addition of vermicompost in peat and coir compared with their respective control, was induced by the high NO₃⁻-N content measured between 18 and 46 DAI (Fig. 2). In a previous study, we also found that partial substitution of peat and coir with vermicompost increased NO₃⁻-N concentrations in the growing media and leachates (Messiga et al. 2020). High N concentrations due to N fertilization tends to exponentially increase N₂O emissions (Shcherbak et al. 2014; Lévesque et al. 2020b). Denitrification of NO₃⁻ could explain part of the increased N₂O emissions particularly under prevailing anaerobic conditions. During vermicomposting, the transit of organic substrates and microorganisms in the gut of earthworms facilitates spore germination which would explain the increasing MBC and MBN in peat amended with vermicompost (Pramanik 2010). Another study also demonstrated that some fungal strains responsible for rapid mineralization of wastes increased after earthworm gut passage (Aira et al. 2005). Vermicompost used in this study was sticky and maintained high moisture content (bulk density of 569.5 kg·m⁻³, dry matter content of 25.15% and coarse particles including 17% >2000 μm and 81% between 2000 and 1000 μm; Table 1) which may have favoured anaerobic conditions and, therefore, increased denitrification after 32 or 46 DAI (Figs. 2, 5, and 6). Studies have shown that N₂O emissions increased dramatically at water-filled pore space >70% (Barton and Schipper 2001). To benefit from the properties of vermicompost including an increased NO₃⁻-N supply as partial substitute of peat, improved water management would be a key issue. Strategies that avoid or limit anaerobic conditions in the growing media will be needed to reduce denitrification and, therefore, losses of N through N₂O-N emissions. The pH of the growing media could also explain the higher N₂O-N emissions observed under peat amended with vermicompost (Figs. 4a, 4b). Peat and vermicompost had acidic pH, 4.03 and 4.22, respectively (Table 1). Messiga et al. (2020) observed that the mixture of peat and vermicompost at the end of one cycle of cabbage and lettuce production had an acidic pH suggesting that the pH of the mixture remained acidic throughout the incubation experiment. Several studies have reported increased N₂O emissions with decreasing soil pH in the range of acidic pH (Šimek and Cooper 2002; Sun et al. 2012). Wang et al. (2018) in a regional study in China reported that soil pH is the main factor explaining regional disparities in N₂O emissions. The main mechanism controlling N₂O emissions across the range of acidic pH is the inhibition of a functional N₂O reductase enzyme (Bakken et al. 2012; Liu et al. 2014). It is, therefore, possible that the acidic pH of peat amended with vermicompost growing inhibited the functional N₂O reductase enzyme, thus increasing N₂O emissions. Raising soil pH of acidic soils to neutrality by liming has resulted to decreased soil N₂O emissions (Hénault et al. 2019). Additional management practices including liming to raise the pH of peat amended with vermicompost close to neutrality could be important to decrease the associated N₂O emissions.