Introduction

Biochar is a promising avenue for carbon (C) sequestration in temperate soils of prairie eco-regions. Biochar is a form of pyrogenic-C produced by O₂-limited thermal decomposition of organic matter (OM) at temperatures <700 °C. Amending soil with biochar, i.e., recalcitrant-C, has been shown to mitigate agricultural greenhouse gas emissions by altering soil biochemical properties, e.g., increasing cation exchange capacity and reducing microbial activity. Biochar stabilizes soil OM, thereby resulting in higher soil C benefits relative to more labile amendments such as manure and compost (Preston and Schmidt 2006; Whitman et al. 2015). Applying biochar to croplands is also frequently associated with improved soil quality and crop productivity; biochar often increases soil pH and nutrient availability for plant uptake (Preston and Schmidt 2006).

Recently, findings that biochar use in animal feeding could lower enteric methane (CH₄) emissions have expanded the prospect of its use in modern-day farming (Whitman et al. 2015). Similarly, biochar–manure manure may also stabilize manure OM, emitting less ammonia (NH₃), nitrous oxide (N₂O), and CH₄ once applied to croplands (Kammann et al. 2017). Romero et al. (2021) demonstrated that biochar increases manure OM recalcitrance and its overall C sequestration potential, yet its effect on manure OM mineralization is mostly unknown.

Pyrogenic-C represents up to 20%–65% of total soil C in prairie soils (Preston and Schmidt 2006). Nevertheless, in cropped sites across central Canada, farming has removed pyrogenic-C by suppressing wildfires, thus altering its cycling in surface soil layers. Restoring pyrogenic-C levels, mainly through biochar additions, could increase crop production, particularly in Gray Luvisols, soils that are often difficult to farm due to their inherent low pH and poor nutrient availability. The objective of this work was to determine the effect of manure amendment, in the presence or absence of biochar, on soil C and N mineralization over a 64 d incubation period. Due to pyrogenic-C’s recalcitrant nature, we hypothesized that biochar would retain some of its properties once excreted by feedlot cattle (Romero et al. 2021), thereby limiting microbial activity and associated soil C and N mineralization (Whitman et al. 2015).

Materials and Methods

Solid manure was retrieved from a 235 d feeding trial conducted at the Lethbridge Research and Development Centre of Agriculture and Agri-Food Canada near Lethbridge, AB, Canada. Briefly, 80 yearling beef steers were fed a typical Canadian feedlot backgrounding diet consisting of 60% barley silage, 35% barley grain, and 5% mineral supplement (Terry et al. 2020). Treatments consisted of adding Southern yellow pine (Pinus echinata) biochar (National Carbon, Oakdale, MN, USA) at 0% (regular manure, RM) and 2% (biochar–manure, BM) of diet dry matter. Manure samples were collected in January 2019 and sent to the University of Alberta Campus in Edmonton, AB, Canada, for incubation and analysis. The biochar had a pH of 7.3, an electrical conductivity of 317 mS·cm⁻¹, an H/C ratio of 0.29, as well as 733 g·kg⁻¹ and 2 g·kg⁻¹ of total C and N, respectively.

Surface soil (0–10 cm) was collected from the Breton Plots (53°07′N, 114°28′W) near Breton, AB, Canada. The soil, classified as a loamy Gray Luvisol (pH 6.3), was amended in four replications with (i) RM at 160 Mg·ha⁻¹, (ii) BM at 160 Mg·ha⁻¹, (iii) raw biochar (BC) at 10 Mg·ha⁻¹, (iv) a combination of (ii) and (iii) (BM + BC), (v) combination of (i) and (iii) (RM + BC), and (vi) a non-amended control (soil without amendments) (CT). The rate of manure application was selected to mimic field applications for barley forage production in Alberta.

After collection, the soil was air-dried at room temperature, passed through a 2 mm sieve to remove plant litter, homogenized, and stored at room temperature (22 °C) for 30 d. Samples were incubated in a Forma Diurnal Growth Chamber-Model 3740 (Thermo Fisher Scientific, Waltham, MA, USA) at 25 °C. Two identical sets of 200 g (dry-weight basis) of air-dried soil were weighed and placed into 500 mL Mason jars. One set was kept undisturbed for carbon dioxide (CO₂) gas measurement. The second set was also kept undisturbed, but small cores were removed for inorganic N measurements at the same sampling intervals as for CO₂. Water-holding capacity (WHC) was determined using a pressure plate analysis at field capacity (−0.33 MPa). Both sets were pre-incubated at 60% WHC for 7 d to avoid the initial flush of respiration after soil disturbance. Immediately after pre-incubation, manure/biochar amendments were applied to the soil. Jars were loosely capped, and caps were removed weekly for 10 min to ensure adequate aeration over the incubation period, during which samples were weighed and water added to maintain the 60% WHC condition.

CO₂-C fluxes and inorganic N, nitrate (NO₃-N), and ammonium (NH₄-N), concentrations were measured every 3 d for the first week, once a week for the following 2 wk, and then biweekly for the remainder of the study (64 d). CO₂-C fluxes were quantified using a LiCor LI-8100 Soil Gas Flux System and multiplexor (LI-COR, Lincoln, NE, USA) plumbed for flask measurements. Soil was extracted with a 2 mol·L⁻¹ KCl solution at a 1:5 (w:v) soil:extract ratio, shaken (250 r·min⁻¹, 1 h), and then filtered using a Whatman No. 42 filter paper. Nitrate and NH₄-N were determined via colorimetry using a Thermo Gallery Plus Autoanalyzer (Thermo Fisher Scientific).

Subsamples of soil and manure/biochar amendments were dried at 105 °C for 48 h, ball-milled (<0.15 mm), and stored in 20 mL scintillation vials. Total C (TC) and N (TN) were analyzed by dry combustion using a Thermo Flash 2000 Organic Elemental Analyzer (Thermo Fisher Scientific). Pyrogenic-C was measured using the benzene polycarboxylic acid method following Wiedemeier et al. (2016). Soil pH was measured in a 1:5 soil:water slurry, after samples were shaken for 1 h, vacuum filtered, and allowed to settle for 30 min.

Statistical analysis

All statistical calculations were performed using R version 1.1 (R Core Team 2020). To predict soil C and N mineralization dynamics, we fit the data to a first-order reaction,

(1)

where Am is the cumulative amount of soil C or N mineralized, Ao represents the labile pool of C or N, t is time, and k is the rate of mineralization constant (Riffaldi et al. 1996).

A one-way analysis of variance was used to analyze differences between manure/biochar amendments and soil properties, as well as the effect of manure/biochar treatments on soil C and N mineralization first-order kinetic curves. Assumptions of normal distribution and equal variance were confirmed using Shapiro and Bartlett’s tests, respectively. Means were compared using Tukey’s honestly significant difference, where F values were significant at α = 0.05.

Results and Discussion

Carbon mineralization

Carbon mineralization (Cm; 528 mg CO₂-C·kg⁻¹) was increased (P < 0.001) with RM + BC relative to the other amendments (>354 mg CO₂-C·kg⁻¹) (Table 1), whereas Co (577 mg CO₂-C·kg⁻¹) was only augmented (P < 0.001) with RM + BC compared with BM (397 mg CO₂-C·kg⁻¹). Mixing RM with BC stimulated a priming effect in soil, whereas BM and BC alone did not (Fig. 1a), supporting our hypothesis that biochar–manure would have a lower soil C mineralization potential relative to manure-only treatments. Carbon mineralization was reduced in BM (217 mg CO₂-C·kg⁻¹) compared with RM (340 mg CO₂-C·kg⁻¹) but was similar to CT (215 mg CO₂-C·kg⁻¹) (Table 1).

Table 1.

Respired soil carbon (C) and mineralized soil nitrogen (N) first-order kinetic parameters to amendment types over a 64 d incubation period and their corresponding P values (means ± standard error) (n = 3).

Fig. 1.

Respired soil carbon (C) (a) and mineralized soil nitrogen (N) (b) as affected by manure/biochar treatments. BC, biochar (10 Mg·ha⁻¹); RM, manure from feedlot cattle on a control diet (160 Mg·ha⁻¹); CT, control (no amendments); BM, manure from feedlot cattle on a control diet with the addition of BC at 2% of diet dry matter (160 Mg·ha⁻¹). Whiskers above each shape indicate standard errors of the means (n = 3).

The reaction rate coefficient (Kc) was increased with RM + BC, BM, and BM + BC relative to CT and BC treatments (Table 1). Apparently, there was a synergistic effect between RM and BC, considering that RM- and BC-only resulted in lower Kc values (Table 1). Lentz et al. (2014) found that applying manure with biochar improves the ability of heterotrophs to degrade biochar-C, which is in agreement with our findings of greater soil C mineralization under RM + BC. Adding biochar to manure may also improve aeration, further increasing microbial activity over labile-C (Whitman et al. 2015). Our observation supports this potential interaction as cumulative CO₂-C emissions in RM + BC did not plateau as quickly as with the other treatments (Fig. 1a).

Interestingly, BM + BC and RM + BC exhibited distinct CO₂-C emission patterns (Fig. 1a), despite soil C mineralization rates being similar between RM and BM treatments (Table 1). Romero et al. (2021) demonstrated that biochar passes through the gastrointestinal tract of feedlot cattle mostly unaltered. Based on this observation, we speculate that manure OM in BM is as labile as in RM, given that manure-C does not interact with biochar-C (Romero et al. 2021). However, biochar–manure is expected to be more aromatic than RM when considering the whole OM mixture (Romero et al. 2021). The latter supports our findings that adding biochar to BM does not increase CO₂-C emissions as much as adding biochar to RM (Table 1).

Conclusions

Application of manure, biochar, and biochar–manure impacted soil C mineralization, but did not affect soil N mineralization in the studied Gray Luvisol. Cumulative CO₂-C emissions were higher with RM + BC than BM + BC and adding BC to RM or BM increased soil C and N mineralization rates. Our results indicate that BC and BM amendment might benefit soil C sequestration by lowering CO₂-C emissions over time without limiting soil N availability. Further research calls for whole-farm studies to validate the cascaded use of BM amendment in agroecosystems. Probing BM properties at a larger scale, utilizing different biochar feedstocks, is critical to identify BM types that maximize soil C benefits in Western Canada.

Conflicts of Interest

The authors declare there are no competing interests.

Contributions

Investigation: T.L.W; writing and editing: C.M.R, M.D.M, T.L.W; resources and supervision, M.D.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by Agriculture and Agri-Food Canada (AGGP2-037), Alberta Agriculture and Forestry (Project No. 2017R036R), Blue Rock Animal Nutrition, National Carbon, and the University of Alberta.