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The co-product of anaerobic digestion, digestate, is nitrogen (N) rich; however, the forms and accessibility of this N by the crops have not been fully explored. This study aimed to determine the mineralization parameters of digestate N and to assess its availability for annual ryegrass (Lolium multiflorum Lam.). Four digestate rates of 0 (control), 38, 75, and 150 mg N kg−1 soil (equal to 0, 90, 180, and 360 kg total N ha−1) were applied to a silty clay loam soil in a completely randomized block design with four replications in a greenhouse study. A 100 d aerobic incubation experiment was also conducted with 0 and 150 mg digestate N kg−1 rates at 25 °C. Digestate feedstock included cattle manure (28%), hay (15%), and silage corn (Zea mays L.; 57%). Total plant biomass and N uptake increased linearly with digestate application rate with average apparent N recovery of 37%. Potentially mineralizable N (N0) and mineralizable N rate constant (k) were not significantly different in digestate and control treatments; however, a flush of digestate organic N (30 mg N kg−1) released right after mixing the digestate with soil. Evidences of N immobilization with digestate application were observed in greenhouse study. Majority of plant-available digestate N was in form of NH4+-N; therefore, NH4+-N can be used for estimation of available digestate N for crops. Results need to be validated for specific feedstock and soil properties under field conditions. Further research is needed to assess how long-term build-up of digestate organic N may impact the N availability for crops.

Effects of biochar-compost mixture (B+Com) and cover crop were assessed on soil and grapevine productivity in an irrigated Merlot (Vitis vinifera L.) vineyard in Okanagan Valley, British Columbia (BC), Canada from 2017 to 2020. The experimental design was a factorial arrangement of control, B+Com, cover crop, and combination of cover crop and B+Com (cover crop/B+Com) treatments in alleys with four replications. The B+Com comprised a 1:1 ratio of biochar and compost and was applied at a rate of 22 Mg ha-1 dry-weight basis in May 2017 and 2019. The cover crop consisted of a dryland forage mixture and birds foot trefoil (Lotus corniculatus L.). Biochar-compost treatment did not affect cover crop biomass or tissue C and N concentrations except for a 12% reduction in 2019 biomass. Biochar-compost and cover crop/B+Com increased soil C content averaged across sampling dates by 11 and 17% (P<0.05) only at the 0-15 cm soil depth compared to the control. Cover crop treatment did not affect (P<0.05) soil C content at two soil depths in all sampling dates. The soil N content was not affected by B+Com, decreased by cover crop by average of 12.5% in both soil depths, and increased with cover crop/B+Com by 4% only at the 0-15 cm soil depth averaged across sampling dates (P<0.05). Grape yield was increased by 32% by cover crop/B+Com relative to control only in 2020. The cover crop reduced petiole N and pruning weights in one or two years out of three.

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