Amending soil with manure from cattle fed biochar (BC) (biochar-manure [BM]) is a potential best management practice to improve plant nutrition in the circular economy. Yet, information concerning the agronomic performance of BM under temperate field conditions is scarce. A 2-year study on a Gray Luvisol was conducted to determine the effect of BM on the crop growth of spring wheat (Triticum aestivum L.), followed by canola (Brassica napus L.), soil fertility, and microbial function. Treatments included (i) no amendments (control), (ii) BC at 5 and 10 Mgha(-1) (BC5 and BC10), (iii) regular stockpiled manure (RM) at 100 kg total Nha(-1), (iv) stockpiled BM at 100 kg total Nha(-1), and (v) BC and RM (BC+RM) or BC and BM (BC+BM) at the aforementioned rates. During the wheat growing season in 2020, which had high precipitation, grain yield was 2.4 times greater in BM+BC10 than in BM alone (1416 vs. 579 kgha(-1), p < 0.001), highlighting synergistic effects of BM together with BC application on agronomic performance. Conversely, lower precipitation and warmer temperatures in the canola growing season in 2021 hampered any statistical differences among treatments. While soil microbial biomass did not change by the end of the experiment, shifts toward amino acid utilization with BC additions in both crops potentially influenced crop growth and nitrogen-use efficiency. In summary, BM+BC at 10 Mgha(-1) performed best in this study during the first cropping season under cold and rainy conditions.
