Biochemical methane potential (BMP) tests were conducted for investigating the effects of alfalfa-derived biochar (AF-BC) on anaerobic digestion (AD) of dairy manure under various loading of AF-BC (0–10 g/L). BMP tests were performed at mesophilic temperature (37 °C) with the addition of AF-BC. Biogas and methane volumes and concentrations, water quality parameters (i.e., COD (chemical oxygen demand)), and volatile fatty acids (VFAs) were measured during the AD process. The addition of 1 and 5 g/L of AF-BC increased the biogas yields by 15.51% and 26.09% and methane yields by 14.61% and 26.88% compared with the control without addition of AF-BC. Additionally, the addition of AF-BC (1–10 g/L) decreased the lag phase by 7.14–22.45% and the CO₂ content of biogas by 13.60–32.48%, while increasing the COD removal efficiency by 19.19–35.94% in the AD of dairy manure. Moreover, the addition of AF-BC also decreased total VFAs and acetic acid concentrations in the AD process. The increase in AD performance was mainly owing to the improvement of buffering ability of the AD system and direct interspecies electron transfer (DIET) among AD microorganisms resulting from the addition of AF-BC. In contrast, the addition of 10 g/L AF-BC did not show any obvious improvement in biogas and methane yields in the AD of dairy manure, possibly because of toxic effects from excessive addition of AF-BC toward the AD microorganisms. Therefore, this study supported practical feasibility of AF-BC-enhanced AD of dairy manure.

Although various studies have investigated biochar (BC) soil amendments for improving soil microbial abundance, functions, and community structure, a comparison of dairy manure biochar (MBC) to wood biochar (WBC) is warranted given the large volume of manure produced in high-intensity dairy production. Additionally, the synergistic effects of different BC sources and loading percentages on microbial functions and community composition using massively parallel 16S DNA sequencing in BC-amended soils with different types of crops are limited. In this study, the synergistic effects of BC type, BC loading percentage, and crop types on soil fertility, prokaryote community diversity, and functions were investigated in a greenhouse study. The MBC and WBC were used to amend sandy loam soils at increasing BC loading percentages (0, 5, and 10%) to grow the cool-season forages crimson clover (Trifolium incarnatum; an annual legume) and Italian ryegrass (Lolium multiflorum Lam.; an annual forage grass) for 120 days. High nutrient concentrations in MBC shifted microbial communities towards r-strategists and alkaliphiles, potentially increasing the rate of nutrient bioremediation from high nitrogen- and phosphorus-containing soil amendments. This study enables emerging biochar agronomic use recommendations with different crops.