Mineral nitrogen availability from co‐digested biodigester products as affected by feedstock pretreatment

Digestion of animal manures with crop residues to produce methane (CH 4 ) gas is a promising technique to generate “green” energy from agricultural wastes while producing biofertilizers. This study was conducted to understand nitrogen (N) release by biofertilizers from the products of solid‐state anaerobic digestion of untreated corn stover and stover treated with aqueous ammonia (NH 4 OH), calcium hydroxide (Ca(OH) 2 ), or Ca(OH) 2 plus Fe 3 O 4 nanoparticles blended with dairy manure. Eight biofertilizer materials (feedstocks and digestates) from anaerobic digestion were incubated in the lab for 28 days and the NH 4 –N and NO 3 –N data obtained were used to predict N release for seasonal plant availability. Of the untreated and alkaline pretreated biofertilizer materials examined, only calcium hydroxide‐pretreated digestate (CaD) had greater N release (220 mg kg −1 ) than the untreated soil control (155 mg kg −1 ) after 28 days of incubation time. In addition, CaD had the most rapid N release rate (0.16 mg kg −1 day −1 ) and shortest lag phase time (41 days) with a predicted mineral N release of 777 mg kg −1 at 120 days. However, including Fe 3 O 4 nanoparticles slightly suppressed N release. Thus, applying calcium hydroxide‐pretreated digestate to soil could complement N supply for crop cultivation as facilitated by the initial carbon to nitrogen (C/N) ratio, despite slight N immobilization when iron nanoparticles were added.