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

Digestion of animal manures with crop residues to produce methane (CH4) 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 (NH4OH), calcium hydroxide (Ca(OH)2), or Ca(OH)2 plus Fe3O4 nanoparticles blended with dairy manure. Eight biofertilizer materials (feedstocks and digestates) from anaerobic digestion were incubated in the lab for 28 days and the NH4–N and NO3–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 Fe3O4 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. Core Ideas: Products from anaerobic biogas digester proved to be a more effective biofertilizer than biodigester feedstocks in this study.Calcium hydroxide pretreated digestate (CaD) released optimal N with the least delay.Adding 20 mg kg−1 Fe3O4 nanoparticles concentration to the CaD reduced NO3–N mineralization. Plain Language Summary: We evaluated four pretreatments of codigested dairy manure and corn stover to determine their suitability as nitrogen (N)‐containing biofertilizer after the digestion process was completed (digestate). The treatments for the study were an un‐pretreated manure stover mixture and pretreatment with calcium hydroxide (CaOH), ammonium hydroxide (NH4OH), and CaOH with Fe3O4 nanoparticles. Pretreatment with calcium hydroxide (CaOH) resulted in a digestate that had the greatest N release and release rate. Adding Fe3O4 nanoparticles to the CaOH‐treated material tended to reduce the availability of the N from the digestate material. [ABSTRACT FROM AUTHOR]