Nitrogen fertilizer replacement value of undigested liquid cattle manure and digestates

• NH4-N in unseparated digested slurry-maize mix (DSMM) was as efficient as ammonium sulphate (AS).
• NH4-N in untreated slurry (US) was less efficient than AS in the first two years.
• NH4-N in DSMM solid fraction (SF) was also less efficient than AS in the first two years.
• SF and US exhibited residual N effects in subsequent years.

Accurate estimation of animal manure nitrogen (N) availability is required to maximize crop N use efficiency and reduce environmental N losses. Many field and laboratory experiments have shown that first-year net mineralization of manure organic N is often negligible, which often causes crop available N to approximate the ammonium N content of the manure. Anaerobic digestion increases the ammonium share and reduces the C to organic N ratio of animal manures, potentially increasing their N fertilizer value.In 2011, we undertook a three-year field experiment in Northern Italy to estimate the N fertilizer value of four manures: undigested cattle slurry, digested cattle slurry-maize mix, and liquid and solid fractions of the digested slurry-maize mix. The experiment also allowed us to test if ammonium recovery was similar among manures, and between manures and ammonium sulphate. Fertilizers were applied annually to plots before silage maize cultivation that was followed by an unfertilized Italian ryegrass crop.Results showed that the recovery of ammonium from manure in maize did not differ significantly compared to ammonium sulphate among all the fertilizers in 2013; however, in 2011 and 2012 it was significantly lower for all manures except digested slurry-maize mix and its liquid fraction in 2011.The increased recovery of applied N in 2012 and 2013 for solid fraction and undigested manure were likely due to the residual effect of previously applied organic N.