Production of mineral nitrogen during composting of dairy cattle slurry solid fraction

Abstract

Screw pressed dairy cattle slurry solid fraction (CSSF) was collected during 2004 and 2005 from two dairy farms with 30% and 22% -24% dry matter (DM) content and the effects of covered either with black polyethylene or with polypropylene, pile dimension, straw addition and of turning frequency on the fate of N were examined. The CSSF with 30% DM was collected at a rate of 1 m³ h^-1 and the material with 22% 24% DM was collected at the rate of 4 and 3 3 _h-1 min 2004 and 2005, respectively. Thermophilic temperatures were attained soon after separation of CSSF with 30% DM. In contrast, temperatures were much cooler in CSSF with 22% DM without straw. The pH ranged from 8 to 9 and C/N ratio was similar amongst treatments on each sampling occasion.

Mineral N production was characterized - by high NH4⁺ and low NO3 contents during the thermophilic phase followed by a decrease of NH4⁺ and an increase of NO3 towards the end of composting. Higher DM, and straw addition were both associated with lower NH4⁺ compost concentrations. Therefore, to minimize the N loss as NH3 gas caused by the alkaline pH during the thermophilic phase of composting, it is suggested: i) to increase DM by slowing the rate of the screw dewatering mechanism; ii) to increase the C/N ratio of composting mixture; and iii) to reduce turning frequency.