Effects of clay and chemical additives on nitrogen retention during cow manure composting

Composting is an environmentally friendly technology to convert livestock manure to stable and nutritional organic fertilizer. However, significant N losses through NH3 emission during composting reduce nutrient value and cause air pollution. Several studies attempt to decrease the nitrogen loss with additives addition, but a comprehensively assessment on N conservation efficiency of additives with different characters is lacking. In this study, chemical substances (calcium oxide (CaO), magnesium oxide (MgO), ferrous sulfate (FeSO4), alum (KAl(SO4)2),) and clay mineral (vermiculite, zeolite, medical stone, bentonite) were used in composting with cow manure and corn stalk as feedstock, with the purpose to study their potential effects on ammonia emission and compost maturity. The addition ratio of additives was 2.5% of compost material (dry weight base). The emissions of NH3 and inorganic nitrogen were monitored during 35-day composting process, together with the pH, EC value, the total organic carbon and the maturity index (Germination Index, GI). Solid samples were taken at various stages (0, 3rd, 7th, 10th, 14th, 21st, 28th, 35th day) of composting. The results showed that the thermophilic phase (above 50 ℃) in all the treatment lasted more than 10 days during composting, suggesting that the product had met the requirements of harmless based the standard GB 7959-2012. Compared with the control treatment, the addition of chemical substances CaO and MgO had no effect on ammonia emission and nitrogen losses, while the addition of FeSO4 and KAl(SO4)2 decreased the ammonia emissions and nitrogen losses by 43.7%, 30.0% and 33.8%, 26.5%, respectively. The addition of vermiculite, zeolite, medical stone, and bentonite decreased the ammonia emissions and nitrogen losses by 24.4%, 29.9%, 7.1%, 20.1% and 15.4%, 22.9%, 2.2%, 13.4%, respectively. Little influences of different additives were found on EC value of compost products. The addition of MgO greatly increased the pH, which resulted in the germination index of the compost product lower than 50% at the end of process, while the other additives had little effect on pH changing and no influence on getting maturity after 35 days composting. In summary, the chemical substrates FeSO4 and KAl(SO4)2 had been considered as more superior additives to conserve nitrogen in composting process without influence on physiochemical characters and compost maturity, when compared than the other six additives. However, the clay minerals are normally cheaper than chemical substrates. So that clay minerals had a higher overall efficiency, but factories could choose different conditioners according to their requirement for the quality or for the cost in practice.