Application of dairy manure amended with mineral nitrogen fertilizer on stubble-covered soil: effects on ammonia emissions.

Mineral fertilizers are the major nitrogen source to crops. However, manure application contributes to the increase of soil organic matter and the addition of beneficial microbes, besides also delivering nutrients to plants. The transport costs within and between farms and higher application rates needed relative to mineral fertilizers are some of the limitations associated with manure application. The mixture of manure and mineral fertilizer might be an alternative to decrease manure application rates, enabling to cover more agricultural lands, improving application efficiency and also to reduce over-fertilization with phosphorus as generally occurred when using manure. The present work aimed to evaluate the effects of mineral nitrogen fertilizer amendment to manure, right before application, on ammonia (NH₃) emissions. A pot experiment using soil covered by wheat stubble was performed at Instituto Superior de Agronomia, Lisbon, Portugal. The treatments considered were: Control (unfertilized treatment), Urea (U), Ammonium nitrate (AN), Manure (MAN), Urea + Manure (UMAN), Ammonium nitrate + Manure (ANMAN). An equivalent amount of total-N was applied in all fertilized treatments. The ammonia emissions were collected through a dynamic chamber system for seven days. The manure amended with mineral fertilizers led to higher NH₃ emissions than each isolated component. This might be motivated by the slightly alkaline pH of the manure. UMAN presented the highest cumulative NH₃ emission, 357% and 87% more than U and MAN, respectively. ANMAN emitted 1,552% more than AN and 54% more than MAN. Thus, it can be concluded that the application of dairy manure mixed with urea or ammonium nitrate on stubble-covered soil stimulates NH₃ emissions relative to isolated application of manure or mineral fertilizer. More studies over raw manure/mineral fertilizer combinations and management strategies that may reduce ammonia emissions from organic-mineral fertilizers are required to allow efficient use of this fertilizing strategy in no-tillage agriculture.