Short‐Term Nitrogen Fertilizing Value of Liquid Dairy Manures is Mainly Due to Ammonium

The efficient use of animal manures is important both for environmental and economic reasons. This study was conducted to determine the mineralization of C and N of five liquid dairy manures incorporated in three soils of increasing clay content (102, 209, and 337 g kg −1 ). The manures represent the variability observed in animal farms in northern Italy. Incubation was performed for 181 d at 25°C and −0.05 MPa of soil water potential; respired CO 2 –C and soil inorganic N were measured on 12 sampling dates. For all soil × manure combinations the dynamics of C respiration showed high rates in the first week, and lower rates thereafter. Differences among manures were greater during the initial phase. At the end of the experiment, the total C respired ranged from 40 to 71% of applied C; model extrapolations indicate that part of manure C will be stored in the soil in the long term. Higher respirations were obtained in the soil with the lowest clay content. For most of the treatments, immobilization of mineral N occurred in the first weeks, followed by slow remineralization of immobilized N, resulting in either no or low net mineralization of organic N at the end of the incubation (−7 to 23%). It is concluded that the variability of manure C and N mineralization is high. Nitrogen fertilizing value of the manures during the first year after application must be mainly attributed to ammonium, while organic N is mineralized slowly, and is therefore available later.