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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

Dep. of Plant Production, Università degli Studi di Milano, Via Celoria 2, 20133 Milano, Italy

^* Corresponding author (luca.bechini{at}unimi.it).

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 CO₂–C and soil inorganic N were measured on 12 sampling dates. For all soil x 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.

Abbreviations: ADF, acid detergent fiber • ANR, apparent N recovery • MN, added manure N • NDF, neutral detergent fiber • NNM, net mineralization of organic N • ON, added organic N with the manure • PAN, plant available N • PCA, principal component analysis • SMN, soil mineral N • VFA, volatile fatty acids