Modeling diffusion and reaction in soils: V. Nitrogen transformations in organic manure-amended soil

Measurements of nitrogen transformation with high temporal and spatial resolution are needed to better understand and predict nitrogen losses from manure-amended soil. Centimeter-scale measurements of nitrogen transport and transformations were carried out in a soil-manure model system corresponding to direct injection of liquid manure into soil. Influence of manure type (cattle or pig manure), initial soil-water, and soil-nitrate content were investigated. The manure type was the dominating factor with respect to both the initial redistribution of water and solutes and the subsequent nitrogen transformation processes. The liquid transport from the pig manure into the soil was rapid and extensive compared with the cattle manure. In both systems, the initial water transport created a low-nitrate zone at the manure-soil interface, possibly limiting denitrification that was found to be insignificant. Nitrification was inhibited initially in the cattle manure systems with high NH₄/⁺ and DOC concentrations. A small N immobilization during the first 2 days of incubation, followed by a net mineralization, was seen in all experiments. An inverse Diffusion-Reaction Model (IDRM) was used to calculate spatial and temporal variations in net nitrate production rates after the initial water transport had ceased. Good agreement was found between IDRM-calculated net nitrate production rates and measured nitrification rates. The net nitrate production rates were higher in the pig manure than in the cattle manure systems in the first 8 to 10 days, but they then decreased rapidly as a result of NH₄/⁺ limitation in the pig manure system. Unlike the frequently used mass balance considerations, the IDRM includes the effects of diffusion and, therefore, seems promising for high resolution analyses of solute transformation processes.