NITROGEN TRANSFORMATIONS IN SOIL-PLANT SYSTEMS IN THREE YEARS OF FIELD EXPERIMENTS USING TRACER AND NON-TRACER METHODS ON AN AMMONIUM-FIXING SOIL

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Abstract
Three years of field experiments showed the interplay of plant uptake of N, N movement, denitrification, fixation of fertilizer NH4+ and its release, and N mineralization in soil–plant systems. The N uptake by barley (Hordeum vulgare L.), averaged over the growing season, ranged between 0.97 and 2.02 kg N/ha/day and the rate depended on initial extractable inorganic N in the soil, and form and timing of N fertilization. The net mineralization rate of this soil, averaged over the growing season, ranged between 0.16 and 1.80 kg N/ha/day and varied with year and N fertilization practices. However, detailed monitoring of plant uptake showed that a maximum rate of uptake occurred early in its growth, decreasing to a negligible rate later in the season. The N mineralization rate was more uniform over the growing season. A pool of inorganic N in the soil at seeding or within the first half of the growing season overcame the seasonal deficit in N supply and resulted in increased crop growth and/or N uptake. Fertilizer N movement was small and never beyond the maximum (75-cm) sampling depth. This supported the assumption that unrecovered fertilizer N in this study was largely due to denitrification. Denitrification was shown to be greatly influenced by the season, with a maximum rate occurring in the spring or early summer, and concurred with the period of maximum rate of plant uptake of N. Denitrifiers were capable of competing with high rates of plant uptake since the rate of denitrification was similar in fallow and cropped systems. The form of N application (NO3−, NH4+, NH4+ plus N-serve) did not significantly affect the denitrification rate. The soil used in this study fixed 34–60% of the 150 kg NH4+/ha fertilizer immediately upon application. The fixed fertilizer N was available to barley, with 71–96% of the recently fixed NH4+ being released over the growth period. The presence of N-serve resulted in less fixed fertilizer NH4+ being released during crop growth.