NITROGEN CYCLING IN PASTURE AND RANGE

Direct recycling of nutrients by grazing animals is often suggested as a means to achieve a sustainable agricultural production system. The mere presence, however, of large herbivores, their management, and uncontrollable site-specific environmental conditions can cause significant nutrient losses. Nitrogen is of particular interest because it is usually the most limiting nutrient to production; it represents a major variable input cost; it has a variety of pathways for input and outflow; and it can have important negative environmental impacts. Large herbivores shorten the N cycle, increase the rate of N cycling, and cause significant redistribution of N. Input of N through symbiotic N2 fixation is greatest in tropical and temperate pastures, whereas wet and dry deposition provide much of the usable N under semiarid conditions. Pathways of N loss also vary with climate, with gaseous losses predominating in dry conditions and NO3- leaching predominating under high rainfall. Spatial and temporal variability in pasture and rangeland N cycles has limited our ability to predict the effects of management decisions on N losses. Development of new experimental methodologies promises substantial advances in our understanding of nutrient cycling processes, from the scale of the microsite to that of the ecosystem.