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 N 2 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 NO − 3 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.