ABSTRACT
The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production. However, the excessive use of artiœcial nitrogen fertilizers, together with fossil fuel combustion and release of nitrogen in wastewater, has dramatically altered the global nitrogen cycle [1]. The extent and effects of the anthropogenically induced doubling of biologically available nitrogen in the soils, waters, and air of the Earth during the past century is still poorly understood. Nitrogen is an essential nutrient for crop and pasture growth. Crop nitrogen status is key information for the application of Variable Rate Technology in precision agriculture, which aims to maximize productivity and, at the same time, to limit the environmental impact of excessive fertilization. For pastures, nitrogen content is a key determinant of the nutritive value, being directly related to the crude protein concentration in herbage or forage [2]. Traditionally, nitrogen content has been estimated through soil testing and plant tissue analysis [3], which involves destructive œeld sampling and laboratory chemical analyses. Besides the effort required, destructive measurements can prevent the study of successive developmental stages. Moreover, they create a lag between sampling, laboratory analyses, and nitrogen treatments [4] or livestock grazing management [5].
