3.2.3 Nitrogen Use Efficiency (NUE)
Historical data and regression results both indicate a positive relationship between NUE and PUE (Table 4). On the global scale, crops with a relatively higher NUE (e.g., cotton and soybean) tend to have a higher PUE, while crops with relatively lower NUE (e.g., rice, fruit, and vegetable) tend to have smaller PUE (Fig. 3b). On a national scale, 99% of the countries (82 countries) with significant results (positive or negative) (Table 4) show a positive relationship.
Improvement in nutrient management leads to NUE and PUE increase. Accumulated residual soil P allows farmers in many developed countries to reduce current P inputs and increase PUE. At the same time, factors such as the adoption of 4R practices lead to NUE improvement (Zhang et al. 2015a). Future studies need to identify and understand factors that can influence the linkage between PUE and NUE, as well as other nutrients.