Exploring options for improving water and nitrogen use efficiency in crop production systems

Water and nitrogen (N) are two key limiting factors in global crop production. However, the optimization of water and N use is often studied separately, and the interactions between water and N use in crop production are often neglected. Lack of systematic and quantitative understanding of the interactions between water and N use may lead to misleading and/or biased recommendations. The main objective of this thesis research was ‘to increase the understanding of interactions between water and N use in crop production’. The specific objectives were (i) to analyse water and N use and their interactions in crop yields and in water and N use efficiencies (WUE and NUE), and (ii) to explore options for increasing crop yields and water and N use efficiencies simultaneously. In this thesis, I combined the use of literature review and meta-analysis, long-term field experiments and soil-crop modelling to quantify relationships between water and N use and their interactions in crop yields, WUE and NUE, and to explore options for improving the productivity and sustainability of two important and contrasting crop systems (i.e., annual cereal and perennial orange systems). In arid and semi-arid regions, such as the Loess Plateau, I found that the water loss via soil evaporation was large as much as 60 to 70% of total rainfall and thereby significantly limited wheat yields. Soil mulching can effectively reduce soil evaporation thereby increasing crop yields by up to 60%. In sub-humid regions, the productivity and sustainability of rainfed wheat-soybean rotation systems can be constrained by soil fertility and other factors besides water and nutrient management. Fertilization and soil organic amendments greatly increased wheat yield and WUE, but decreased NUE. In advanced fertigated orange production systems, a main challenge is to optimize water and N use for optimal yield while minimizing environmental pollution. Fertigation provides opportunities to simultaneously optimize water and N use in orange production systems. Optimization of water and N supply is, however, complicated in practice due to erratic and uncontrollable rainfall, which may lead to large incidental losses. Improving water and N input can significantly and simultaneously increase orange yield, WUE and NUE (by 20, 30 and 40% respectively). Importantly, even with optimal water and N input levels, fractionation (split) strategies can significantly increase orange yield and reduce N losses. In conclusion, rainfed cereal crop yields, WUE and NUE in dryland agriculture can be increased by up to 60%, with currently available knowledge and soil mulching techniques. In many regions in the world, there is still a large gap between observed farm yield and attainable yield by best management. Given large potentials in low-performing agricultural systems, investments in knowledge and education, infrastructures and subsidies should also be prioritized for these regions. Potentially, this will significantly contribute to local and global food security at large.