Exploring optimal nitrogen management practices within site-specific ecological and socioeconomic conditions

The “4R” fertilizer management principles of using the right rate, right source, right timing and right place is emergently required for enhancing crop production and improving crop Nitrogen Use Efficiency (NUE) in intensive small farming system. However, the methodology is still vague to design and implement technologies and management practices (TMPs) following the “4R” principles in these regions. This study designs various TMPs and evaluates their agronomic, environmental, and economic impacts in two typical intensive cereals cropping systems in China to explore how TMPs interact with biophysical conditions, and finally establish methodologies to recommend local optimal TMPs. Among 5 designed TMPs, the optimal TMPs for each site and cropping system were selected, which achieved the sustainable targets for productivity (80% of the achievable yield potential), NUE (0.60), and economic profitability. But several TMPs failed to satisfy the integrated targets, which implied the TMPs must be carefully designed to fit site-specific biophysical contexts. This work provided some basic guidelines for determining each “R”: The “Right rate” can be determined by balancing with crop aboveground N uptake where soil testing is not always available. The “Right timing” must consider the fertilizer products and local climatic features. Split application is not always better than one-time application, e.g., applying urea at 11th-leaf stage decreased spring maize yield by 9% in Lishu due to a 20-day sustained high temperature and drought condition. For the “Right source”, the adoption of enhanced efficiency fertilizers should take soil properties (especially soil pH) into consideration, e.g., urease inhibitors are effective in increasing NUE (by 44%) on alkaline soils, but the efficacy on acid soils is greatly reduced (by 19%). The “Right place” needs to be designed according to characteristics of crop root distribution and fertilizer sources, e.g., surface application of urease inhibitors in winter wheat gained greater NUE (0.62) and grain yield (8.3 Mg ha−1) than deep placement of urea and calcium ammonium nitrate. Considering the biophysical condition variations and its great influence on TMPs, a long-term trial research network to design, test and reshape the site-specific optimal TMPs following “4R” principles will be necessary.