Critical period and pathways of water borne nitrogen loss from a rice paddy in northeast China.

Rice paddy nitrogen (N) loss is a great concern leading to a high risk of receiving water pollution. Various models have been applied as practical tools for simulation of the nutrient loss amount, and pathways or yield change affected by management factors in previous studies. However, N loss features of rice paddies in northern regions have received less attention and few model simulation studies have combined crop yield and N loss to simultaneously meet the needs of yield maintenance and environmental protection. To consider benefits to local farmers and to assess the paddy N loss features and factors in northeast China, rice yields and water borne N losses in 2013-2017 were simulated using the APSIM-Oryza model applied to Xingkai Lake Farm. Different from subtropical regions, high field ridges and lower rainfall limit local paddy overflow occurrence except after unexpected storms after irrigation in dry years or serial rainfall events, which result in subsurface N loss during stages of tillering (Ti) to flowering (Fl) which comprise the dominant pathway accounting for 50.03-69.99% of the total water borne N loss. The correlation analysis results also indicate irrigation and the applied N amount more significantly affect local paddy N loss than does precipitation. In each year, stimulated by an increase in the applied N amount, increasing rice yield (symbolizing crop growth status) indicated N loss implicitly rose. But under similar applied N amount range, inter-annual N loss results showed weaker growth status result in a higher N loss. Based on local N loss features, nutrient conservation practices including planting density increase or side strip application, and net N loss reduction practices including intermittent or recycling irrigation are recommended to limit nutrient loss from a paddy field which would be helpful for optimization of local nutrient conservation and surrounding water environment protection.