Coupling of a nitrate production model with HYDRUS to predict nitrate leaching

Losses of nitrogen fertilizer from agricultural watersheds are among the main causes of water quality degradation in Quebec. Under maize crop, leached nitrate can reach up to 185 kg N ha−1. The synchronization of the nitrogen fertilization with crop uptake would be a way to optimize the use of nitrogen fertilizer and reduce nitrate leaching. Coupling an empirical nitrogen cycle model with a physical soil water dynamics model could simplify the optimization of nitrogen fertilizer use. The objective of this study was to couple the soil surface empirical nitrate production model adapted to the regional agro-climatic conditions of study area with HYDRUS to determine the most suitable combination in order to predict nitrate leaching into subsurface drains. In this study, we used estimation equations of nitrogen pools and transformations in nitrate. The equations, using historical field data, were combined into 60 N-release patterns taking into account a dissolution function of nitrate, 4 transformation and dissolution functions for the ammonium, 5 organic nitrogen pools and 3 N organic nitrogen from the soil and manure release functions. These N release patterns were applied in a mineral fertilizer treatment and four organic fertilizer treatments for a total of 300 unique nitrate release patterns. The results demonstrate the potential of a method to evaluate the contributions of nitrogen based on atmospheric data, crop rotation or soil particle sizes as 5% of the 300 combination gave a Nash–Sutcliffe efficiency coefficient (NSE) above 0.70. The prediction method must be different according to the source of the fertilization, organic or mineral as only 1% of the combination gave NSE above 0.70 in more than one treatment. In addition, the study showed that the soil organic nitrogen contribution to nitrate leaching tends to decrease as the organic fertilizer applications rate increases as 75% of the combination giving NSE above 0.70 in high fertilization treatment used the lower organic N pool and only 17% used the lower organic N pool for the agronomic fertilization rate and mineral fertilizer. The simulations also showed that the nitrate-leached masses are closely related to the fertilizer supply nitrogen content.