Assessing Climate, Irrigation, Nitrogen, and Soil Effects on Nitrate Leaching in Potato Production Systems in the Columbia Basin of the United States Pacific Northwest.

Potato is an important crop in the United States Pacific Northwest. Within this region, the Columbia Basin in the southeastern part of Washington State is one of the most productive areas for high-quality processing potatoes. Here, water and nitrogen are major inputs to potatoes. Because the costs of these inputs relative to the expected value of the crop are less, farmers use high rates of these inputs. In this region, therefore, nitrate is the most prevalent and frequently documented groundwater contaminant. Despite reports of nitrate pollution and the decades of research, little is known about the dynamics of nitrate leaching as influenced by environment and management. This modeling study demonstrated that the magnitude of nitrate leaching could be influenced by climate, irrigation, nitrogen fertilization, and soil. The results are based on the study involving the combinations of three sites, two climate years (cold and hot), two soils (well-drained (WDS) and excessively-drained (EDS)), five irrigation intervals (1-, 2-, 3-, 4-, and 5-day), five irrigation amounts (400, 500, 600, 700, and 800 mm), and five nitrogen fertilization rates (NFR; 168, 252, 336, 420, and 504 kg/ha). Generally, leaching was not significantly different between hot and cold years. The leaching difference between the two years, however, increased with an increase in irrigation amount and also with an increase in NFR. The rates of leaching with respect to irrigation amount and NFR were greater in a hot year. The difference between the years was larger in WDS than in EDS. In WDS, hot year had more leaching than cold year. In EDS, no difference between the years was found. Generally, leaching in EDS was higher than that in WDS. The leaching difference between the soils was smaller for a larger irrigation and larger for a higher NFR and in the cold year. The increase in leaching in WDS was larger than that in EDS when moved from cold to hot year. Generally, leaching was not influenced by irrigation interval. For irrigations of <600 mm, however, leaching increased with an increase in irrigation interval. The difference among intervals was smaller for a larger irrigation, but the same for all NFRs, soils, and years. Leaching was higher for a larger irrigation amount. The differences among amounts decreased with an increase in irrigation interval. The leaching rates with respect to irrigation interval were greater with smaller irrigations than with larger irrigations (≥ 600 mm). The leaching rates with respect to NFR were greater with larger irrigations. The differences among irrigation amounts were larger in WDS. When moved from WDS to EDS, the increase in leaching was less with a larger irrigation. The leaching differences among amounts were about the same in each year. Leaching increased with an increase in NFR. The differences among NFRs were the same for all irrigation intervals and years. The differences, however, increased with an increase in irrigation amount. The leaching rates with respect to irrigation amount were higher with a higher NFR. The leaching rate was higher with a higher NFR when moved from WDS to EDS. These findings might be helpful to potato growers in this region in identifying sustainable production practices aimed toward maximizing potato yields, while minimizing the nitrate contamination of groundwater.