Evaluation of Soil Mineral Nitrogen Content in Different Irrigation Systems during Potato Crop Development.

Sandy soils with shallow water table induce soil nutrients leaching in response to irrigation and rainfall, in particular NO₃^- and NH₄⁺. The objective of this study was to determine the spatial and temporal distribution of soil N fertilizer applied at different timing and rates strategies in a sandy soil under seepage, tile drainage, subsurface drip (SDI) and sprinkler irrigation. A factorial randomized complete block design with three N-rates (0, 56, 112 kg.ha^-1) applied at planting and two N-rates (56 and 112 kg.ha^-1) applied at emergence and tuber initiation were setup within each irrigation system in 2015 and 2016 potato seasons. Soil-N content (NO₃^- and NH₄⁺) was determined at five-potato growth stages for 0-15, 15-30 and 30-60 cm of soil depth. During both potato seasons, the soil water flux influence the soil N distribution. Water table control irrigation systems had higher soil-N content measured at 0-15 cm of soil than deeper soils, while the sprinkler irrigation had higher soil-N at 15-30 cm of soil than other irrigation systems. Regarding growth stages, soil-N was higher at tuber initiation than other growth stages when potato was overhead sprinkler irrigated, but water table control irrigation systems had the highest soil-N at maturity. Overall, N-rates above 56 kg.ha^-1 had no significant differences in soil-N with planting application timing. N-rate treatments applied at emergence required 112 kg ha^-1 to increase soil mineral N in the 0-15 cm of soil, while 112 kg.ha^-1 of N applied at tuber initiation had higher soil-N than 56 kg.ha^-1 only for seepage at maturity and harvest. Soil N distribution varied according to weather conditions, and decision-makers should consider rainfall events before N fertilizer managements.