Evaluation of nitrate redistribution in surface and subsurface drip irrigation systems

Nitrogen compounds added to the soil may convert to nitrate and cause contamination. The distribution and uniformity of soil nitrate in surface vs. subsurface drip irrigation systems were compared in a physical model consisting of a transparent glass box (1.20 x 0.5 x 1 m) and sandy loam soil, and considering emitter installation depths of 0 and 30 cm, discharge rates of Q1 = 2, Q2 = 4, Q3 = 8 L/h, and fertilizer levels of S1 = 125, S2 = 250, S3 = 375 mg/L. Irrigation continued for 6 h and nitrate and moisture sampling was performed for 68 h after the initiation of water front advance. The result showed that doubling the discharge caused the wetted area to triple in size in the subsurface drip irrigation system whereas it only doubled in size in the surface drip irrigation system. Thus in the subsurface system, when increasing the fertilizer level, the nitrate spread out extensively and therefore its concentration was greatly reduced. Also, by increasing discharge, the difference in soil nitrate concentration between the two systems increases because of increasing non-uniformity of nitrate distribution in the surface system, such that by increasing the fertilizer concentration form 125 to 375 mg/L, the difference in nitrate concentration increases from 22% to 500% (for Q1 = 2 L/h), 43% to 352% (for Q2 = 4 L/h), and 14% to 166% (for Q3 = 8 L/h). Thus the subsurface drip irrigation system has a more uniform trend of nitrate distribution than the surface drip irrigation system. Also, treatment with maximum flow and fertilizer level will create the most optimal nitrate concentration in the soil.