Identification of efficient transport pathways from the soil surface to field drains by smoke injection

Surface‐applied agrochemicals have been measured soon after application in drainage water from agricultural fields in various studies. The bypass flow from the soil surface into drains may result from direct macropore connections. In order to identify such macropores, smoke was injected into three 1.15‐m deep tile drains in a sandy loam soil using ‘smoke oil’ and an air blower. Smoke‐emitting macropores (SEM) at the soil surface were characterized as having either strong or weak plumes when compared with reference plumes from 3‐ and 6‐mm wide tubes. A total drain length of 93 m, at three separate sub‐drain lines, was investigated after harvest of wheat in autumn 2010 and in spring 2011. Smoke only reached the soil surface layer via earthworm burrows, located in a 1‐m wide belt directly above the drain lines. The distributions of SEM across the drain line were similar in autumn and spring. The average number of SEM along the drain lines was 2.3 SEM m⁻¹drain line. Ponded water in 6‐cm wide rings was applied above 52 burrows, including 17 reference burrows that did not emit smoke, and 13 pathways in the soil were examined with dye tracer. Strong SEM marked the entrance of efficient transport pathways conducting surface‐applied water and dye tracer into the drain. Water infiltration rates were significantly larger (P < 0.05) in strong SEM (geometric mean, 195 ml minute⁻¹; n = 19) than in weak SEM (geometric mean, 63 ml minute⁻¹; n = 16) and for the reference burrows (geometric mean, 39 ml minute⁻¹; n = 17). The results suggest that the smoke injection technique is a valuable means of identifying potentially efficient pathways for surface‐applied contaminants to enter drains, such pathways being associated primarily with strong SEM.