Root Distribution of Lettuce under Commercial Production: Implications for Crop Uptake of Nitrogen

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Abstract
Crop root distribution is important for management of soil fertility and for development of practice that minimize leaching of nitrate (NO3 −-N) below the root zone. Objectives of this study were to describe lettuce (Lactuca sativa L.) rooting patterns under actual management practices in growers' fields and to relate rooting patterns to soil NO3 −-N distribution in lettuce production systems. Three conventional farms (Farms 1, 2 and 3) and one organic farm (Farm 4) were evaluated: 1) a direct-seeded crisphead lettuce field which was sprinkler irrigated until thinning and later furrow irrigated; 2) a similarly managed site planted with both direct-seeded and transplanted crisphead lettuce; 3) a direct-seeded crisphead lettuce field under subsurface drip irrigation; and 4) a high-density planting of leaf lettuce grown under surface drip irrigation. The ‘Salinas’ or a closely-related crisphead cultivar was planted at the first three sites and leaf lettuce was grown at the organic farm. Root characteristics were measured during the pre-harvest period of maximum water and N uptake at the four sites and included root biomass, N allocation and root length density in depth increments to 75cm. Measurements of root length and biomass in cores were used to estimate totals per m2, based on calculations derived for each type of management system. Roots were most abundant in the top 50 cm of soil; root length density decreased to < 0.5 cm cm−3 below this depth in all management systems. Total root length was estimatd to vary from 2,060 to 17,700 m m−2 (0–75 cm depth). At the furrow-irrigated sites, roots were dispersed throughout the top 50 cm of the beds and furrows, with more root length and biomass present in the 0–15 cm layer of the beds. At the site with subsurface drip irrigation, the root system was small with more roots in the beds than in the furrows and a greater proportion of roots at 0–15 cm depth. The high-density leaf lettuce under surface drip-irrigation had the highest root length density and root biomass, with most of the roots in the 0–15 cm soil layer. The response of lettuce root distribution to availability of water and N in the top 50 cm of the profile suggests that lettuce can potentially increase its capacity for N uptake in N3 −-N enriched soil zones. Below this depth, however, roots did not follow the NO3 −-N distribution in the soil profile. Management practices to increase rooting depth and density are discussed.