Management of soil nitrate heterogeneity resulting from crop rows in a lettuce-tomato rotation under a greenhouse

Vegetable crops grown under plastic tunnels in the Mediterranean region are intensively managed. Generally, a spring row crop, such as tomatoes, is followed by one or two lettuce crops in the autumn and winter. Lettuce cropping in these systems can generate significant nitrate losses. Thus, nitrogen must be properly managed in such systems. Drip fertigation of the tomato crops prior to lettuce cropping creates an uneven distribution of nitrate in the soil including its accumulation between the rows. However, the fate of this residual, unevenly distributed nitrate and its impact on the following crop nitrogen (N) budget is unknown. Thus, a 2-year experiment was conducted to study N dynamics in a tomato-lettuce rotation. Two different management systems were tested: one with typical current farm practices ("C") and a second system with decisions based on continuous monitoring of soil nitrate and water content ("L") that resulted in the requirement of less water and N inputs. We revealed a recurring annual sequence in the two systems: strong N accumulation occurred during the spring crop and autumn fallow followed by soil nitrate exhaustion during lettuce growth as a consequence of crop uptake and leaching. We showed that leaching during lettuce growth appeared unavoidable even in the "L" management system. Between the former tomato rows (in the inter-rows), the higher initial soil N residues resulted in significantly higher N uptake (an increase of 14-25 kg ha(-1)) and higher leaching (an increase of 2-32 kg ha(-1)) compared with uptake and leaching within the tomato rows. Therefore, to limit substantial N losses, nitrogen fertiliser recommendations should take into account the residual nitrate distribution in the soil by adding different amounts of fertiliser depending on the position relative to the previous row crop.