Intercropping leek (Allium porrum L.) with dyer's woad (Isatis tinctoria L.) increases rooted zone and agro-ecosystem retention of nitrogen

Nitrate leaching can be high in organic vegetable production. Late-harvested crops like leek limit the use of autumn catch crops. The aim of this study was to investigate the growing of a combination of a deep-rooted catch crop and a shallow-rooted vegetable to reduce the risk of nitrate leaching. We compared a leek sole crop (S) with two intercropped systems of leek and early-sown dyer's woad (five weeks after leek planting) (IE) or late-sown dyer's woad (eight weeks after leek planting) (IL) in two seasons: 2012 and 2013. To reveal root and resource competition, leek with dyer's woad rows left empty (Semp), and early and late-sown dyer's woad with leek rows left empty (DEemp, DLemp) were included. Yield, dry above-ground biomass, aboveground N accumulation and soil inorganic N (Ninorg) were measured as well as root growth by use of minirhizotrons to 2.3 m soil depth. Results showed that the marketable yield of leek in IE and IL systems was comparable with the yield in the S system when calculated per length of leek row. The Relative Competition Index (RCI) revealed that interspecific competition facilitated the growth of leek but hampered that of dyer's woad. The rooted zone increased from 0.5 m in the S system to more than 2 m depth in those of the intercropped systems. Dyer's woad ceased growing above ground but kept growing below ground after crop harvest and extended roots under the leek root system in 2012. Intercropping increased the root intensity of late-sown dyer's woad after leek harvest in the 0.75-1.75 m soil layer compared to dyer's woad growing alone (DLemp), while the root depth was not affected. The intercropped system with early-sown dyer's woad reduced soil Ninorg by 52 kg ha−1 relative to the sole-cropped system, and dyer's woad accumulated 48 kg N ha−1 in aboveground biomass at harvest in 2013. Late-sown dyer's woad had fewer roots, left higher soil Ninorg and had lower aboveground N accumulation than early-sown dyer's woad until the following spring. Therefore, early-sown dyer's woad is applicable in an organic intercropped system with high yields of leek to decrease the risk of nitrate leaching.