The spatial and temporal rooting pattern of Brussels sprouts and leeks

Abstract

The vegetables Brussels sprouts (Brassica oleracea L. var gemmifera) and leeks (Allium porrum L.) differ widely in the degree of nitrogen utilisation under field conditions. Brussels sprouts usually take up nitrogen in a relative short period after planting and leaves a profile depleted of nitrogen. Leeks, however, utilise in general only half of the available nitrogen. This implies that for this crop large amounts of nitrogen can be subject to losses to the environment during or after the growing period. In order to investigate the role of the root system in nitrogen utilisation, rooting intensity and distribution of the two crops were assessed in field experiments and in the Wageningen Rhizolab, a rhizotron facility built in 1990. In field experiments rooting was quantified in the various layers of the profile as the volumetric Root Length Density (RLD, cm cm-3) and in the Rhizolab with horizontal glass minirhizotrons as the Number of Roots per cm2 (NR). A regression procedure of RLD and NR on thermal time (accumulated average daily temperature above ground) after planting revealed that the rooting depth of Brussels sprouts increased faster with thermal time than rooting depth of leeks, e.g. in one of the experiments in the Rhizolab 0.13 cm (°C day)-1 and 0.08 cm (°C day)-1 respectively. Furthermore, leeks showed an unusual distribution of roots in the profile with maximal rooting intensity at depths of 10-20 cm. In all experiments the proliferation of roots (calculated as the increase in NR or RLD with thermal time) was slower in leeks than in Brussels sprouts, especially in the deeper layers of the soil profile. Therefore leeks can be considered as a shallow rooting crop. Compared to the Rhizolab, the field experiments showed in general the same difference between the crops: for Brussels sprouts a higher root density, a deeper rooting and a faster proliferation of roots. For both crops, however, the proliferation of roots at the deeper layers was in the field much slower, probably due to unfavourable conditions in the subsoil caused by higher bulk densities or temporary high water tables. The consequences of different rooting patterns for nitrogen utilisation are discussed.