The root depths of annual crops vary from 0.2 m to more than 2 m depending on root growth
rate and length of growing season. However, studies of root growth and N uptake
are often restricted to a depth of 1 m or less, as root biomass is assumed to be negligible
below this depth. We have studied the importance of root growth and N uptake to
a depth of 2.5 m
in fully grown field vegetables and cover crops by use of minirhizotrons
and deep point placement of 15N. Deep rooted crucifereous
crops were found to have high root densities to a depth of 1.5-2 m and high 15N
uptake to this depth. Uptake was significant near the bottom of the root zone at
a depth of 2-2.5 m
where root density was low. Here N uptake rates were higher per unit root
length compared to more shallow depths. Thus, deep roots were more important to
crop N uptake than indicated by their density. In studies where subsoil nitrate
was high, deep rooted crops could reduce soil nitrate content by up to 100 kg N ha-1 at
a depth of 1-2.5 m
over the growth season compared to shallow rooted crops. The effects on crop N
uptake, when matching deep rooted crops with high subsoil nitrate in crop
rotations, will be illustrated by modelling. The work shows that knowledge of
the interactions between root growth and soil N below a depth of 1 m is important, to understand
crop N uptake and nitrate leaching from agro-ecosystems.