Roots below one meters depth are important for nitrate uptake by annual crops

Summary

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 are important to understand crop N uptake and nitrate leaching from agro-ecosystems.