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. These data were used for calculation of the potential rate of nitrate uptake below 1 m depth. The rate was found to be significant for the six crops studied. The effect of root depth on soil N left by an annual crop was simulated with the EU-rotate N model. The results showed that deep rooted crops exploit deep N when it is available and thereby reduce the amount of nitrate that is prone to leaching. Therefore deep roots and N uptake need to be considered to understand and model N cycling in agro-ecosystems.