Capacity of Deep Rooted Species to Take Up Water and Nutrients from Deep Soil Layers

Summary

Introduction
Agricultural cropping systems generally have a limited use of water and nutrients from the soil below 1m depth. However, several plant species have much deeper root systems, and if included in cropping systems, these would allow for exploitation of otherwise unused resources from deep soil layers. In a unique facility consisting of 12, 4m high ‘root towers’ we examined the root development and root activity in different soil layers of different biannual/perennial plant species.
Methods
Four plant species lucerne (Medicago sativa), rosinweed (Silphium integrifolium), intermediate wheatgrass (Thinopyrum intermedium), and dyers voad (Isatis tinctoria) were grown in soil in large rhizotrons (root towers) (4m×1.2m×0.6 m). Root growth was continuously followed during the growing season by imaging. Uptake of a range of nutrient tracers, Cs, Li, and Se, from in-growth cores placed in the soil for 1 month at 0.5, 2.3 and 3.5 m depth respectively, was determined. Furthermore, uptake of 2H-enriched water and 15N, was determined after injection in 2.9 m depth. Differences in water content in the soil was determined by TDR sensors.
Results and discussion
All species showed the capacity to grow to 3.6-4.0 m depth within the first year after transplanting. Dyers voad had the highest root growth rate, reached 4m depth after 4 months, and had the highest uptake of subsoil nutrient tracer as well. Despite sufficient water being available in more shallow layers, all species were seen to take up water from 2.9 m depth except for rosinweed, which had limited root growth in that depth. Water content sensors showed that Lucerne was able to use most of the available water down to 3.5 m depth, while intermediate wheatgrass used significantly less water despite similar aboveground biomass and rooting depth. This indicated differences in the capacity of the deep roots to use deep resources.