Spatial distribution of roots and water uptake of maize (Zea mays L.) as affected by soil structure

The incomplete soil exploration by roots is a possible cause of reduced water uptake and the occurrence of strong gradients in water potential between root clusters and the bulk soil. Therefore, the hydraulic or root signal-elicited behaviors of plants might correspond to a lower water content than that of the bulk soil. Evidence supporting this hypothesis is scarce and often circumstantial due to the lack of appropriate techniques. This work studies the relations of root clustering caused by localized soil compaction and the spatial patterns of water uptake in plants grown on stored water. Five soil structure treatments were compared in 100-cm high containers: clay-loam soil (C), sandy-clay soil (S), sandy clay with large clay-loam peds (S + LA), sandy clay with small clay-loam peds (S + SA), and compacted clay-loam soil (CC). Root length density (RLD) and volumetric water content (thetav) (time-domain reflectometry) were measured in bulk soil and across peds in 2-cm increments. In the S treatment, plant growth was rapid initially, but green leaf area declined to zero when water was exhausted. Root length density was quite uniform in each layer. In the CC treatment, root density and water uptake were limited and leaf area remained stable throughout the experiment. suggesting that incomplete water extraction was a consequence rather than a cause of reduced plant growth. In the other treatments, when plants had lost all leaves, the water content of peds ranged from 0.23 cm(3) cm(-1) in the outer 2 cm, where roots were present. to more than 0.30 cm(3) cm(-1) in the center, scarcely penetrated by roots. Thus, there was incomplete water extraction in the C and S + LA, and to a lesser extent in S + SA treatments. This study shows that large gradients in soil water content occur in cases of root clustering caused by localized soil compaction. These gradients are associated with a reduction in water uptake.