Rhizosphere Processes in a Deep-Rooted Eucalypt Plantation in Brazil as Affected by Rainfall Reduction: Root Functioning is Altered at Considerable Depth

Climate change is a major threat for forest production, and the question of adaptation to such change, especially increased drought, is crucial, as well as its possible mitigation via carbon sequestration. We tackled these issues in highly productive eucalypt plantations in nutrient-poor, deep soils in Brazil. For testing the impact of climate change in such deep-rooted trees, we used a unique rainfall reduction experiment (37% of the throughfall removed through gutters), and sampled roots, rhizosphere and bulk soil down to 4 meters.

Available potassium and phosphorus concentrations as well as pH of the bulk soil were not influenced by rainfall reduction. For phosphorus, there was no rhizosphere effect, whatever the rainfall treatment and depth. The rhizosphere pH was significantly lower than bulk soil pH only in the reduced rainfall plot. In contrast, the concentrations of available potassium in the rhizosphere at all depths were significantly larger than those of bulk soil in both treatments, but this increase was significantly greater in the reduced rainfall plot, notably below 2-m depth. Our results suggested that roots induced some weathering of potassium-bearing silicates down to 4-m depth, partly as a consequence of rhizosphere acidification, which was stimulated when trees were exposed to greater drought.

While no impact of rainfall reduction was observed for organic carbon concentration in the bulk soil, substantial differences were found in the rhizosphere. The rhizosphere carbon concentrations in the reduced rainfall treatment were 27% higher in average compared to those in bulk soil. In contrast, rhizosphere C concentrations increased by 8% in average in the control conditions. Our results suggested a significant capacity of deep roots of these fast-growing trees to contribute to significant carbon input at considerable depth. This process was stimulated when eucalypts were exposed to greater drought, especially at depth, probably due to enhanced rhizodeposition.