Distance from the trunk and depth of uptake of labelled nitrate for dominant and suppressed trees in Brazilian Eucalyptus plantations: Consequences for fertilization practices

Climate changes will increase the probability of drought, which is likely to dramatically increase tree mortality. The capacity of trees to withdraw water in deep soil layers is an important trait likely to account for tree survival over prolonged droughts. Our study aimed to gain insight into the maximum distance from the trunk where Eucalyptus fine roots take up water and mobile nutrients in deep sandy soils during thy periods. NO3--N-15 was injected in the soil at the end of the rainy season in commercial Eucalyptus stands planted with the same E. urophylla x E. grandis clone. The N-15 tracer was applied in the middle of the inter-row (replicated in 3 plots): at 5 depths (from 0.1 to 6 m) at age 0.6 year, at 4 depths (from 0.1 to 9 m) at age 1.2 years, at 5 depths (from 0.1 to 12 m) at age 2.2 years, and at 6 depths (from 0.1 to 15 m) at age 6.4 years. delta N-15 was determined in leaves sampled in dominant and suppressed trees at different distances from each injection area, 4-5 months after NO3--N-15 injection (after the dry season). While dominant trees took up NO3--N-15 down to a depth of 6 m between 7 and 12 months after planting, the maximum depth of NO3--N-15 uptake for suppressed trees was between 3 and 4.5 m. From 1.5 to 6 years after planting, a foliar enrichment in N-15 was mainly detected when the NO3--N-15 tracer was injected in the upper soil layers and only for a few trees at a depth of 6 m. Most of the uptake of N-15 occurred within 2m of horizontal distance from the injection site, whatever tree age and tree social status. Low amounts of NO3--N-15 were taken up for injection sites located between 2 m and 5 m from the trunk, and N-15 uptake was never detected at horizontal distances greater than 6 m from the trunk. Eucalyptus fine roots can take up nitrates at depths between 6 and 8 m the first year after planting. However, the NO3--N-15 tracer injected at a depth of 6 m was only taken up by dominant trees and a N-15 foliar enrichment of suppressed trees was only detected when the tracer was injected in the upper 3 m. Fertilizers must be applied within 2 m of the trunks in Eucalyptus plantations to be taken up by all trees, regardless of their social status. When fertilizations are concentrated the first months after planting in sandy soils, nutrient leaching in deep layers might increase the heterogeneity of the stands since mobile nutrients could only be taken up by dominant trees.