Fertilization increases the functional specialization of fine roots in deep soil layers for young Eucalyptus grandis trees

Functional specialization of fine roots was found for Eucalyptus grandis trees at harvesting age (6 years) on tropical soils. Aiming to elucidate whether functional specialization is a ubiquitous feature of eucalypts, we focused on its changes with ontogeny, tree nutrient status and soil depth. We studied the potential uptake of N, K and Ca by 2-year-old E. grandis trees, as a function of soil depth and NPK fertilization. We injected NO-3-N-15, Rb+ (K+ analogue) and Sr2+ (Ca2+ analogue) tracers simultaneously in a solution at depths of 10, 50, 150 and 300 cm in a sandy Ferralsol soil. A complete randomized block design was set up with three replicates of paired trees per injection depth, in fertilized and non-fertilized plots. Recently expanded leaves were sampled at 70 days after tracer injection. Determination of foliar Rb, Sr concentrations and x(N-15) allowed estimating the relative uptake potential (RUP) and the specific RUP (SRUP), defined as the ratio between RUP and fine root length density (RLD) in the corresponding soil layer. Various root traits were measured at each depth. Foliar N and K concentrations were higher in fertilized than in non-fertilized trees. The RUP of NO3--N-15 decreased sharply with soil depth and the highest values of the SRUP of NO3--N-15 were found at a depth of 50 cm. The RUP of Rb+ and Sr2+ did not change with soil depth, whilst the SRUP of Rb+ and Sr2+ were higher at the depth of 300 cm than in the topsoil, concomitant with an increase in root diameter and a decrease in root tissue density with depth. The SRUP of Rb+ and Sr2+ at a depth of 300 cm were on average 136 and 61% higher for fertilized trees than for non-fertilized trees, respectively. Fine roots of young E. grandis trees showed contrasting potential uptake rates with soil depth depending on the nutrient. Fertilization increased the uptake rate of Rb+ and Sr2+ by unit of root length in deep soil layers. Functional specialization of fine roots for cations of low mobility depending on depth previously shown at harvesting age also occurs in young E. grandis plantations and increases with fertilization application. This mechanism helps explaining very low amounts of cations lost by leaching in Eucalyptus plantations established in deep tropical soils, even in highly fertilized stands.