Consequences of mineral fertilization and biosolid application on nutrient leaching over an entire rotation in Brazilian eucalypt plantations

Large amounts of fertilizers are applied in tropical planted forests managed in short rotations to sustain high biomass productions in highly weathered soils. Our study aimed to assess the consequences of fertilizer and biosolid additions on groundwater quality and nutrient leaching in Eucalyptus grandis plantations. Soil solutions were continuously sampled in 9 plots using plate lysimeters and ceramic cups at depths of 15, 50, 100 and 300 cm. The solutions were collected every week over 6 years to determine the concentrations of the main anions and cations. Three treatments were studied in 3 blocks: no nitrogen addition (control), 120 kg N ha-1 applied as ammonium sulphate, and 30 t ha-1 of biosolid added containing 1260 kg N ha-1. The main biogeochemical processes controlling the chemistry of gravitational solutions throughout their transfer were identified. The chemical composition of soil solutions in the topsoil was driven by the mineralization of soil organic matter and harvest residues as well as the dissolution of the fertilizers. The peaks of nutrient concentrations reached a depth of 1 m about 9 months after planting but high concentrations never reached a depth of 3 m, except after the last biosolid application. E. grandis trees exhibited a remarkable filter capacity against nutrient leaching as a result of i) a relatively slow transfer of mobile ions in deep soil layers, ii) a very fast root growth (down to > 6 m at 1 year after planting) and iii) a high demand of trees in water and nutrients from 6 months after planting onward. Our results suggest that the number of fertilizations could be reduced in commercial eucalypt plantations growing in deep Ferralsols and that biosolid applications could be adjusted to reach high growth rates without polluting deep water tables. (Resume d'auteur)