Effect of understory vegetation management through liming and sewage sludge fertilisation on soil fertility and Pinus radiata D. Don growth after reforestation

Tree-understory competition is one of the most important aspects that control tree growth after reforestation. The relationship between trees and the understory can be modified by improving acidic soils with lime and by fertilisation. This experiment aims to evaluate the effect of soil improvements on the pasture-tree relationship by liming and fertilisation on different dates in a Pinus radiata-reforested area. Both lime and sewage sludge improved soil fertility by increasing Ca and reducing Al in the soil. Initially, tree development was reduced by lime, which improved the establishment of competitive grasses. Tree growth in limed treatments did not initially respond to sludge inputs, likely because both tree and grass roots shared the same soil depth layer. Three years after establishment, the use of high doses of sewage sludge in limed plots caused a growth rate similar to the best treatments of unlimed plots, which grew with a poorly sown grass establishment. After 2 years of the experiment, the presence of Erica woody shrub diminished tree development. High doses of sewage sludge with lime, as well as high doses of sewage sludge without lime, applied in April and low doses of sewage sludge without lime added in early February improved tree growth. From a practical point of view, lime and sewage sludge dose close to 100 kg total N ha(-1) should be recommended if a silvopastoral system is established, as it enhances pasture production and tree growth.