Biosolids Application For Passive Vegetalization of a Lateritic Soil Previously Used For Pine (Pinus caribaea) Production

Sewage sludge may be used to increase the organic matter concentration into the soil, and, subsequently increase the N and P availability for plants. The amendment for forestry represents a valorization way, in particular, for the pine production. Soils from New Caledonia are usually characterized by a poor nutrient content and large quantities of heavy metals like Ni, Cr and Co, resulting in a slow installation and development of vegetal species. Some Pinus caribaea plantations on soil from the south of New Caledonia have not presented a great production success. An experiment was conducted to introduce different level of biosolids into the topsoil of an old pine forest. The aim of this study consisted in verifying that these biosolids represent an interesting response to promote the vegetal establishment on a lateritic soil.A part of the old forestry area, in which were delimited 25 experimental plots (5×5m), was mechanically cleaned in order to remove all vegetal and pine stumps. On all these plots, amendments were operated in order to obtain five loads from 0 to 30 t dw.ha-1 of dewatered sewage sludge (15% dw). Biosolids came from the Koutio wastewater treatment plant (Dumbéa, New Caledonia) and are characterized by a low level of heavy metals (largely under the European limits) and 1.0% and 4.6% for P and N respectively. All these 5 experimental treatments were replicated 5 times and let for a passive vegetal colonization (no plantation).During the following three years period, it was observed that vegetal species were installed in each zone and the more the amendment is important, the more is the colonization. Nevertheless, in the highest level of biosolids, it was observed a high grass density. Numerous pine seeds from the topsoil have germinated and the highest trees were observed in the highest level of biosolids. The mean high for pine trees were 9.1±0.5, 41±4, 44±4, 78±7 and 122±17 for 0, 0.5, 2, 8 and 30 t dw.ha-1 respectively after 2.5 years. This results are in accordance with previous works with other pinus species like Pinus pinea, Pinus radiata or Pinus elliottii. In the other hand, the trees density is significantly more important in the lowest amended zone (1.1 and 0.2 pinus.m-2 for 0 and 30 t dw.ha-1 respectively). The low value in highest amended zones may partly be due to grass shading.Chemical analysis on pine shoots presents low nitrogen and phosphorus concentrations into the biomass both in needles and branches. These values are under the observed literature values with other pine species.Fields results indicate that the 8 t dw.ha-1 dose seems the most interesting for the pine installation.