Sewage sludge application for spontaneous plant restoration of a New Caledonian Ferralsol

Abstract

Soils from New Caledonia typically present poor nutrient content and large quantities of heavy metals such as nickel, chromium and cobalt, resulting in slow plant development. We evaluated the impact of sewage sludge application (0–30 t dry weight (DW) ha–1) on the passive revegetation of a former pine plantation. The spontaneously regenerated plant cover was mainly represented by the dominant Pinus caribaea, a shrub (Sannantha leratii), and a grass (Costularia comosa). The density of pine seedlings was significantly higher in the moderately amended zone (1.2 and 0.2 pines m–2 for 0.5 and 30 t DW ha–1 respectively). The same tendency was observed for S. leratii, but for C. comosa, no net change was observed. With no amendment, after 5 years many species were present, although aboveground biomass (0.3 kg m–2) was low, whereas, with sludge amendment, aboveground biomass was high (5 kg m–2) but diversity low. Amendment increased pine tree heights from 0.15 to 3.92 m with increased amendment from 0 to 30 t DW ha–1. The uptake of nitrogen (N) by pine trees was also improved with sludge supply, as was the uptake of phosphorus (P). Regarding S. leratii, N and P levels were highest at the sludge dosage of 2 t DW ha–1. Carbon storage in P. caribaea biomass increased from 0.40 to 180 kg m–2 with increased amendment applied. Five years after spreading at the highest amendment levels, available soil P remained enhanced. For heavy metal uptake by pine trees, no significant effect of sewage sludge was observed. The optimal dosage to stimulate biodiversity was 0.5–2 t DW ha–1 but maximal biomass was reached at 8–30 t DW ha–1.