Sewage sludge as a soil amendment in a Larix decidua plantation: Effects on tree growth and floristic diversity

Sewage sludge application in forest plantations is an interesting complementary alternative practice to sewage sludge reutilization and recycling, with a significant and sustainable net effect in climate change mitigation. However, to optimize it a detailed knowledge of its effects on ecosystem components such as plants, soil, water and fauna is needed. We investigated the effects of sewage sludge application on soil, tree growth and floristic diversity in a ten-year-old plantation of European larch (Larix decidua Mill.). Our one-hectare study site, located at Mélisey, Haute-Saône, France (47°753' Lat., 6°580' Long.), was subdivided into six plots. Three plots, alternating with three control plots (no sewage sludge application), were amended in June 2008 with 0.4tDWha^-1 obtained from a municipal urban wastewater treatment plant in Mélisey. Within each plot, one subplot was delimited and sludge was again manually applied at 3t of DWha^-1 in July 2009 and March 2010 to the soil surface of the amended subplots without incorporation. The results showed no effect on radial and height growth of European larch amended with 0.4tDWha^-1. While a significant temporary increase in pH, macro-element contents (N, P and Ca) and the trace metal (Cu and Zn) concentration in the soil was observed, it had no significant effect on needles and sporocarp contents. The number of species in the amended subplots with 3tDWha^-1year^-1 increased by 80% compared to the control. However, the relative species abundance present only in amended subplots remains <1, except for Hypericum humifusum.