Soil solution chemistry in 11 monitoring plots of Douglas-fir plantations in France: implications for soil fertility.

Purpose: Douglas-fir occupies a growing role in the European timber industries but its effects on soils and the environment are still not well understood. We monitored the biogeochemical cycle of major elements in 11 representative mature stands throughout France over the 2013 to 2020 period to better understand repercussions on soil processes. Methods: The present study is focused on throughfall and soil solution chemistry: concentrations were measured monthly at 15 cm and 60 cm depth and fluxes of major anions and cations were computed. Results: Our results show that nitrate strongly dominated the soil solution chemistry: concentrations were elevated (>20 mg L−1) for 8 out of 11 sites with mean annual NO3− values ranging from 2 to 253 mg L−1. Fluxes of nitrate at 60 cm of depth, ranging from 23 to 309 kg ha−1 yr−1, were much higher than those in throughfall for nine studied sites. Nitrate, which was the dominant anion in soil solution, associated either to Ca2+, Mg2+ and K+, or to Al3+ was lost out of the soil-plant system through leaching. Conclusions: These results suggest a production of NO3− in the soil exceeding tree uptake and microbial immobilization resulting in strong ongoing soil acidification at many sites and potential degradation of water quality. Significant negative correlations were found between nitrate concentration in soil solution and sandy texture, soil C and N, site elevation and, annual rainfall, but further research is necessary to clarify the role of these factors in reducing the negative impacts on terrestrial and aquatic ecosystems. [ABSTRACT FROM AUTHOR]