Elevated nitrate concentrations in soil solution under pure Douglas fir stands can be lowered by mixing with European beech and by site selection

Forest management aims for productive and stable forests that continuously provide ecosystem goods and services, including balancing nutrient fluxes. Increasing heat and frequent droughts in temperate European forests make the introduction of non-native Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) an increasingly relevant climate change adaptation strategy, particularly as an admixture to native tree species, such as European beech (Fagus sylvatica [L.]). Douglas fir can alter biogeochemical processes in forest soils, potentially leading to an excess of nitrogen in the ecosystem, but the biotic and abiotic controls of this effect need further examination (species interactions, soil type). Here we studied nitrate leaching on plots of two contrasting textures (southern loamy and northern sandy site) planted with either pure Douglas fir, pure Norway spruce (Picea abies [L.] Karst.), pure European beech or a mixture of beech with either of the conifers. We used P80 suction lysimeters at 5 and 60 cm soil depth and collected soil solution over two continuous years to estimate nitrate leaching risks. We found highest concentrations of nitrate in soil solution in lower soil layers under Douglas fir (29.14 mg/L), which corroborates the findings of some studies conducted in pure stands. Comparisons of concentrations below the litter layer and below the main rooting zone implied that accumulation and microbial production of nitrate is taking place under Douglas fir. In Douglas fir-beech mixed stands, however, we found sustained significantly lower nitrate concentrations in soil solution below the main rooting zone (1.68 mg/L), implying a mixture effect. Furthermore, site played a key role in controlling nitrate concentrations in soil solution under Douglas fir on sites with finer soil texture and a lower C:N ratio. Elevated nitrate concentrations were associated with a P-limitation found in the trees, which likely caused reduced nitrate uptake. We discuss the results with regards to throughfall, litter, soil and microbial characteristics. We conclude that increased nitrate concentrations under Douglas fir stands may pose a relatively higher nitrate leaching risk than Norway spruce and a considerably higher risk compared to beech stands. However, the low susceptibility to leaching under beech stands seems to be a strong effect trait in mixtures, diminishing the high leaching potential Douglas fir induces on some sites. Low leaching potential is key to sustaining adequate nutrition in temperate forests and reducing pollution of groundwater. Our findings strongly urge forestry experts to carefully assess site conditions and foster mixtures with European beech when planting Douglas fir.