Dynamic of nitrogen and dissolved organic carbon in an alpine forested catchment: atmospheric deposition and soil solution trends

A number of studies have reported decreasing trends of acidifying and N deposition inputs to forest areas throughout Europe and the USA in recent decades. There is a need to assess the responses of the ecosystem to declining atmospheric pollution by monitoring the variations of chemical species in the various compartments of the forest ecosystem on a long temporal scale. In this study, we report on patterns and trends in throughfall deposition concentrations of inorganic N, dissolved organic N (DON) and C (DOC) over a 20-year (1995–2015) period in the LTER site -Val Masino (1190 m a.s.l.), a spruce forest, in the Central Italian Alps. The same chemical species were studied in the litter floor leachates and mineral soil solution, at three different depths (15, 40 and 70 cm), over a 10-year period (2005–2015). Inorganic N concentration was drastically reduced as throughfall and litter floor leachates percolated through the topsoil, where the measured mean values (2 µeq L-1) were much lower than the critical limits established for coniferous stands (14 µeq L-1). The seasonal temperature dependence of throughfall DOC and DON concentration suggests that the microbial community living on the needles was the main source of dissolved organic matter. Most of DOC and DON infiltrating from the litter floor were retained in the mineral soil. The rainfall amount was the only climatic factor exerting a control on DOC and N compounds in throughfall and forest floor leachates over a decadal period. Concentration of SO4 and NO3 declined by 50% and 26% respectively in throughfall deposition. Trends of NO3 and SO4 in forest floor leachates and mineral soil solution mirrored declining depositions. No trends in both DON and DOC concentration and in DOC/DON ratio in soil solutions were observed. These outcomes suggest that the declining NO3 and SO4 atmospheric inputs did not influence the dynamic of DON and DOC in the Val Masino forest. The results of this study are particularly relevant, as they are based on a comprehensive survey of all the main compartments of the forest ecosystem. Moreover, this kind of long-term research has rarely been carried out in the Alpine region.

Climatic factors influencing fluxes of dissolved organic carbon from the forest floor in a continuous-permafrost Siberian watershed

Fluxes of dissolved organic carbon (DOC) in forested watersheds underlain by permafrost are likely to vary with changes in climatic regime that increase soil moisture and temperature. We examined the effects of temporal and spatial variations in soil temperature and moisture on DOC fluxes from the forest floor of contrasting north- and south-facing slopes in central Siberia. DOC fluxes increased throughout the growing season (June–September) on both slopes in 2002 and 2003. The most favorable combination of moisture content and temperature (deepest active soil layer) occurred in September, and we believe this was the primary driver of increased DOC concentrations and flux in autumn. Total DOC flux for June–September was 12.6–17.6 g C·m–2 on the south-facing slope and 4.6–8.9 g C·m–2 on the north-facing slope. DOC concentrations in forest floor leachates increased with increasing temperature on the north-facing slope, but were almost unaffected by temperature on the south-facing slope. Our results suggest that water input in midseason from melting of ice or precipitation events is the primary factor limiting DOC production. Significant positive correlations between amounts of precipitation and DOC flux were found on both slopes. Dilution of DOC concentrations by high precipitation volumes was observed only for the forest floor leachates collected from the north-facing slope. Our results suggest that global warming will result in increased DOC production in forest floors of permafrost regions, and that precipitation patterns will play an important role in determining the magnitude of these changes in DOC flux as well as its interannual variability. However, the longer-term response of soils and DOC flux to a warming climate will be driven by changes in vegetation and microbial communities as well as by the direct results of temperature and moisture conditions.

Throughfall nitrogen in a white spruce forest in southwest Alberta, Canada

Rates of transfer of nitrogen in throughfall were measured in a Piceaglauca (Moench) Voss forest in southwest Alberta, Canada. Total annual flux of N in throughfall was estimated to be 6.8 kg•ha−1•year−1. The highest rates of N deposition in throughfall occurred during the spring. The pattern of seasonal variation of N in throughfall suggested that different processes of N accumulation in and loss from the canopy may have been operating at different times of the year. Annual leaching of N from the canopy was estimated to be 4.09 kg•ha−1•year−1 and represented 24% of the total amount of N returned to the forest floor (leachates plus litter fall).