Data from: Soil solution in Swiss forest stands: a 20 year's time series

Soil solution chemistry is influenced by atmospheric deposition of air
pollutants, exchange processes with the soil matrix and
soil-rhizosphere-plant interactions. In this study we present the results
of the long-term Intercantonal Forest Observation Program in Switzerland
with soil solution measurements since 1998 on a current total of 47 plots.
The forest sites comprise two major forest types of Switzerland including
a wide range of ecological gradients such as different nitrogen (N)
deposition and soil conditions. The long-term data set of 20 years of soil
solution measurements revealed an ongoing, but site-specific soil
acidification. In strongly acidified soils (soil pH below 4.2),
acidification indicators changed only slowly over the measured period,
possibly due to high buffering capacity of the aluminum buffer (pH 4.2 –
3.8). In contrast, in less acidified sites we observed an increasing
acidification rate over time, reflected, for example, by the continuous
decrease in the ratio of base cations to aluminum (BC/Al ratio). Nowadays,
the main driver of soil acidification is the high rate of N deposition,
causing cation losses and hampering sustainable nutrient balances for tree
nutrition. Mean nitrate leaching rates for the years 2005-2017 were 9.4 kg
N ha‑1 yr‑1, ranging from 0.04 to 53 kg N ha‑1 yr‑1. Three plots with high
N input had a remarkable low nitrate leaching. Both N deposition and
nitrate leaching have decreased since 2000. However, the latter trend may
be partly explained due to increased drought in recent years. Nonetheless,
those high N depositions are still affecting the majority of the forest
sites. Taken together, this study gives evidence of anthropogenic soil
acidification in Swiss forest stands. The underlying long-term
measurements of soil solution provides important information on nutrient
leaching losses and the impact climate change effects such as droughts..
Furthermore, this study improves the understanding of forest management
and tree mortality regarding varying nitrate leaching rates.