Soil solution in Swiss forest stands: A 20 year's time series

Atmospheric deposition and soil water chemistry in Swedish forests since 1985 - Effects of reduced emissions of sulphur and nitrogen

Trends for the atmospheric deposition of sulphur (S) and inorganic nitrogen (inorg-N) to forests and changes in the forest soil water chemistry in Sweden have been assessed since 1985, with special focus on the last 25 years, based on measurements within the Swedish Throughfall Monitoring Network (SWETHRO). The reductions in the deposition of S and inorg-N in the southern part of Sweden corresponded relatively well with the pollutant emission reductions for S and inorg-N from both EU27 + UK and Sweden during 1996/97-2021/22. For northern Sweden the deposition of S and inorg-N decreased to a lesser extent than both European and Swedish emissions. The bulk deposition of NO3-N has decreased more than the deposition of NH4-N over the last 25-year period, which is consistent with the much larger emission reductions for NOx compared to NH3 from EU27 + UK and Sweden. The S concentrations in the soil water, at 50 cm below soil surface, have decreased during the last 25 years, however somewhat less than the S deposition. At sites with low ANC and pH in the beginning of the period, the increase in ANC was generally greater and the increase in pH was smaller, but at sites with high pH and ANC above zero, the increase in pH was dominant, in line with the nonlinear relationship between pH and ANC in the soil water. The incidence of elevated concentrations of NO3-N in the soil water was highest in southwest Sweden, ranging between 4 and 19 % of all measuring occasions since 1985/86. The reduced deposition of N over the 35-year period was not reflected in the incidence of elevated concentrations of NO3-N in the soil water over time.

The Influence of Tree Species on the Recovery of Forest Soils from Acidification in Lower Saxony, Germany

Atmospheric acid deposition has increased sharply since the beginning of industrialization but has decreased considerably since the 1980s owing to clean-air policies. Soil acidification induced by an input of acidity has been demonstrated in numerous studies using repeated forest-soil inventories. So far, relatively few data have been sampled to analyze long-term soil trends and only a few studies show the recovery of forest soils from acidification, whereas the recovery of surface waters following declining acid deposition is a widespread phenomenon. To assess a possible recovery from acid deposition, soil resampling data from 21 forested permanent soil-monitoring sites in Lower Saxony (Germany) were evaluated. For most sites, at least three repetitions of inventories from a period of 30 to 50 years were available. Trend analyses of indicators for the acid-base status of unlimed forest soils using generalized additive mixed models (GAMM) show either a trend reversal or a stagnation of the acid-base status at a strong acidification level. The recovery, if indicated by an increase of soil pH and base saturation, of soils from plots with deciduous trees appears to have occurred faster than in coniferous forest stands. This observation may be attributed to a larger amount of temporarily stored sulfur in the soil because of the higher atmospheric input into coniferous forests. As indicators for the acid-base status still show considerable soil acidification, mitigation measures such as forest liming still appear to be necessary for accelerating the regeneration process.

Water Limitation in Forest Soils Regulates the Increase in Weathering Rates under Climate Change

Climate change is generally expected to have a positive effect on weathering rates, due to the strong temperature dependence of the weathering process. Important feedback mechanisms such as changes in soil moisture, tree growth and organic matter decomposition can affect the response of weathering rates to climate change. In this study, the dynamic forest ecosystem model ForSAFE, with mechanistic descriptions of tree growth, organic matter decomposition, weathering, hydrology and ion exchange processes, is used to investigate the effects of future climate scenarios on base cation weathering rates. In total, 544 productive coniferous forest sites from the Swedish National Forest Inventory are modelled, and differences in weathering responses to changes in climate from two Global Climate Models are investigated. The study shows that weathering rates at the simulated sites are likely to increase, but not to the extent predicted by a direct response to elevated air temperatures. Besides the result that increases in soil temperatures are less evident than those in air temperature, the study shows that soil moisture availability has a strong potential to limit the expected response to increased temperature. While changes in annual precipitation may not indicate further risk for more severe water deficits, seasonal differences show a clear difference between winters and summers. Taking into account the seasonal variation, the study shows that reduced soil water availability in the summer seasons will strongly limit the expected gain in weathering associated with higher temperatures.

Forest Soils—What’s Their Peculiarity?

Mankind expects from forests and forest soils benefits like pure drinking water, space for recreation, habitats for nature-near biocenoses and the production of timber as unrivaled climate-friendly raw material. An overview over 208 recent articles revealed that ecosystem services are actually the main focus in the perception of forest soil functions. Studies on structures and processes that are the basis of forest soil functions and ecosystem services are widely lacking. Therefore, additional literature was included dealing with the distinct soil structure and high porosity and pore continuity of forest soils, as well as with their high biological activity and chemical soil reaction. Thus, the highly differentiated, hierarchical soil structure in combination with the ion exchange capacity and the acid buffering capacity could be described as the main characteristics of forest soils confounding the desired ecosystem services. However, some of these functions of forest soils are endangered under the influence of environmental change or even because of forest management, like mono-cultures or soil compaction through forest machines. In the face of the high vulnerability of forest soils and increased threads, e.g., through soil acidification, it is evident that active soil management strategies must be implemented with the aim to counteract the loss of soil functions or to recover them.

Effects of Moderate Nitrate and Low Sulphate Depositions on the Status of Soil Base Cation Pools and Recent Mineral Soil Acidification at Forest Conversion Sites with European Beech (“Green Eyes”) Embedded in Norway Spruce and Scots Pine Stands

High N depositions of past decades brought changes to European forests including impacts on forest soil nutrition status. However, the ecosystem responses to declining atmospheric N inputs or moderate N depositions attracted only less attention so far. Our study investigated macronutrient (N, S, Ca2+, Mg2+, K+) pools and fluxes at forest conversion sites over 80 years old in Central Germany with European beech (so-called “Green Eyes” (GE)). The GE are embedded in large spruce and pine stands (coniferous stands: CS) and all investigated forest stands were exposed to moderate N deposition rates (6.8 ± 0.9 kg ha−1 yr−1) and acidic soil conditions (pHH2O < 4.7). Since the understanding of forest soil chemical and macronutrient status is essential for the evaluation of forest conversion approaches, we linked patterns in water-bound nutrient fluxes (2001–2018) and in predicted macronutrient storage in the herbaceous and tree layer to patterns in litter fall (2016–2017) and in forest floor and mineral soil macronutrient stocks at GE and CS assessed in 2018. Our results exhibited 43% (Nt) and 21% (S) higher annual throughfall fluxes at CS than at GE. Seepage water at 100 cm mineral soil depth (2001–2018) of CS is characterized by up to fivefold higher NO3− (GE: 2 ± 0.7 µmolc L−1; CS: 9 ± 1.4 µmolc L−1) and sevenfold higher SO42− (GE: 492 ± 220 µmolc L−1; CS: 3672 ± 2613 µmolc L−1) concentrations. High base cation (∑ Ca2+, Mg2+, K+) concentrations in CS mineral soil seepage water (100 cm depth: 2224 ± 1297 µmolc L−1) show significant positive correlations with SO42−. Tree uptake of base cations at GE is associated especially with a Ca2+ depletion from deeper mineral soil. Foliar litter fall turns out to be the main pathway for litter base cation return to the topsoil at GE (>59%) and CS (>66%). The litter fall base cation return at GE (59 ± 6 kg ha−1 yr−1) is almost twice as large as the base cation deposition (30 ± 5 kg ha−1 yr−1) via throughfall and stemflow. At CS, base cation inputs to the topsoil via litter fall and depositions are at the same magnitude (24 ± 4 kg ha−1 yr−1). Macronutrient turnover is higher at GE and decomposition processes are hampered at CS maybe through higher N inputs. Due to its little biomass and only small coverage, the herbaceous layer at GE and CS do not exert a strong influence on macronutrient storage. Changes in soil base cation pools are tree species-, depth- and might be time-dependent, with recently growing forest floor stocks. An ongoing mineral soil acidification seems to be related to decreasing mineral soil base cation stocks (through NO3− and especially SO42− leaching as well as through tree uptake).

A Combined Measurement and Modelling Approach to Assess the Sustainability of Whole-Tree Harvesting—A Swedish Case Study

The demand of renewable energy has increased the interest in whole-tree harvesting. The sustainability of whole-tree harvesting after clear-cutting, from an acidification point of view, depends on two factors: the present acidification status and the further loss of buffering capacity at harvesting. The aims of this study were to investigate the relationship between these two factors at 26 sites along an acidification gradient in Sweden, to divide the sites into risk classes, and to examine the geographical distribution of them in order to provide policy-relevant insights. The present status was represented by the acid neutralizing capacity (ANC) in soil solution, and the loss of buffering capacity was represented by the estimated exceedance of critical biomass harvesting (CBH). The sites were divided into three risk classes combining ANC and exceedance of CBH. ANC and exceedance of CBH were negatively correlated, and most sites had either ANC < 0 and exceedance (high risk) or ANC > 0 and no exceedance (low risk). There was a geographical pattern, with the high risk class concentrated to southern Sweden, which was mainly explained by higher historical sulfur deposition and site productivity in the south. The risk classes can be used in the formulation of policies on whole-tree harvesting and wood ash recycling.