Forests support a broad range of ecosystem services. These services cannot simply be traced back to the individual biotic and abiotic components of the forest ecosystems. Rather, they stem from complex interactions between these components. CON.ECO.FOR, the Italian branch of the ICP-Forests (International Co-operative Program on Assessment and Monitoring of Air Pollution Effects on Forests) network, established in 1994, has shown to be a unique data source for understanding forest interactions and processes. This contribution aims to draw a comprehensive picture of the Italian forest soil conditions by reviewing the main issues that have arisen and the results obtained over the last 30 years of forest soil monitoring. Forest health is often controlled by soil-mediated processes. Thus, we have evidenced how including soil parameters in environmental studies has proven to be of high value for a better understanding of forest ecosystem conditions and trends. Here, we are reviewing all of the results obtained concerning soils from the analyses of the impacts of atmospheric deposition on forest soils to the study of the relationships between organism diversity and the soil.

As a large and persistent carbon sink, forest soils have an essential role in the carbon cycle, thus performing valuable services to society. This paper aims to investigate the role of several environmental factors in driving soil organic carbon (SOC) storage variability in forest soils. The Italian ICP-Forests (International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests) dataset provides an excellent opportunity to analyse an extensive range of variables in a land spanning different ecological regions and cli-matic gradients, thus being a case of broad interest. We estimated SOC storage for mineral forest soils, consid-ering the topsoil (0-20 cm; SOCM02) and the whole soil profile (0-80 cm; SOCM08). Boosted regression trees (BRTs) were applied to explore the relative influence of environmental predictors and to model SOC storage responses. The reference soil groups (RSG-WRB) and the total subsoil phosphorus (P) content were the factors with the highest performance in explaining SOC storage for both models (SOCM02-SOCM08). Parameters related to vegetation, such as tree species, biodiversity and plant traits, clearly influenced topsoil SOC storage, while their impact was reduced with depth. Climate directly controls SOC storage, but indirect influences via plant or soil characteristics were also identified. Interactions between different environmental factors were discussed to elucidate and analyse SOC responses evidenced by the models. Our results highlighted how soil types (RSGs) can be an effective environmental factor in explaining SOC storage variability, which would likely improve SOC models on national and global scales. Interactions between different environmental factors were seen to be most important, pointing out the strong links between SOC storage and the general ecological context.

Although acidification is a natural soil formation process, atmospheric emissions of nitrogen and sulphur have exposed forest ecosystems to accelerated anthropogenic acidification for many decades. In Italy nitrogen deposition loads are high, and among the highest in Europe. Data from the Italian ICP-Forests monitoring network have shown that nitrogen deposition increases tree growth, and consequently the organic carbon sequestration. However, the accumulation of nitrogen in the ecosystem could cause nutrient imbalances and contribute to soil acidification. Within this context, we investigated the temporal changes of atmospheric deposition and soil solution concentration in the Italian ICP-Forests using the non-parametric Seasonal Mann Kendall (SMK) test. Further, we applied input-output models, to evaluate the capabilities of the soil-forest system to retain deposited nitrogen and thus protect underground waters. Increase of soil solution pH was observed in most of the sites, likely due to sulphate deposition decrease with a concomitant high and stable natural exchangeable base deposition. For the sites around the Po plain, however, high levels of nitrogen deposition impede pH increase. The site with the maximum mineral N deposition showed signs of active acidification. The analysis of the fluxes of nitrogen compounds demonstrates a complete retention of the ammonium forms, which further contribute to acidification through nitrification. Furthermore, the Italian monitoring network showed that the retention of nitrogen in the soil-forest system was effective since the observed N fluxes in mineral soil were strongly reduced compared to mineral N input in most of the plots. In spite of this, significant NO3- fluxes from the subsoil were observed in sites with high deposition, and also in the southernmost site which is exposed to relatively low mineral N loads. Drivers other than pollution should also be considered since N can follow different pathways at different time scales, influencing N leaching independently from the amount of deposition.

Acidification is a major soil-forming process. Emission of nitrogen (N) and sulphur (S) compounds has exposed forest ecosystems to acidifying anthropogenic deposition for several decades. Since the 1980s, the reduction of air emissions following the environmental regulations has been resulted in a decline of pollutant depositions. Nevertheless, in Italy, N depositions are still high, among the highest in Europe. Nitrogen from depositions has been found to favour plant growth, contributing to organic carbon uptake. However, a continuous accumulation of deposited inorganic N causes plant nutrient unbalances, and likely contributes to soil acidification more than commonly reckoned. At the same time, depositions of buffer elements are high and constant in Italy due to the proximity of the Saharan source. Within this context, our researches aim to explore the specific impacts of peculiar atmospheric deposition, hypothesising a strong influence of aeolian dusts, on forest soil ecosystem in Italy. Our main objectives are a) to evaluate the long term trends in soil solution elemental fluxes in relation to depositions trends; b) to investigate the soil response to pollution change and recovery, individuating whether soil acidification is, or not, an active process; c) to identify sites where the transfer of reactive N from atmospheric N pollution to freshor ground-waters is a real concern. We analysed data on deposition and elemental concentrations in soil solutions recorded at forest plots of the ICP Forests intensive monitoring network in Italy. We used a conceptual model based on input-output budgets estimating atmospherically derived fluxes of elements that enter terrestrial ecosystem through deposition and losses of elements through drainage water. For the base cation (BCE) budget, an estimate of rock-derived elements due to chemical weathering were also included in the model.

The environmental factors driving humus form differentiation in Italian forest soils were investigated for Moder, Mull and Amphi models, highlighting the high relative weight of tree species as a predictor. Specific soil-plant-litter- nutrient flow effects were evidenced, representing a direct influence of tree species on humus form development. The effect of parent material on pedofauna activity and composition appeared to be fundamental with key role played by calcium and phosphorus. Specific pathways leading to the evolution of Amphi, a major humus form in Italy, were discussed.

Acid deposition arising from sulphur (S) and nitrogen (N) emissions from fossil fuel combustion and agriculture has contributed to the acidification of terrestrial ecosystems in many regions globally. However, in Europe and North America, S deposition has greatly decreased in recent decades due to emissions controls. In this study, we assessed the response of soil solution chemistry in mineral horizons of European forests to these changes. Trends in pH, acid neutralizing capacity (ANC), major ions, total aluminium (Al-tot) and dissolved organic carbon were determined for the period 1995-2012. Plots with at least 10 years of observations from the ICP Forests monitoring network were used. Trends were assessed for the upper mineral soil (1020 cm, 104 plots) and subsoil (40-80 cm, 162 plots). There was a large decrease in the concentration of sulphate (SO42-) in soil solution; over a 10-year period (2000-2010), SO42- decreased by 52% at 10-20 cm and 40% at 40-80 cm. Nitrate was unchanged at 10-20 cm but decreased at 40-80 cm. The decrease in acid anions was accompanied by a large and significant decrease in the concentration of the nutrient base cations: calcium, magnesium and potassium (Bc = Ca2+ + Mg2+ + K+) and Altot over the entire dataset. The response of soil solution acidity was nonuniform. At 10-20 cm, ANC increased in acid-sensitive soils (base saturation <= 10%) indicating a recovery, but ANC decreased in soils with base saturation >10%. At 40-80 cm, ANC remained unchanged in acid-sensitive soils (base saturation <= 20%, pH(CaCl2) <= 4.5) and decreased in better-buffered soils (base saturation >20%, pH(CaCl2) >4.5). In addition, the molar ratio of Bc to Altot either did not change or decreased. The results suggest a long-time lag between emission abatement and changes in soil solution acidity and underline the importance of long-term monitoring in evaluating ecosystem response to decreases in deposition.

Dissolved organic carbon (DOC) in surface waters is connected to DOC in soil solution through hydrological pathways. Therefore, it is expected that long-term dynamics of DOC in surface waters reflect DOC trends in soil solution. However, a multitude of site studies have failed so far to establish consistent trends in soil solution DOC, whereas increasing concentrations in European surface waters over the past decades appear to be the norm, possibly as a result of recovery from acidification. The objectives of this study were therefore to understand the long-term trends of soil solution DOC from a large number of European forests (ICP Forests Level II plots) and determine their main physico-chemical and biological controls. We applied trend analysis at two levels: (1) to the entire European dataset and (2) to the individual time series and related trends with plot characteristics, i.e., soil and vegetation properties, soil solution chemistry and atmospheric deposition loads. Analyses of the entire dataset showed an overall increasing trend in DOC concentrations in the organic layers, but, at individual plots and depths, there was no clear overall trend in soil solution DOC. The rate change in soil solution DOC ranged between −16.8 and +23 % yr−1 (median = +0.4 % yr−1) across Europe. The non-significant trends (40 %) outnumbered the increasing (35 %) and decreasing trends (25 %) across the 97 ICP Forests Level II sites. By means of multivariate statistics, we found increasing trends in DOC concentrations with increasing mean nitrate (NO3−) deposition and increasing trends in DOC concentrations with decreasing mean sulfate (SO42−) deposition, with the magnitude of these relationships depending on plot deposition history. While the attribution of increasing trends in DOC to the reduction of SO42− deposition could be confirmed in low to medium N deposition areas, in agreement with observations in surface waters, this was not the case in high N deposition areas. In conclusion, long-term trends of soil solution DOC reflected the interactions between controls acting at local (soil and vegetation properties) and regional (atmospheric deposition of SO42− and inorganic N) scales.

We aim to investigate the occurrence of forest humus forms (Moder, Amphi and Mull) in relation to environmental factors describing parent material, climate and tree species. Boosted regression trees (BRTs) were applied as modeling tool to analyze data of 238 plots of the BioSoil database covering the whole Italian forest territory. Though predictive ability was not very high, especially for the Amphi form, we could gain significant insight into factors controlling humus form differentiation. In the BRT analysis, the diversity of tree species was the most important predictor for Moder and Mull models and specific plant effects were evidenced. However, our results showed that the geographic distribution of Italian forest species was influenced by soil and climate conditions, partly explaining the high weight of tree species as factor. The importance of the soil nutritional status, due to parent material properties, in driving humus form differentiation was stated, highlighting the key role played by pH and calcium content, with the hitherto understated importance of phosphorus. This study further clarified the functioning of the still poorly understood Amphi form. Reduced effective soil volume (EfVol) combined with seasonality appeared to constrain pedofauna activity in otherwise favorable and nutrient rich systems, favoring the evolution of Amphi instead of Mull forms.

We studied forest monitoring data collected at permanent plots in Italy over the period 2000–2009 to identify the possible impact of nitrogen (N) deposition on soil chemistry, tree nutrition and growth. Average N throughfall (N‐NO3+N‐NH4) ranged between 4 and 29 kg ha−1 yr−1, with Critical Loads (CLs) for nutrient N exceeded at several sites. Evidence is consistent in pointing out effects of N deposition on soil and tree nutrition: topsoil exchangeable base cations (BCE) and pH decreased with increasing N deposition, and foliar nutrient N ratios (especially N : P and N : K) increased. Comparison between bulk openfield and throughfall data suggested possible canopy uptake of N, levelling out for bulk deposition >4–6 kg ha−1 yr−1. Partial Least Square (PLS) regression revealed that ‐ although stand and meteorological variables explained the largest portion of variance in relative basal area increment (BAIrel 2000–2009) ‐ N‐related predictors (topsoil BCE, C : N, pH; foliar N‐ratios; N deposition) nearly always improved the BAIrel model in terms of variance explained (from 78.2 to 93.5%) and error (from 2.98 to 1.50%). N deposition was the strongest predictor even when stand, management and atmosphere‐related variables (meteorology and tropospheric ozone) were accounted for. The maximal annual response of BAIrel was estimated at 0.074–0.085% for every additional kgN. This corresponds to an annual maximal relative increase of 0.13–0.14% of carbon sequestered in the above‐ground woody biomass for every additional kgN, i.e. a median value of 159 kgC per kgN ha−1 yr−1 (range: 50–504 kgC per kgN, depending on the site). Positive growth response occurred also at sites where signals of possible, perhaps recent N saturation were detected. This may suggest a time lag for detrimental N effects, but also that, under continuous high N input, the reported positive growth response may be not sustainable in the long‐term.

Soil solution monitoring aims to understand various temporal scales of soil processes. The first eight years of observation in ABR1 Level II site have brought significant elements of understanding about the shorter temporal scales. It is suggested that certain solutes, regularly produced by forest floor microbial processes, are transferred to the highly mobile portion of the soil solution by a non linear process, producing irregular pulses and creating a strong high frequency component. Seasonal processes remain nonetheless detectable after simple and rough filtering. A multi-year trend of diminished nitrate mineralization and increased pH of forest floor solutions is possible. It is estimated that much more accurate analysis will be possible in a relatively short time span of further monitoring.

There is a growing interest in the links between humus forms and soil biota, and little is known about these links in Mediterranean ecosystems. Culture-independent techniques, such as DNA extraction followed by DGGE and enzyme activities, allowed us to compare microbial communities in two horizons of a forest soil in different seasonal conditions. Direct in situ lysis was applied for extraction of DNA from soil; intracellular DNA was separated from extracellular and used to represent the composition of microflora. The aims were to describe how biochemical and microbiological parameters correlate with topsoil properties in typical Mediterranean Moder humus. Changes in bacterial and fungal community composition were evident from DGGE profiles. Degrees of similarity and clustering correlation coefficients showed that the seasonal conditions may affect the composition and activity of bacterial and fungal communities in the OH horizon, while in the E horizon the two communities were hardly modified. In the same season, OH and E horizons showed a different composition of bacterial and fungal communities and different enzyme activities, suggesting similar behaviour of eubacteria and fungi relatively to all the variables analysed. Evidently, different organic carbon content in soil horizons influenced microflora composition and microbial activities involved in the P and N cycles.

As soil solution represents the major phase of soil chemical reactions, its study is a powerful tool for ecological investigations. Soil solution chemical composition gives a realistic idea about the soil chemical components immediately available in the environment, mainly in relation to the soil ecosystem reaction to the disturbance due to acidifying loads. Within the CONECOFOR Program, the monitoring of forest soil conditions was performed in a level II plot (ABR I), under a Fagus sylvatica (European beech) stand, through the study of throughfall and soil solutions collected from depths ranging between the base of the litter layers and 90 cm. To be able to investigate solution contents of nutrients, acidifying agents and DOC throughout the profile, both zero tension and tension lysimeters were used. The first ones were inserted below the organic horizons, while tension lysimeters were placed within the mineral horizons at 15, 25, 55 and 90 cm depth. Sampled solutions were analyzed for Na, K, Ca, Mg, NH4, Cl, F, NO3, SO4, and DOC. The results evidence a clear seasonal pattern, mainly for macronutrients and inorganic N components. Acidic pulses were mostly evident below the organic horizons, in relation to strong nitric N releases from litter; these last were not always immediately neutralized by basic cations. Acid solutions leaving the organic horizons were invariably neutralized in the surface mineral horizons, within 15 cm depth. Temporal patterns of sulphate retention and release suggest that the soil has low retention capability for this anion. Such behaviour can be explained by the composition of the solid phase, where potential anion adsorbants appear strongly linked with organic matter in long residence time complexes. Sulphate and nitrate loading of this soil appear, anyway, to be mostly non-anthropogenic, but rather linked to natural mineralization pulses and, for sulphate, to aeolian solid transport from the south.

The clay fraction of a Spodosol and its parent rock in the Apennine mountains of central Italy were studied by powder X-ray diffraction (XRD) and infrared (IR) spectroscopy, to evaluate the possibility of transformation of chlorite into low-charge expandable minerals. Results indicated that the main phyllosilicate in the rock was a slightly weathered trioctahedral chlorite, rich in both Mg and Fe, together with dioctahedral mica and minor amounts of kaolinite. In the BC horizon, chlorite has undergone partial transformation into 2 vermiculitic components, in 1 of which the interlayer could be removed by hot Na-citrate treatment; the presence of a regular interstratified mineral (high-charge corrensite) was also observed. Further changes in the structure of chlorite were detected in the Bsl horizon, becoming more evident towards the soil surface. The first stage of weathering of chlorite involved Fe oxidation and partial expulsion of Mg from the hydroxide sheet, followed by deposition of Al in the interlayer space. Iron is also removed from the interlayer sheet, possibly remaining, in the oxidized state, in the 2:1 octahedral sheet, and so contributing to the lowering of layer charge and transformation to a dioctahedral structure. When approaching the surface, Al removal from the interlayers is enhanced by complexing agents, and further charge reduction leads to the formation of 2:1 minerals with a smectite nature. Illite, because of its low content in the soil clay fraction, contributes marginally to this weathering sequence, forming the high charged expandable component observed in the Bhs horizon. At the soil surface, a randomly interstratified vermiculite/illite was detected, which probably originated from K fixation by the higher-charged expandable minerals. This study of weathering in a natural soil strongly supports the hypothesis, previously ascertained by laboratory experiments, that chlorite can transform into a low-charge expandable mineral.