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Given the ongoing climate change, estimating the amount of less degradable plant compounds that can be stored in the soil, such as lignin, is a topic of primary importance. There are few methods applicable to soils for the determination of lignin, such as the copper oxide (CuO) oxidation method (CuOL). Acetyl bromide spectrophotometric lignin (ABSL) could be a valid alternative providing information that is less detailed compared to CuOL, but it offers data on the bulk amount of lignin and may offer a valid, fast, and cheap alternative to the CuO method. The aim of this work was to compare ABSL with the CuO method on several soils receiving plant residues from different trees. Mineral soil samples from 0 to 10 cm depth were obtained from a former agricultural site in northern Italy (Brusciana, Tuscany), where different tree plantations were established 22 years ago. The plantations were white poplar and common walnut, which were also intercropped with other species such as hazelnut, Italian alder, and autumn olive. Soil samples under these plantations were also compared to soil under an adjacent agricultural field. In general, the amount of lignin in the afforested stands was approximately double than in the agricultural field as determined by either method. The two methods returned a largely different scale of values due to their different mechanisms of action. The acid-to-aldehyde ratio of syringyl structural units highlights that forest plantation provides a plant input material that is more slowly oxidatively degraded compared to arable soil. A linear mixed model proved that ABSL performed well in relation to CuOL, especially when considering the random variation in the model given by the plantation field design. In conclusion, ABSL can be considered a valid proxy of soil C pool derived from structural plant component, although further analyses are needed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

In Italy, beech forests are the most important ecosystem for their carbon stock, but they are also threatened by climate change. In fact, one study showed a considerable reduction of beech extent between 64.5% and 98.8% in different IPCC scenarios along with a reduction in C stock. In further detail, the C stock in the first 5 cm of soil was influenced by temperature seasonality and elevation. On this basis, we carried out an elevation transept in 10 beech forests in Italy, sampling 3 stands at an increasing distance of 200 m from each other. In each of these 30 stands we collected litter and topsoil. A preliminary analysis of the relationship between the organic carbon and temperature seasonality and elevation as predictors showed a much stronger effect of the former, together with spatial autocorrelation. As a consequence, we added uncorrelated bioclimatic variables used as predictors in GLS modelling. Our models have been projected on current, potential and future (2070 RCP6) beech extent. Considering the sum of leaf litter and topsoil, our results showed that, on the one hand, taking into account a stock in the potential conditions of 410 Tg, there can be a change of the organic C stock to 225 Tg in 2070, while under the current extent of beech the estimate is 121 Tg. On the other hand, the concentration of organic C is expected to increase from 80 Mg/ha under current and potential conditions up to 122 Mg/ha for 2070. This increase will be much more evident in the southern Apennines (up to 4 times) than in the Alps (almost no change). These results can be explained by temperature seasonality changes, which is expected to increase from 6.2 to 6.9 ° C for 2070 in beech’s area of occurrence.

Summary: The origin and fate of humus, a soil organic matter fraction that is most resistant to decomposition, are not yet completely understood. Humus derives mainly from plant structural components; that is, lignin and cellulose, together with several other primary and secondary compounds. There are several methods to estimate lignin and cellulose in plant material; however, the applications to soil are limited and usually complicated and expensive. Conversely, humus can be estimated in soil by chemical titration methods, which estimate the carbon linked to humic and fulvic acids (HAC and FAC). Therefore, we carried out analyses that aimed to compare, within beech forest soils, spectrophotometric data for lignin and cellulose with those of HAC and FAC. We used soil samples from six different beech woods in Italy, sampling in four layers from 0–5 to 30–40 cm. The relations between spectrophotometric cellulose and lignin and HAC and FAC were tested by linear mixed models. Our results showed that both lignin and cellulose were significant predictors for HAC and FAC, even if cellulose had a larger conditional coefficient of determination (R2c). The best fit was between cellulose and the sum of HAC and FAC (R2c= 0.675). Our novel approach proved that spectrophotometric lignin and cellulose provide reliable results in both organic and mineral beech forest soils, even though lignin had a greater random variation than cellulose. Thus, spectrophotometric lignin and cellulose provide reliable and rapidly obtained predictors of humic substances in beech forest soils, with appropriate conversion factors to estimate HAC and FAC. Highlights: Humic substances are of pivotal importance in the global carbon cycle. There is a need for simple and rapid methods to estimate humic substances. Spectrophotometric methods provide fast results for cellulose and lignin in soils. Lignin and cellulose function as good predictors for humic substances in forest soils.

The issue of soil exhaustion or soil sickness, referring to the progressive loss of soil quality and decrease in fertility, has increased considerably in the last decades. Recent models predicted that around 200,000 km2 of degraded agricultural land in Europe will be abandoned between 2000 and 2040. Accordingly, this research aims at identifying an adequate forest management that might recover low fertility soils through an increase of soil organic matter (SOM) and soil quality, improving the ecosystem services of those areas. Thus, a former agricultural site was identified in Brusciana (FI) where, for the past 18 years, various forest associations for wood production have been established, namely white poplar and common walnut (K), which were also planted intercropped with other species such as hazelnut (C), Italian alder (A), and autumn olive (E). These forest stands were compared also to an adjacent agricultural field (X). Preliminary analyses on the topsoil (0-10 cm) showed a general low concentration of SOM (10-70 mg/g), although lowest values were found in X and highest in A, while the others were intermediate. The same trend was followed by soil urease enzyme, while β-glucosidase was lower in the agricultural field in comparison to the other stands, where no significance difference were reported Regarding the acid phosphomonoesterase, significant differences were found between A and X, while the remaining stands showed similar values. We have computed the AI3 index as a reference for soil quality, which showed a better values in A compared to X, indicating that soil quality increased up to 4 times compared to the beginning of the experiment, while the other values were comparable. In conclusion, our preliminary results showed that afforestation of tired agricultural soils improved soil organic matter and soil quality, in particular in the stands where Italian alder were planted.

Litter decomposition is an ecosystem process that is regulated by a multitude of factors and by their complex interactions. Current decomposition paradigms do not always offer a coherent view of the process because it can be hardly understood without a thorough analysis of interacting factors. Thus, there is a need to further understand the mechanics of litter decay with a comprehensive approach, especially in temperate forest ecosystems where decomposition plays a crucial role in regulating them as source or sink of CO2. Therefore, the aim of this work was to identify the interactions between chemical, biological and soil fauna variables in order to discern driving variables and the changes in their interactions during long-time (1300 days) beech leaf litter decomposition. In order to investigate patterns of variation and co-variation within and between datasets, we used Two-block Partial Least Squares, helping us to interpret the decomposition process with a systemic approach. Our key findings showed that the decomposition process of beech litter in two Mediterranean forests was driven by litter quality at the beginning and in the later stages of decomposition, while edaphic and climatic factors were implied in the central steps, with a dramatic change of scenario around 2.5 years. Simultaneous and interacting changes in chemical variables, extracellular enzyme activities, and soil fauna were shown, with a significant role of lignocellulosic components and enzymes involved in their degradation, Mn residual weight, and abundance of Collembola.

Forests are the most important land ecosystems that can mitigate the earth’s ongoing climate change through their ability to sequester CO2 as C stock in forest biomass and soil. Short-rotation deciduous hardwoods or N2-fixing species are ideal candidates for afforestation and reforestation, given that most of the carbon accumulates in the first 30 years. Alders match both of the above-mentioned features, and Italian alder, which is less dependent on riparian habitats and more drought tolerant, is an ideal candidate. Despite this, few studies exist of this tree species and its effect on soil organic matter. In this study, we focused on litter input and leaf litter decomposition dynamics, forest floor leaf litter and topsoil (0–5 cm) organic matter, and patterns of covariation from litter to topsoil. The leaf litter was rich in nitrogen and decomposed quickly (k = 0.002 day−1). There was a large organic carbon stock, which varied in the leaf litter (from 1.7 ± 0.3 Mg/ha in January to 0.4 ± 0.1 Mg/ha in July) and was stable in the topsoil (on average 28.6 ± 1.5 Mg/ha). Stocks for total nitrogen, cellulose, lignin, water and ethanol extractables, and total phenols were also evaluated. In order to investigate patterns of covariation in these stocks from litter to soil, we used two-block partial least squares. The first axis showed that from January to July there was a reduction of total nitrogen, lignin and cellulose in the forest floor leaf litter, while in the topsoil there was a decrease in water extractables and total organic carbon. The second axis showed minor phenomena involving phenols, water and ethanol extractables, and total N. The fast turnover of dissolved organic matter fractions (water and ethanol extractables), linked with cellulose and lignin dynamics, might suggest that within the Italian alder ecosystem there is a reasonably fast formation of stable C compounds in the soil. Thus, Italian alder is an ideal species for afforestation and reforestation, which could be particularly interesting for land-use policies.

Soil is the largest terrestrial carbon pool, of which about 20% is stored in forest soils. The dominant species of each forest typology strongly influence structure, function and activity of soil microbial communities. There is a substantial lack of studies integrating analyses on soil chemical-physical features and biological activities in soil deep layers under different canopies. Accordingly, the present work aims to (a) characterize and compare organic matter quality and quantity along the profiles of soils developed on the same parent material, but under different canopies and (b) evaluate the distribution and activity of the microbial biomass along the soil profiles. The research was carried out in a mountain ecosystem in southern Italy, along the decomposition’s continuum. In order to reach our goals we evaluated Soil Organic Matter (SOM), Soil Total Nitrogen and Soil Total Carbon and different C fractions, microbial and active fungal biomass, enzyme activities involved in SOM degradation. The results showed that in the soil, the chemical-physical variables made the difference in the A horizons, while some enzyme activities explained most of the diversity in the deepest horizons. Overall, the two stands showed differences, highlighting a separation of the two profiles, despite the same parent material. Soil is the largest terrestrial carbon pool, of which about 20% is stored in forest soils. The dominant species of each forest typology strongly influence structure, function and activity of soil microbial communities. There is a substantial lack of studies integrating analyses on soil chemical-physical features and biological activities in soil deep layers under different canopies. Accordingly, the present work aims to (a) characterize and compare organic matter quality and quantity along the profiles of soils developed on the same parent material, but under different canopies and (b) evaluate the distribution and activity of the microbial biomass along the soil profiles. The research was carried out in a mountain ecosystem in southern Italy, along the decomposition's continuum. In order to reach our goals we evaluated Soil Organic Matter (SOM), Soil Total Nitrogen and Soil Total Carbon and different C fractions, microbial and active fungal biomass, enzyme activities involved in SOM degradation. The results showed that in the soil, the chemical-physical variables made the difference in the A horizons, while some enzyme activities explained most of the diversity in the deepest horizons. Overall, the two stands showed differences, highlighting a separation of the two profiles, despite the same parent material.