Comment on egusphere-2022-306

In their paper "Unrecognized threat to global soil carbon by a widespread invasive species" O'Bryan et al. (2021) suggested that wild boar (also named feral pigs or wild pigs) and their grubbing reduce global soil organic carbon (SOC) stocks. In this study models were used to estimate global wild boar abundance and postulated additional CO2 emissions due to wild boar bioturbation. However, the authors ignored experimental evidence about the effects of wild boar on SOC that points in a completely different direction altogether.

<strong class="journal-contentHeaderColor">Abstract.</strong> Soil organic matter (SOM) plays an important role in the global carbon cycle, especially in alpine ecosystems. However, ongoing forest expansion in high elevation systems potentially alters SOM storage through changes in organic matter (OM) inputs and microclimate. In this study we investigated the effects of an <em>Picea abies</em> L. afforestation chrono-sequence (40&ndash;130 years) of a former subalpine pasture in Switzerland on soil organic carbon (SOC) stocks and SOM dynamics. We found that SOC stocks remained relatively constant throughout the chrono-sequence, with comparable SOC stocks in the mineral soils after afforestation and previous pasture (SOC₄₀-year-old forest = 11.6 &plusmn; 1.1 kg m^&minus;2, SOC₁₃₀-year-old forest = 11.0 &plusmn; 0.3 kg m^&minus;2, and SOC pasture = 11.5 &plusmn; 0.5 kg m^&minus;2). However, including the additional carbon of the organic horizons in the forest, reaching up to 1.7 kg m^&minus;2 in the 55-year-old forest, resulted in a slight in-crease in overall SOC stocks following afforestation. We found that the soil C:N ratio in the mineral soil increased in the topsoil (0&ndash;5cm) with increasing forest stand age, from 11.9 &plusmn; 1.3 in the grassland to 14.3 &plusmn; 1.8 in the 130-year-old forest. In turn, we observed a decrease in soil C:N ratio with increasing depth in all forest stand ages. This suggests that litter-derived organic matter (C:N from 35.1 &plusmn; 1.9 to 42.4 &plusmn; 10.8) is likely incorporated and translocated from the organic horizon to the mineral topsoil (0&ndash;10 cm) of the profiles. As roots had very high C:N ratios (pasture 63.5 &plusmn; 2.8 and forests between 54.7 &plusmn; 3.9 and 61.2 &plusmn; 2.9), particulate root-derived organic matter seems to have a minor influence on forest soil C:N ratio and thereby on SOC stock accumulation in the mineral soil. These results suggest that, although the afforestation only moderately affected the SOC stock, there is an apparent alteration in the SOC dynamics through changes of the litter composition caused by the vegetation shift. We conclude that spruce afforestation on a former subalpine pasture does not necessarily change the total SOC stock and that consequently there is no SOC sequestration on a decadal to centennial scale.

Abstract. Soil organic matter (SOM) plays an important role in the global carbon cycle, especially in alpine ecosystems. However, ongoing forest expansion in high-elevation systems potentially alters SOM storage through changes in organic matter (OM) inputs and microclimate. In this study, we investigated the effects of an Picea abies L. afforestation chrono-sequence (0 to 130 years) of a former subalpine pasture in Switzerland on soil organic carbon (SOC) stocks and SOM dynamics. We found that SOC stocks remained constant throughout the chrono-sequence, with comparable SOC stocks in the mineral soils after afforestation and previous pasture (SOC forest40 = 11.6 ± 1.1 kg m−2, SOC forest130 = 11.0 ± 0.3 kg m−2 and SOC pasture = 11.5 ± 0.5 kg m−2). However, including the additional carbon of the organic horizons in the forest, reaching up to 1.7 kg m−2 in the 55-year old forest, resulted in an increase in the overall SOC stocks following afforestation. We found that the soil C:N ratio in the mineral soil increased in the topsoil (0–5 cm) with increasing forest stand age, from 11.9 ± 1.3 in the pasture to 14.3 ± 1.8 in the 130-year old forest. In turn, we observed a decrease in the soil C:N ratio with increasing depth in all forest stand ages. This suggests that litter-derived organic matter (C:N from 35.1 ± 1.9 to 42.4 ± 10.8) is likely to be incorporated and translocated from the organic horizon to the mineral topsoil (0–10 cm) of the profiles. Due to the high root C:N ratio (pasture 63.5 ± 2.8 and forests between 54.7 ± 3.9 and 61.2 ± 2.9), particulate root-derived organic matter seems to have a rather small effect on forest soil C:N ratios, as well as on SOC accumulation in the mineral soil. These results suggest that, although afforestation does not change the SOC stock in the mineral soil, there is an apparent alteration in the SOM dynamics through changes in the litter composition caused by the vegetation shift. We conclude that, at our study site, spruce afforestation on a former subalpine pasture does not change the total SOC stock and that, consequently, there is no additional SOC sequestration on a decadal to centennial scale.

Although soil organic matter (SOM) forms a small portion of the soil body. Nevertheless, it is the most important component of the soil ecosystem, as well as of the carbon global cycle. In the semi-arid environment, there has been little research on the spatial distribution of SOM and soil organic carbon (SOC) stock. In this study, stratified random samples of total 30 soils were collected from two different soil depth (topsoil, subsoil) of Al Balikh plain and used for mapping the spatial variability of SOC and to estimating the SOC stock. The result showed that the values were relatively homogenate, with the normal decreasing trend with increasing the depth. The standard deviation (Std. D) for both SOC and SOC stock indicates homogeneous and absence of outliers values, whereas the coefficient of variation (C.V) indicates non-dispersion and clustering of values around the average. SOC was 0.38%, 0.17% in topsoil and subsoil respectively; the corresponding averages of SOC stock were 1.23 kg·m-2 and 1.14 kg·m-2 respectively, these values reflecting typical characteristics of poor SOC semi-arid soil. The correlation between SOC and SOC stock was (R2 = 0.996, p < 0.001) in topsoil and it was (R2 = 0.941, p < 0.001) for subsoil. The semivariograms were indicated that both SOC and SOC stock were best fitted to the exponential model. Nugget, range, and sill were equal to 0.002, 0.036, and 0.044, respectively for SOC in topsoil, and 0.014, 0.071, and 0.081, for SOC in the subsoil. For SOC stock, it was 0.0, 0.036, and 0.0508, respectively in topsoil. In the subsoil, the values were 0.1899, 0.086, and 4.159, respectively. SOC and SCO stock in both two layers are shown a strong spatial dependence, for which were 4.3, 17.2 for SOC in topsoil and subsoil respectively, and 0.0, 4.5 for SOC stock in topsoil and subsoil respectively, thus, which can be attributed to intrinsic factors.

Tools used by agencies and organizations like the Forest Service (FS), the Natural Resource Conservation Service (NRCS), the Nature Conservancy (TNC), and the Central Appalachian Spruce Restoration Initiative (CASRI) to help guide red spruce (Picea rubens) ecosystem restoration within Central Appalachia could better address outcomes from management practices implemented in terms of soil organic carbon (SOC) stock changes. These high-elevation landscapes have a natural capacity to produce diverse ecosystem services that affect humans, animals, and plants alike. Ecological site descriptions (ESD) are an important tool used to restore impacted landscapes and provide detailed management prescriptions specific to red spruce ecological sites (ES) and ecological states occurring in the Monongahela National Forest (MNF). Previous studies have evaluated ESD utility for identifying ecologic communities and restoration pathways primarily in western rangelands, but none have focused on Central Appalachian landscapes. Research associated with SOC stocks and forest ESD is minimal. Studies have analyzed how SOC can benefit ecosystem services, yet none seek to compare SOC stocks across multiple ecological states to address both restoration pathways and management outcomes that could potentially increase SOC sequestration capacity while restoring impaired ecosystem services. 120 individual plots within the dual extent of the Spodic Shale Upland Conifer Forest (SSUCF) and Spodic Intergrade Shale Upland Hardwood and Conifer Forest (SISUHCF) ES were analyzed using soil profiles and ecosystem descriptions sampled between 2009 and 2021. Soil samples were analyzed using dry combustion to determine SOC percent weight and further used to calculate the SOC stock to 100 cm in depth where applicable. Here, mean SOC stock, SOC stock variance, and the relationship between percent conifer canopy cover and SOC stocks of ecological states of both ES were compared and discussed. Analyses showed differences between ES total SOC (TSOC) stock (p < 0.0001), O horizon SOC (OSOC) stock (p < 0.0001), and spodic horizon SOC (SPSOC) stocks (p = 0.001), while mineral SOC (MSOC) showed no difference (p = .628) (Table 4.1). At the ecological state level, there were only two significant differences when examining TSOC (p = 0.038) and OSOC (p = 0.001) stocks (Table 4.2). The SSUCF demonstrated higher variance than the SISUHCF in TSOC stock (p-value < 0.0001) and OSOC stock (Table 4.3, p-value < 0.0001). Conversely, there was no significant difference between ES when comparing MSOC stock variance (p-value = 0.971) and SPSOC stock variance (p-value = 0.126). Regression analysis used a fixed model and showed

Switchgrass ( Panicum virgatum) and miscanthus ( Miscanthus spp.) are C4 perennial biomass crops (PBCs) commonly cultivated in Ontario. The 13C natural abundance technique was used to assess the following: (i) quantify the proportion of C4-derived soil organic carbon (SOC) stock (Mg C ha−1) within the SOC stocks of the whole soil, (ii) determine the C4-derived SOC sequestration rate, and (iii) quantify C4-derived SOC stocks in soil particle size fractions. This was achieved by analyzing the δ13C values in SOC of both the whole soil and various soil particle size fractions in PBC fields that were converted from agriculture. At Guelph after 11 years of PBC, C4-derived SOC stock as a proportion of SOC stock in the whole soil accounted for 30% in switchgrass and 31% in miscanthus fields, while at Elora it comprised 41% in switchgrass and 50% in miscanthus. However, in Burlington after 4 years, C4-derived SOC represented 10%–12% in PBC fields compared to 17% in agriculture fields. The C4-derived SOC sequestration rates were 0.8, 0.7, and 0.4 Mg C ha−1 year−1 in switchgrass, miscanthus, and agriculture fields, respectively at the Guelph site. In miscanthus fields, lower C4-derived SOC stock in macro soil particles were observed, yet higher percentages were observed in micro soil particles and the silt plus clay fractions compared to switchgrass and agriculture fields. Agriculture fields had higher C4-derived SOC in macro soil particles, indicating lower SOC stability. Therefore, cultivating PBCs on low-productive agricultural lands could contribute to climate change mitigation practices due to increased SOC stability.

Changes in quality and quantity of soil organic matter stocks resulting from wastewater irrigation in formerly forested land

Quantifying changes in soil organic carbon (SOC) stocks and other soil properties is essential for understanding how soils will respond to land management practices and global change. Although they are widely used, comparisons of SOC stocks at fixed depth (FD) intervals are subject to errors when changes in bulk density or soil organic matter occur. The equivalent soil mass (ESM) method has been recommended in lieu of FD for assessing changes in SOC stocks in mineral soils, but ESM remains underutilized for SOC stocks and has rarely been used for other soil properties. In this paper, we draw attention to the limitations of the FD method and demonstrate the advantages of the ESM approach. We provide illustrations to show that the FD approach is susceptible to errors not only for quantifying SOC stocks but also for soil mass-based properties such as SOC mass percent, C:N mass ratio, and δ13 C. We describe the ESM approach and show how it mitigates the FD method limitations. Using bulk density change simulations applied to an empirical dataset from bioenergy cropping systems, we show that the ESM method provides consistently lower errors than FD when quantifying changes in SOC stocks and other soil properties. To simplify the use of ESM, we detail how the method can be integrated into sampling schemes, and we provide an example R computer script that can perform ESM calculations on large datasets. We encourage future studies, whether temporal or comparative, to utilize sampling methods that are amenable to the ESM approach. Overall, we agree with previous recommendations that ESM should be the standard method for evaluating SOC stock changes in mineral soils, but we further suggest that ESM may also be preferred for comparisons of other soil properties including mass percentages, elemental mass ratios, and stable isotope composition.

Benchmark values for forest soil carbon stocks in Europe: Results from a large scale forest soil survey

Changes in soil organic and inorganic carbon stocks in deep profiles following cropland abandonment along a precipitation gradient across the Loess Plateau of China

Studies of tree species effects on soils have revealed a significant impact on soil organic carbon (SOC) stocks and the carbon (C) distribution between forest floor and mineral soil, but the underlying mechanisms including the roles of litter traits, soil properties, and microbiome remain unclear. To address this challenge, we tested the effect of six common European tree species on the quality and nutrient availability of soil organic matter (SOM) as perceived by the naturally assembled microbial communities and explored the possible links between soil enzyme activities (EAs), microbial resource limitation, and microbial community with SOC stocks. The six studied tree species (Acer pseudoplatanus L., Fraxinus excelsior L, Fagus sylvatica L., Quercus robur L., Tilia cordata L., and Picea abies L.) were planted in common garden monocultures more than 40 years ago at six sites across Denmark. In forest floor, microbial biomass C, fungal and total microbial biomass and fungi to bacteria (F/B) ratios decreased with decreasing litter quality. Ecoenzymatic stoichiometry and relative EAs indicated that microbes in spruce forest floor were more limited by phosphorus (P) than in maple, lime, and beech, while microbes in mineral soils were less P limited in spruce than in other monocultures. Mineral soil under the tree species associated with arbuscular mycorrhizal (AM) fungi had higher microbial C and P limitation, bacterial biomass and total microbial biomass than under tree species associated with ectomycorrhizal (EcM) fungi. Our results indicated that tree species with high-quality litter (i.e. AM-associated trees) had (i) higher microbial biomass and less nutrient limitation that were conducive to higher decomposition rates and lower C stocks in the forest floor, and (ii) such tree species could lead to both greater stabilization of mineral soil C by mineral-associated OM formation and greater microbial mineralization of SOM with higher microbial resource demand. The results suggest that tree species-mediated EA, microbial resource limitation and microbial community composition are important drivers of stocks and vertical distribution of SOC among tree species and between the two types of associated mycorrhiza.

&amp;lt;p&amp;gt;Drought-sensitive European beech forests are increasingly challenged by climate change. Admixing other, preferably more deep-rooting, tree species has been proposed to increase the resilience of beech forests to summer drought. This might not only alter soil water dynamics and availability, but also soil organic carbon (SOC) and total nitrogen (TN) storage in soils. Since information of these effects is scattered, our aim was to synthesize results from studies that compared SOC/TN stocks of beech monocultures with those of mixed beech stands as well as of other monocultures. We conducted a meta-analysis including 40 studies with 208, 231 and 166 observations for forest floor, mineral soil and the total soil profile, respectively. Pure conifer stands had higher SOC stocks compared to beech in general, especially in the forest floor with up to 200% (larch forests). Other broadleaved tree species (ash, oak, lime, maple, hornbeam) showed in comparison to beech lower SOC storage in the forest floor, with little impact on total stocks. &amp;amp;#160;Similarly, for mixed beech-conifer stands we found significantly increased SOC stocks of &amp;gt;10% and a small increase in TN stocks of approx. 4% compared to beech monocultures, which means a potential SOC storage increase of &amp;gt;0.1 t ha&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt;yr&amp;lt;sup&amp;gt;-1 &amp;lt;/sup&amp;gt;(transformation of mineral soil to 100 cm depth). In contrast, mixed beech-broadleaved stands did not show a significant change in total SOC stocks. Currently, the influence climatic and soil parameters on SOC changes due to admixture of other tree species is analyzed based on this dataset. This is expected to facilitate an assessment which mixtures with beech have the largest potential towards increasing SOC stocks.&amp;lt;/p&amp;gt;

&amp;lt;p&amp;gt;&amp;amp;#8216;Strategic objective 1&amp;amp;#8217; of the United Nations Convention to Combat Desertification (UNCCD) aims to improve conditions of affected ecosystems, combat desertification/land degradation, promote sustainable land management, and contribute to land degradation neutrality. The indicator &amp;amp;#8216;Proportion of land that is degraded over total land area&amp;amp;#8217; (SO1) is compiled from three sub-indicators: &amp;amp;#8216;Trends in land cover&amp;amp;#8217; (SO1-1), &amp;amp;#8216;Trends in land productivity or functioning of the land&amp;amp;#8217; (SO1-2), &amp;amp;#8216;Trends in carbon stocks above and below ground&amp;amp;#8217; (SO1-3).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Soil organic carbon (SOC) stock can be adopted as the metric of SO1-3, until globally accepted methods for estimating the total terrestrial system carbon stocks will be elaborated. SOC can be considered as one of the most important properties of soil, which shows not just spatial but temporal variability. According to our previous results in the topic, UNCCD default data of SOC stock for Hungary is strongly recommended to be replaced with country specific estimation of SOC stock.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;SOC stock maps were compiled in the framework of DOSoReMI.hu (Digital, Optimized, Soil Related Maps and Information in Hungary) initiative, predicted by proper digital soil mapping (DSM) method. Reference soil data were derived from a countrywide monitoring system. The selection of environmental covariates was based on the SCORPAN model. The elaborated SOC stock mapping methodology have two components: (1) point support modelling, where SOC stock is computed at the level of soil profile, and (2) spatial modelling (quantile regression forest), where spatial prediction and uncertainty quantification are carried out using the computed SOC stock values.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We analyzed how SOC stock changed between 1998 and 2016. &amp;amp;#160;Nationwide SOC stock predictions were compiled for the years 1998, 2010, 2013, and 2016. For the intermediate years, we do not recommend to calculate SOC stock values, because we have no information on the dynamics of change in the intervening years. Based on the 1998 SOC stock prediction, we compiled a SOC stock map for 2018, using only land use conversion factors, according to the default data conversion values.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;According to the elaborated scheme during the respective period, significant changes cannot be detected, only tendentious SOC stock changes appear. Based on our results, we recommend to use spatially predicted layers for all years when data are available, rather than calculating SOC stock change based on land use conversion factors.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Acknowledgment:&amp;lt;/strong&amp;gt; Our research was supported by the Hungarian National Research, Development and Innovation Office (NKFIH; K-131820) and by the Premium Postdoctoral Scholarship of the Hungarian Academy of Sciences (PREMIUM-2019-390) (G&amp;amp;#225;bor Szatm&amp;amp;#225;ri).&amp;lt;/p&amp;gt;

Improvements in farmland soil organic carbon (SOC) stock enhance crop yield and soil fertility while mitigating climate change. Rational fertilization in agricultural production is crucial for safeguarding SOC stock. In this study, field experiments were conducted with different ratios of chemical fertilizer reduction and organic fertilizer substitution for three consecutive years (2018-2020) to explore their effects and interlinkages on SOC fractions, soil properties and SOC stock. The results showed that organic fertilizer substitution increased SOC and its fractions content, SOC stock (by 3.98-12.98% and 7.15-18.13%) and soil fertility index (by 11.76-49.26% and 33.33-91.47%) compared to conventional fertilization in 2019 and 2020, while chemical fertilizer reduction had the opposite effect. Moreover, soil properties (except total nitrogen to total phosphorus ratio, N/P) and SOC fractions significantly affected SOC stock, with SOC fractions contributing more than soil properties. The high sensitivity of microbial biomass carbon (MBC) and dissolved organic carbon (DOC) can indicate changes in soil carbon pool. Structural equation modeling (SEM) revealed that organic fertilizer substitution increased SOC content and stock by increasing SOC fractions [recalcitrant organic carbon (ROC) and labile organic carbon (LOC) fractions] content and soil fertility. Our study revealed the corresponding mechanisms of the two fertilization modes affecting SOC stock changes. The use of organic fertilizer substitution is recommended to increase SOC stocks and soil fertility in wheat fields. © 2023 Society of Chemical Industry.

Reduced carbon assimilation by trees is often considered to lower the overall carbon sink function of drought-stressed forests. However, soil organic carbon (SOC) stocks may respond differently to drought than ecosystem carbon flux dynamics, leading to imprecise predictions of soil carbon sequestration when one value is inferred from the other. As a major component of soil organic matter, SOC is the largest actively cycling terrestrial carbon reservoir, and thus fulfills various important ecosystem services. Yet, there is uncertainty about how SOC quantity and quality respond to drought in temperate forests. This study addressed the depth distribution of SOC stocks and soil organic matter stability in a forest exposed to artificial drought for five consecutive growing seasons below clusters of temperate mature deciduous beech (Fagus sylvatica L.) and coniferous spruce (Picea abies (L.) Karst.). In addition to SOC stock determination, we measured concentrations of water-extractable organic carbon (WEOC), performed density fractionation, and determined beta values of SOC (slopes of linear regressions between δ13C of soil and log-transformed SOC content throughout soil depth profiles). Following drought, SOC stocks down to 30 cm depth increased by a factor of 1.5 under P. abies while they did not change with drought under F. sylvatica. Under both species, SOC stocks in the mineral topsoil (0–5 cm soil depth) increased by >80 % with drought, increasing the relative contribution of this thin depth section to total SOC from 5 % to >30 %. At 5–15 cm soil depth, SOC stocks decreased with drought under F. sylvatica but not under P. abies. With drought, carbon in the free light fraction (fLF) increased under F. sylvatica but declined marginally under P. abies. Results from density fractionation and beta values suggest decreased soil organic matter stability under F. sylvatica and increased stability under P. abies. Greater SOC accumulation suggests that the belowground carbon sink strength of drought-stressed forests increases, which contrasts with reduced ecosystem carbon uptake under drought.