Soil Stocks Research Articles

Impact of Soil Organic Layer Thickness on Soil-to-Atmosphere GHG Fluxes in Grassland in Latvia

Drained organic soils in agricultural land are considered significant contributors to total greenhouse gas (GHG) emissions, although the temporal and spatial variation of GHG emissions is high. Here, we present results of the study on soil-to-atmosphere fluxes of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) from drained organic (fen) soils in grassland. A two-year study (from July 2021 to June 2023) was conducted in three research sites in Latvia (Europe’s hemiboreal zone). Soil total respiration (Rtot), CH4 and N2O fluxes were determined using a manual opaque chamber technique in combination with gas chromatography, while soil heterotrophic respiration (Rhet) was measured with a portable spectrometer. Among research sites, the thickness of the soil organic layer ranged from 10 to 70 cm and mean groundwater level ranged from 27 to 99 cm below the soil surface. Drained organic soil in all research sites was a net source of CO2 emissions (mean 3.48 ± 0.33 t CO2-C ha−1 yr−1). No evidence was obtained that the thickness of the soil organic layer (ranging from 10 to 70 cm) and OC stock in soil can be considered one of the main affecting factors of magnitude of net CO2 emissions from drained organic soil. Drained organic soil in grassland was mostly a source of N2O emissions (mean 2.39 ± 0.70 kg N2O-N ha−1 yr−1), while the soil both emitted and consumed atmospheric CH4 depending on the thickness of the soil organic layer (ranging from −3.26 ± 1.33 to 0.96 ± 0.10 kg CH4-C ha−1 yr−1).

Root-derived carbon stocks in formerly deep-ploughed soils – A biomarker-based approach

Roots can add significant amounts of carbon (C) to the subsoil, which improves soil physical properties and can mitigate climate change. About 5% of croplands in Germany have been deep-ploughed (30–120 cm) at least once. This can provide better root access to the subsoil and may increase yields, but little is known on the fate of root-derived C in the subsoil (at depth greater than 30 cm) after deep-ploughing. We hypothesized that five decades after deep-ploughing, root-derived C stocks were higher than conventionally ploughed treatments due to better root development. We analysed suberin and cutin monomers as tracers for root- and shoot-derived C at three former deep-ploughed sites in Northern Germany with different soil textures and deep-ploughing depths. Concentrations of suberin monomers in the soil positively correlated with root biomass; this was most pronounced at one sandy site, but had higher variability at the other two sites due to crops with different root systems in the crop rotation, lower root development, and more favourable conditions for C decomposition. Suberin contributed more to the bulk soil organic carbon (SOC) stocks than cutin throughout the soil profile at all sites. The contribution of suberin monomers to the bulk SOC stock at silty site Banteln and the sandy site Essemühle was 38% higher in the deep-ploughed plots than at the reference plot, respectively, these differences were most visible in the subsoil of Essemühle. We conclude that as C stocks and root development increase, suberin SOC stocks also increase, especially in sandy subsoils with low pH.

Human population density and blue carbon stocks in mangroves soils

Mangrove soils provide many important ecosystem services such as carbon sequestration, yet they are vulnerable to the negative impacts brought on by anthropogenic activities. Research in recent decades has shown a progressive loss of blue carbon in mangrove forests as they are converted to aquaculture, agriculture, and urban development. We seek to study the relationship between human population density and soil carbon stocks in urban mangrove forests to quantify their role in the global carbon budget. To this end, we conducted a global analysis, collecting mangrove soil carbon data from previous studies and calculating population density for each study location utilizing a recent database from the European Commission. Results indicate population density has a negative association with mangrove soil carbon stocks. When human population density reaches 300 people km−2, which is defined as ‘urban domains’ in the European Commission database, mangrove soil carbon is estimated to be lower than isolated mangrove forests by 37%. Nonetheless, after accounting for climatic factors in the model, we see the negative relationship between population density and soil carbon is reduced and is even non-significant in mixed effects models. This suggests population density is not a good measure for the direct effects of humans on mangrove ecosystems and further implies mangrove ecosystems in close proximity to very high population density can still possess valuable carbon stocks. Our work provides a better understanding of how soil carbon stocks in existing mangrove forests correlate with different levels of population density, underscores the importance of protecting existing mangroves and especially those in areas with high human population density, and calls for further studies on the association between human activities and mangrove forest carbon stocks.

Carbon and nitrogen stock in soils of subtropical urban forests: Isotopic δ13C and δ15N indicators for nature-based solutions in a megacity

Urban forest soils play a pivotal role in enhancing the environmental sustainability of cities, contributing to various natural processes, including plant–microbe interactions, microbial activity, and the decomposition of organic matter. Consequently, urban forest soils emerge as effective NBS, underscoring their potential to mitigate environmental challenges and foster sustainable urban ecosystems. In these sense, this manuscript aimed at evaluating how soil attributes of the urban forests of São Paulo, Brazil, with different adjacent land uses, influence the capacity to store the excess of C and N from anthropogenic emissions, making this ecosystem an important reservoir of urban anthropogenic emissions. Three hundred urban forest soil samples were collected from the surface to a depth of 50 cm. All samples were analyzed for C and N content (and their stable isotopes). In addition, granulometric tests were also carried out to classify the soils. It was found that the most central forest fragment has the highest C and N contents at all analyzed depths, probably due to the association with physical aspects of soil texture. For all layers, the most central fragment soil sample, the only one with a clay soil, presented approximately twice as many elements when compared to the soil samples of the other sites. In general, C and N stocks (and their stable isotopes, δ13C and δ15N, respectively) varied significantly in forests located in the center-periphery direction (%N - F = 24.58, p < 0.05; %C - F = 22.48, p < 0.05; δ15N - F = 4.27, p < 0.05; δ13C - F = 19.8, p < 0.05; C/N - F = 14.56, p < 0.05). This more central forest fragment with higher vehicle emissions showed greater potential to store these atmospheric elements and with greater neutralizing efficiency than the other forest fragments. δ13C and δ15N contents together with the C:N ratio indicated the efficiency of biogeochemical cycling, through decomposition, in different urban forest fragments. More peripheral fragments showed high efficiency in C and N cycling along the analyzed soil profiles, while in the most superficial layer, the most central fragment was highly efficient. These shed light results how integrating NbS principles into strategic urban planning and city-level climate policies can bolster the effectiveness of urban green areas. The integration not only promotes carbon sequestration and efficient nutrient cycling in the soil but also fosters sustainable practices, contributing to a more resilient urban landscape.

Conversion of Marginalized Tea Lands into Timber Plantations and its Impact on Soil Organic Carbon Content: A Case Study in Central Highlands of Sri Lanka

Tea (Camellia sinensis) plantations can be in production for about 100 years under well-managed conditions. However, soil degradation makes the tea lands marginalized and uneconomical. Land use conversion is an option for marginalized tea lands that could gain numerous environmental and economic benefits. This study estimated and compared the soil organic carbon stocks (SOC) in the surface soil layer (0–30 cm) of marginalized tea lands against those converted to timber plantations in the Badulla and Nuwaraeliya districts. Disturbed and undisturbed soil samples were collected from the top layer (0-30 cm) of the selected sites of marginalized tea lands and those converted to timber plantations for analysis. Organic matter percentage in the site-level dry soil samples was analysed using the loss on ignition method. The final carbon stocks (t C ha-1) for samples from both land use types were then estimated using the SOC percentages and bulk density estimates. The carbon stocks for the soil samples collected at marginalized tea lands and the marginalized tea lands converted to timber plantations were then statistically analysed (descriptive statistics, normality tests and two-sample t-tests) using Minitab 21.2 software. The pH of marginal tea lands (5.34±0.14) and the marginal tea lands converted to timber plantations (5.20±0.13) was slightly acidic in nature. The bulk density estimates of marginalized tea lands and marginalized tea lands converted to timber plantations were very close and estimated as 1.15±0.02. The statistical analysis showed that overall mean SOC was significantly higher under the marginalized tea lands converted to timber plantations (155.3±10.0 t C ha-1) compared to the marginalized tea lands (126.7±5.8 t C ha-1). Land use conversion has increased the SOC stocks gradually after forest establishment. The increased carbon stocks could be attributed to consistent addition of litter layer on the soil, less soil disturbance, and retarded soil erosion contributing to the buildup of soil organic matter and SOC. On the other hand, low carbon stocks in the marginalized tea land could be attributed to unsustainable agronomical practices. Our findings suggest that converting marginalized tea lands in central highlands into timber plantations could enhance the sequestration of atmospheric CO2, significantly contributing to climate change mitigation.&#x0D; Keywords: Soil organic carbon, Land use change, Climate change, Tea plantations

VIS-NIR spectroscopy and environmental factors coupled with PLSR models to predict soil organic carbon and nitrogen

Soil profile organic carbon (OC) and total nitrogen (TN) are influenced by topographic attributes, and land use. The visible and near-infrared (Vis-NIR) spectroscopy method can be used for the prediction of OC and TN because it is reliable, nondestructive, fast, and cost-effective. VIS-NIR soil spectral and environmental data were combined with the Partial least squares regression (PLSR) model to examine the effect of topography attributes and land use on topsoil and subsoil OC and TN stocks. After this, based on the soil depth, 114 soil samples were collected from 0 to 20 cm (topsoil) and 20–50 cm (subsoil) under three land uses, as well as OC and TN, along with several soil properties including soil particles (sand, silt, clay), pH, and bulk density in both topsoil and subsoil samples were measured. A DEM with a resolution of 30 m was used to derive the topography factors and remote sensing data was used to calculate the vegetation index. Soils (0–50 cm) under orchard land use had the highest stock of SOC (7.4 kg m−2) as well as TN (2.4 kg m−2). There was a significant increase in the organic matter stock of soils located on the south aspect (8.3 kg m−2) compared to soils located on other aspects, particularly on the north aspect (3.9% increase). Soils on the south aspect contain higher soil-water contents and lower temperatures, resulting in a decrease in the decomposition of soil organic matter. A strong positive correlation was demonstrated between topography wetness index (0.57–0.63) and topography TN stocks (0.54–0.66) as well as the highest loading score among terrain attributes, suggesting that topography is the primary factor controlling SOC stocks, particularly subsoil stocks. Additionally, we found that soils on the south-facing aspects (N aspects) had the highest spectra. Additionally, the PLSR, which showed an R2 of 0.82, a RMSE of 0.15 %, and a RPD of 0.39 indicated excellent prediction capabilities for the OC content. We concluded that the PLSR model coupled with Vis-NIR spectroscopy is able to predict topsoil and subsoil OC and N content under different aspect slopes.

A random forest model to predict soil organic carbon storage in mangroves from Southern Colombian Pacific coast

Mangrove ecosystems serve as significant reservoirs of carbon stocks, particularly within the soil. However, these ecosystems face widespread degradation globally, underscoring the necessity for comprehensive technical insights to facilitate effective management. Enhancing the precision of soil organic carbon estimates within mangroves necessitates access to precise field data. Hence, this study aimed to assess the soil organic carbon stored in mangrove ecosystems along the southern Colombian Pacific coast. Employing a Random Forest model, constructed with data from 28 soil profiles spanning a depth of 2 m and 16 predictor variables, we evaluated the model's performance using repeated 10-fold cross-validation (with 5 random repeats). Uncertainty in carbon storage predictions was quantified through 95% confidence intervals. Our model successfully accounted for 88% of the variability in soil organic carbon storage. Depth, cation exchange capacity, pH, and sand content emerged as the most influential predictors of soil organic carbon storage variation. The study estimated a mean of 270.32 ± 52.75 t C ha−1, with a total carbon reserve of 25458088 t in the study area. Notably, this value falls below global estimates, suggesting a potential overestimation of carbon stocks in mangrove soils by existing global models.

Carbon and nitrogen stocks in Lithic soils: Environmental drivers and land-use implication in Brazilian dry ecosystems

Carbon and nitrogen stocks in Lithic soils: Environmental drivers and land-use implication in Brazilian dry ecosystems

Carbon and Nitrogen Stocks in Agricultural Soils under Different Natural Conditions and Management in Slovakia

Soil is a natural capital which supplies valuable ecosystem services including carbon and nitrogen storage. Agroecosystems play an important role in soil organic carbon (SOC) and soil total nitrogen (NT) accumulation. The aim of this study was to analyse SOC stock (SOCS) and NT stock (NTS) in relation to land use (arable land-AL, permanent grasslands-PG), management, soil depth, and selected soil properties of six soil subtypes (Rendzic Leptosol—LPrz, Dystric Cambisol—CMdy, Stagnic Cambisol—CMst, Haplic Fluvisol—FLha, Gleyic Fluvisol—FLgl, Haplic Chernozem—CHha) which are the most widespread in Slovakia. SOCS for a 50 cm deep soil profile ranged from 161 t.ha−1 in CHha to 59 t.ha−1 in FLgl in grasslands, and in arable lands from 111 t.ha−1 in CHha to 38 t.ha−1 in CMst. In grasslands, FLs and CMst showed the significantly lowest SOCS and NTS in comparison to CMdy, LPrz, and CHha. The mean soil NT content in arable land and grasslands was 2.21 g.kg−1 and 2.82 g.kg−1, respectively. ANOVA showed that soil subtype, land use, and site have significantly affected SOCS but not NTS. The correlation analysis revealed correlations between SOCS and NTS. SOCS was also correlated with C:N, pH, P, and K. This study should help to encourage practices to maintain soil C and soil properties and to ensure the sustainability of the functions of many soil types in Slovakia.

Effect of Forest Reclamation on Carbon Stocks and Respiration of Soils of Natural and Technogenic Ecosystems of Southern Karelia

Effect of Forest Reclamation on Carbon Stocks and Respiration of Soils of Natural and Technogenic Ecosystems of Southern Karelia

Relative and absolute difference in soil organic carbon stocks in grassland soils in Ireland: Impact of rock fragments, bulk density and calculation methods

On-farm management measures can sequester carbon by removing carbon dioxide from the atmosphere and storing it securely for long periods, in our soils. Estimates of soil organic carbon (SOC) stocks differ by calculation methods in terms of how they account for the coarse >2 mm fraction (i.e. rock fragments) of the soil. Irish grassland soils have variable volumetric stone contents and the extent of overestimation is unknown. Without best practice guidance, calculations of SOC in these soils are vulnerable to overestimation. The objectives of the present study for grassland soils in Ireland were to: 1) calculate mean SOC stocks using different methods by soil horizon and volumetric stone content class; 2) highlight relative differences in SOC stocks between these calculation methods and 3) compare normalised layer (Ly, fixed depth) versus horizon (Hz) type derived sampling data in terms of SOC stocks, to establish a best practice standardised sampling protocol. Five methods (M, 1–3 overestimate SOC stocks, whereas 4–5 are equivalent and are closest to reality) to calculate SOC stocks were used in combination with three soil databases i.e. Irish Soil Information System (I-SIS), the Heavy Soils Programme (HSP) and the Soil Quality Assessment Research Project (SQUARE). Objectives 1 and 2 were assessed using all three databases across methods, whereas objective 3 used the SQUARE database and M4. These data were sorted into volumetric stone content (%) classes (< 5%, 5–10%, 10–20%, 20–30% and > 30%) and sampling horizon (O, A, E, A/B, B, B/C, and C) with SOC stocks calculated using M1–5 equations. Results confirmed the high volumetric stone content assumption of grassland soils in Ireland (exception O horizons). Volumetric stone contents ranged from 0% to 36% for all horizons. When M1–5 are considered the average relative difference in SOC stocks between M1 and M5 was 18.9% (max: 388%), M2 and M5 was 7.7% (max: 60.7%) and M3 and M5 was 8.9 (max: 310%). In terms of Ly versus Hz sampling, no significant differences were found for the A horizons sampled. However, in Ireland, best practice should still utilise elements from both Ly (using an auger to get fixed depth soil samples; analysed for organic C) and Hz (using a soil profile pit to extract representative BD cores and to gather bulk samples for volumetric stone content analysis) approaches in the field. Laboratory methods used must fit with the needs and assumptions of equations M4/5 to calculate SOC stocks (thereby minimising overestimation and indeed underestimation).

Regulation of soil nitrogen cycling by shrubs in grasslands

Globally, grasslands are experiencing shrub expansion, but the effects of such plant community shifts on soil N cycling are seldom investigated. We explored the spatial heterogeneity of total soil N content by grid sampling down to a soil depth of 1.2 m, and analyzed net ammonification and nitrification rates in the top 0.2 m, during the growing season, in response to the patchiness of grasses and four shrub species in a grazed meadow on the Tibetan Plateau. The δ15N analyses of plant, soil, and mineral N (NH4+ and NO3−) revealed differences in N absorption strategies between shrubs and grasses. Heterogeneity in the total soil N distribution was common to at least a depth of 1.2 m. The lower δ15N values of the 1.2 m soil profile under shrubs were consistent with the lower δ15N values of leaves, aboveground litter, and roots of shrubs compared to grasses, reflecting the imprint of long-term N cycling in the soil as affected by shrubs. Increases in total soil N stock in the entire 1.2 m profile by 28%–32% under non-leguminous shrubs relative to grasses were ascribed to N accumulation in the lower 0.8 m. Consequently, deep sampling is essential for accurate estimation of soil N storage in shrubby meadows. The total N stock in soil under the leguminous shrub species was similar to that under grasses, which showed that the effect of shrubification on soil N storage depends on the shrub species. Nitrification rates in the topsoil under all shrubs were greater than under grasses, producing higher 15N-depleted NO3− contents under shrubs. Therefore, the lower δ15N signatures of shrub tissues compared to grasses reflected a preferential or adaptive uptake of NO3− by shrubs. The expansion of shrubs in grasslands creates high heterogeneity in N cycling and storage down to a considerable soil depth.

Can natural undisturbed revegetation restores soil organic carbon to levels under native climax vegetation under tropical semiarid climate?

AbstractLand‐use change has driven soil carbon stock losses in ecosystems worldwide. Implementing agricultural crops and exploiting forest resources trigger the breakdown of soil aggregates, thus exposing organic matter to microbial decomposition and enhancing carbon dioxide emissions, especially in biomes more susceptible to climate extremes as in the tropical semiarid regions. This study was based on the hypothesis that the undisturbed soil from the dry forest (Caatinga biome under natural revegetation in Brazilian semiarid) would have an improvement in the mass of macroaggregates and recover more than 50% of the soil C stock within 10 years. Thus, a field experiment was conducted to investigate soils from the Caatinga biome under native vegetation, “cowpea cropping” for over 30 years, and soil under natural revegetation for over 10 years, after conventional soil cultivation of maize and cowpea, to determine soil and soil‐aggregates carbon stocks and to estimate the recovery rate of these stocks. The proportional mass of aggregates of different sizes and the total stock of particulate organic carbon (POC) were also quantified. The results showed that soil under preserved native vegetation of dry forest Caatinga biome had higher total soil C stock (50.9 Mg ha−1) than that under cowpea cropping (23.2 Mg ha−1) and natural revegetation (45.1 Mg ha−1). The proportional mass of large macroaggregates was higher in soil under native vegetation for all depths. However, soil under cowpea cropping had lower C stocks in macroaggregates, and recovered roughly 63% of the original C stocks, while revegetation recovered 78% of the stock in 10 years. Although the conventional management system for cowpea monoculture aggravated losses in soil carbon stock by more than 50% of the original C stocks, dry forest under natural revegetation recovered 79% of this stock and almost 100% of POC stock in 10 years (~12 Mg ha−1). Furthermore, soil under undisturbed Caatinga dry forest achieved C stock levels equivalent to that of the global average range for semiarid tropical environments. The high recovery rate of C stock in forest soil under natural revegetation indicates the resilience potential of organisms responsible for structural protection of aggregates and the encapsulated soil organic matter content.

Variations in soil physico-chemical properties, soil stocks, and soil stoichiometry under different soil layers, the major forest region Liupan Mountains of Northwest China.

Liupan Mountains are an important region in China in the context of forest cover and vegetation due to huge afforestation and plantation practices, which brought changes in soil physio-chemical properties, soil stocks, and soil stoichiometries are rarely been understood. The study aims to explore the distribution of soil nutrients at 1-m soil depth in the plantation forest region. The soil samples at five depth increments (0-20, 20-40, 40-60, 60-80, and 80-100 cm) were collected and analyzed for different soil physio-chemical characteristics. The results showed a significant variation in soil bulk density (BD), soil porosity, pH, cation exchange capacity (CEC), and electric conductivity (EC) values. More soil BD (1.41 g cm-3) and pH (6.97) were noticed in the deep soil layer (80-100 cm), while the highest values of porosity (60.6%), EC (0.09 mS cm-1), and CEC (32.9 c mol kg-1) were reflected in the uppermost soil layer (0-20 cm). Similarly, the highest contents of soil organic carbon (SOC), total phosphorus (TP), available phosphorus (AP), total nitrogen (TN), and available potassium (AK) were calculated in the surface soil layer (0-20 cm). With increasing soil depth increment a decreasing trend in the SOC and other nutrient concentration were found, whereas the soil total potassium (TK) produced a negative correlation with soil layer depth. The entire results produced the distribution of SOCs and TNs (stocks) at various soil depths in forestland patterns were 0→20cm > 20→40cm > 40→60cm ≥ 60→80cm ≥ 80→100 cm. Furthermore, the stoichiometric ratios of C, N, and P, the C/P, and N/P ratios showed maximum values (66.49 and 5.46) in 0-20 cm and lowest values (23.78 and 1.91) in 80-100 cm soil layer depth. Though the C/N ratio was statistically similar across the whole soil profile (0-100 cm). These results highlighted that the soil depth increments might largely be attributed to fluctuations in soil physio-chemical properties, soil stocks, and soil stoichiometries. Further study is needed to draw more conclusions on nutrient dynamics, soil stocks, and soil stoichiometry in these forests.

Agroecology and Sustainable Citriculture

Sustaining citriculture amidst climate change has become a more daunting challenge considering simultaneous occurrence of multiple nutrient deficiencies triggered by depleting carbon stock of soil coupled with glaring impact of global warming on crop performance. Despite paradigm shift from purely inorganic to either organic or the combination of two, agroecological applications have always occupied a center-stage in sustainable citriculture. Development of microbial consortium (microbial concoction), rhizosphere hybridization, identification of microbial corridor and microbial hot spot in a field, besides storing carbon in plant vegetative framework, are some of the upcoming ways to realize the multiple dimensions of agroecology in sustaining the citriculture in years to come with emphasis on on-farm research. These efforts have to be further matched with other natural resource conserving options like conservation agriculture, organic farming, natural farming to name a few important ones

Productivity and Carbon Sequestration Potential of Arecanut Cultivars under Malnad Region of Karnataka

A field experiment was conducted at Agricultural and Horticultural Research Station, Thirtahhalli, KSNUAHS, Shivamogga, Karnataka, India, where arecanut palms of eight cultivars were evaluated for growth and yield performance for two consecutive years. The above-ground carbon sequestration potential and the soil carbon sequestration potential were assessed. The cultivar SAS-1 performed superior over other cultivars for all the growth parameters under study viz., palm height (9.02 m), stem diameter (18.07 cm), number of leaves (9.84) and internodal distance (17.38cm). However highest yield of fruit bunch per palm was recorded in the cultivar Mohithnagar (20.28 kg), followed by Thirthalli local (17.92 kg), Sreemangala (17.46 kg) and Tarikere Local (15.31 kg). Maximum chali/ dried ripe nut yield was recorded in Thithalli local (3.29 kg), Mohithnagar (3.10 kg) and Sreemangala (2.80 kg). Significantly higher values for standing biomass (15.49 t/ha), carbon stock (7.75 t/ha) and carbon sequestration (28.43 t/ha) were recorded in cultivar SAS-1. Soil parameters viz., bulk density, soil organic carbon and carbon stock in soil was analyzed in the rhizosphere of respective cultivars and maximum soil organic carbon (13.10g/kg and 11.43g/kg at 0-30 &amp; 30-60 cm depths respectively) and carbon stock (52.77 t/ha at 0-30 cm and 46.41 t/ha at 30-60 cm depth) were recorded in that of cultivar Sumangala.

Organic carbon stock of loess soil in the Lower Orkhon River

We aimed to study the soil organic carbon (SOC) stock of the loess soil and conducted samplings on a non-sandy loess terrace at the Lower Orkhon River in Shaamar Soum of Selenge Province. We made a soil profile with a 200 cm depth and collected 11 soil samples from each layer. The SOC was determined according to the Turin method in the Soil Research Laboratory at the Institute of Geography and Geoecology of the Mongolian Academy of Sciences. Results indicated that the mean SOC of the 200 cm soil profile ranges between 1.78-7.52 g/kg and the organic carbon content of the paleosol was higher than that of loess layers. The SOC was 0.7 kg/m-2 at 0-26 cm, 1.1 kg/m-2 at 26-42 cm, 1.8 kg/m-2 at 42-60 cm, 1.2 kg/m-2 at 60-80 cm, 0.7 kg/m-2 at 80-92 cm, 0.6 kg/m-2 at 92-113 cm, 0.8 kg/m-2 at 113-130 cm, 0.7 kg/m-2 at 130-140 cm, 0.7 kg/m-2 at 140-150 cm, 0.7 kg/m-2 at 140-150 cm, 1.1 kg/m-2 at 150-170 cm, and 3.3 kg/m-2 at 170-200 cm depth. The SOC in the topsoil layer was 9.9 t/ha at 0-30 cm, 36.6 t/ha at 0-60 cm, 57.7 t/ha at 0-100 cm, and 128.4 t/ha at 0-200 cm layer. The loess soil distributed in the northern part of Mongolia had a dark color and paleosol layer with high organic contents. Therefore, there was no tendency for SOC reduction when the depth increased. The amount of the SOC at a depth of less than 1 m was higher than the loess soil distributed in the Plateau region of China but the SOCs were similar at a depth of up to 1 m. Орхон голын адаг орчмын хэмэрлэг хөрсний органик нүүрстөрөгчийн нөөц ХУРААНГУЙ: Энэхүү судалгаа нь хэмэрлэг хөрсний органик нүүрстөрөгч (SOC)-ийн нөөцийг судлахад чиглэсэн бөгөөд хөрсний хээрийн судалгааг Сэлэнгэ аймгийн Шаамар сумын нутаг Орхон голын зүүн дэнжийн чулуугүй-элсэрхэг хэмэрлэг дээр гүйцэтгэсэн болно. Бид 200 см-ийн гүнтэй хөрсний зүсэлт хийж, хөрсний давхарга бүрээс нийт 11 ширхэг хөрсний дээж авч лабораторийн шилжилгээнд хамруулсан. SOC-ийг Тюрины аргын дагуу ШУА-ийн Газарзүй, геоэкологийн хүрээлэнгийн Хөрс судлалын лабораторид толорхойлсон. Судалгааны үр дүнгээр 200 см-ийн хөрсний SOC-ийн агууламж нь 1.78-7.52 г/кг-ийн хооронд хэлбэлзэх ба органик нүүрстөрөгчийн агууламжаар дарагдмал үе давхарга нь хэмэрлэг үе давхаргынхаас өндөр байна. SOC-ийн нөөц хөрсний 0-26 см-ийн давхаргад 0.7 кг/м-2 байсан бол 26-42 см-т 1.1 кг/м-2, 42-60 см-т 1.8 кг/м-2, 60-80 см-т 1.2 кг/м-2, 80-92 см-т 0.7 кг/м-2, 92-113 см-т 0.6 кг/м-2, 113-130 см-т 0.8 кг/м-2, 130-140 см-т 0.7 кг/м-2, 140-150 см-т 0.7 кг/м-2, 150-170 см-т 1.1 кг/м-2, ба 170-200 см-т 3.3 кг/м-2. Хэмэрлэх хөрсний 0-30 см өнгөн давхарга дахь SOC нөөц 9.9 тн/га, 0-60 см-т 36.6 тн/га, 0-100 см-т 57.7 тн/га бол 0-200 см-т 128.4 тн/га-гийн SOC нөөцтэй. Монгол орны хойд хэсгээр тархсан хэмэрлэг нь бараан өнгийн органикийн агууламжаар өндөр дарагдмал үеүдтэй учраас хөрсний гүн нэмэгдэхэд органикийн нөөц буурах зүй тогтол илрэхгүй, 1 метрээс дооших гүнд байгаа органик нүүрстөрөгчийн нөөцөөр Хятадын тэгш өндөрлөгт тархсан хэмэрлэг хөрснийхөөс өндөр. Харин 1 метр хүртэл гүн дэх нөөцөөр ойролцоо байна. Түлхүүр үгс: Хэмэрлэг хөрс, Хөрсний органик, Органик нүүрстөрөгчийн нөөц, Орхон гол, Шаамар

Pyrogenic carbon stocks are controlled by hydro-edaphic conditions in different forest types of the Northern Brazilian Amazon

Pyrogenic carbon (PyC) is considered one of the most stable components found in soil, corresponding to an expressive amount of refractory organic matter. Despite PyC significantly affect soil properties and processes, it has not been considered in global carbon cycle models due to uncertainties related to its quantification. To provide a greater understanding of PyC importance and its role in the Amazon carbon cycle, we estimate PyC soil stocks dispersed in the vertical soil column of different forest types (ombrophilous, deciduous and semideciduous) which occur in a hydro-edaphic gradient in the Maracá Island, northern Brazilian Amazon. Soil profiles measuring one meter in depth were sampled at 30 permanent plots arranged along the eastern portion of Maracá Island. The soil samples were fractioned in 10 subsamples corresponding to each 10 cm layer of the 1 m column and all of them were analyzed by the thermal method. We observed that PyC contributed to a mean of 1.92 % of the TOC (total organic carbon) present in the samples, and the PyC/TOC ratios have also evidenced a tendency to increase with depth. For absolute values, deciduous forests occurring in seasonal flooding soils had the highest stock of PyC (1.27 Mg PyC ha−1) when compared with ombrophilous (non-flooding) and semideciduous (predominance of deep rock fragments) forests. Our results indicate that atmospheric deposition was the major source of PyC entrances into the soil and that hydro-edaphic features control the spatial and vertical distribution of PyC in the study area. These findings are key points for reducing uncertainties regarding PyC stocks and improves our understanding on the carbon stocks variability in the Amazonian soil.

Improving soil thickness estimations and its spatial pattern on hillslopes in karst forests along latitudinal gradients

Soils provide storage space for nutrients and water, which governs many hydrological processes and functions in the natural ecosystems. Due to the complex soil-rock structures and localized climate, soil thickness and stock vary distinctly in different karst areas in southwest China. The difficulties of soil detection and the non-transparent underground soil-rock structures limit a deeper understanding of the spatial distribution of soils and their ecological functions. In this study, we employed electrical resistivity tomography (ERT) combined with soil probes to detect the spatial distribution of soils on hillslopes in karst forests in southwest China. The ERT surveys were conducted at six sites with 19 plots (20 × 30 m2) along latitudinal gradients. The results show that the ERT method performs well in visualizing the complex soil–bedrock interfaces and patchy distribution of soils in karst hillslopes, with remarkable consistency with the observed soil thickness and rock outcrops. Soil probes often hit rocks in soils (which we call a “rock barrier effect”), resulting in an underestimation of the mean soil thickness within the one-meter exploration depth by ∼0.18 m. This indicates that soil stock estimates derived from soil probes may be obviously underestimated in karst areas where the rock barrier effects are widespread. Meanwhile, the concept of equivalent soil thickness is proposed to remove the impact of scattered bedrock on soil stock estimation. For the six selected sites, the ERT-inferred mean soil thickness, equivalent soil thickness, and stock are 0.86 m, 0.60 m, and 0.10 m3/m3 respectively, with a trend of decreasing from high to low latitudes. Moreover, the rock outcrop coverage rate explains 52 % (60 %) of the variation in soil thickness (stock), followed by the annual mean temperature. This study emphasizes the importance of considering bedrock topography and its distribution in improving the detection of soil distribution, and the implications of the findings in this study are perhaps most profound for soil thickness-related soil erosion, soil water storage, and carbon storage in karst critical zones.

Stabilization of organic matter in soils: drivers, mechanisms, and analytical tools – a literature review

ABSTRACT Soils are the largest terrestrial carbon (C) reservoir, and most of this C is retained as soil organic matter (SOM). Due to its ability to capture, stabilize, and store C for extended periods, soils are considered important allies in decarbonizing the atmosphere. The term ‘C stabilization’ includes a series of mechanisms or processes by which soil C is protected within soils and its losses are reduced through microbial decomposition or leaching. Due to their relevance in the global C cycle, C stabilization mechanisms have received intensive attention from the scientific community. As new analytic technologies push the boundaries of what was previously possible to know, new paradigms emerge. This literature review summarizes the current knowledge of the main mechanisms that may promote SOM stabilization. Factors that govern accumulation of SOM are also addressed. We highlight the role of organo-mineral associations and spatial inaccessibility of SOM due to occlusion within soil aggregates to understand the relative contribution of these mechanisms in different soil conditions (e.g., soil texture, mineralogy, and land- use). In addition, the contribution of cutting-edge approaches and analytical techniques to advance the understanding of SOM protection is presented. Modern techniques to evaluate SOM on a micro, nano, and molecular scale can contribute to the mechanistic understanding of SOM stabilization and the study and adoption of management strategies that maintain and increase C stocks in soils.