Effects of forest management intensity on carbon and nitrogen content in different soil size fractions of a North Florida Spodosol

Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to advanced growing systems, studies have shown little or even negative effects on the C content of the extremely sandy soils of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest management on soil organic carbon (SOC) sequestration. This study was conducted to examine the C profile in a 4-year-old loblolly pine (Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000–250 μm, 250–150 μm, 150–53 μm and <53 μm). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure and water-soluble SOC components such as esters and amides and did not affect the N distribution. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which showed the presence of recently added organic matter in the largest sand fraction, as well as more decomposed organic matter in the <53 μm fraction. Intensive forest management reduced SOC in all three 2000–53 μm fractions, most likely due to reduced root input of understory plants that were controlled using herbicides. The 2000–250 μm fractions contained nearly half of the total SOC and showed a 23% decrease in C content due to the intensive management regime. Results from this study indicated the significance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reductions in SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and the mechanisms responsible. Responsible Editor: Barbara Wick     

Aboveground productivity and soil carbon storage of biofuel crops in Ohio

Biofuel crops may help achieve the goals of energy‐efficient renewable ethanol production and greenhouse gas (GHG) mitigation through carbon (C) storage. The objective of this study was to compare the aboveground biomass yields and soil organic C (SOC) stocks under four crops (no‐till corn, switchgrass, indiangrass, and willow) 7 years since establishment at three sites in Ohio to determine if high‐yielding biofuel crops are also capable of high levels of C storage. Corn grain had the highest potential ethanol yields, with an average of more than 4100 L ha^?1, and ethanol yields increased if both corn grain and stover were converted to biofuel, while willow had the lowest yields. The SOC concentration in soils under biofuels was generally unaffected by crop type; at one site, soil in the top 10 cm under willow contained nearly 13 Mg C ha^?1 more SOC (or 29% more) than did soils under switchgrass or corn. Crop type affected SOC content of macroaggregates in the top 10 cm of soil, where macroaggregates in soil under corn had lower C, N and C : N ratios than those under perennial grasses or trees. Overall, the results suggest that no‐till corn is capable of high ethanol yields and equivalent SOC stocks to 40 cm depth. Long‐term monitoring and measurement of SOC stocks at depth are required to determine whether this trend remains. In addition, ecological, energy, and GHG assessments should be made to estimate the C footprint of each feedstock.     

Quantification of soil organic carbon sequestration potential in cropland: A model approach

Agroecosystems have a critical role in the terrestrial carbon cycling process. Soil organic carbon (SOC) in cropland is of great importance for mitigating atmospheric carbon dioxide increases and for global food security. With an understanding of soil carbon saturation, we analyzed the datasets from 95 global long-term agricultural experiments distributed across a vast area spanning wide ranges of temperate, subtropical and tropical climates. We then developed a statistical model for estimating SOC sequestration potential in cropland. The model is driven by air temperature, precipitation, soil clay content and pH, and explains 58% of the variation in the observed soil carbon saturation (n=76). Model validation using independent data observed in China yielded a correlation coefficient R ² of 0.74 (n=19, P<0.001). Model sensitivity analysis suggested that soils with high clay content and low pH in the cold, humid regions possess a larger carbon sequestration potential than other soils. As a case study, we estimated the SOC sequestration potential by applying the model in Henan Province. Model estimations suggested that carbon (C) density at the saturation state would reach an average of 32 t C ha⁻¹ in the top 0–20 cm soil depth. Using SOC density in the 1990s as a reference, cropland soils in Henan Province are expected to sequester an additional 100 Tg C in the future.     

Bomb <Superscript>14</Superscript>C enrichment indicates decadal C pool in deep soil?

Studies of changes in soil organic carbon (SOC) stocks normally limit their focus to the upper 20–30 cm of soil, yet 0–20 cm SOC stocks are only ∼40% of 0–1 m SOC. Accounting for only the upper 20–30 cm of SOC has been justifiable assuming that deeper SOC is unreactive since it displays ¹⁴C-derived mean residence times of hundreds or thousands of years. The dramatic increase in the ¹⁴C content of the atmosphere resulting from thermonuclear testing circa 1963 allows the unreactivity of deep SOC to be tested by examining whether deep soils show evidence of ‘bomb-¹⁴C’ incorporation. At depths of 40–100 cm, a well-studied New Zealand soil under stable pastoral management displays progressive enrichment of over 200‰ across samplings in 1959, 1974 and 2002, indicating substantial incorporation of bomb ¹⁴C. This pattern of deep ¹⁴C enrichment—previously observed in 2 well-drained California grassland soils—leads to the hypothesis that roots and/or dissolved organic C transport contribute to a decadally-reactive SOC pool comprising ∼10–40% of SOC below 50 cm. Deep reactive SOC may be important in the global C cycle because it can react to land-use or vegetation change and may respond to different processes than the reactive SOC in the upper 20–30 cm of soil.     

Deep soil flipping increases carbon stocks of New Zealand grasslands

Sequestration of soil organic carbon (SOC) has been recognized as an opportunity to off‐set global carbon dioxide (CO₂) emissions. Flipping (full inversion to 1–3 m) is a practice used on New Zealand's South Island West Coast to eliminate water‐logging in highly podzolized sandy soils. Flipping results in burial of SOC formed in surface soil horizons into the subsoil and the transfer of subsoil material low in SOC to the “new” topsoil. The aims of this study were to quantify changes in the storage and stability of SOC over a 20‐year period following flipping of high‐productive pasture grassland. Topsoils (0–30 cm) from sites representing a chronosequence of flipping (3–20 years old) were sampled (2005/07) and re‐sampled (2017) to assess changes in topsoil carbon stocks. Deeper samples (30–150 cm) were also collected (2017) to evaluate the changes in stocks of SOC previously buried by flipping. Density fractionation was used to determine SOC stability in recent and buried topsoils. Total SOC stocks (0–150 cm) increased significantly by 69 ± 15% (179 ± 40 Mg SOC ha^‐1) over 20 years following flipping. Topsoil burial caused a one‐time sequestration of 160 ± 14 Mg SOC ha^‐1 (30–150 cm). The top 0–30 cm accumulated 3.6 Mg SOC ha^‐1 year^‐1. The chronosequence and re‐sampling revealed SOC accumulation rates of 1.2–1.8 Mg SOC ha^‐1 year^‐1 in the new surface soil (0–15 cm) and a SOC deficit of 36 ± 5% after 20 years. Flipped subsoils contained up to 32% labile SOC (compared to <1% in un‐flipped subsoils) thus buried SOC was preserved. This study confirms that burial of SOC and the exposure of SOC depleted subsoil results in an overall increase of SOC stocks of the whole soil profile and long‐term SOC preservation.     

Overgrazing and soil carbon dynamics in eastern Inner Mongolia of China

Eastern Inner Mongolia of China is a typical ecotone between sandy forests and steppe. Little is known about the effect of overgrazing on carbon loss from soil in semiarid steppe and sandy forests of the north of China. The soil carbon parameters were measured in a 10,000 ha natural reserve in eastern Inner Mongolia of China (43°30′–43°36′N, 117°06′–117°16′E). Three situations were compared: primary protected (PP), moderately protected (MP) and highly degraded (HD). Soil and litter samples were recovered in spring and summer. Soil organic carbon (SOC) and CO₂–C values decreased from the PP (9.23 kg m⁻² and 157 g m⁻²) to the HD (1.69 kg m⁻² and 57 g m⁻²) sites whereas the C mineralization rate increased toward the less restored sites (1.06–2.37). Surface-litter C was different in both sites under protection (PP 648 and MP 408 g m⁻²), an was low at the HD site (17 g m⁻²). Leaves from woody species dominated the surface litter at the PP site, whereas grass material was predominant at the MP site. During summer, both CO₂–C and SOC decreased, whereas the C mineralization rate increased. We calculated that C loss since the introduction of cattle into the forest was 77 M g ha⁻¹, reaching a total of 1.1×10¹⁵ g for eastern Inner Mongolia. These values are higher than those caused by the conversion of steppe and other ecosystems into agriculture or cultivated pastures. The amount of C fixed at the PP site (650 g ha⁻¹year⁻¹) indicates that the sandy soils have a significant potential as atmospheric carbon sinks. Foundation item: The National Natural Science Foundation of China (No. 39900019, 30070129).     

Long-term effects of continuous direct seeding mulch-based cropping systems on soil nitrogen supply in the Cerrado region of Brazil

In the Cerrado region of Brazil conventional soybean monoculture is since the 1980s being replaced by direct seeding mulch-based cropping (DMC) with two crops per year and absence of tillage practices. The objective of this study was to assess the long-term impact of DMC on soil organic matter accumulation and nitrogen (N) mineralization. Measurements of soil organic carbon (C) content, soil total N content and soil N mineralization, both under laboratory conditions using disturbed soil samples and under field conditions using intact soil cores were conducted on a chronosequence of 2-, 6-, 9- and 14-year-old DMC fields (DMC-2, DMC-6, DMC-9 and DMC-14, respectively). The average increase of organic C in the 0–30 cm topsoil layer under DMC was 1.91 Mg C ha⁻¹ year⁻¹. Soil total N increased with 103 kg N ha⁻¹ year⁻¹ (0–30 cm). The potential N mineralization rate under laboratory conditions (28°C, 75% of soil moisture at field capacity) was 0.27, 0.28, 0.39 and 0.36 mg N kg soil⁻¹ day⁻¹ for, respectively, the DMC-2, DMC-6, DMC-9 and DMC-14 soils. The corresponding specific N mineralization rates were 0.16, 0.15, 0.22 and 0.17 mg N g N⁻¹ day⁻¹. There was no obvious explanation for the higher specific N mineralization rate of soils under DMC-9, given the similar soil conditions and land-use history before DMC was introduced. Results from the in situ N incubation experiments were in good agreement with those from the laboratory incubations. We estimated that soil N mineralization increases with about 2.0 kg N ha⁻¹ year⁻¹ under DMC. The increase was mainly attributed to the larger soil total N content. These results indicate that even in the medium term (10 years), continuous DMC cropping has limited implications for N fertilization recommendations, since the extra soil N supply represents less than 20% of the common N fertilization dose for maize in the region.     

Threshold responses of soil organic carbon concentration and composition to multi-level nitrogen addition in a temperate needle-broadleaved forest

Responses of soil organic carbon (SOC) cycling and C budget in forest ecosystems to elevated nitrogen (N) deposition are divergent. Little is known about the N critical loads for the shift between gain and loss of SOC storage in the old-growth temperate forest of Northeast China. The objective of this study was to investigate the nonlinear responses of SOC concentration and composition to multiple rates of N addition, as well as the microbial mechanisms responsible for SOC alteration under N enrichment. Nine rates of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha^?1 year^?1) with 4 replicates for each treatment were conducted. Soil samples in the 0–10 cm mineral layer were taken after 3 years of N fertilization. Soil aggregate size distribution and SOC physical fractionation were performed to examine SOC dynamics. Phospholipid fatty acid (PLFA) technique was used to measure the abundance and structure of microbial community. Three years of N addition led to significant increases in the concentrations of soil particulate organic C and aggregate-associated organic C fractions only. The responses of total N and each labile SOC fraction to the rates of N addition followed Gaussian equations, with the N critical loads being estimated to be between 80 and 100 kg N ha^?1 year^?1. The change in SOC concentration (ΔSOC) was positively correlated with the changes in aggregate associated OC (r² > 0.80) and POC concentrations (r² > 0.50). Significant correlations among the concentrations of labile SOC fractions, the percentages of soil aggregates, and the abundances of microbial PLFAs were observed, which implies a close linkage between microbial community structure and SOC accumulation and stability. Our results suggest that increase in soil moisture and shift of microbial community structure could control the critical N load for the switch between C accumulation and loss. The current N deposition rate (~ 11 kg N ha^?1 year^?1) to the northeast China’s forests is favorable for soil C accumulation over the short term.     

Regulatory effects of soil properties on background N2O emissions from agricultural soils in China

The background nitrous oxide (N₂O) emission (BNE) from agricultural soils originates from microbial nitrification and denitrification processes of soil nitrogen supplies, excluding emissions from nitrogen fertilizers applied in the current year. It is of great necessity to quantify BNE accurately at various spatial scales since BNE contributes considerably to the overall N₂O emissions from croplands. Annual BNE rates across various soil/climate regions and major cropping systems of China were determined by network observations during 2002–2006 using the static chamber technique. The observations show BNE rates ranging from 0.1 to 3.67 kg N ha⁻¹ year⁻¹, with a mean of 1.35 kg N ha⁻¹ year⁻¹. Empirical functions are derived for cultivated mineral soils and describe the dependences of annual BNE rates upon soil total nitrogen (TN) content, soil organic carbon (SOC) content, bulk density (BD) and clay fraction (CF), separately or collectively. These empirical functions provide simple approaches to scale up estimated national/regional BNE inventories using available database of soil properties surveys and cropland area statistics. The national BNE of China is estimated to be 0.114–0.184 Tg (1 Tg = 10¹² g) N year⁻¹ in 2000, with the range being due to the use of different approaches. However, the available observations of annual BNE rates do not cover the entire range of soil properties on a national scale. Further work is needed to verify the empirical models for a complete range of soil types. In addition, a predictive empirical relationship between annual BNE rates and TN or SOC is established for cultivated mineral soil at the global scale. However, the empirical models could not accurately predict the BNE rates of cultivated organic soils. Further studies are required to understand the regulatory effects of soil properties on annual BNE rates of cultivated organic soils.     

Impact of long-term nitrogen addition on carbon stocks in trees and soils in northern Europe

The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0–10 cm mineral soil) by analysing data from 15 long-term (14–30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30–50 kg N ha⁻¹ year⁻¹) were always more efficient per unit of N than high application rates (50–200 kg N ha⁻¹ year⁻¹). Addition of a cumulative amount of N of 600–1800 kg N ha⁻¹ resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg⁻¹ (N added) (“N-use efficiency”), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg⁻¹ (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg⁻¹ (N) at C/N 35 and decreased again to about 20 kg (C) kg⁻¹ (N) at C/N 50 when N only was added. In contrast, addition of NPK resulted in high (40–50 kg (C) kg⁻¹ (N)) N-use efficiency also at N-rich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at N-rich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3–4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg⁻¹ (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha⁻¹ year⁻¹ higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m⁻² more tree C and 1.3 ± 0.5 kg m⁻² more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70–80% of the difference in SOC can be explained by different N deposition.     

Legacy effects of grassland management on soil carbon to depth

The importance of managing land to optimize carbon sequestration for climate change mitigation is widely recognized, with grasslands being identified as having the potential to sequester additional carbon. However, most soil carbon inventories only consider surface soils, and most large‐scale surveys group ecosystems into broad habitats without considering management intensity. Consequently, little is known about the quantity of deep soil carbon and its sensitivity to management. From a nationwide survey of grassland soils to 1 m depth, we show that carbon in grassland soils is vulnerable to management and that these management effects can be detected to considerable depth down the soil profile, albeit at decreasing significance with depth. Carbon concentrations in soil decreased as management intensity increased, but greatest soil carbon stocks (accounting for bulk density differences), were at intermediate levels of management. Our study also highlights the considerable amounts of carbon in subsurface soil below 30 cm, which is missed by standard carbon inventories. We estimate grassland soil carbon in Great Britain to be 2097 Tg C to a depth of 1 m, with ~60% of this carbon being below 30 cm. Total stocks of soil carbon (t ha^?1) to 1 m depth were 10.7% greater at intermediate relative to intensive management, which equates to 10.1 t ha^?1 in surface soils (0–30 cm), and 13.7 t ha^?1 in soils from 30 to 100 cm depth. Our findings highlight the existence of substantial carbon stocks at depth in grassland soils that are sensitive to management. This is of high relevance globally, given the extent of land cover and large stocks of carbon held in temperate managed grasslands. Our findings have implications for the future management of grasslands for carbon storage and climate mitigation, and for global carbon models which do not currently account for changes in soil carbon to depth with management.     

Stabilization of recent soil carbon in the humid tropics following land use changes: evidence from aggregate fractionation and stable isotope analyses

Quantitative knowledge of stabilization- and decomposition processes is necessary to understand, assess and predict effects of land use changes on storage and stability of soil organic carbon (soil C) in the tropics. Although it is well documented that different soil types have different soil C stocks, it is presently unknown how different soil types affect the stability of recently formed soil C. Here, we analyze the main controls of soil C storage in the top 0.1 m of soils developed on Tertiary sediments and soils developed on volcanic ashes. Using a combination of fractionation techniques with ¹³C isotopes analyses we had the opportunity to trace origin and stability of soil carbon in different aggregate fractions under pasture and secondary forest. Soil C contents were higher in volcanic ash soils (47–130 g kg⁻¹) than in sedimentary soils (19–50 g kg⁻¹). Mean residence time (MRT) of forest-derived carbon in pastures increased from 37 to 57 years with increasing silt + clay content in sedimentary soils, but was independent from soil properties in volcanic ash soils. MRTs of pasture-derived carbon in secondary forests were considerably shorter, especially in volcanic ash soils, where no pasture-derived carbon could be detected in any of the four studied secondary forests. The implications of these results are that the MRT of recently incorporated organic carbon depends on clay mineralogy and is longer in soils dominated by smectite than non-crystalline minerals. Our results show that the presence of soil C stabilization processes, does not necessarily mean that recent incorporated soil C will also be effectively stabilized.     

Soil organic carbon dynamics 75 years after land-use change in perennial grassland and annual wheat agricultural systems

The dynamics of roots and soil organic carbon (SOC) in deeper soil layers are amongst the least well understood components of the global C cycle, but essential if soil C is to be managed effectively. This study utilized a unique set of land-use pairings of harvested tallgrass prairie grasslands (C₄) and annual wheat croplands (C₃) that were under continuous management for 75 years to investigate and compare the storage, turnover and allocation of SOC in the two systems to 1 m depth. Cropland soils contained 25 % less SOC than grassland soils (115  and 153 Mg C ha^?1, respectively) to 1 m depth, and had lower SOC contents in all particle size fractions (2000–250, 250–53, 53–2 and <2 μm), which nominally correspond to SOC pools with different stability. Soil bulk δ¹³C values also indicated the significant turnover of grassland-derived SOC up to 80 cm depth in cropland soils in all fractions, including deeper (>40 cm) layers and mineral-associated (<53 μm) SOC. Grassland soils had significantly more visible root biomass C than cropland soils (3.2 and 0.6 Mg ha^?1, respectively) and microbial biomass C (3.7 and 1.3 Mg ha^?1, respectively) up to 1 m depth. The outcomes of this study demonstrated that: (i) SOC pools that are perceived to be stable, i.e. subsoil and mineral-associated SOC, are affected by land-use change; and, (ii) managed perennial grasslands contained larger SOC stocks and exhibited much larger C allocations to root and microbial pools than annual croplands throughout the soil profile.     

The soil organic carbon in particle-size separates under different regrowth forest stands of north eastern Costa Rica

Despite the importance of the secondary forest (SF) in tropical areas, few studies have quantified the soil organic carbon (SOC) pool in Costa Rica. Most of the studies conducted to date in this country have focused mainly on changes in the soil C pool following conversion of forests to pastures, which is the predominant land use in the tropics. The aim of this study was to measure SOC concentration and pool in particle-size fractions down to 50 cm depth in four SF stands regenerating from different intensities of prior land use in loamy sand and sandy loam soils of northeast Costa Rica: (i) a gallery forest (GF), (ii) a 15-year-old SF enriched with commercially planted native trees (15SF), (iii) a 25-year-old SF (25SF), and (iv) an abandoned Theobromma cacao plantation >60 years old (60SF). Additional objectives were (1) to determine the relationship of SOC concentration with selected physical and chemical soil properties, and (2) to establish the key determinants of the depth distribution of SOC in order to identify meaningful trends in the SOC pool. The SOC pool was highest under the 60SF (221.4 Mg C ha⁻¹) followed by the 15SF (212.1 Mg C ha⁻¹), the 25SF (195.9 Mg C ha⁻¹) and the lowest in the GF (183.5 Mg C ha⁻¹). The SOC concentration decreased significantly from 59.7 to 94.1 g kg⁻¹ in the 0–10 cm layer down to 31.0 to 45.5 g kg⁻¹ in the 40–50 cm layer in all forest stands. The fine silt + clay fraction contained the highest values of SOC concentration in all forest stands. Soil texture and the age of the SF were identified as the main factors that explained the variability in SOC. The age of SF stand influenced the distribution of size class aggregates and SOC.     

引黄灌区土壤有机碳密度剖面特征及固碳速率

为揭示灌溉耕作对土壤有机碳密度剖面(0—100 cm)分布产生的影响,通过在宁夏引黄灌区进行实地调查与采样,以无灌溉耕作的自然土壤作为对照,研究不同灌溉耕作时间序列下灌区土壤有机碳密度的剖面分布特征,并估算其平均固碳速率。结果表明:灌区土壤有机碳含量具有随土层深度增加而下降的剖面分布特征,灌溉耕作对增加表层土壤有机碳含量作用最明显;灌区土壤剖面碳密度与灌溉耕作时间和土壤类型均显著相关(P0.01),相关系数分别为0.63和0.74,且因灌溉耕作时间和土壤类型的不同,土壤有机碳密度差异性显著(P0.05);灌溉耕作影响的土层深度及剖面土壤有机碳密度的增加量因灌溉耕作时间长短的不同而异;引黄灌区5类土壤的平均固碳速率为0.53 MgC·hm-2·a-1。引黄灌溉耕作在增加农田土壤固碳中发挥着重要作用。     

Contemporary carbon stocks of mineral forest soils in the Swiss Alps

Soil organic carbon (SOC) has been identified as the main globalterrestrial carbon reservoir, but considerable uncertainty remains as toregional SOC variability and the distribution of C between vegetationand soil. We used gridded forest soil data (8–km × 8–km)representative of Swiss forests in terms of climate and forest typedistribution to analyse spatial patterns of mineral SOC stocks alonggradients in the European Alps for the year 1993. At stand level, meanSOC stocks of 98 t C ha^–1 (N = 168,coefficient of variation: 70%) were obtained for the entiremineral soil profile, 76 t C ha^–1 (N =137, CV: 50%) in 0–30 cm topsoil, and 62 t Cha^–1 (N = 156, CV: 46%) in0–20 cm topsoil. Extrapolating to national scale, we calculatedcontemporary SOC stocks of 110 Tg C (entire mineral soil, standarderror: 6 Tg C), 87 Tg C (0–30 cm topsoil, standarderror: 3.5 Tg C) and 70 Tg C (0–20 cm topsoil, standarderror: 2.5 Tg C) for mineral soils of accessible Swiss forests(1.1399 Mha). According to our estimate, the 0–20 cm layers ofmineral forest soils in Switzerland store about half of the Csequestered by forest trees (136 Tg C) and more than five times morethan organic horizons (13.2 Tg C).At stand level, regression analyses on the entire data set yielded nostrong climatic or topographic signature for forest SOC stocks in top(0–20 cm) and entire mineral soils across the Alps, despite thewide range of values of site parameters. Similarly, geostatisticalanalyses revealed no clear spatial trends for SOC in Switzerland at thescale of sampling. Using subsets, biotic, abiotic controls andcategorial variables (forest type, region) explained nearly 60%of the SOC variability in topsoil mineral layers (0–20 cm) forbroadleaf stands (N = 56), but only little of thevariability in needleleaf stands (N = 91,R ² = 0.23 for topsoil layers).Considerable uncertainties remain in assessments of SOC stocks, due tounquantified errors in soil density and rock fraction, lack of data onwithin-site SOC variability and missing or poorly quantifiedenvironmental control parameters. Considering further spatial SOCvariability, replicate pointwise soil sampling at 8–km × 8–kmresolution without organic horizons will thus hardly allow to detectchanges in SOC stocks in strongly heterogeneous mountain landscapes.     

Effects of cropland reclamation on soil organic carbon in China's black soil region over the past 35 years

The long-term use of cropland and cropland reclamation from natural ecosystems led to soil degradation. This study investigated the effect of the long-term use of cropland and cropland reclamation from natural ecosystems on soil organic carbon (SOC) content and density over the past 35 years. Altogether, 2140 topsoil samples (0–20 cm) were collected across Northeast China. Landsat images were acquired from 1985 to 2020 through Google Earth Engine, and the reflectance of each soil sample was extracted from the Landsat image that its time was consistent with sampling. The hybrid model that included two individual SOC prediction models for two clustering regions was built for accurate estimation after k-means clustering. The probability hybrid model, a combination between the hybrid model and classification probabilities of pixels, was introduced to enhance the accuracy of SOC mapping. Cropland reclamation results were extracted from the land cover time-series dataset at a 5-year interval. Our study indicated that: (1) Long-term use of cropland led to a 3.07 g kg⁻¹ and 6.71 Mg C ha⁻¹ decrease in SOC content and density, respectively, and the decrease of SOC stock was 0.32 Pg over the past 35 years; (2) nearly 64% of cropland had a negative change in terms of SOC content from 1985 to 2020; (3) cropland reclamation track changed from high to low SOC content, and almost no cropland was reclaimed on the “Black soils” after 2005; (4) cropland reclamation from wetlands resulted in the highest decrease, and reclamation period of years 31–35 decreased when SOC density and SOC stock were 16.05 Mg C ha⁻¹ and 0.005 Pg, respectively, while reclamation period of years 26–30 from forest witnessed SOC density and stock decreases of 8.33 Mg C ha⁻¹ and 0.01 Pg, respectively. Our research results provide a reference for SOC change in the black soil region of Northeast China and can attract more attention to the area of the protection of “Black soils” and natural ecosystems.     

The role of soil in storing carbon in tropical rainforests: the case of Ankasa Park, Ghana

Tropical primary rainforests of Africa are an enormous reservoir of carbon (C), most of which, in the common perception, is stored in the biomass. We studied one of these forests, Ankasa, in the south-western part of Ghana, in terms of quantity and ¹⁴C activity of soil organic carbon (SOC) to elucidate the little known important role of soil in storing carbon in such biomass-rich environments. The stock of carbon in the mineral soil to a depth of 1 m was measured to be 151?±?20 Mg C ha^?1, a similar value in magnitude to the one of the aboveground biomass being 138–170 Mg C ha^?1, including live and dead wood. Surface litter C is roughly 10% (15?±?9 Mg C ha^?1) of the C in the biomass and soil. The radiocarbon measurements indicate that SOC was significantly affected by “bomb C” enrichment, so that “Modern C”, namely with a mean radiocarbon age lower than 200 years, is present also deeper than 45 cm in the Bo2 horizon. The mean residence time (MRT) estimated from radiocarbon content are of the order of a few decades in the topsoil and a few centuries in the deeper horizons. Altogether, the MRT values indicate a fast recycle of C compared to temperate or boreal forests, but not as fast as usually believed for tropical forest soils. Making a pondered mean, in the Ankasa forest the time an atom of C resides in soil is not much different from one atom of C in the woody aboveground biomass. Hence, the contribution of soil in storing C is substantial, implying that in primary rainforests it is mandatory to determine the SOC stock and its dynamics, too often neglected or underestimated.     

Elevated [CO2], temperature increase and N supply effects on the accumulation of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l⁻¹ [CO₂]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha⁻¹ year⁻¹) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO₂ concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO₂] reduced it. At the last sampling date, elevated [CO₂] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO₂2] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha⁻¹, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha⁻¹. There was no significant effect of a 3 °C temperature increase under elevated [CO₂] on C accumulation in the OM fractions above 50 μm. This revised version was published online in June 2006 with corrections to the Cover Date.     

Wildfire effects on carbon and nitrogen in inland coniferous forests

A ponderosa pine/Douglas-fir forest (Pinus ponderosa Dougl., Pseudotsuga menziesii (Mirb.) Franco; PP/DF) and a lodgepole pine/Engelmann spruce forest (Pinus contorta Loud., Picea engelmannii Parry ex Engelm.; LP/ES) located on the eastern slopes of the Cascade Mountains in Washington state, USA, were examined following severe wildfire to compare total soil carbon and nitrogen capitals with unburned (control) forests. One year after fire, the average C content (60 cm depth) of PP/DF and LP/ES soil was 30% (25 Mg ha^-1) and 10% (7 Mg ha^-1) lower than control soil. Average N content on the burned PP/DF and LP/ES plots was 46% (3.0 Mg ha^-1) and 13% (0.4 Mg ha^-1) lower than control soil. The reduction in C and N in the PP/DF soil was largely the result of lower nutrient capitals in the burned Bw horizons (12–60 cm depth) relative to control plots. It is unlikely that the 1994 fire substantially affected nutrient capitals in the Bw horizons; however, natural variability or past fire history could be responsible for the varied nutrient capitals observed in the subsurface soils. Surface erosion (sheet plus rill) removed between 15 and 18 Mg ha^-1 of soil from the burned plots. Nutrient losses through surface erosion were 280 kg C ha^-1 and 14 kg N ha^-1 in the PP/DF, whereas LP/ES losses were 640 and 22 kg ha^-1 for C and N, respectively. In both forests, surface erosion of C and N was _∼1% to 2% of the A-horizon capital of these elements in unburned soil. A bioassay (with lettuce as an indicator plant) was used to compare soils from low-, moderate- and high-severity burn areas relative to control soil. In both forests, low-severity fire increased lettuce yield by 70–100% of controls. With more severe fire, yield decreased in the LP/ES relative to the low-intensity burn soil; however, only in the high-severity treatment was yield reduced (14%) from the control. Moderate- and high-severity burn areas in the PP/DF were fertilized with _∼56 kg ha^-1 of N four months prior to soil sampling. In these soils, yield was 70–80% greater than the control. These results suggest that short-term site productivity can be stimulated by low-severity fire, but unaffected or reduced by more severe fire in the types of forests studied. Post-fire fertilization with N could increase soil productivity where other environmental factors do not limit growth. This revised version was published online in June 2006 with corrections to the Cover Date.