Dynamics and Sources of Soil Organic C Following Afforestation of Croplands with Poplar in a Semi-Arid Region in Northeast China

Afforestation of former croplands has been proposed as a promising way to mitigate rising atmospheric CO₂ concentration in view of the commitment to the Kyoto Protocol. Central to this C sequestration is the dynamics of soil organic C (SOC) storage and stability with the development of afforested plantations. Our previous study showed that SOC storage was not changed after afforestation except for the 0–10 cm layer in a semi-arid region of Keerqin Sandy Lands, northeast China. In this study, soil organic C was further separated into light and heavy fractions using the density fractionation method, and their organic C concentration and ¹³C signature were analyzed to investigate the turnover of old vs. new SOC in the afforested soils. Surface layer (0–10 cm) soil samples were collected from 14 paired plots of poplar (Populus × xiaozhuanica W. Y. Hsu & Liang) plantations with different stand basal areas (the sum of the cross-sectional area of all live trees in a stand), ranging from 0.2 to 32.6 m² ha⁻¹, and reference maize (Zea mays L.) croplands at the same sites as our previous study. Soil Δ_C stocks (Δ_C refers to the difference in SOC content between a poplar plantation and the paired cropland) in bulk soil and light fraction were positively correlated with stand basal area (R ² = 0.48, p<0.01 and R ² = 0.40, p = 0.02, respectively), but not for the heavy fraction. SOC_crop (SOC derived from crops) contents in the light and heavy fractions in poplar plantations were significantly lower as compared with SOC contents in croplands, but tree-derived C in bulk soil, light and heavy fraction pools increased gradually with increasing stand basal area after afforestation. Our study indicated that cropland afforestation could sequester new C derived from trees into surface mineral soil, but did not enhance the stability of SOC due to a fast turnover of SOC in this semi-arid region.     

Role and Variation of the Amount and Composition of Glomalin in Soil Properties in Farmland and Adjacent Plantations with Reference to a Primary Forest in North-Eastern China

The glycoprotein known as glomalin-related soil protein (GRSP) is abundantly produced on the hyphae and spores of arbuscular mycorrhizal fungi (AMF) in soil and roots. Few studies have focused on its amount, composition and associations with soil properties and possible land-use influences, although the data hints at soil rehabilitation. By choosing a primary forest soil as a non-degraded reference, it is possible to explore whether afforestation can improve degraded farmland soil by altering GRSP. In this paper, close correlations were found between various soil properties (soil organic carbon, nitrogen, pH, electrical conductivity (EC), and bulk density) and the GRSP amount, between various soil properties and GRSP composition (main functional groups, fluorescent substances, and elements). Afforestation on farmland decreased the EC and bulk density (p < 0.05). The primary forest had a 2.35–2.56-fold higher GRSP amount than those in the plantation forest and farmland, and GRSP composition (tryptophan-like and fulvic acid-like fluorescence; functional groups of C–H, C–O, and O–H; elements of Al, O, Si, C, Ca, and N) in primary forest differed from those in plantation forest and farmland (p < 0.05). However, no evident differences in GRSP amount and composition were observed between the farmland and the plantation forest. Our finding highlights that 30 years poplar afforestation on degraded farmland is not enough to change GRSP-related properties. A longer period of afforestation with close-to-nature managements may favor the AMF-related underground recovery processes.     

Association of Soil Aggregation with the Distribution and Quality of Organic Carbon in Soil along an Elevation Gradient on Wuyi Mountain in China

Forest soils play a critical role in the sequestration of atmospheric CO₂ and subsequent attenuation of global warming. The nature and properties of organic matter in soils have an influence on the sequestration of carbon. In this study, soils were collected from representative forestlands, including a subtropical evergreen broad-leaved forest (EBF), a coniferous forest (CF), a subalpine dwarf forest (DF), and alpine meadow (AM) along an elevation gradient on Wuyi Mountain, which is located in a subtropical area of southeastern China. These soil samples were analyzed in the laboratory to examine the distribution and speciation of organic carbon (OC) within different size fractions of water-stable soil aggregates, and subsequently to determine effects on carbon sequestration. Soil aggregation rate increased with increasing elevation. Soil aggregation rate, rather than soil temperature, moisture or clay content, showed the strongest correlation with OC in bulk soil, indicating soil structure was the critical factor in carbon sequestration of Wuyi Mountain. The content of coarse particulate organic matter fraction, rather than the silt and clay particles, represented OC stock in bulk soil and different soil aggregate fractions. With increasing soil aggregation rate, more carbon was accumulated within the macroaggregates, particularly within the coarse particulate organic matter fraction (250–2000 μm), rather than within the microaggregates (53–250μm) or silt and clay particles (< 53μm). In consideration of the high instability of macroaggregates and the liability of SOC within them, further research is needed to verify whether highly-aggregated soils at higher altitudes are more likely to lose SOC under warmer conditions.     

Carbon Sequestration Efficiency of Organic Amendments in a Long-Term Experiment on a Vertisol in Huang-Huai-Hai Plain,China

Soil organic carbon (SOC) sequestration is important for improving soil fertility of cropland and for the mitigation of greenhouse gas emissions to the atmosphere. The efficiency of SOC sequestration depends on the quantity and quality of the organic matter, soil type, and climate. Little is known about the SOC sequestration efficiency of organic amendments in Vertisols. Thus, we conducted the research based on 29 years (1982–2011) of long-term fertilization experiment with a no fertilizer control and five fertilization regimes: CK (control, no fertilizer), NPK (mineral NPK fertilizers alone), NPK+1/2W (mineral NPK fertilizers combined with half the amount of wheat straw), NPK+W (mineral NPK fertilizers combined with full the amount of wheat straw), NPK+PM (mineral NPK fertilizers combined with pig manure) and NPK+CM (mineral NPK fertilizers combined cattle manure). Total mean annual C inputs were 0.45, 1.55, 2.66, 3.71, 4.68 and 6.56 ton/ha/yr for CK, NPK, NPKW1/2, NPKW, NPKPM and NPKCM, respectively. Mean SOC sequestration rate was 0.20 ton/ha/yr in the NPK treatment, and 0.39, 0.50, 0.51 and 0.97 ton/ha/yr in the NPKW1/2, NPKW, NPKPM, and NPKCM treatments, respectively. A linear relationship was observed between annual C input and SOC sequestration rate (SOCsequestration rate  = 0.16 Cinput –0.10, R = 0.95, P<0.01), suggesting a C sequestration efficiency of 16%. The Vertisol required an annual C input of 0.63 ton/ha/yr to maintain the initial SOC level. Moreover, the C sequestration efficiencies of wheat straw, pig manure and cattle manure were 17%, 11% and 17%, respectively. The results indicate that the Vertisol has a large potential to sequester SOC with a high efficiency, and applying cattle manure or wheat straw is a recommendable SOC sequestration practice in Vertisols.     

Soil Organic Carbon Pools and Stocks in Permafrost-Affected Soils on the Tibetan Plateau

The Tibetan Plateau reacts particularly sensitively to possible effects of climate change. Approximately two thirds of the total area is affected by permafrost. To get a better understanding of the role of permafrost on soil organic carbon pools and stocks, investigations were carried out including both discontinuous (site Huashixia, HUA) and continuous permafrost (site Wudaoliang, WUD). Three organic carbon fractions were isolated using density separation combined with ultrasonic dispersion: the light fractions (<1.6 g cm⁻³) of free particulate organic matter (FPOM) and occluded particulate organic matter (OPOM), plus a heavy fraction (>1.6 g cm⁻³) of mineral associated organic matter (MOM). The fractions were analyzed for C, N, and their portion of organic C. FPOM contained an average SOC content of 252 g kg⁻¹. Higher SOC contents (320 g kg⁻¹) were found in OPOM while MOM had the lowest SOC contents (29 g kg⁻¹). Due to their lower density the easily decomposable fractions FPOM and OPOM contribute 27% (HUA) and 22% (WUD) to the total SOC stocks. In HUA mean SOC stocks (0–30 cm depth) account for 10.4 kg m⁻², compared to 3.4 kg m⁻² in WUD. 53% of the SOC is stored in the upper 10 cm in WUD, in HUA only 39%. Highest POM values of 36% occurred in profiles with high soil moisture content. SOC stocks, soil moisture and active layer thickness correlated strongly in discontinuous permafrost while no correlation between SOC stocks and active layer thickness and only a weak relation between soil moisture and SOC stocks could be found in continuous permafrost. Consequently, permafrost-affected soils in discontinuous permafrost environments are susceptible to soil moisture changes due to alterations in quantity and seasonal distribution of precipitation, increasing temperature and therefore evaporation.     

Effects of Conservation Tillage on Topsoil Microbial Metabolic Characteristics and Organic Carbon within Aggregates under a Rice (Oryza sativa L.) –Wheat (Triticum aestivum L.) Cropping System in Central China

Investigating microbial metabolic characteristics and soil organic carbon (SOC) within aggregates and their relationships under conservation tillage may be useful in revealing the mechanism of SOC sequestration in conservation tillage systems. However, limited studies have been conducted to investigate the relationship between SOC and microbial metabolic characteristics within aggregate fractions under conservation tillage. We hypothesized that close relationships can exist between SOC and microbial metabolic characteristics within aggregates under conservation tillage. In this study, a field experiment was conducted from June 2011 to June 2013 following a split-plot design of a randomized complete block with tillage practices [conventional intensive tillage (CT) and no tillage (NT)] as main plots and straw returning methods [preceding crop residue returning (S, 2100−2500 kg C ha⁻¹) and removal (NS, 0 kg C ha^-1)] as subplots with three replications. The objective of this study was to reveal the effects of tillage practices and residue-returning methods on topsoil microbial metabolic characteristics and organic carbon (SOC) fractions within aggregates and their relationships under a rice–wheat cropping system in central China. Microbial metabolic characteristics investigated using the Biolog system was examined within two aggregate fractions (>0.25 and <0.25 mm). NT treatments significantly increased SOC concentration of bulk soil, >0.25 aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer by 5.8%, 6.8% and 7.9% relative to CT treatments, respectively. S treatments had higher SOC concentration of bulk soil (12.9%), >0.25 mm aggregate (11.3%), and <0.25 mm aggregate (14.1%) than NS treatments. Compared with CT treatments, NT treatments increased MBC by 11.2%, 11.5%, and 20%, and dissolved organic carbon (DOC) concentration by 15.5%, 29.5%, and 14.1% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer, respectively. Compared with NS treatments, S treatments significantly increased MBC by 29.8%, 30.2%, and 24.1%, and DOC concentration by 23.2%, 25.0%, and 37.5% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer, respectively. Conservation tillage (NT and S) increased microbial metabolic activities and Shannon index in >0.25 and <0.25 mm aggregates in the 0−5 cm soil layer. Redundancy analysis showed that the SOC and its fractions (DOC and MBC) were closely correlated with microbial metabolic activities. Structural equation modelling showed that the increase in microbial metabolic activities directly improved SOC by promoting DOC in >0.25 mm aggregate in the upper (0−5 cm) soil layer under conservation tillage systems, as well as directly and indirectly by promoting DOC and MBC in <0.25 mm aggregate. Our results suggested that conservation tillage increased SOC in aggregates in the topsoil by improving microbial metabolic activities.     

Soil carbon stock change following afforestation in Northern Europe: a meta‐analysis

Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land‐use changes during the past decades. However, this region has not been well represented in previous large‐scale syntheses of land‐use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta‐analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0–10 cm and 0–20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land‐use, forest type, and soil textural class. Three major improvements were incorporated in the meta‐analysis: analysis of major interaction groups, evaluation of the influence of nonindependence between samples according to study design, and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0–10 cm layer. Afforestation of former grasslands had a small negative (nonsignificant) effect indicating limited SOC change following this land‐use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta‐estimates calculated for the periods <30 years and >30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta‐estimates in former land‐use, forest type, and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30‐year perspective.     

Fine-scale spatial distribution of soil organic carbon and its fractions after afforestation with Pinus sylvestris and Salix psammophila in a semiarid desert of China

半干旱沙漠中樟子松和沙柳造林后土壤有机碳及其组分的小尺度空间分布 半干旱沙漠造林有助于改善土壤功能以及增加土壤有机碳(SOC)固定,但人们对造林后SOC及其不稳定(LOC)组分的小尺度空间分布了解甚少。本研究以毛乌素沙地东南缘樟子松(Pinus sylvestris)和沙柳 (Salix psammophila)为研究对象,量化了距离树体20、80、150和240 cm处SOC、LOC组分及其相关变量的小尺度空间分布。研究结果表明,沙柳和樟子松造林显著提高了SOC、总氮(TN)、可溶性有机碳 (DOC)、微生物碳(MBC)和易氧化有机碳(ROOC)含量;在距离树体20 cm处,0–100 cm土层樟子松SOC 储量比沙柳高27.21%;在距离树体80和150 cm处,沙柳SOC储量分别比樟子松高5.50%和5.66%;与流 沙地相比,在距离树体20、80、150 和240 cm处,沙柳和樟子松SOC储量显著增加了94.90%、39.50%、 27.10%和18.50%;沙柳和樟子松ROOC分别占SOC的14.09%和18.93%。总之,造林促进了半干旱流沙地SOC的积累,樟子松比沙柳分配更多的有机质到距离树体<80 cm范围内的土体中。     

Decomposition of Organic Carbon in Fine Soil Particles Is Likely More Sensitive to Warming than in Coarse Particles: An Incubation Study with Temperate Grassland and Forest Soils in Northern China

It is widely recognized that global warming promotes soil organic carbon (SOC) decomposition, and soils thus emit more CO₂ into the atmosphere because of the warming; however, the response of SOC decomposition to this warming in different soil textures is unclear. This lack of knowledge limits our projection of SOC turnover and CO₂ emission from soils after future warming. To investigate the CO₂ emission from soils with different textures, we conducted a 107-day incubation experiment. The soils were sampled from temperate forest and grassland in northern China. The incubation was conducted over three short-term cycles of changing temperature from 5°C to 30°C, with an interval of 5°C. Our results indicated that CO₂ emissions from sand (>50 µm), silt (2–50 µm), and clay (<2 µm) particles increased exponentially with increasing temperature. The sand fractions emitted more CO₂ (CO₂-C per unit fraction-C) than the silt and clay fractions in both forest and grassland soils. The temperature sensitivity of the CO₂ emission from soil particles, which is expressed as Q₁₀, decreased in the order clay>silt>sand. Our study also found that nitrogen availability in the soil facilitated the temperature dependence of SOC decomposition. A further analysis of the incubation data indicated a power-law decrease of Q₁₀ with increasing temperature. Our results suggested that the decomposition of organic carbon in fine-textured soils that are rich in clay or silt could be more sensitive to warming than those in coarse sandy soils and that SOC might be more vulnerable in boreal and temperate regions than in subtropical and tropical regions under future warming.     

Soil Aggregates and Associated Organic Matter under Conventional Tillage,No-Tillage,and Forest Succession after Three Decades

Impacts of land use on soil organic C (SOC) are of interest relative to SOC sequestration and soil sustainability. The role of aggregate stability in SOC storage under contrasting land uses has been of particular interest relative to conventional tillage (CT) and no-till (NT) agriculture. This study compares soil structure and SOC fractions at the 30-yr-old Horseshoe Bend Agroecosystem Experiment (HSB). This research is unique in comparing NT and CT with adjacent land concurrently undergoing forest succession (FS) and in sampling to depths (15–28 cm) previously not studied at HSB. A soil moving experiment (SME) was also undertaken to monitor 1-yr changes in SOC and aggregation. After 30 years, enhanced aggregate stability under NT compared to CT was limited to a depth of 5 cm, while enhanced aggregate stability under FS compared to CT occurred to a depth of 28 cm and FS exceeded NT from 5–28 cm. Increases in SOC concentrations generally followed the increases in stability, except that no differences in SOC concentration were observed from 15–28 cm despite greater aggregate stability. Land use differences in SOC were explained equally by differences in particulate organic carbon (POC) and in silt-clay associated fine C. Enhanced structural stability of the SME soil was observed under FS and was linked to an increase of 1 Mg SOC ha⁻¹ in 0–5 cm, of which 90% could be attributed to a POC increase. The crushing of macroaggregates in the SME soil also induced a 10% reduction in SOC over 1 yr that occurred under all three land uses from 5–15 cm. The majority of this loss was in the fine C fraction. NT and FS ecosystems had greater aggregation and carbon storage at the soil surface but only FS increased aggregation below the surface, although in the absence of increased carbon storage.     

Changes in Soil Carbon and Nitrogen following Land Abandonment of Farmland on the Loess Plateau,China

The revegetation of abandoned farmland significantly influences soil organic C (SOC) and total N (TN). However, the dynamics of both soil OC and N storage following the abandonment of farmland are not well understood. To learn more about soil C and N storages dynamics 30 years after the conversion of farmland to grassland, we measured SOC and TN content in paired grassland and farmland sites in the Zhifanggou watershed on the Loess Plateau, China. The grassland sites were established on farmland abandoned for 1, 7, 13, 20, and 30 years. Top soil OC and TN were higher in older grassland, especially in the 0–5 cm soil depths; deeper soil OC and TN was lower in younger grasslands (<20 yr), and higher in older grasslands (30 yr). Soil OC and N storage (0–100 cm) was significantly lower in the younger grasslands (<20 yr), had increased in the older grasslands (30 yr), and at 30 years SOC had increased to pre-abandonment levels. For a thirty year period following abandonment the soil C/N value remained at 10. Our results indicate that soil C and TN were significantly and positively correlated, indicating that studies on the storage of soil OC and TN needs to focus on deeper soil and not be restricted to the uppermost (0–30 cm) soil levels.     

Carbon Storages in Plantation Ecosystems in Sand Source Areas of North Beijing,China

Afforestation is a mitigation option to reduce the increased atmospheric carbon dioxide levels as well as the predicted high possibility of climate change. In this paper, vegetation survey data, statistical database, National Forest Resource Inventory database, and allometric equations were used to estimate carbon density (carbon mass per hectare) and carbon storage, and identify the size and spatial distribution of forest carbon sinks in plantation ecosystems in sand source areas of north Beijing, China. From 2001 to the end of 2010, the forest areas increased more than 2.3 million ha, and total carbon storage in forest ecosystems was 173.02 Tg C, of which 82.80 percent was contained in soil in the top 0–100 cm layer. Younger forests have a large potential for enhancing carbon sequestration in terrestrial ecosystems than older ones. Regarding future afforestation efforts, it will be more effective to increase forest area and vegetation carbon density through selection of appropriate tree species and stand structure according to local climate and soil conditions, and application of proper forest management including land-shaping, artificial tending and fencing plantations. It would be also important to protect the organic carbon in surface soils during forest management.     

Carbon Sequestration in Reclaimed Minesoils

Minesoils are drastically influenced by anthropogenic activities. They are characterized by low soil organic matter (SOM) content, low fertility, and poor physicochemical and biological properties, limiting their quality, capability, and functions. Reclamation of these soils has potential for resequestering some of the C lost and mitigating CO₂ emissions. Soil organic carbon (SOC) sequestration rates in minesoils are high in the first 20 to 30 years after reclamation in the top 15 cm soil depth. In general, higher rates of SOC sequestration are observed for minesoils under pasture and grassland management than under forest land use. Observed rates of SOC sequestration are 0.3 to 1.85 Mg C ha^{? 1} yr^{? 1} for pastures and rangelands, and 0.2 to 1.64 Mg C ha^{? 1} yr^{? 1} for forest land use. Proper reclamation and postreclamation management may enhance SOC sequestration and add to the economic value of the mined sites. Management practices that may enhance SOC sequestration include increasing vegetative cover by deep-rooted perennial vegetation and afforestation, improving soil fertility, and alleviation of physical, chemical and biological limitations by fertilizers and soil amendments such as biosolids, manure, coal combustion by-products, and mulches. Soil and water conservation are important to SOC sequestration. The potential of SOC sequestration in minesoils of the US is estimated to be 1.28 Tg C yr^?1, compared to the emissions from coal combustion of 506 Tg C yr^{? 1}.     

Faster accumulation and greater contribution of glomalin to the soil organic carbon pool than amino sugars do under tropical coastal forest restoration

Microbial metabolic products play a vital role in maintaining ecosystem multifunctionality, such as soil physical structure and soil organic carbon (SOC) preservation. Afforestation is an effective strategy to restore degraded land. Glomalin-related soil proteins (GRSP) and amino sugars are regarded as stable microbial-derived C, and their distribution within soil aggregates affects soil structure stability and SOC sequestration. However, the information about how afforestation affects the microbial contribution to SOC pools within aggregates is poorly understood. We assessed the accumulation and contribution of GRSP and amino sugars within soil aggregates along a restoration chronosequence (Bare land, Eucalyptus exserta plantation, native species mixed forest, and native forest) in tropical coastal terraces. Amino sugars and GRSP concentrations increased, whereas their contributions to the SOC pool decreased along the restoration chronosequence. Although microaggregates harbored greater microbial abundances, amino sugars and GRSP concentrations were not significantly affected by aggregate sizes. Interestingly, the contributions of amino sugars and GRSP to SOC pools decreased with decreasing aggregate size which might be associated with increased accumulation of plant-derived C. However, the relative change rate of GRSP was consistently greater in all restoration chronosequences than that of amino sugars. The accumulation of GRSP and amino sugars in SOC pools was closely associated with the dynamics of soil fertility and the microbial community. Our findings suggest that GRSP accumulates faster and contributes more to SOC pools during restoration than amino sugars did which was greatly affected by aggregate sizes. Afforestation substantially enhanced soil quality with native forest comprising species sequestering more SOC than the monoculture plantation did. Such information is invaluable for improving our mechanistic understanding of microbial control over SOC preservation during degraded ecosystem restoration. Our findings also show that plantations using arbuscular mycorrhizal plants can be an effective practice to sequester more soil carbon during restoration.     

毛乌素沙地杨柴灌木林恢复演替过程中土壤活性有机碳组分变化特征

土壤活性有机碳是土壤有机碳(SOC)的活性部分,是衡量土壤质量和健康状况的重要指标,能够反映植被恢复演替过程中土壤环境的早期变化。但在SOC贫瘠的沙地,长期恢复演替如何影响土壤活性有机碳组分尚不清楚。本研究以毛乌素沙地杨柴人工灌木林为研究对象,分别选取未造林(CK)与造林年限9 a、18 a和30 a的杨柴人工灌木林,探究毛乌素沙地杨柴人工灌木林恢复演替过程中土壤可溶性有机碳(DOC)、微生物量碳(MBC)、易氧化有机碳(ROC)和SOC变化规律。结果表明：(1)毛乌素沙地杨柴灌木林随恢复演替年限增加土壤固碳能力增强,但在恢复演替18 a时出现转折点,恢复演替18—30 a时土壤固碳速率相对减缓;(2)表层0—10 cm土壤DOC、MBC和ROC对恢复演替响应较为敏感,恢复演替过程中表层土壤活性有机碳各组分含量逐渐升高;(3)恢复演替年限并未对土壤活性有机碳占SOC比例产生显著影响,同时也未显著改变碳库活度。综上所述,毛乌素沙地杨柴灌木林恢复演替有助于土壤活性有机碳和SOC积累,但长期恢复演替是否持续对土壤活性有机碳固持产生积极作用仍需进一步研究。     

Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.     

Global patterns of the dynamics of soil carbon and nitrogen stocks following afforestation: a meta-analysis

? Afforestation has been proposed as an effective method of carbon (C) sequestration; however, the magnitude and direction of soil carbon accumulation following afforestation and its regulation by soil nitrogen (N) dynamics are still not well understood. ? We synthesized the results from 292 sites and carried out a meta-analysis to evaluate the dynamics of soil C and N stocks following afforestation. ? Changes in soil C and N stocks were significantly correlated and had a similar temporal pattern. Significant C and N stock increases were found 30 and 50 yr after afforestation, respectively. Before these time points, C and N stocks were either depleted or unchanged. Carbon stock increased following afforestation on cropland and pasture, and in tropical, subtropical and boreal zones. The soil N stock increased in the subtropical zone. The soil C stock increased after afforestation with hardwoods such as Eucalyptus, but did not change after afforestation with softwoods such as pine. Soil N stocks increased and decreased, respectively, after afforestation with hardwoods (excluding Eucalyptus) and pine. ? These results indicate that soil C and N stocks both increase with time after afforestation, and that C sequestration through afforestation depends on prior land use, climate and the tree species planted.     

Enhancement of Carbon Sequestration in Soil in the Temperature Grasslands of Northern China by Addition of Nitrogen and Phosphorus

Increased nitrogen (N) deposition is common worldwide. Questions of where, how, and if reactive N-input influences soil carbon (C) sequestration in terrestrial ecosystems are of great concern. To explore the potential for soil C sequestration in steppe region under N and phosphorus (P) addition, we conducted a field experiment between 2006 and 2012 in the temperate grasslands of northern China. The experiment examined 6 levels of N (0–56 g N m^-2 yr^-1), 6 levels of P (0–12.4 g P m^-2 yr^-1), and a control scenario. Our results showed that addition of both N and P enhanced soil total C storage in grasslands due to significant increases of C input from litter and roots. Compared with control plots, soil organic carbon (SOC) in the 0–100 cm soil layer varied quadratically, from 156.8 to 1352.9 g C m^-2 with N addition gradient (R² = 0.99, P < 0.001); and logarithmically, from 293.6 to 788.6 g C m^-2 with P addition gradient (R² = 0.56, P = 0.087). Soil inorganic carbon (SIC) decreased quadratically with N addition. The net C sequestration on grassland (including plant, roots, SIC, and SOC) increased linearly from -128.6 to 729.0 g C m^-2 under N addition (R² = 0.72, P = 0.023); and increased logarithmically, from 248.5 to 698 g C m^-2under P addition (R² = 0.82, P = 0.014). Our study implies that N addition has complex effects on soil carbon dynamics, and future studies of soil C sequestration on grasslands should include evaluations of both SOC and SIC under various scenarios.     

Assessment of the Soil Organic Carbon Sink in a Project for the Conversion of Farmland to Forestland: A Case Study in Zichang County,Shaanxi, China

The conversion of farmland to forestland not only changes the ecological environment but also enriches the soil with organic matter and affects the global carbon cycle. This paper reviews the influence of land use changes on the soil organic carbon sink to determine whether the Chinese “Grain-for-Green” (conversion of farmland to forestland) project increased the rate of SOC content during its implementation between 1999 and 2010 in the hilly and gully areas of the Loess Plateau in north-central China. The carbon sink was quantified, and the effects of the main species were assessed. The carbon sink increased from 2.26×10⁶ kg in 1999 to 8.32×10⁶ kg in 2010 with the sustainable growth of the converted areas. The black locust (Robinia pseudoacacia L.) and alfalfa (Medicago sativa L.) soil increased SOC content in the top soil (0–100 cm) in the initial 7-yr period, while the sequestration occurred later (>7 yr) in the 100–120 cm layer after the “Grain-for-Green” project was implemented. The carbon sink function measured for the afforested land provides evidence that the Grain-for-Green project has successfully excavated the carbon sink potential of the Shaanxi province and served as an important milestone for establishing an effective organic carbon management program.     

The impact of agricultural land use changes on soil organic carbon dynamics in the Danjiangkou Reservoir area of China

Aims

Over recent decades, a large uncultivated area has been converted to woodland and shrubland plantations to protect and restore riparian ecosystems in the Danjiangkou Reservoir area, a water source area of China’s Middle Route of the South-to-North Water Transfer Project. Besides water quality, afforestation may alter soil organic carbon (SOC) dynamics and stock in terrestrial ecosystems, but its effects remain poorly quantified and understood.

Methods

We investigated soil organic C and nitrogen (N) content, and δ ¹³C and δ ¹⁵N values of organic soil in plant root-spheres and open areas in an afforested, shrubland and adjacent cropped soil. Soil C and N recalcitrance indexes (RIC and RIN) were calculated as the ratio of unhydrolyzable C and N to total C and N.

Results

Afforestation significantly increased SOC levels in plant root-spheres with the largest accumulation of C in the afforested soil. Afforestation also increased belowground biomass. The C:N ratios in organic soil changed from low to high in the order the cropped, the shrubland and the afforested soil. The RIC in the afforested and shrubland were higher than that in cropped soil, but the RIN increased from the afforested to shrubland to cropped soil. The δ¹⁵N values of the organic soil was enriched from the afforested to shrubland to cropped soil, indicating an increased N loss from the cropped soil compared to afforested or shrubland soil. Changes in the δ¹³C ratio further revealed that the decay rate of C in the three land use types was the highest in the cropped soil.

Conclusions

Afforestation increased the SOC stocks resulted from a combination of large C input from belowground and low C losses because of decreasing soil C decomposition. Shifts in vegetation due to land use change could alter both the quantity and quality of the soil C and thus, have potential effects on ecosystem function and recovery.